1. 1
PLANT DESIGN FOR THE PRODUCTION OF 400,000
METRIC TONNES OF NITRIC ACID PER ANNUM
FROM AIR OXIDATION OF AMMONIA GAS
BY
ANDREW OFOEDU
DEPARTMENT OF CHEMICAL ENGINEERING
FEDERAL UNIVERSITY OF TECHNOLOGY, OWERRI.
SEPTEMBER 2013
2. 2
EXECUTIVE SUMMARY
This report describes the detailed design of a plant to produce 400000 tonnes of
nitric acid per year by Ostwald Process. The single pressure process was selected
as the most advantageous, having considered several factors one of which is
efficient energy management. The process begins with the vaporization of
ammonia at 1000 kPa and 35°C using process heat. Steam is then used to
superheat the ammonia up to about 80°C. Filtered air is compressed in an axial
compressor to a discharge pressure of about 740kPa and temperature of 155°C.
Part of the air is diverted for acid stripping. This preheated air and the ammonia
vapour are then mixed and passed through the platinum/rhodium catalyst gauze
in a converter for oxidation. The reaction gas flows through a series of heat
exchangers for recovery of energy as either high-pressure superheated steam, or
as shaft horsepower from the expansion of hot tail gas in the turbine. Considering
the proximity to market, sea port and source of raw materials, it was decided to
site the plant in Eleme, Rivers State. The plant’s estimated capital investment is
₦5.41 billion. The rate of return on investment is 26.25% and the payback period
is estimated to be 3 years and 7 months. Thus, the project is both technically and
economically feasible.
3. 3
TABLE OF CONTENT
Title page -----------------------------------------------------------------------------------i
Executive Summary---------------------------------------------------------------------------ii
Table of content-------------------------------------------------------------------------------iii
CHAPTER ONE
1.0 Introduction ----------------------------------------------------------------------1
1.3 Design justification-------------------------------------------------------------------3
1.4 Design Objectives---------------------------------------------------------------------4
CHAPTER TWO
2.0 Literature review------------------------------------------------------------------------5
2.1 History of Nitric acid production-------------------------------------------------------5
2.2 Ammonia oxidation chemistry----------------------------------------------------------8
2.3 Emission and Control-----------------------------------------------------------------------14
2.4 Structureand bonding---------------------------------------------------------------------15
2.5 Reactions-------------------------------------------------------------------------------------16
2.6 Uses---------------------------------------------------------------------------------------------19
2.7 Safety-------------------------------------------------------------------------------------------21
2.8 Pinch technology in modern plant------------------------------------------------------22
2.9 Plant Location ---------------------------------------------------------------------- 24
2.9.5 Plant layout ------------------------------------------------------------------------------29
2.9.6 Process routes for theproduction of nitric acid-----------------------------------33
CHAPTER THREE
3.0 Material balance --------------------------------------------------------------------42
3.1 Conservation of mass --------------------------------------------------------------42
3.2 Methods of material balancing --------------------------------------------------43
3.3 Materials balance assumptions --------------------------------------------------44
3.4 Summary of material balance calculations-------------------------------------44
3.5 Material balance for each unit ---------------------------------------------------44
4. 4
CHAPTER FOUR
4.0 Energy balance ----------------------------------------------------------------------53
4.1 Conservation of energy------------------------------------------------------------54
4.2 Energy balance assumptions -----------------------------------------------------56
4.3 Summary for energy balances ----------------------------------------------------56
CHAPTER FIVE
5.0 Chemical Engineering design--------------------------------------------------------61
5.1 Process units of Nitric acid Production--------------------------------------------61
CHAPTER SIX
6.0 Equipment design and specification --------------------------------------------66
6.1 Problem specification --------------------------------------------------------------67
6.2 Analyzing the problemsolution --------------------------------------------------68
6.3 Preliminary design-----------------------------------------------------------------------68
6.4 Material Selection-----------------------------------------------------------------------69
6.5 Design optimization---------------------------------------------------------------------69
6.6 Summary of design and equipment specification calculation---------------70
CHAPTER SEVEN
7.0 Process controland instrumentation -------------------------------------------73
7.1 Objective-----------------------------------------------------------------------------------73
7.2 Plant control instrumentation----------------------------------------------------74
7.3 Alarms and safety trips ------------------------------------------------------------77
7.4 Lining, piping, valves and pumps ------------------------------------------------78
7.5 Pipe support -------------------------------------------------------------------------81
CHAPTER EIGHT
8.0 Safety and environmentalconsiderations---------------------------------------82
8.1 Safety------------------------------------------------------------------------------------82
5. 5
8.2 Hazard and Operability (HAZOP) study-------------------------------------------89
8.3 Environmental impact assessment-------------------------------------------------97
CHAPTER NINE
9.1 Overview -----------------------------------------------------------------------------103
9.2 Economic Consideration-----------------------------------------------------------103
9.3 Cost estimation---------------------------------------------------------------------------106
9.6 Economic analyses calculation ---------------------------------------------------108
CHAPTER TEN
10.0 Start up and shut down procedure --------------------------------------------113
10.1 Emergency shutdown and emergency depressurization -----------------114
10.2 Notification -------------------------------------------------------------------------114
10.3 Record keeping --------------------------------------------------------------------115
10.4 Startup operation -----------------------------------------------------------------116
CHAPTER ELEVEN
11.0 Conclusion/Recommendation----------------------------------------------------118
11.1 Conclusion------------------------------------------------------------------------------118
11.2 Recommendation -------------------------------------------------------------------119
REFERENCES -------------------------------------------------------------------------------120
APPENDIXI
Tables and Charts--------------------------------------------------------------------------------123
APPENDIXII
Material Balance Calculation------------------------------------------------------------------126
APPENDIXIII
7. 7
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND INFORMATION
Nitric acid is a strong acid and a powerful oxidizing agent with enormous
possibilities for applications in the chemical processing industry. It has
commercial uses as a nitrating agent, oxidizing agent, solvent, activating agent,
catalyst and hydrolyzing agent. In relation to world production, approximately
65% of all nitric acid produced is used for the production of ammonium nitrate
(specifically for fertilizer manufacture).
Nitric acid is now produced commercially using the stepwise, catalytic oxidation
of ammonia with air, to obtain nitrogen monoxide and nitrogen dioxide. These
nitrogen oxides are subsequently absorbed in water to yield between 50% and
68% strength nitric acid by weight. For applications requiring higher strengths,
severalmethods of concentrating the acid are used.
The traditional methods are:
(a) Extractive distillation with dehydrating agents such as sulphuric acid or
magnesiumnitrate;
(b) Reaction with additional nitrogen oxides.
The latter technique has the greatestapplication in industry.
The chemistry of ammonia oxidation is remarkably simple with only six main
reactions that need to be considered.
8. 8
1.1.1 PROPERTIES AND USES
Nitric acid is an oxidizing mineral acid with physical and chemical properties that
make it one of the most useful inorganic minerals. It is a colorless liquid at room
temperature and atmospheric pressure. It is soluble in water in all proportions
and there is a release of heat of solution upon dilution. Its high solubility in water
is the basis for the process methods used for commercial nitric acid manufacture.
It is a strong acid that almost completely ionizes when in dilute solution. It is also
a powerful oxidizing agent with the ability to passivate some metals such as iron
and aluminum. A compilation of many of the physical and chemical properties of
nitric acid are presented in the Appendix. Arguably the most important physical
property of nitric acid is its azeotropic point, this influences the techniques
associated with strong acid production. The constant-boiling mixture occurs at
121.9°C, for a concentration of 68.4%(wt) acid at atmospheric pressure.
Nitric acid has enormously diverse applications in the chemical industry. It has
commercial uses as a nitrating agent, oxidizing agent, solvent, activating agent,
catalyst and hydrolyzing agent. The most important use is undoubtedly in the
production of ammonium nitrate for the fertilizer and explosives industries, which
accounts for approximately 65% of the world production of nitric acid.
Nitric acid has a number of other industrial applications. It is used for pickling
stainless steels, steel refining, and in the manufacture of dyes, plastics and
synthetic fibers. Most of the methods used for the recovery of uranium, such as
ion exchange and solvent extraction, usenitric acid.
An important point is that for most uses concerned with chemical production, the
acid must be concentrated above its azeotropic point to greater than 95%(wt).
9. 9
Conversely, the commercial manufacture of ammonium nitrate uses nitric acid
below its azeotropic point in the range 50 -65 %(wt.). If the stronger chemical
grade is to be produced, additional process equipment appropriate to super-
azeotropic distillation is required.
There is a potential health hazard when handling, and operating with, nitric acid.
Nitric acid is a corrosive liquid that penetrates and destroys the skin and internal
tissues. Contact can cause severe burns. The acid is a potential hazard, the various
nitrogen oxides present as product intermediates in the process are also toxic. An
assessment of the health risk must be fundamental to the design of any process.
Further consideration and recommendations for the operating health risk and
environmental impact of the plant are presented in the Appendix.
1.2 DESIGN JUSTIFICATION
At present, there is no Nitric acid plant in Nigeria. The little Nitric acid produced
mainly by fertilizer plants in the country is used up immediately by them to make
their fertilizer. This means that most of the all Nitric acid used in the country is
imported.
A Nitric acid plant sited in the country producing Nitric acid made available to the
Nigerian market will not only reduce importation of the acid but also encourage
fertilizer production, create job opportunities as well as develop the area in which
it is sited.
10. 10
1.3 DESIGN OBJECTIVES
To design a plant that will deliver 400000 metric tonnes of 60%(wt) Nitric
Acid per annum.
To determine the technical and economic feasibility of the plant.
11. 11
CHAPTER TWO
LITERATURE REVIEW
2.1 HISTORY OF NITRIC ACID PRODUCTION
Until the beginning of the 20th century, Nitric acid (HNO3), also known as aqua
fortis and spirit of niter was prepared commercially by reacting sulphuric acid
with either potassium nitrate (saltpetre) or with sodium nitrate (Chile saltpetre or
nitre). Up to four tonnes of the two ingredients were placed into large retorts and
heated over a furnace (Kirk 1996). The volatile product vapourized and was
collected for distillation. An acid of 93-95 %( wt) was produced (Gregory 1999).
In 1903 the electric-arc furnace superseded this primitive original technique. In
the arc process, nitric acid was produced directly from nitrogen and oxygen by
passing air through an electric-arc furnace (Ray 1990).
Gregory (1999, p.40) argues that ‘Although the process benefitted from an
inexhaustible supply of free feed material (air), the power consumption for the
arc furnacewas costprohibitive’
According to Ray (1989, p.8) Researchers returned to the oxidation of ammonia in
air, (recorded as early as 1798) in an effort to improve production economics. In
1901 Wilhelm Ostwald had first achieved the catalytic oxidation of ammonia over
a platinum catalyst. The gaseous nitrogen oxides produced could be easily cooled
and dissolved in water to produce a solution of nitric acid. This achievement
began the search for an economic process route.
12. 12
By 1908 the first commercial facility for production of nitric acid, using this new
catalytic oxidation process, was commissioned near Bochum in Germany (Ray et
al 1989). The Haber-Bosch ammonia synthesis process came into operation in
1913, leading to the continued development and assured future of the ammonia
oxidation process for the production of nitric acid. (Ray et al 1989)
During World War 1, the intense demand for explosives and synthetic dyestuffs
created an expansion of the nitric acid industry.
Many new plants were constructed, all of which employed the ammonia
oxidation process. This increased demand served as the impetus for several
breakthroughs in process technology.
These included:
(a) The development of chrome-steel alloys for tower construction, replacing the
heavy stoneware and acid-proof bricks. This enabled process pressures above
atmospheric levels to be used.
(b) The improved design of feed preheaters enabled higher process temperatures
to be attained. Higher temperatures improved the yields and capacities, and also
reduced equipment requirements (Ohrueet al 1999).
(c) Early developments in automatic process control improved process
performanceand reduced labor requirements.
All of these factors helped to improve the process efficiency. The increasing
availability of ammonia reduced processing costs stillfurther.
13. 13
In the late 1920’s the development of stainless steels enabled manufacturers to
use higher operating pressures. The increase in yield and lower capital
requirements easily justified the use of high pressure operation despite increased
ammonia consumption.
The introduction of higher pressure processes resulted in a divergence of
operating technique within the industry. The United States producers opted for a
high-pressure system, using a constant high pressure throughout the process. The
European manufacturers opted for a split-pressure system. This latter system
entails operating the ammonia oxidation section at atmospheric pressure, while
the absorption unit is operated at higher pressures, thus capitalizing on improved
absorption rates. (Harvin et al 1979)
Recent developments in the ammonia oxidation process have included efforts to
reduce catalyst losses in the process. Platinum recovery filters have been installed
at various stages in the process. (Ohrueetal 1999)
Gold/palladium gauze filter pads have been added on the exit side of the catalyst
bed, inside the reactor/converter units. These filters have reportedly ensured a
platinum recovery of 80% (Anon 1979). Another trend has been for the use of
additional filters in the downstream units. These filters are of alumino-silicate
construction.
Perhaps the greatest progress in nitric acid production technology has been in the
manufacture of strong nitric acid (>90% by weight). Advances in the areas of
super-azeotropic distillation and in high pressure absorption are most significant.
(Ohkubo et al 1999)
14. 14
Research work is continually being performed in an effort to reduce nitrogen
oxide emissions from nitric acid plants. The Humphreys and Glasgow/Bolme nitric
acid process is just one example of a new philosophy being applied to the
absorption systems of weak nitric acid plants (50-68% by weight). Nitrogen oxide
emissions have been reduced from 2000-5000 ppm to less than 1000 ppm (Ray et
al 1989).
For the production of stronger nitric acid, tail gases are now being treated by
selective or non-selective catalytic combustion systems. These innovative units
have reduced the nitrogen oxide emissions to below 400 ppm(Ray et al 1989).
2.2 AMMONIA OXIDATION CHEMISTRY
Notably, all commercial nitric acid production methods used today are centered
on the oxidation of ammonia. It is therefore appropriate to investigate the
chemistry of this process, in the knowledge that it is directly applicable to any of
the production processes available. (Chilton 1960)
The chemistry of the oxidation of ammonia is surprisingly simple. It begins with a
single pure compound, plus air and water, and ends with another pure compound
in aqueous solution, with essentially no by-products. The process may be
described by justsix major reactions as shown as follows:
1. 𝑁𝐻3(𝑔) + 2𝑂2 → 𝐻𝑁𝑂3(𝑎𝑞) + 𝐻2 𝑂(𝑙)
2.4𝑁𝐻3(𝑔) + 5𝑂2(𝑔) → 4𝑁𝑂(𝑔) + 6𝐻2 𝑂(𝑙)
3. 2𝑁𝑂(𝑔) + 𝑂2 → 2𝑁𝑂2(𝑔)
4. 2𝑁𝑂2(𝑔) ⇌ 𝑁2 𝑂4
15. 15
5. 3𝑁2 𝑂4 + 2𝐻2 𝑂(𝑙) → 4𝐻𝑁𝑂3 + 2𝑁𝑂(𝑔)
6. 3𝑁𝑂2(𝑔) + 𝐻2 𝑂(𝑙) → 2𝐻𝑁𝑂3(𝑎𝑞) + 𝑁𝑂(𝑔)
Reaction 1 is the overall reaction for the process. This net result is achieved from
three separate, and distinct, chemical steps. The first is the oxidation of ammonia
to nitrogen monoxide (Reaction 2). The second is the further oxidation of nitrogen
monoxide to nitrogen dioxide (Reaction 3), then nitrogen dioxide to nitrogen
tetroxide (Reaction 4). The third and final stage involves the absorption of these
nitrogen-based oxides into water to form the nitric acid product (Reactions 5 and
6). In most commercial processes, each of these three stages is conducted in
separate process units. (Chilton 1960)
The first step in the process is the heterogeneous, highly exothermic, gas-phase
catalytic reaction of ammonia with oxygen (Reaction 2). The primary oxidation of
ammonia to nitric acid (over a catalyst gauze of 9:l platinum/rhodium alloy)
proceeds rapidly at process temperatures between 900-970°C. (Kent1983)
The second step in the process involves two reactions (Reactions 3 and 4). These
are the oxidations of nitrogen monoxide to the dioxide and tetroxide forms. The
equilibrium mixture is loosely referred to as nitrogen peroxide. Both reactions are
homogenous, moderately exothermic, gas-phase catalytic reactions. All reactions
shown arehighly exothermic. (Chilton 1960)
The third step in the process involves cooling the reaction gases below their dew
point, so that a liquid phase of weak nitric acid is formed. This step effectively
promotes the state of oxidation and dimerization (Reactions 3 and 4), and
16. 16
removes water from the gas phase. This in turn increases the partial pressure of
the nitrogen peroxide component. (Chilton 1960)
Finally, nitric acid is formed by the reaction of dissolved nitrogen peroxide with
water (Reactions 5 and 6).
Nitric acid is produced by 2 methods. The first method utilizes oxidation,
condensation, and absorption to produce a weak nitric acid. Weak nitric acid can
have concentrations ranging from 30 to 70 percent nitric acid. The second
method combines dehydrating, bleaching, condensing, and absorption to produce
a high-strength nitric acid from a weak nitric acid. High-strength nitric acid
generally contains more than 90 percent nitric acid. The following text provides
more specific details for each of these processes. (Chilton 1960)
2.2.1 WEAKNITRIC ACID PRODUCTION
According to Ray(1989, Nearly all the nitric acid produced in the U. S. is
manufactured by the high-temperature catalytic oxidation of ammonia. This
process typically consists of 3 steps: (1) ammonia oxidation, (2) nitric oxide
oxidation, and (3) absorption. Each step corresponds to a distinct chemical
reaction.
1. AMMONIAOXIDATION
First, a 1:9 ammonia/air mixture is oxidized at a temperature of 1380 to 14700
F as
it passes through a catalytic convertor, according to the following reaction:
4𝑁𝐻3 + 5𝑂2 → 4𝑁𝑂 + 6𝐻2 𝑂
The most commonly used catalyst is made of 90 percent platinum and 10 percent
rhodium gauze constructed fromsquares of fine wire. Under these conditions, the
oxidation of ammonia to nitric oxide (NO) proceeds in an exothermic reaction
17. 17
with a range of 93 to 98 percent yield. Oxidation temperatures can vary from
1380O
F to 16500
F. (Chilton 1960) Higher catalyst temperatures increase reaction
selectivity toward NO production. Lower catalyst temperatures tend to be more
selective toward less usefulproducts: nitrogen (N2) and nitrous oxide (N2O).
Nitric oxide is considered to be a criteria pollutant and nitrous oxide is known to
be a global warming gas. The nitrogen dioxide/dimmer mixture then passes
through a waste heat boiler and a platinum filter. (Chilton 1960)
2. NITRIC OXIDE OXIDATION
The nitric oxide formed during the ammonia oxidation must be oxidized. The
process stream is passed through a cooler/condenser and cooled to 1000
F or less
at pressures up to 116 pounds per square inch absolute (psia). The nitric oxide
reacts non-catalytically with residual oxygen to form nitrogen dioxide (NO2) and
its liquid dimmer, nitrogen tetra-oxide:
2𝑁𝑂2 + 𝑂2 → 2𝑁𝑂2 ⇌ 𝑁2 𝑂4
This slow, homogeneous reaction is highly temperature and pressure dependent.
Operating at low temperatures and high pressures promotes maximum
production of NO2 within a minimum reaction time (Kent 1983).
3. ABSORPTION
The final step introduces the nitrogen dioxide/dimmer mixture into an absorption
process after being cooled. The mixture is pumped into the bottom of the
absorption tower, while liquid dinitrogen tetra-oxide is added at a higher point.
De-ionized process water enters the top of the column. Both liquids flow
countercurrent to the nitrogen dioxide/dimmer gas mixture. Oxidation takes
place in the free space between the trays, while absorption occurs on the trays.
18. 18
The absorption trays are usually sieve or bubble cap trays. The exothermic
reaction occurs as follows:
3𝑁𝑂2 + 𝐻2 𝑂 → 2𝐻𝑁𝑂3 + 𝑁𝑂
A secondary air stream is introduced into the column to re-oxidize the NO that is
formed in Reaction 3. This secondary air also removes NO2 from the product acid.
An aqueous solution of 55 to 65 percent (typically) nitric acid is withdrawn from
the bottom of the tower. The acid concentration can vary from 30 to 70 percent
nitric acid. The acid concentration depends upon the temperature, pressure,
number of absorption stages, and concentration of nitrogen oxides entering the
absorber.
There are 2 basic types of systems used to produce weak nitric acid: single-stage
pressure process and dual-stage pressure process (Harvin et al 1979). In the past,
nitric acid plants have been operated at a single pressure, ranging from
atmospheric pressure to 14.7 to 203 psia. However, since Reaction 1 is favored by
low pressures and Reactions 2 and 3 are favored by higher pressures, newer
plants tend to operate a dual stage pressure system, incorporating a compressor
between the ammonia oxidizer and the condenser. The oxidation reaction is
carried out at pressures from slightly negative to about 58 psia, and the
absorption reactions are carried out at 116 to 203 psia. (Harvn et al 1979)
In the dual-stage pressure system, the nitric acid formed in the absorber
(bottoms) is usually sent to an external bleacher where air is used to remove
(bleach) any dissolved oxides of nitrogen. The bleacher gases are then
compressed and passed through the absorber. The absorber tail gas (distillate) is
sent to an entrainment separator for acid mist removal. Next, the tail gas is
reheated in the ammonia oxidation heat exchanger to approximately 3920
F. The
19. 19
final step expands the gas in the power-recovery turbine. The thermal energy
produced in this turbine can be used to drive the compressor.
2.2.2 HIGH STRENGTH NITRIC ACID PRODUCTION
A high-strength nitric acid (98 to 99 percent concentration) can be obtained by
concentrating the weak nitric acid (30 to 70 percent concentration) using
extractive distillation. (Imai et al 1999) The weak nitric acid cannot be
concentrated by simple fractional distillation. The distillation must be carried out
in the presence of a dehydrating agent. Concentrated sulfuric acid (typically 60
percent sulfuric acid) is most commonly used for this purpose. The nitric acid
concentration process consists of feeding strong sulfuric acid and 55 to 65 percent
nitric acid to the top of a packed dehydrating column at approximately
atmospheric pressure. The acid mixture flow downward, countercurrent to
ascending vapors. Concentrated nitric acid leaves the top of the column as 99
percent vapor, containing a small amount of NO2 and oxygen (O2) resulting from
dissociation of nitric acid. The concentrated acid vapor leaves the column and
goes to a bleacher and a countercurrent condenser system to effect the
condensation of strong nitric acid and the separation of oxygen and oxides of
nitrogen (NO2) byproducts. (Ohkubo et al 1999) These byproducts then flow to an
absorption column where the nitric oxide mixes with auxiliary air to form NO2,
which is recovered as weak nitric acid. Inert and un-reacted gases are vented to
the atmosphere from the top of the absorption column. Emissions from this
process are relatively minor. A small absorber can be used to recover NO2. (Kirk et
al 1981)
20. 20
2.3 EMISSIONS AND CONTROL
Emissions from nitric acid manufacture consist primarily of NO, NO2 (which
account for visible emissions), trace amounts of HNO3 mist, and ammonia (NH3).
By far, the major source of nitrogen oxides (NO2) is the tail-gas from the acid
absorption tower. In general, the quantity of NO2 emissions is directly related to
the kinetics of the nitric acid formation reaction and absorption tower design. NO2
emissions can increase when there is (1) insufficient air supply to the oxidizer and
absorber, (2) low pressure, especially in the absorber, (3) high temperatures in
the cooler-condenser and absorber, (4) production of an excessively high-strength
product acid, (5) operation at high throughput rates, and (6) faulty equipment
such as compressors or pumps that lead to lower pressures and leaks, and
decrease plant efficiency. (Leray et al 1979)
Roudier (1979) states that the two most common techniques used to control
absorption tower tail gas emissions are extended absorption and catalytic
reduction. Extended absorption reduces NO2 emissions by increasing the
efficiency of the existing process absorption tower or incorporating an additional
absorption tower. An efficiency increase is achieved by increasing the number of
absorber trays, operating the absorber at higher pressures, or cooling the weak
acid liquid in the absorber. The existing tower can also be replaced with a single
tower of a larger diameter and/or additional trays.
In the catalytic reduction process (often termed catalytic oxidation or
incineration), tail gases from the absorption tower are heated to ignition
temperature, mixed with fuel (natural gas, hydrogen, propane, butane, naphtha,
carbon monoxide, or ammonia) and passed over a catalyst bed. In the presence of
the catalyst, the fuels are oxidized and the NO2 are reduced to N2. The extent of
21. 21
reduction of NO2 and NO to N2 is a function of plant design, fuel type, operating
temperature and pressure. Space-velocity through the comparatively small
amounts of nitrogen oxides is also lost from acid concentrating plants. These
losses (mostly NO2) are from the condenser system, but the emissions are small
enough to be controlled easily by inexpensive absorbers. Acid mist emissions do
not occur from the tail-gas of a properly operated plant. The small amounts that
may be present in the absorber exit gas streams are removed by a separator or
collector prior to entering the catalytic reduction unit or expander. (Kent 1983)
The acid production system and storage tanks are the only significant sources of
visible emissions at most nitric acid plants. Emissions from acid storage tanks may
occur during tank filling.
2.4 STRUCTURE AND BONDING
Fig 2: Two major resonancerepresentations of HNO3.
The molecule is planar. Two of the N-O bonds are equivalent and relatively short
(this can be explained by theories of resonance. The canonical forms show double
bond character in these two bonds, causing them to be shorter than typical N-O
bonds.), and the third N-O bond is elongated because the O is also attached to a
proton.
22. 22
2.5 REACTIONS
2.5.1 ACID-BASEPROPERTIES
Nitric acid is normally considered to be a strong acid at ambient temperatures.
The pKa value is usually reported as less than −1. This means that the nitric acid in
solution is fully dissociated except in extremely acidic solutions. The pKa value
rises to 1 at a temperature of 250 °C.
Nitric acid can act as a basewith respect to an acid such as sulfuric acid.
HNO3 + 2H2SO4 NO2
+
+ H3O+
+ 2HSO4
–
The nitronium ion, NO2
+
, is the active reagent in aromatic nitration reactions.
Since nitric acid has both acidic and basic properties it can undergo an
autoprotolysis reaction, similar to the self-ionization of water
2HNO3 NO2
+
+ NO3
–
+ H2O
2.5.2 REACTIONS WITH METALS
Nitric acid reacts with most metals but the details depend on the concentration of
the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in
its reaction with most metals. Magnesium, manganese and zinc liberate H2.
Others give the nitrogen oxides. (Ababio 2007)
Nitric acid can oxidize non-active metals such as copper and silver. With these
non-active or less electropositive metals the products depend on temperature
and the acid concentration. For example, copper reacts with dilute nitric acid at
23. 23
ambient temperatures with a 3:8 stoichiometry to produce nitric oxidewhich may
react with atmospheric oxygen to give nitrogen dioxide.
3 Cu + 8 HNO3 → 3 Cu2+
+ 2 NO + 4 H2O + 6 NO3
-
With more concentrated nitric acid, nitrogen dioxide is produced directly in a
reaction with 1:4 stoichiometries.
Cu + 4 H+
+ 2 NO3
−
→ Cu2+
+ 2 NO2 + 2 H2O
Upon reaction with nitric acid, most metals give the corresponding nitrates. Some
metalloids and metals give the oxides, for instance, Sn, As, Sb, Ti are oxidized into
SnO2, As2O5, Sb2O5 and TiO2 respectively.
Some precious metals, such as pure gold and platinum group metals do not react
with nitric acid, though pure gold does react with aqua regia, a mixture of
concentrated nitric acid and hydrochloric acid. However, some less noble metals
(Ag, Cu, ...) present in some gold alloys relatively poor in gold such as colored gold
can be easily oxidized and dissolved by nitric acid, leading to color changes of the
gold-alloy surface. Nitric acid is used as a cheap means in jewelry shops to quickly
spotlow-gold alloys (< 14 carats) and to rapidly assess the gold purity.
Being a powerful oxidizing agent, nitric acid reacts violently with many non-
metallic compounds and the reactions may be explosive. Reaction takes place
with all metals except the noble metals series and certain alloys. As a general rule,
oxidizing reactions occur primarily with the concentrated acid, favoring the
formation of nitrogen dioxide (NO2). (Ababio 2007) However, the powerful
oxidizing properties of nitric acid are thermodynamic in nature, but sometimes its
24. 24
oxidation reactions are rather kinetically non-favored. The presence of small
amounts of nitrous acid (HNO2) greatly enhances the rate of reaction.
Although chromium (Cr), iron (Fe) and aluminum (Al) readily dissolve in dilute
nitric acid, the concentrated acid forms a metal oxide layer that protects the bulk
of the metal from further oxidation. The formation of this protective layer is
called passivation. Typical passivation concentrations range from 20–50% by
volume (ASTM A967-05 2000). Metals which are passivated by concentrated nitric
acid are Iron, Cobalt, Chromium, Nickel, and Aluminum.
2.5.3 REACTIONS WITH NON-METALS
Being a powerful oxidizing acid, nitric acid reacts violently with many organic
materials and the reactions may be explosive. (Kent 1983)
Reaction with non-metallic elements, with the exceptions of nitrogen, oxygen,
noble gases, silicon and halogens, usually oxidizes them to their highest oxidation
states as acids with the formation of nitrogen dioxide for concentrated acid and
nitric oxide for dilute acid. (Ababio 2007)
C + 4 HNO3 → CO2 + 4 NO2 + 2 H2O
OR
3 C + 4 HNO3 → 3 CO2 + 4 NO + 2 H2O
Concentrated nitric acid oxidizes I2, P4 and S8 into HIO3, H3PO4 and H2SO4
respectively.
25. 25
2.5.4 XANTHOPROTEIC TEST
Nitric acid reacts with proteins to form yellow nitrated products. This reaction is
known as the xanthoproteic reaction (Gregory 1999). This test is carried out by
adding concentrated nitric acid to the substance being tested, and then heating
the mixture. If proteins that contain amino acids with aromatic rings are present,
the mixture turns yellow. Upon adding a strong base such as liquid ammonia, the
color turns orange. These color changes are caused by nitrated aromatic rings in
the protein. Xanthoproteic acid is formed when the acid contacts epithelial cells
and is indicative of inadequate safety precautions when handling nitric acid
2.6 USES
2.6.1 NITRIC ACID IN A LABORATORY.
The main use of nitric acid is for the production of fertilizers. Nitric acid is
neutralized with ammonia to give ammonium nitrate. According to Gregory
(1999, p.408) this application consumes 75-80% of the 26M tons produced
annually. The other main applications are for the production of explosives, nylon
precursors, and specialty organic compounds.
2.6.2 PRECURSOR TO ORGANIC NITROGEN COMPOUNDS
In organic synthesis, industrial and otherwise, the nitro group is a versatile
functionality. Most derivatives of aniline are prepared via nitration of aromatic
compounds followed by reduction. Nitrations entail combining nitric and sulfuric
acids to generate the nitronium ion, which electrophilically reacts with aromatic
26. 26
compounds such as benzene. (Gregory 1999) Many explosives, e.g. TNT, are
prepared in this way.
The precursor to nylon, adipic acid, is produced on a large scale by oxidation of
cyclohexanone and cyclohexanol with nitric acid.
1.6.3 ROCKET FUEL
Nitric acid has been used in various forms as the oxidizer in liquid-fueled rockets.
These forms include red fuming nitric acid, white fuming nitric acid, mixtures with
sulfuric acid, and these forms with HF inhibitor. IRFNA (inhibited red fuming nitric
acid) was one of 3 liquid fuel components for the BOMARC missile. (Gregory
1999)
2.6.4 ANALYTICAL REAGENT
In elemental analysis dilute nitric acid (0.5 to 5.0%) is used as a matrix compound
for determining metal traces in solutions. Ultrapure trace metal grade acid is
required for such determination, because small amounts of metal ions could
affect the resultof the analysis. (Kirk 1981)
It is also typically used in the digestion process of turbid water samples, sludge
samples, solid samples as well as other types of unique samples which require
elemental analysis via flame atomic absorption spectroscopy. Typically these
digestions use a 50% solution of the purchased HNO3 mixed with deionized water.
In electrochemistry, nitric acid is used as a chemical doping agent for organic
semiconductors, and in purification processes for raw carbon nanotubes.
27. 27
2.6.5 WOODWORKING
In a low concentration (approximately 10%), nitric acid is often used to artificially
age pine and maple. The color produced is a grey-gold very much like very old wax
or oil finished wood (wood finishing).
2.6.6 ETCHANTAND CLEANING AGENT
The corrosive effects of nitric acid are exploited for a number of specialty
applications, such as pickling stainless steel. A solution of nitric acid, water and
alcohol, Nital, is used for etching of metals to reveal the microstructure (Gregory
1999). Commercially available aqueous blends of 5–30% nitric acid and 15–40%
phosphoric acid are commonly used for cleaning food and dairy equipment
primarily to remove precipitated calcium and magnesium compounds (either
deposited from the process stream or resulting from the use of hard water during
production and cleaning). The phosphoric acid content helps to passivate ferrous
alloys against corrosion by the dilute nitric acid.(Anon 1979) Nitric acid can be
used as a spot test for alkaloids, giving a variety of colors depending on the
alkaloid.
2.7 SAFETY
Nitric acid is a strong acid and a powerful oxidizing agent. The major hazard posed
by it is chemical burns as it carries out acid hydrolysis with proteins (amide) and
fats (ester) which consequently decomposes living tissue (e.g. skin and flesh).
Concentrated nitric acid stains human skin yellow due to its reaction with the
28. 28
keratin. These yellow stains turn orange when neutralized. Systemic effects are
unlikely, however, and the substanceis not considered a carcinogen or mutagen.
The standard first aid treatment for acid spills on the skin is, as for other corrosive
agents, irrigation with large quantities of water. Washing is continued for at least
ten to fifteen minutes to cool the tissue surrounding the acid burn and to prevent
secondary damage. Contaminated clothing is removed immediately and the
underlying skin washed thoroughly. (Othmer et al 1981)
Being a strong oxidizing agent, reactions of nitric acid with compounds such as
cyanides, carbides, metallic powders can be explosive and those with many
organic compounds, such as turpentine, are violent and hypergolic (i.e. self-
igniting). Hence, it should be stored away from bases and organics.
2.8 PINCH TECHNOLOGY IN MODERN PLANTS
One of the most successful and generally useful techniques is that developed by
Bodo Linnhoff and other workers: pinch technology. The term derives from the
fact that in a plot of the system temperatures versus the heat transferred, a pinch
usually occurs between the hot stream and cold stream curves. (Sinnot 2005)
Pinch technology is a relatively modern engineering tool developed in the late
1970s and early 1980s. This new approach to evaluating the energy requirements
of a site quickly identified ways of improving the overall energy use. The name
“pinch technology” was applied because the technique identified the point or
points in the energy flow where restrictions applied and hence limited one’s
ability to reuselow grade energy.
29. 29
The major difference between this new technology and the previous engineering
approaches was the formalized methodology involving the rigorous application of
thermodynamic principles. Pinch technology was initially adopted by major
chemical companies and petrochemical energy. Beet sugar was quite quick to
adopt it because of the industry’s energy profile and it is now being adopted by
the cane industry too. It has also been shown that the pinch represents a distinct
thermodynamic break in the system and that, for minimum energy requirements,
heat should not be transferred across thepinch, (Linnhoff et al 1983)
2.8.1 APPLICATIONS
Pinch technology is equally applicable to Greenfield project and refurbishments.
In either case, their objectives are to achieve:
1. Minimum energy consumption
2. Optimization of utilities
3. Minimum capital expenditure to achieve these
Minimizing energy consumption implies minimizing cooling water requirements
too because all of the energy used ultimately has to be rejected again in some low
grade form. ( Sinnot 2005)
The technology strength are its overall approach to process integration (rather
than optimizing a single station) and its blend of thermodynamics with
commercial requirements. It also takes into account the operational requirements
of the site and does reduce flexibility or availability.
30. 30
2.9 PLANT LOCATION
Plant location refers to the choice of a region or the selection of a particular site
for settling up the business or a factory. However, the choice is made only after
considering alternative sites. It is a strategic decision that cannot be changed once
it is taken. Therefore, careful care must be taken before a decision is made on the
location of the plant site (Ray et al 1989).
2.9.1 IDEAL PLANTLOCATION
An ideal plant location is one where the cost of the production is minimal, with a
large market availability, least risk involved and maximum gain obtainable. It is a
place of maximum net advantage or with lowest unit cost of production and
distribution. For achieving this objective, small and large scale entrepreneur can
make useof local analysis.
2.9.2 LOCAL ANALYSIS
Local analysis is a dynamic process where the entrepreneur analyses and
compares the feasibility of different sites with the aim of selecting the best site
for a given enterprise. Itconsiders the following:
a. Demographic analysis: it involves the study of the population in the area in
terms of total number of people in the area, age composition, per capital
income, educational level and occupational structures etc.
b. Trade area analysis: it is an analysis of the geographic area that provides
continued clientele to the industry. It is advisable to also see the feasibility
of accessing the trade area fromalternative sites. (Ray et al 1989)
31. 31
c. Competitive analysis: it helps to judge the nature, location, size and quality
of competition in a given trade area.
d. Traffic analysis: this is done to have a rough idea about the number of
potential customers passing by the proposed site during the working hours
of the industry. The traffic analysis aims at judging the alternative sites in
terms of pedestrian and vehicular traffic passing by the site.
e. Site economics: alternative sites are evaluated in terms of establishments,
costs and operational costs under this. Cost of establishment of a plant is
basically cost incurred for permanent physical facilities but operation costs
are incurred for running the plant.
2.9.3 SELECTION CRITERIA
According to Ray (1989, p. 76) the importantconsiderations for selecting a
suitable location are as follows:
I. Nature or climate conditions
II. Availability and nearness to the sources of raw materials
III. Transport costs: this should be considered both for obtaining raw
material and also distribution or marketing finished products to the
ultimate users.
IV. Close proximity to the anticipated market: the industry’s warehouse
should be located within the vicinity of densely populated areas.
V. Availability of infrastructural facilities such as developed industrial shed
or site, link roads, nearness to railway stations, airports or seaports,
32. 32
availability of electricity, water, public utilities, civil amenities and means
of communication are important.
VI. Availability of skilled and non-skilled labor and technically qualified and
trained managers.
VII. Banking and financial institutions should be located nearby.
VIII. Safety and security should be given due consideration
IX. Government influences: tax relief, subsidies, liberation and other
positive policies of the government to support the start off of any
industry should be duly considered before any industry is set up. Also,
negative government influences like restrictions for setting up industries
in an area for reason of pollution control and decentralization of
industries should be considered.
X. Utility costs and availability.
2.9.4 SELECTION OF PLANTLOCATION FOR THENITRIC ACID PLANT
There were three plant locations proposed. Each was evaluated and the final
decision based on maximum net advantagewas made.
2.9.4.1 LOCATION ONE:AGBARA INDUSTRIAL ESTATE(OGUN STATE)
Advantages
1. Relatively cheap available land and labor cost.
2. Relatively close to market (Lagos Nylon and plastic market).
3. Relatively close to sea (Lagos Apapa) for import of raw material and export
of productif need be.
4. Availability of infrastructuralfacilities such as link roads, public utilities etc.
33. 33
5. Availability of financial institution.
6. Relatively secure.
7. Availability of social amenities and means of communication.
8. Disadvantages
1. No local source of raw material nearby meaning all raw materials have to
be transported to the plant location.
2. The major roads that will be used for transportation (i.e form Apapa to
Agbara) are bad and one is prone to experience hold up on it.
3. Transport cost will be very high for both bringing in of raw material and
marketing finished product as the target market is Lagos and things are
known to be very expensive there.
4. The Nylon and plastic market in Lagos is not large enough to exhaust all
nitric acid produced by the plant.
5. Additional cost of providing water and electricity for the plant.
2.9.4.2 LOCATION TWO:ABA (ABIA STATE)
Advantages
1. Relatively cheap available land and labor cost.
2. Availability of market (plastic and Nylon market)
3. Availability of financial institution.
4. Relatively secure.
5. Availability of social amenities and means of communication.
34. 34
Disadvantages
1. Not close to sourceof raw material
2. Additional cost of providing water and electricity for the plant.
3. Market available not enough to exhaust all nitric acid produced in the plant.
4. Lack of infrastructural facilities such as sea port, airport and railway stations
nearby.
2.9.4.3 LOCATION THREE:ELEME, PORT-HARCOURT(RIVERS STATE)
Advantages
1. Close to source of raw material: National Fertilizer Company of Nigeria
(NAFCON), an ammonia and fertilizer plant at Onne, Port-Harcourt, Rivers
State bought over by Notore started operation in Jan 2009. Their
production of ammonia per day of ammonia was 1,000MT as at 2009 of
anhydrous ammonia (more than enough raw material for our nitric acid
plant). Eleme Petrochemical located in Eleme, Port-harcourt, Rivers State is
also billed to come up with an ammonia plant in 2014 which will make
available to the Nigeria market2300MT.
2. Availability of market in Port-Harcourt, closeness to sea for export of
productif necessary.
3. Availability of public utilities such as water, sea port, airport, etc.
4. Availability of both skilled and unskilled labor.
5. Availability of banking and financial institutions.
6. Availability of social amenities and means of communication.
7. Relatively secure.
35. 35
Disadvantages
2. High costof land
3. No regular power supply
2.9.5 PLANT LAYOUT
Having selected a suitable site for the chemical plant, it is possible and necessary
to make a preliminary decision regarding the layout of the plant equipment. (Ray
et al 1989) Although the equipment has not been designed in detail, preliminary
estimates of the physical size of each item should be available in the equipment
list. Any sizing differences between the initial and final estimates should not be
too excessive, and appropriate areas should be allowed around the plant items
when determining the layout.
A preliminary determination of the plant layout enables consideration of pipe
runs and pressure drops, access for maintenance and repair and in the event of
accidents and spills, and location of the control room and administrative offices.
The preliminary plant layout can also help to identify undesirable and unforeseen
problems with the preferred site, and may necessitate a revision of the site
selection. (Baasel 1989) The proposed plant layout must be considered early in
the design work, and in sufficient detail, to ensure economical construction and
efficient operation of the completed plant. The plant layout adopted also affects
the safe operation of the plant, and acceptance of the plant (and possibly any
subsequentmodifications or extensions) by the community.
36. 36
There are two schemes that can be adopted for determination of the plant layout.
(Buckhurst & Harker 1973) First, the ‘flow-through’ layout (or ‘flow-line’ pattern)
where plant items are arranged (sequentially) in the order in which they appear
on the process flow sheet. This type of arrangement usually minimizes pipe runs
and pressure drops (and is often adopted for small plants). Second, the
equipment is located on site in groupings of similar plant items, e.g. distillation
columns, separation stages, reactors and heat exchanger pre-heaters, etc. The
grouped pattern is often used for larger plants and has the advantages of easier
operation and maintenance, lower labor costs, minimizing transfer lines and
hence reducing the energy required to transfer materials. These two schemes
represent the extreme situations and in practice some compromise arrangement
is usually employed. The plant layout adopted depends upon whether a new
(‘grass roots’) plant is being designed or an extension/modification to an existing
plant. Space restrictions are the most common constraints; however, space
limitations are usually imposed even with new sites. Other factors to be
considered are:
(a) Siting of the control room, offices, etc., away from areas of high accident risk,
and upstreamof the prevailing winds.
(b) Location of reactors, boilers, etc., away fromchemical storage tanks.
(c) Storage tanks to be located for easy access, and a decision made as to whether
all tanks (for raw materials and product) should be located together or dispersed
around the site.
(d) Labor required for plant operation.
37. 37
(e) Elevation of equipment.
(f) Requirements of specific plant items, e.g. pumps.
(g) Supply of utilities, e.g. electricity, water, steam, etc.
(h) Minimizing plant piping systems.
(i) Suitable access to equipment requiring regular maintenance or repair.
(j) Plant layout to facilitate easy clean-up operations and dispersion of chemicals
in the event of a spillage.
(k) Access to the plant in the event of an accident.
(1) Siting of equipment requiring cooling water close to rivers, estuaries, etc.
(m) Location of plant waste and water drainage systems (separate or combined?)
and treatment tanks.
(n) Adopting a plant layoutthat will act to contain any fires or explosions.
(o) Spacing between items of equipment (insurance companies specializing in the
insurance of chemical plants have specific recommendations for the distances
required between particular items of equipment).
The layout of plant equipment should aim to minimize:
(i) damage to persons and property due to fire or explosion;
(ii) Maintenance costs;
(iii) Number of plant personnel;
38. 38
(iv) Operating costs; construction costs;
(v) Cost of plant expansion or modifications.
Some of these aims are conflicting, e.g. (i) and (iv), and compromises are usually
required when considering the plant layout to ensure that safety and economic
operation are both preserved. The final plant layout will depend upon the
measures for energy conservation within the plant and any subsequent
modifications, and the associated piping arrangements.
The process units and ancillary buildings are laid out in such a way to give the
most economical flow of materials and personnel around the site. Hazardous
processes are located a safe distance from other buildings. Consideration for
future expansion is also put in place. The ancillary buildings and service required
on the site include:
Administrativeblock
Laboratory
Storagefor both raw materials and products
Maintenance workshop
Utilities (generator, steam boiler, transformer station)
Store for maintenance and operation supplies
Other amenities like car park, restaurantand clinic.
39. 39
Tank Farm
Waste
Incinerator
Roads
Plant Area
Expansion
Utilities
Fire Station
Workshop
Stores
Emergency
Water
Canteen
Car Pack
Laboratory
Offices
Auditorium
Medical
Center
Roads
Fig 1.1 Expected plant layout.
2.9.6 PROCESS ROUTES FOR THE PRODUCTION OF NITRIC ACID
CHILE SALTPETRE/NITRATEPROCESS
Chile saltpetre is material which contains sodium nitrate NaNO3 with percentage
around 35-60%, and remaining percentage compounds with KNO3 and NaCl. This
raw material Chile saltpetre is concentrated by crystallization in pre-treatment of
ore to attain 95% NaNO3 and remaining KNO3 as feed raw material. (Kent 1983)
Sulphuric acid with 93% is mixed with the refined Chile saltpetre as per the ratio
required as per stoichiometry and sent into a retort which is made with cast iron
and the mixture is heated to 200o
C with help of furnace flue gasses and coal fire.
40. 40
Thus at this temperature, the following reaction is carried forward to produce
HNO3, nitric acid vapors.
NaNO3 + H2SO4 →NaHSO4 + HNO3
All hot vapors of nitric acid are sent to cool down in water circulated cooled silica
pipes, condensed HNO3 are collected in receiver which has material resistance to
nitric acid. Uncondensed gas which escapes from the collector is scrubbed with
cooled water in packed bed tower to collect nitric acid in dilute format. Liquid
sodiumbi-sulphate is collected fromthe bottom outlet of the retort.
Advantage: it was one of the firstmethods used in the manufactureof nitric acid.
Disadvantage: sourceof raw material can be exhausted.
Fig 1.2: Manufactureof nitric acid fromChile Saltpetre.
41. 41
BIRKELAND-EYDEPROCESS/ARC PROCESS
This process is based upon the oxidation of atmospheric nitrogen by atmospheric
oxygen to nitric oxide at very high temperature. An electric arc is used to provide
the high temperatures, and yields of up to 4% nitric oxide were obtained. ( Ohrue
1999)
N2 + O2 →2NO
The nitric oxide was cooled and oxidized by the remaining atmospheric oxygen to
nitrogen dioxide
2 NO + O2 →2NO2
This nitrogen dioxide is then dissolved in water to give dilute nitric acid.
3 NO2 + H2O → 2HNO3 + NO
Advantage: unlimited sourceof raw material (air)
Disadvantage: The process is very energy intensive and is only feasible when
electricity is available and cheap.
WINSCONSIN PROCESS/NITROGENFIXATION PROCESS
Atmospheric oxygen and nitrogen are combined in a high temperature
regenerative furnace operating at about 2000o
C. Nitric oxide is formed with a
yield of nearly 2%.
Advantage: it does not use electricity to provide the high temperature and
therefore does not have the disadvantageof the Birkeland-Eyed process.
42. 42
Disadvantage: cannotcompete favorably with the Ostwald process.
Another method of production of nitric acid via nitrogen fixation is the nuclear
nitrogen fixation route. This method directly combines oxygen and nitrogen.
Yields of nitrogen oxide of 5-15% have been reported by exposing air at 150 and
400o
F to radiation fromUranium235.
Advantage: gives a greater yield of nitrogen oxide than the Winsconsin process
Disadvantage: with this method comes all the disadvantages of nuclear reaction
(problemof managing the radiation which is harmful to living things)
OSTWALD PROCESS
In this process, anhydrous ammonia is oxidized to nitric oxide, in the presence of
platinum or rhodium gauge catalyst at high temperature of about 500K and a
pressureof 9bar. (Ray et al 1989)
4 NH3 (g) + 5 O2 (g) →4 NO (g) + 6 H2O (g) (∆H=-905.2KJ)
Nitric acid is then reacted with oxygen in air to formnitrogen dioxide.
2 NO (g) + O2 (g) → 2NO2 (g) (∆H=-114KJ/mol)
This is subsequently absorbed in water to formnitric acid and nitric oxide
3 NO2 (g) + H2O (l) →2 HNO3 (aq) +NO (g) (∆H=-117KJ/mol)
The nitric oxide is cycled back for re-oxidation. Alternately, if the last step is
carried out in air:
4 NO2 (g) + O2 (g) + 2H2O (l) → 4HNO3 (aq)
43. 43
The aqueous HNO3 obtained can be concentrated by distillation up to about 68%
by mass.
There are 2 basic types of systems used to produceweak nitric acid:
Both processes follow the basic Ostwald process for the catalytic oxidation of
ammonia. In summary, this involves an oxidation stage whereby ammonia is
reacted with air in a catalytic converter at temperatures in the range of 850-
950°C. Reaction gases pass through a series of energy recovery stages before
entering an absorption column. The bottoms from the column are bleached of
dissolved nitrogen peroxide using air, and the resulting solution is the weak nitric
acid product(Roudier et al 1979).
The major difference between the two processes lies in the initial conversion
stage. The dual-pressure process employs a conversion stage operating in the
range l00-350kPa, and a reactor temperature of about 865°C. The single-pressure
process however operates the converter at 800-1100 kPa, with a reactor
temperature closer to 940°C. ( Harvin et al 1979)
1. Single-stage pressure process: in this case, the plant is operated at a single
pressurethroughout.
44. 44
Fig 1.3. Process flow diagramfor single-stagepressureprocess.
Advantage:
Less expensive as less equipment’s are used.
The single-pressure process uses a higher ammonia conversion pressure.
This higher pressure provides advantages in terms of equipment design,
e.g. smaller converter dimensions and a single heat-exchanger-train
layout.( Leray et al 1979)
45. 45
The higher temperature and the favorable pressure both increase the
energy recovery fromthe process.
Limited spaceavailability may favor the single-pressureprocess
Disadvantage:
Less efficient as the overall process is favored by varying pressure.
Experimental work indicates that the rate loss of catalyst (without a catalyst
recovery system) is approximately three times more rapid at 973°C than at
866°C. This means that more catalyst is lost in the single-stage pressure
process ( Harvin et al 1979).
Absorber efficiency is reduced prompting the need for larger absorber
thereby increasing cost.
2. Dual-stage pressure process: here, the plant is operated at different
pressures and differentstages.
Advantages:
The first reaction (catalytic conversion of anhydrous ammonia to nitric
oxide) is favored by lower pressure while the remaining reactions are
favored by higher pressures. This variation in pressure is achieved in dual-
stage pressureprocess. (Harvin etal 1979)
Capacities of 1130-1360 tonnes per day favor the dual-pressure process,
because of the possibility of absorption up to 1550 KPa.
Less catalyst is lost because of lower operating temperature
47. 47
The process selected for this design of nitric acid is single-stage pressure Ostwald
process becauseof its abovementioned advantages.
FILTER
COMPRESSOR
MIXER
0-100
deg
mV
10-50
CONVERTER
NH3 SUPERHEATER
NH3 VAPORISER
ABSOBER
PURIFIER
COMBUSTION CHAMBER
FILTER
TURBINE
E-27
CHILLED WATER
REFRIGERATION
OXIDATION VESSEL
STEAM
CW
HOT AIR
ATMOSPHERIC AIR
E-30
CW
BOILER
CW
CW
STEAM
E-33
WATER
DEIONISER
STEAM
CW
DESOBED
NITROUS ACID
CW
FILTER
LIQUID AMMONIA
STEAM
FOR SALE WEAK AMMONIA
SOLUTION
CW
STEAM AIR
CONDEN-
SATE
STEAM
E-35
NITRIC ACID
STRIPPER
STEAM
SAGK
GAS HEATER
CW
WASTE
HEAT BOILER
WASTE HEAT
BOILER
AIR HEATER
Fig1.5: Selected Process flow diagramfor Nitric acid plant.
48. 48
CHAPTER THREE
MATERIAL BALANCE
Material balance is one of the most important components of a process design.
Overall raw material of the entire process determines the qualities of raw
materials required and the products produced in the process.
Balance over individual process units determines the process stream flows and
their compositions and also the sizes of the various process equipment used in
the process.
Material balance on the plant used in the production of 400000 tonnes of Nitric
acid per year.
Mass flow rate = 400000 x1000
𝑘𝑔
𝑦𝑒𝑎𝑟⁄ =50000
𝑘𝑔
ℎ𝑟⁄
3.1 CONSERVATION OF MASS
For a steady state process, the accumulation term will be zero; but if a chemical
reaction takes place, particular chemical specie may be formed or consumed in
the process. When there is chemical reaction, the material balance equation is
given as,
Input+ Generation = Output+ Consumption
If there is no chemical reaction, the steady state balance reduces to;
Input= Output
49. 49
A balance equation can be written for any identifiable specie present, elements or
compound; and for the total material.
3.2 METHODS OF MATERIAL BALANCE
There are two basic methods of material balance and they are;
(a) Algebraic Method
The algebraic method of material balancing is one of the simplest and most
common methods applied in balancing the materials that flow through a system.
It involves the systematic and sequential technique in indentifying some variable
sets which are related by some sets of linear or non-linear equations whose
solution depends on the resulting degree of freedom for the system. This degree
of freedom provides us with the limit of freedom for which we can set values for
some of the variable which is referred to as the design variables. A choice of
values for the design variables result in a corresponding value for the remaining
variables. The solutions to the equation set are obtained by the various method of
solution for simultaneous equations, most appreciably the methods of
substitution and elimination. The algebraic method is most efficient for simple
system but it may be inappropriate for complex systems involving large number
of units. The split fraction and method is recommended for such systems.
(b). Split FractionMethod
This method is based on the theory of recycle processes published by Magier
(1964). The method is based on the realization that the basic function of most
chemical processing units (Unit Operation) is to divide the inlet flow of a
50. 50
component between two or more outlet streams. This method is ideal in carrying
out material balancing of complex of multi-unit plants.
3.3 MATERIALS BALANCE ASSUMPTIONS
The following assumptions weremade during the material balance calculations:
1. The system is operating at steady state i.e. there is no accumulation of any
sortin the system.
2. There is negligible amount of inert in the process air.
3. Reasonably high conversion in the reactors.
4. Effect of side reactions is minimal.
3.4 SUMMARY OF MATERIAL BALANCE CALCULATIONS
From the steady state material balance equation, the flow rates of each stream
are calculated as follows.
3.5 MATERIAL BALANCE FOR EACH UNIT
Basis:1hr
THE COMPRESSOR
1a 1a
Stream 1 Stream 2
Stream3
51. 51
Components Stream 1( kg/hr) Composition Stream 1a( kg/hr)
O2 49720. O2 49720.
N2 187080 N2 187080
Total 236750 Total 236750
THE MIXER
Stream 2 Stream 5
Stream 4
60. 60
CHAPTER FOUR
ENERGY BALANCE
The Energy balance gives the account of all the energy requirement of the process
which is based on the principle of conservation of energy. The principle states
that energy can either be create nor destroyed but can be transformed from one
formto another. Also energy can be transferred fromone body to another.
If a plant uses more energy than its competitor, its product could be priced out of
the market. Accountability of the energy utilization of a process plant is
necessary in every design project.
The conservation of energy however differs from the mass in that energy can be
generated (or consumed) in a chemical process. Material can change form; new
molecular specie can be formed in a process unit and must be equal to the one
out at steady state. The same is not true for energy. The total enthalpy of the
outlet stream will not be equal to that of the inlet stream if energy is generated or
consumed in the processes, such as thatdue to heat of reaction.
Energy can exist in various forms: head, mechanical, electrical energy, and it is the
total energy that is conserved. In plant operation, an energy balance on the plant
will show the patterns of energy usage and suggest area for conservation and
saving.
61. 61
4.1 CONSERVATION OF ENERGY
As for materials balance, a general equation can be written for energy balance;
Energy out – Energy in + Generation – Consumption = Accumulation
This is a statement of the first law of thermodynamics. An energy balance can be
written for any process step. Chemical reactions will evolve energy (exothermic)
or consume energy (endothermic). For steady state processes, the accumulation
of both mass and energy will be zero (0).
Energy exists in many forms; the basic forms are listed below:
Potential Energy: This is due to position or height due to motion
Internal Energy: This is the energy associated with molecules and is dependent on
temperature.
Work: This is achieved when a force gets through a distance. Work done on a
systemis positivewhile work doneby a systemis negative
Kinetic Energy: This is the energy due to motion.
For unit mass of material
W Q
Z1
Z2
62. 62
𝑈1 + 𝑃1 𝑉1 +
𝑈1
2
𝑔⁄ + 𝑍1 𝑔 + 𝑄 = 𝑈2 + 𝑃2 𝑉2 +
𝑈2
2
𝑔
+ 𝑍2 𝑔 + 𝑊
Where, Q = Heat transferred across thesystemboundary
W = Work done by the system
P1P2 = Pressurein PressureOut
V1V2 = Volume in, Volume out
U1U2 = Velocity in, Velocity out
Z1Z2 = Height in, Height out
g = Acceleration due to gravity (9.81m/s2
)
In chemical processes the kinetic energy factor ( 𝑈2
𝑔⁄ ) and the Potential energy
factor (zg) are small and negligible and the relation between U and PV is
correlated in terms of enthalpy (H)
H = U + PV
H2 – H1 = Q – w
Also, the work term can be negligible in many chemical engineering systems.
Hence,
H2 - H1 = Q
63. 63
4.2 ENERGY BALANCE ASSUMPTIONS
1. The process is at steady state
2. No heat is lost from the vessel and from the pipe i.e. there is proper
lagging.
3. Effect of pressureon enthalpy is ignored .
4. Potential and kinetic energy changes are negligible.
4.3 SUMMARY OF ENERGY BALANCE
THE COMPRESSOR
Tin= 20°C Tout=155°C
TABLE4.1: HEATBALANCEAROUND COMPRESSOR.
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
N2 187030 0 187030 140.4
O2 49720 87.56 49720 87.56
PROPERTIES QUANTITY/VALUE
Inlet Temperature( °C ) 20
Outlet Temperature( °C ) 155
Heat duty( KJ/hr ) 26259012
Power and Actual Shaft work,
repectively.(KJ/hr and KJ)
399515.49 and 475613.68
64. 64
TABLE4.2 HEAT BALANCE ABOUTTHEAIR HEATER
For air component that passes throughthe air heater
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
N2 187030 0 187030 80.79
O2 49720 152.49 49720 152.49
Inlet Temperature (o
C) 155
Outlet Temperature (o
C) 200
Heat Duty( KJ/hr ) 15107946.75
For nitrous gases recycledback tothe air heater
PROPERTIES QUANTITY/VALUE
Inlet Temperature( °C ) 350
Outlet Temperature( °C ) 200
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
N2 160860 -161.10 16080 -161.10
NO 1460 0 1460 -155.11
NO2 33530 -196.5 3350 -196.5
Heat Duty( KJ/hr ) -226460.6
TABLE4.3 HEAT BALANCEAROUND THECONVETER
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
NH3 13500 0 270 1902.99
O2 42760 612.58 11660 612.58
NO2 - - 23320 610.38
N2 160860 693.63 160860 693.63
H2O 65 1309.44 21060 1309.44
Heat Duty( KJ/hr ) 20579273.83 KJ/hr
65. 65
TABLE4.4 HEAT BALANCEAROUND THEWASTEHEATBOILER (Unit 9)
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
O2 11660 -669.8 11660 -669.8
N2 160860 -719.25 160860 -719.25
NO 23320 0 23320 -685.09
TABLE4.5 HEAT BALANCEAROUND THEOXIDIZING VESSEL
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
O2 11660 100.27 - -
N2 160860 107.80 160860 107.80
NO 23320 0 1460 103.96
NO2 - - 33530 131.00
PROPERTIES QUANTITY/VALUE
Inlet Temperature( °C ) 250
Outlet Temperature( °C ) 350
Heat Duty( KJ/hr ) 1240891.54
TABLE4.6 HEAT BALANCEAROUND THESTACKGAS HEATER
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
N2 160860 -52.909 160860 -52.909
NO 1460 0 1460 -719.25
NO2 33530 -65.50 33530 -65.50
PROPERTIES QUANTITY/VALUE
Inlet Temperature( °C ) 890
Outlet Temperature( °C ) 250
Heat Duty( KJ/hr ) -15976252
Outlet Temperature of Steam (°C ) 410
66. 66
TABLE4.7 HEAT BALANCEAROUND THEABSORPTION COLUMN
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
NH3 270 0 270 576.48
H2O 28120 117597.84 23830 99657.0
N2 160860 167616.12 160860 167616.12
NO 1460 1437.66 8600 8586.24
HNO3 - - 30000 51600.0
NO2 33530 43924.39 670 880.32
PROPERTIES QUANTITY/VALUE
Inlet Temperature( °C ) 50
Outlet Temperature( °C ) 54
Heat Duty( KJ/hr ) -53280.03
PROPERTIES QUANTITY/VALUE
Inlet Temperature( °C ) 150
Outlet Temperature( °C ) 50
Heat Duty( KJ/hr ) -149781.4
Heat Duty of Steam( KJ/hr ) -74322.76
Temperature of Steam (°C ) 118.48
67. 67
HEAT BALANCE AROUND THE AMMONIA VAPORIZER, SUPERHEATER AND
STRIPPER.
AMMONIAVAPORIZER
Heat Duty = 14978 KJ/hr
Outlet Temperature = -28.20 °C
THE AMMONIA SUPPERHEATER
Heat Duty = -1596252KJ/hr
Outlet Temperature = 26.65 °C
THE STRIPPER
Heat Duty = -25873200 KJ/hr
Inlet Temperature = 250 °C
Outlet Temperature = 120°C
68. 68
CHAPTER FIVE
CHEMICAL ENGINEERING DESIGN
The equipment used in chemical process industries can be divided into two
classes: proprietary equipment such as pumps, centrifuge, etc which are designed
and manufactured by specialist firms; non-proprietary equipment which includes
the reactor, heat exchanger, evaporators, still, condensers and bleaching vessels.
The proprietary equipment will only be selected and specified while the non-
proprietary equipment will be designed as special, one-off, items for the
particular processes and purposes they are expected to serve.
The chemical Engineer’s part in the design of “non-proprietary” equipment is
usually limited to “selecting” and “sizing” the equipment. Same will be done in
this design work.
5.1 PROCESS UNITS OF NITRIC ACID PRODUCTION PLANT
The nitric acid process plantcomprises:
1. Ion- Exchange Unit
This unit consists of series of packed beds containing various organic polymer
resins for the removal of unwanted divalent and monovalent ions. Used for the
generation of de-ionized water.
2. De-ionizedwater Cooler
Consistof finned fan-typecooler for cooling the circulating de-ionized water.
69. 69
3. Air Compressor
Here air is compressed in two stages. The first-stage compression is a low-
pressure compression from atmospheric pressure up to 310 kPa. An axial
compressor is used which takes its shaft drive from a gas turbine. The second
compression utilizes a centrifugal-type compressor. The centrifugal compressor is
more efficient for the air flow-rate (36 000 kg/h) and outlet pressure (1090 kPa).
The centrifugal compressor takes its shaft drive from the expansion of tail gas.
Intermediate to the two compression stages is an intercooler which allows the air
temperature to be lowered from 180°C to 45°C, with a pressure loss of 10 kPa.
The temperature drop enables a more efficient second compression stage.
4. Ammonia Vaporizer
This unit consists of a shell and tube-type heat exchanger with two passes per
shell on the tube side. Operating pressure is 1240 kPa. The exchanger is made
frommild steel.
5. Ammonia Super-heater
It consists of a shell and tube-type heat exchanger of similar mechanical
construction to the ammonia vaporizer. Itis constructed frommild steel.
6. Reactor
The reactor is a pressure vessel operating in the range 1050 kPa to 1100 kPa. The
bottom section of the reactor is jacketed. Air is preheated in this jacket prior to
mixing with ammonia. The bottom section of the reactor also contains a shell and
tube-type heat exchanger. This exchanger provides the final stage of tail-gas
70. 70
preheating. Tail gas enters at 235°C and the reaction gases leave the exchanger
section of the reactor at 645°C.
7. SteamSuper-heater
This unit superheats saturated steam from 250°C (and 4000kPa) to 380°C. The
product steam is of medium pressure and suitable quality for ‘in-house’
application and also for export. The super-heater cools the reaction gases from
the reactor exit temperature of 645°Cto 595°C.
8. Waste-heat Boiler
A shell and tube-type exchanger required to heat pressurized (4000 kPa) hot
water from 117°C to a saturated vapour at 250°C. The waste-heat boiler cools
reaction gases from595°Cto 280°C.
9. Tail-gas Pre-heater
Also comprises of shell and tube-type exchanger. It takes reaction gases leaving
the platinum filter at about 315°C and 1020 kPa, and subsequently reduces their
temperature to 185°C. The cooling medium is tail gas. It enters at about 50°C and
leaves the tail-gas pre-heater at 235°C.
10. Cooler/Condenser
This unit condenses weak nitric acid from the gaseous mixture and cools the
remaining gases from an inlet temperature of 185°C to 60°C. The shell and tube-
type heat exchanger uses de-ionized water as its cooling medium.
71. 71
11. OxidationUnit
The oxidation unit is an empty pressure vessel that takes input reaction gases and
blends in additional air from the bleaching column. The extra oxygen provided
enables further oxidation to occur and raises the gas mixture temperature to
140°C. At the top of the oxidation unit is a mist eliminator to prevent carry-over of
acid vapor by entrainment. At the bottom of the vesselis the weak-acid drain.
12. Secondary Cooler
The secondary cooler takes the exit gases from the oxidation unit at 140°C and
cools them down to 65°C, a suitable temperature for entry into the absorption
column. The cooling medium is circulating warm water from the warm-water
loop. The inlet temperature is 50°C and the exit temperature is about 80°C.
13. Absorber
The absorber is usually a sieve tray-type column. It has an operating pressure
around 990 kPa. A bursting disc is used for pressure relief. Each tray is provided
with cooling coils to allow the cooling of the absorption liquor. There are two
independent cooling circuits, each uses de-ionized water. The top section has an
inlet temperature of 7°C and an outlet temperature of 20°C. The bottom section
cooling loop has an inlet temperature of 20°C and an exit of 40°C. The use of two
cooling circuits provides greater flexibility in manipulating absorption conditions
in the column. The tail gas leaves the column at about 10°C. Weak acid from the
cooler/condenser is added to an appropriate tray midway up the column, and
make-up water at 7°C is added to the top tray. The acid drained from the bottom
of the column contains somedissolved nitrogen oxides.
72. 72
14. Stripping Column
The bleaching column is a smaller sieve tray-type column. Impure acid runs down
the column from the top tray and air is bubbled up through the liquor to remove
dissolved nitrogen oxides. The acid from the base of the column is the final
desired 60% (wt.) product.
15. Storage Tank
Stores the supply of nitric acid produced fromthe process plant.
73. 73
CHAPTER SIX
EQUIPMENT DESIGN
The need to design process equipmentmay arise as a result of the desire to:
i. Modify an existing process equipment or
ii. Develop new equipment.
Modification of existing equipment may be required as a result of poor
performance or the need of scale up (or down). For example, increased market
success of a product may lead to increased production. It may be more
economical to increase the capacity of the existing equipment rather than add
another line of equipment. This is usually the case when operational cost costs
(man power, energy etc.) are high.
New equipment, on the other hand may be desired as a result of successful
laboratory research and pilot plant studies or as a result of satisfactory process
simulation using the computer.
In either situation (new or existing equipment), the actual design commences
with the assessment of the characteristics of the feed materials, the products and
the physical and chemical processes required to convert the raw material to
products. The overall satisfactory performance and reliability of the equipment
would depend on the following factors.
I. Optimum processing conditions
II. Appropriatematerials of construction
III. Strength and rigidity of components
74. 74
IV. Satisfactory performanceof mechanical part
V. Reliable methods of fabrication
VI. Ease of maintenance and repairs
VII. Ease of operation and control.
VIII. Safety requirements
IX. Environmental impact
The typical process equipmentdesign procedurewill involve:
1. Specifying the problem
2. Analyzing the probably solution
3. Preliminary design, applying chemical engineering process, principles and
theories of mechanics relevant to the problem.
4. Selecting appropriatematerials of construction.
5. Evaluating and optimizing the design, the possible application of computer
aided design (CAD) systemlike HYSIS, Aspen Plus etc
6. Preparing the drawings and specifications
6.1. PROBLEM SPECIFICATION
The specification of the problem is the key stone in the quest to design an
equipment to meet the needs of the customer. Specification of a problem may
include:
1. The quantity of material to be processed in a given time such as the
proposed capacity of the equipment.
2. The physicaland chemical properties of the product.
75. 75
Constraints such as:
a. Availability and cost of materials of construction
b. Availability and cost energy, water, oil etc.
c. Budget for production
d. Availability and cost of manpower with relevant skill for fabrication
e. Space to be occupied by the equipment
f. Environmental issues
g. Safety issues
h. Number of working days in the year
i. Ergonomics
6.2. ANALYZING THE PROBLEM SOLUTION
A thorough analysis will reduce the list for example if the equipment is to be used
for small scale processing. All the constraint listed above will need to be
considered.
6.3. PRELIMINARY DESIGN: APPLYING CHEMICAL ENGINEERING
PROCESS PRINCIPLE AND THEORIES OF MECHANICS.
Probably the most important expression in the design of process equipment is
that of mass and energy balance which may be expressed in general term as;
Input+ generation – output – consumption = accumulation
This expression is found in various forms in thermodynamics, fluid mechanics,
transport phenomena, heat transfer, separation process and other subject areas.
76. 76
It is simply an expression of indestructibility of matter and energy. This expression
applies to all raw materials, intermediate and product.
6.4 MATERIAL SELECTION
Materials are critical in the design of process equipment. Materials must be
selected to take care of possible corrosion problems. Materials of construction
should also possess adequate mechanical properties to withstand tensile,
compressive, shear and impact stresses.
Stainless steel of various grades finds wide application in process equipment
design especially for parts in contact with raw materials and product. Glass,
plastic and rubber lined vessels are also used are also used when materials tend
to react with steel. Steel of various carbon contents are used for compounds such
as shaft, springs and gears and for supportstructure.
6.5 DESIGN OPTIMIZATION
The calculation process in the design of equipment may require simple arithmetic,
algebraic, differential calculus or integral calculus. In many cases an exact solution
may not be feasible thus necessitating the use of various approximation
techniques such as graphical or numerical methods.
In many cases also, only some parts of the equipment are designed on the basis of
analytical calculations. Practical conditions are used to determine the
specifications of the remaining part. It is thus not unusual to have several feasible
solutions. There is thus the need to select the best solution. The ultimate goal is
to minimize costor maximize profit.
77. 77
In chemical process industries, equipment used are classified into two;
Proprietary equipment such as pumps, centrifuges which are designed and
manufactured by a specialistfirm.
Non- proprietary equipment such as reactors, heat exchangers, condenser,
bleaching vessels etc are designed as specially requested.
6.6 SUMMARY OF THE DESIGN AND SPECIFICATION OF EQUIPMENT
CALCULATION.
In designing and specifying of equipment for chemical industries, the variables
/parameters involved namely; pressure, temperature, density, volume, area,
diameter, height, heat duties, heat capacities etc must be carefully calculated.
This gives the designer exact data for fabrication and manufacturing. For the
production of Nitric Acid; the following equipment are designed and specified;
Nitric Acid storage tank, Ammonia storage tank, Absorber, Converter, Oxidation
vessel, heat exchangers.
FOR REACTORS
The operating intensity is given for the reactors=11296.324kg/m2/24hrs
=11296.324kg/m2/day
Equipment Mass of reactant
(kg/h)
Area(m2) Diameter(m)
Converter 13500 28.68 6.04
Oxidation Vessel 23320 49.55 7.94
Absorber 33530 71.24 9.52
The stripper column has 10 plates
78. 78
FOR STORAGE TANKS
Equipment Type Nitric Acid storagetank
Shape Cylindrical
Nature Insulated
Material of Construction Stainless Steel
Capacity 50000000kg/hr
Volume(m3) 23.8
Diameter(m) 4.6
Height(m) 13.9
Equipment Type Ammonia Storagetank
Shape Cylindrical
Nature Insulated
Material of Construction Stainless Steel
Capacity 13565kg/hr
Volume(m3) 66.8
Diameter(m) 6.5
Height (m) 19.6
FOR HEAT EXCHANGERS
Using the formulae; Q=AUDTm
A= Q/UdTm
Where
Q= Heat Duty of the heat exchanger(KW)
A= area(m2)
U= OverallHeat Transfer Coefficient(KW/m2)( This is assumed for all)
79. 79
DTm=Log Mean Temperature Difference(Celsius)
Using a countercurrentflow; DTM= DT1−DT2/ ln(DT1 /DT2)
DT1 = Thin-Tcout
DT2 = Thout –TCin
Equipment Q (KJ/hr) Thin ThoutTCoutTcout
Waste Heat Boiler (1) 15976252 890 250 30 410
Air Heater 350 200 150 250
Stack Gas
Heater
74323 200 150 30 118.5
Waste Heat
Boiler(2)
149781.4 208 50 150 32
NH3 Super
Heater
4437.85 410 330 26.6 28.2
NH3
Vaporizer
149781.4 208 167.2 28.2 33.4
Table 6.1: Table showing the heat transfer area of some equipment
Equipment Q(KW/S) Area(M2)
Stack Gas Heater 20.65 2
Waste Heat Boiler(1) 4437.85 130.5
Waste Heat Boiler(2) 41.61 11.83
NH3 Super Heater 4437.85 127.6
NH3 Vaporizer 41.61 1.87
Air Heater 4133.7 588.7
80. 80
CHAPTER SEVEN
PROCESS CONTROL AND INSTRUMENTATION
Instruments are provided to monitor the key process variables during plant
operation. They may be incorporated in automatic control loops, or used for the
manual monitoring of the process operation. They may also be part of an
automatic computer data logging system. Instruments monitoring critical process
variables will be fitted with automatic alarms to alert the operators to critical and
hazardous situations.
It is desirable that the process variable to be monitored be measured directly;
often, however, this is impractical and some dependent variable, that is easier to
measure, is monitored in its place.
7.1 OBJECTIVES
The primary objectives of the designer when specifying instrumentation and
control schemes are:
1. Safe plant operation:
(a) To keep the process variables within known safeoperating limits.
(b) To detect dangerous situations as they develop and to provide alarms and
automatic shut-down systems.
(c) To provideinterlocks and alarms to prevent dangerous operating procedures.
2. Production rate: To achieve the design productoutput.
3. Product quality: To maintain the product composition within the specified
quality standards.
4. Cost: To operate at the lowest production cost, commensurate with the other
objectives.
81. 81
These are not separate objectives and must be considered together. The order in
which they are listed is not meant to imply the precedence of any objective over
another, other than that of putting safety first. Product quality, production rate
and the cost of production will be dependent on sales requirements. For example,
it may be a better strategy to producea better-quality product at a higher cost.
In a typical chemical processing plant these objectives are achieved by a
combination of automatic control, manual monitoring and laboratory analysis.
7.2 PLANT CONTROL CONFIGURATION
The plant will be designed for manned operation and will be linked to the
adjacent fertilizer manufacturing plant. Certain configurations will be put in place
to monitor some key parameters of the plant.
The acid plant process control will be embedded in the plant DCS. The
instruments of the individual process units will be terminated in junction boxes
located at the unit’s skid limits. From here these instruments will be connected to
instrumentcabinets in the auxiliary roomand integrated in the PAS.
The plant safety instrument system (SIS) will be independent of the PAS. There
will be a link between the PAS and the SIS for data monitoring/logging and
maintenance/operational override control purposes. Fire and gas monitoring will
also be a dedicated module integrated in the safeguarding system.
The process controlschemes of some vital units are discussed as follows:
Absorption column
The process controlscheme for the absorption column is presented in fig Itwas
designed fromthe recommendations presented in the HAZOP analysis.
82. 82
It features ratio control on the make-up water stream. The signals from flow
transmitters on this line and on the gas input line are fed to the ratio controller,
whereby the make-up water stream is adjusted.
Other control features include a pressure controller on the tail-gas outlet stream
so that the column absorption pressure can be maintained at the design
operating value of 950 kPa. A temperature transmitter on the tail-gas outlet
stream provides the signal for control of the overall cooling-water flow rate. This
is the temperature which is most useful in determining good absorption. The
cooling circuit itself is fed from a common line (on which the overall flow rate is
controlled). Small block valves on each of the tray cooling-coil feed lines enable
flow rate regulation to each of the coils. These valves feature a removable top
whereby a magnetic flow meter may be inserted to read the flow rate. The valves
need only be set initially and then periodically adjusted manually.
There is no automatic control on the flow rate of the gas inlet stream or weak-
acid condensate stream, since both of these flows are predetermined by feed
flow rates earlier in the process. Isolation valves and provision for spectacle blinds
are included to enable the column to be isolated during shutdown periods.
The product-acid solution is withdrawn from the column using a level control
valve on this line. The liquid level in the base of the column must be maintained
slightly above the level of the plate downcomer to prevent incoming gas from by-
passing the sieve plates.
All controllers suggested for the absorption column feature HIGH and LOW alarms
for good control.
The final safety requirement is a relief line with a relief valve protected by a
bursting disc.
83. 83
Air heater
The process control scheme suggested for the air heater is shown in Fig. This flow
scheme features a control valve on the compressed air inlet line. A temperature
controller taking its signal from the heater outlet line ensures the flow is
regulated to maintain the heater temperature of 250°C. Air pressure is controlled
prior to entry into the unit and is kept constant at 7.3 atm.
A pressure indicator on both inlet and outlet steam lines enables this parameter
to be adequately monitored.
The nitrogen oxide reaction gas stream cannot be directly controlled from the air
heater. Instead the flow rate, temperature and pressure are predetermined by
the reactor feed conditions.
Both inlet and outlet lines possess isolation valves for plant shutdown. These lines
would be blanked before any platinum recovery work was attempted on the
heater. Inlet and outlet lines also feature temperature indicators, consistent with
the policy of constant monitoring of this parameter throughoutthe process.
Ammonia Vaporiser and Superheater
Pressure indicator and controller will be installed to maintain ammonia vapor at
7.3 atm. Temperature indicator and controller is required to ensure that the
outlet temperature of 250°C is achieved in the superheater. The control scheme is
shown in the figure below.
Ammonia Converter
Temperature control system is needed within the converter to ensure that the
temperature in the converter does not drop below the reaction temperature of
890-900°C, to avoid loss of heat.
84. 84
7.3 ALARMS, SAFETY TRIPS AND INTERLOCKS
Alarm systems need to be installed in specific areas to alert operators of serious,
and potentially hazardous, deviations in process conditions. Key instruments are
fitted with switches and relays to operate audible and visual alarms on the control
panels and annunciator panels. Where delay or lack of response, by the operator
is likely to lead to the rapid development of a hazardous situation, the instrument
would be fitted with a trip system to take action automatically to avert the
hazard; such as shutting down pumps, closing valves, operating emergency
systems.
The basic components of an automatic trip systemare:
1. A sensor to monitor the control variable and provide an output signal when a
preset value is exceeded (the instrument).
2. A link to transfer the signal to the actuator, usually consisting of a system of
pneumatic or electric relays.
3. An actuator to carry out the required action; close or open a valve, switch off a
motor.
The high-temperature alarm operates a solenoid valve, releasing the air on the
pneumatic activator, closing the valveon high temperature.
7.3.1 INTERLOCKS
Where it is necessary to follow a fixed sequence of operations for example, during
a plant start-up and shut-down, or in batch operations interlocks are included to
prevent operators departing from the required sequence. They may be
incorporated in the control system design, as pneumatic or electric relays, or may
be mechanical interlocks. Various proprietary special lock and key systems are
available.
85. 85
Table 7.1: Letter Code for Instruments Symbols
Property
measured
Firstletter Indicating only Controlling only
Flow – rate F FI FC
Level L LI LC
Pressure P PI PC
Temperature T TI TC
Humidity H HI HC
I - Indicator C - Controller
L - Level T - Temperature
F - Flow rate P - Pressure
H - Humidity
(Source: Sinnott, R.R1999).
7.4 LINING, PIPING, VALVES AND PUMPS
In Fig.7.1, which is the piping and instrument diagrams, there are various
mechanical component introduced in the plant to obtain maximum efficiency
some of which includes, flanges, valves, piping lines, blinds, gaskets and so on.
7.4.1 VALVES
The valves used for chemical process plant can be divided into two broad classes,
depending on their primary function:
Shut-off valves (block valves), whosepurposeis to closeoff the flow.
86. 86
Control valves, both manual and automatic, used to regulate flow.
The table below shows some of the valves used in the P and I diagram (figure 5),
their symbols, and functions.
Table 7.2: Types of Valves and SymbolUsed In PID
NAME SYMBOL FUNCTIONS
Used to control flow in lines.
Fitted on sensitive lines and are either
pneumatically or digitally controlled.
Fitted in lines of relatively high pressure or
velocity
Used for control of gas or vapour flows
Control
Valves
AutomaticValves
Check Valves
Butterfly
Valves
87. 87
7.4.2 JOINTS
There are various joints used in fig 3.0 either as flow reducers, or to aid the
carrying property of pipe. And effective transport of fluids in the piping flow.
Below is a table of the various elbows and joints used in the P and I diagram:
Table 7.3: Joints
JOINTS AND
ELBOWS
SYMBOLS FUNCTIONS
EQUAL ‘T” REDUCER
JOINT
90o
T – CONNECTOR
ELBOW
LONG – RADIUS
ELBOW
Used to reduce a flow line
into three equal lines
Used in joining a running line
to a flow line.
Used in channeling lines also
reduces flow speed.
Used in channeling lines in
pipe support.
Used in branching lines.
88. 88
45o
LATERAL
REDUCER
Used in reducing pressure
flow.
7.5 PIPE SUPPORT
The Design of a plant’s P and I is not complete without the use of supports. Pipe
supports in plant piping helps in reducing cost and number of pump required to
maintain line flow parameter and safety of personnelthrough operation zone.
Below is some major type of support:
I – BEAM Support to carry pipe lines
H – BEAM Supportabove2m
U – CHANNEL
PLATES TO ALIGNVALVES
SHOES TO HOISTPIPEINTO PROPERORIENTATION
89. 89
CHAPTER 8
SAFETY AND ENVIRONMENTAL CONSIDERATIONS
8.1 SAFETY
Safety is the condition of being protected against any danger. Every organization
has a legal and moral obligation to safeguard the health and welfare of its
employees and the general public. The good management practices needed to
ensure safe operation will also ensure efficient operation. In a chemical processing
industry, the chemicals used or produced can be hazardous to humans or the
environment if not properly handled and this could equally lead to a lot of damage
structurally and financially.
The best organizations are those that have come to the realization that provision
of safety is not only the right thing to do for their employees, it is also profitable.
Advantages of a safe working environment
1. Ultimately, safety leads to more profit as less money is spent taking care of
legal bills, hospitalbills, and repair of equipment.
2. Itgives the company a good name.
3. Happy employees which increases their job performance.
Safety is usually considered in three classes:
I. Safety of the environment
II. Safety of the personnel
III. Safety of the plant and equipment.
90. 90
The term “engineering safety” covers the provision in the design of control
systems alarms, trips, pressure relief devices, automatic shutdown system and
duplication of key equipment, firefighting equipment and service; personnel
protect equipment and so on.
8.1.1 SAFETY OF THE ENVIRONMENT
There are several hazards associated with industrial process. These hazards need
to be prevented and kept in check in order to protect the environment.
Environment in this context refers to the immediate surroundings around the
plant. For the safety of the environment to be ensured, the following points
should be noted and applied;
1. Flaring of gases should be done minimally.
2. The level of toxicity of effluent should be monitored regularly and kept in
check.
3. Storagetanks should be situated in areas away fromvehicle traffic.
4. The control room should be attended to at all times to ensure that there is
an immediate responseif an alarm is triggered.
5. There should be a way of informing the community around the facility if
there is danger that might affect them e.g Fire. An alarm is suggested, and
this should be tested regularly.
6. Protect pipe racks and cable trays fromfire.
7. Fire-fighting systemmust be provided within the complex. This consist:
Fire water pipe network throughout the facility supported by necessary
hydrants. Hoses should bepermanently placed near these hydrants.
