Propane propylene splitter

PROPANE-PROPYLENE SPLITTER UNIT
DESIGN
ING. LUCA BARBAGALLO
CHEMICAL ENGINEERING AND
SIMULATION……………………………..

Dedicated to
my family and my girlfriend
with love and dedication
to achieve huge goals.

Eng. Luca Barbagallo
: lucabarbagallo40@gmail.com
: https://it.linkedin.com/in/ing-luca-barbagallo-770b29113
 L.B. was born into 8° August, 1987. He was graduated from the Department of Industrial Engineering of Catania (Italy) with
specialization in chemical engineering. He obtained the qualification of industrial engineer. He worked as a sales engineer in
the field of renewable energies, he continued his career working as an inspector of facilities for sorting and storage of natural
gas is currently an intern with the planner job in industrial production in a rubber compounds industry.
CHEMICAL ENGINEERING AND SIMULATION

 PREFACE

An overall chemical process is a complex building of different
kind of unit operations in which are allowable transformations
and separations techniques very more sophisticated. At this
point there are some important and several technologies that
try separating very complex mixture of a several types of
chemical components, such as hydrocarbons from refineries
and petrochemicals ones or by different plants of natural gas
treatments. Without forgetting biomasses treatments and
renewable resource facilities, from these are just possible
splitting gases and vapors and steams based on light
hydrocarbons, water, carbon monoxide, carbon dioxide,
methane, nitrogen and more and more others.

We depend largely on crude, the gases associated with it and
natural gas (mainly methane) as the source of liquid fuels
(petrol, diesel) and the feedstock for the chemical industry.
Oil, and the gases associated with it, consists of a mixture of
hundreds of different hydrocarbons, containing any number
of carbon atoms from one to over a hundred. Most of these
are straight chain, saturated hydrocarbons which, except for
burning, have relatively little direct use in the chemical
industry or as fuel for cars. Thus the various fractions
obtained from the distillation of crude oil and the associated
gases have to be treated further in oil refineries to make
them useful. The most valuable fractions for the chemical
industry, and for producing petrol, are liquefied petroleum
gas (LPG), naphtha, kerosene and gas oil.

Petrol (gasoline) contains a mixture of hydrocarbons, with 5 to
10 carbon atoms. The mixture of C5-C10 hydrocarbons
obtained directly from the distillation of crude oil contains a
high proportion of straight-chain alkanes. However, if this
mixture is used as petrol, it does serious damage to a car's
engine. Petrol containing a high proportion of straight chain
alkanes tends to ignite in the cylinder of the car engine as the
piston increases the pressure and before the cylinder reaches
the optimum position. Ideally, the mixture of petrol vapour and
air is ignited with a spark at a predetermined position of the
piston in the cylinder.
This problem of premature ignition is referred to as pre-
ignition and also as engine knock. The term knock is used as
pre-ignition can be heard. Severe knock can cause serious
engine damage. However, branched-chain alkanes,
cycloalkanes and aromatic hydrocarbons are much more
resistant to knock and straight-chain alkanes are converted into
them in a series of processes in the refinery which are
described in this unit.

The resistance of petrol to knock is measured in terms of an “octane rating” (octane number). The higher the number, the less likely is
a fuel to pre-ignite. The octane rating is on a scale where heptane is given an arbitrary score of 0 and 2,2,4-trimethylpentane (iso-
octane) one of 100 %.
Thus a petrol with the same knocking characteristics as a mixture of 95% 2,2,4-trimethylpentane and 5% heptane has an octane rating
of 95. A rating of 95 does not mean that the petrol contains just iso-octane and heptane in these proportions, but that it has the same
tendency to knock as this mixture. The octane rating of petrols usually available for cars range from 95 upwards and contain a mixture
of straight-chain, branched, cyclic and aromatic hydrocarbons, produced by the processes described below. These processes are also
used to convert staight-chain hydrocarbons to hydrocarbons which are much more useful to make chemicals which are then used to
make a huge range of compounds from polymers to pharmaceuticals.

Petrochemical Refinery

An important unit operation in the chemical process industries (CPI) is to separate a mixture into its components. A typical chemical
plant, as illustrated in the schematic flow sheet of Figure 1 will consist of both reaction and separation units. The raw materials are first
purified in a separating unit and then fed to the reactor. A factor representing the efficiency with which raw materials are converted to
products is the selectivity.
This is given by moles of primary product produced divided by the moles of limiting reactant consumed. It can vary between 0 to 1,
depending on the stoichiometry, the molar feed ratio of reactants, reactor temperature(s), reactor configuration, the catalyst if required
etc. Any unreacted feed that remains is separated from the reaction products and recycled back to the reactor. The products are further
separated and purified, before being marketed or used in subsequent processes. If the products contain ‘‘non-condensable’’
components, such as methane, hydrogen, argon, these must be separated by flash separation or similar process. Before flash separation,
the process stream is usually cooled and depressurized.
Afterwards, it may be fed to another separator to remove and purify useful components. Any remaining raw materials are recycled to the
chemical process. The arrangement of Figure 1 is typical of many petrochemical processes, and is illustrated in (Hydrocarbon Processing
Magazine).

Figure 1: An illustrative scheme of a petrochemical plant.

Gas separation, in the upper side of the flow chart, will be main the object of discussion in this elaborate based on separation of light
ends gases. These go out from upper side (top) of the topping unit operation, such as atmospheric distillation. The mixture include a
mixture of C1-C4 hydrocarbons and are separated by fractional distillation systems. Some of the columns are:
 1) A debutanizer which separates the C4 hydrocarbons from the C1-C3 hydrocarbons
 2) A depropanizer which separates out the C3 hydrocarbons
 3) A deethaniser which separates out the C2 hydrocarbons
 4) A demethanizer which separates out the methane
 5) A C3 splitter which separates propylene from propane
 6) A C2 splitter which separates ethane from ethane.
Figure 2: Gas separation train in a refinery plant (By kind permission of SABIC
Europe.)

In this elaborate will be pointed out the features of the C3 splitter column by using a theoretical model for a multicomponent mixture
and simulations with computational simulator as three different kind of them. They are very noted and commercial software such as
Chemcad, Aspen plus and Chemsep. The results will be compared both of them to evaluate and validate the current theoretical model
for calculating final response, more useful in designing the overall column.

Propylene is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, is the second simplest
member of the alkene class of hydrocarbons, and is also second in natural abundance. Propylene is produced primarily as a by-product
of petroleum refining and of ethylene production by steam cracking of hydrocarbon feedstocks. Also, it can be produced in an on-
purpose reaction (for example, in propane dehydrogenation, metathesis or syngas-to-olefins plants). It is a major industrial chemical
intermediate that serves as one of the building blocks for an array of chemical and plastic products, and was also the first
petrochemical employed on an industrial scale. Commercial propylene is a colorless, low-boiling, flammable, and highly volatile gas.
Propylene is traded commercially in three grades:
 Polymer Grade (PG): min. 99.5% of purity.
 Chemical Grade (CG): 90-96% of purity.
 Refinery Grade (RG): 50-80% of purity.
Separation train of light end gases

The three commercial grades of propylene are used for different applications. RG propylene is obtained from refinery processes. The
main uses of refinery propylene are in liquefied petroleum gas (LPG) for thermal use or as an octane-enhancing component in motor
gasoline. It can also be used in some chemical syntheses (e.g., cumene or isopropanol). The most significant market for RG propylene is
the conversion to PG or CG propylene for use in the production of polypropylene, acrylonitrile, oxo-alcohols and propylene oxide. While
CG propylene is used extensively for most chemical derivatives (e.g., oxo-alcohols, acrylonitrile, etc.), PG propylene is used in
polypropylene and propylene oxide manufacture. PG propylene contains minimal levels of impurities, such as carbonyl sulfide, that can
poison catalysts. Propylene has a calorific value of 45.813 kJ/kg, and RG propylene can be used as fuel if more valuable uses are
unavailable locally (i.e., propane – propene splitting to chemical-grade purity). RG propylene can also be blended into LPG for
commercial sale. Also, propylene is used as a motor gasoline component for octane enhancement via dimerization – formation of poly-
gasoline or alkylation. Propylene is commercially generated as a co-product, either in an olefins plant or a crude oil refinery’s fluid
catalytic cracking (FCC) unit, or produced in an on-purpose reaction (for example, in propane dehydrogenation, metathesis or syngas-
to-olefins plants).

Globally, the largest volume of propylene is produced in NGL (Natural Gas Liquids) or naphtha steam crackers, which generates ethylene
as well. In fact, the production of propylene from such a plant is so important that the name “olefins plant” is often applied to this kind
of manufacturing facility (as opposed to “ethylene plant”). In an olefins plant, propylene is generated by the pyrolysis of the incoming
feed, followed by purification. Except where ethane is used as the feedstock, propylene is typically produced at levels ranging from 40
to 60 wt% of the ethylene produced. The exact yield of propylene produced in a pyrolysis furnace is a function of the feedstock and the
operating severity of the pyrolysis. The pyrolysis furnace operation usually is dictated by computer optimization, where an economic
optimum for the plant is based on feedstock price, yield structures, energy considerations and market conditions for the multitude of
products obtained from the furnace. Thus, propylene produced by steam cracking varies according to economic conditions. In an olefins
plant purification section, also called separation train, propylene is obtained by distillation of a mixed C3 stream, i.e., propane,
propylene, and minor components, in a C3-splitter tower (also called propylene-propane splitter, or simply P-P splitter). It is produced
as the overhead distillation product, and the bottoms are a propane enriched stream. The size of the C3-splitter depends on the purity
of the propylene product.

The propylene produced in refineries also originates from other cracking processes. However, these processes can be compared to only
a limited extent with the steam cracker for ethylene production because they use completely different feedstocks and have different
production objectives. Refinery cracking processes operate either purely thermally or thermally – catalytically. By far the most important
process for propene production is the fluid catalytic cracking (FCC) process, in which the powdery catalyst flows as a fluidized bed
through the reaction and regeneration.
Figure 3: Separation of natural gas train

The LPG production rate from the deethanizer , depropanizer and debutanizer depends on the quality of the crude. Since the crude
slates are different in different seasons, the LPG product rates are different. Using industrial data, the weight and volume of LPG are
calculated from the weight and volume of the debutanizer feed. The weight of the debutanizer bottom steam is then estimated
according to material balance. According to industrial data, the total volume loss is negligible in the debutanizer, less than 1 %. In other
words, the summation of the volumetric flow rates of LPG and debutanizer bottom product is almost equal to the volumetric flow rate of
the debutanizer feed. Assuming that there is no volume loss, the volumetric flow rate of the debutanizer bottom stream is then
calculated. Separations are “big businesses” in chemical processing. It has been variously estimated that the capital investment in
separation equipment is 40-50% of the total for a conventional fluid processing unit. Of the total energy consumption of an average
unit, the separation steps accounts for about 70%. And of the separation consumption, the distillation method accounts for about 95%.
In general, initial design of a distillation tower involves specifying the separation of a feed of known composition and temperature.
Constraints require a minimum acceptable purity of the overhead and/or bottoms product.

The desired separation can be achieved with relatively low energy requirements by using a large number of trays, thus incurring larger
capital costs with the reflux ratio at its minimum value. On the other hand, by increasing the reflux ratio, the overhead composition
specification can be met by a fewer number of trays but with higher energy costs.
In particular, the optimization of reflux ratio is attractive for distillation columns that operate with:
1. high reflux ratio;
2. high differential product values between overhead and bottom;
3. high utility costs;
4. low relative volatility;
5. feed light key far from 50%.

Figure 4 shows a typical olefins plant in which a propylene splitter is used for separating propane and propylene. The lighter
component (propylene) is more valuable than propane. The overhead stream has to be at least 95% propylene.
Figure 4: A schematic flow-sheet of a C3 splitter location in a light end gases plant.

Figure 5: Another schematic implementation of C3 splitter for LPG facilities.
Propylene and propane are two isomeric compounds
with high affinity and similar volatility and due to this
feature they are very hard to separate in a single step
of stripping process. It will require different stages of
separation in column. So in this way unit operation
shall have high pressures about 13-17 bar from top
up to bottom side, consequently temperature are side
by side both 60 and 50 degree Celsius. It will be more
easy separating these compounds in a distillation
tower in which will be present other hydrocarbons as
well as ethane, n-butane or iso-butane and pentanes
and different kind of octanes and iso-octanes without
forgetting exanes and heptanes.

Figure 6: 3D-Modelling of two kind of
splitters based on hydrocarbons (C2 and C3 ).
FEED
SPECIFICATIONS:
P= 16,55 atm
T= 41,66 °C
C2 = 0,0397 % mol
C3
= = 72,81 % mol
C3 = 26,48 %mol
C4 = 0,662 %mol

Figure 7: Model of propylene-propane splitter.
In figure 7 has been seen a model of tower taken from Chemsep
software and this chapter will be considered the main features of
designing of distillation tower in valve fixed trays. At the first will
be adopted a theoretical model to evaluate theoretical
compositions as taken off products by two streams from the top
and the bottom from distillation column. Then, by using
commercial software, as CHEMCAD, ASPEN PLUS and CHEMSEP will
be able comparing these results with an accurate sense of
validation and relability in order to make real a preliminary design
of the same tower.

SHORT-CUT METHOD FOR A MULTICOMPONENT SYSTEM
Short-cut methods were developed for the design of separation columns for hydrocarbon systems in the petroleum and petrochemical
systems industries, and caution must be exercised when applying them to other systems. They usually depend on the assumption of
constant relative volatility, and should not be used for severely non-ideal systems. If the presence of the other components does not
significantly affect the volatility of the key components, the keys can be treated as a pseudo-binary pair. The number of stages can then
be calculated using a McCabe-Thiele diagram, or the other methods developed for binary systems. This simplification can often be
made when the amount of the non-key components is small, or where the components form near-ideal mixtures. Where the
concentration of the non-keys is small, say less than 10 per cent, they can be lumped in with the key components. For higher
concentrations the method proposed by Hengstebeck (1946) can be used to reduce the system to an equivalent binary system.

Propane propylene splitter

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