Reactor Arrangement for Continuous Vapor Phase Chlorination

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Process Engineering Guide:
GBHE-PEG-RXT-814
Reactor Arrangement for
Continuous Vapor Phase Chlorination
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Process Engineering Guide:
Reactor Arrangement for Continuous Vapor Phase Chlorination
CONTENTS
1 BACKGROUND
2 REACTOR
3 CHEMICAL SYSTEM
4 PROCESS CHEMISTRY
5 KINETICS EXPERIMENTS AND MODELLING
6 INTERPRETATION OF KINETICS INFORMATION
7 OPERATING CONDITIONS AND REACTOR DESIGN
8 REACTOR STABILITY AND CONTROL

FIGURES
1 POSTULATED REACTION PATHS FOR PROGRESSIVE
CHLORINATION OF B-PICOLINE 3
2 CHLORINATION OF b-PICOLINE: MODEL PREDICTIONS OF
PRODUCT DISTRIBUTION IN FULLY-MIXED REACTOR
3 TWO-STAGE REACTOR: RATE OF CHLORINATION OF b-PICOLINE
DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE

1 BACKGROUND
This Guide summarizes the method used to design a reactor for the
manufacture of CCMP by the continuous vapor phase chlorination of b
picoline (5-methylpyridine). The CCMP production required from the
reactor is 22 kg/hr or 193 tpa for a 8760 hr year.
2 REACTOR
Backmixed reactor followed by plug flow reactor. Continuous. Gas Phase.
Operating temperature 375° C, operating pressure 3 bar a.
3 CHEMICAL SYSTEM
Non-catalytic, free radical chlorination. Inert diluent (carbon tetrachloride)
used as heat sink. Overall reaction is:
Reaction occurs above 300° C. Reaction fast (seconds) and highly
exothermic (430 kJ/kg mol b-picoline). Molar feed ratio of b-picoline
chlorine : carbon tetrachloride is 1:10:20.
4 PROCESS CHEMISTRY
The primary reaction mechanism is one of progressive addition of chlorine
free radicals to both the side chain and the pyridine ring. Each reaction
can be expressed in the following form:

Clearly the number of possible reactions is large. A proportion of these
reactions can be discounted on the basis of product distributions obtained
in laboratory experiments. The resultant matrix of elementary reactions is
shown in Figure 1, along with the alphanumeric computer coding of
reaction species.
The range of possible by-products includes under-chlorinated picolines
(less than 4 chlorine additions per molecule), over-chlorinated picolines
(more than 4 chlorine additions per molecule), CCMP isomers (e.g.
P6M53) and demethylated chloropyridines.
Under-chlorinated intermediates, which are thought to be highly toxic,
cannot be recycled because they are inherently unstable (they are basic
and can self-quaternise to form tars, or can form high melting point
hydrochloride salts) and cause problems at the product purification stage.
There is, of course, no point in recycling over-chlorinated by-products.
However, they are less toxic than the under-chlorinated by-products, and
the downstream purification is easier.
The strategy adopted is to operate the reactor with excess chlorine; by-
products are then more stable, less toxic and easier to separate from the
CCMP. In addition, unreacted chlorine can be absorbed and separated
from the reactor off gases with the CTC, and then recycled.
Since the by-products cannot be recycled, by-product formation leads to a
direct loss of process efficiency. Thus, optimization of selectivity, via a
kinetics model, is obviously of paramount importance.

FIGURE 1 POSTULATED REACTION PATHS FOR PROGRESSIVE
CHLORINATION OF b-PICOLINE

5 KINETICS EXPERIMENTS AND MODELLING
The reaction modeling work was based on GBHE-PEG-RXT-818, a
kinetics simulation package.
Initial laboratory feasibility runs were carried out in a 10 cm inside
diameter tubular reactor. The heat losses from the reactor were very large
and no meaningful exothermicity data could be obtained. The flow pattern
in the reactor was ill-defined, plug flow with some backmixing. The reactor
was operated at between 350° C and 400° C; the shortest possible
residence time was 12 seconds. In general, the reactor product was
always CCMP, P6M53 and over-chlorinated picolines. Isothermal reaction
heat effects were swamped by heat losses. A plug flow reactor model, set
up in GBHE-PEG-RXT-818, predicted the product distributions reasonably
well. The model gave a fair appreciation of the reaction mechanism but
obviously it could not be used to predict the required temperature and
residence time for optimum CCMP yield.
Kinetic experiments with a 2.5 cm inside diameter plug flow reactor, with
the feeds jet mixed prior to injection into the reactor were unsuccessful,
mainly because of reaction instability. However, these experiments,
together with experiments in a 250 cc batch reactor, indicated that
the initiation and early chain propagation reaction steps were extremely
fast. The static experiments also showed that the rate of reaction slowed
down significantly as more chlorine atoms were substituted into the
picoline molecule. Side chain chlorination occurred before ring
chlorination.
The speed and exothermicity of the reaction suggested the study of at
least partial reaction in a backmixed reactor; the reaction temperature can
be more easily controlled in this type of reactor, and it is very useful if
reaction heat effects can lead to instability and runaway, e.g. the
experiments with the 2.5 cm inside diameter tubular reactor.
Experiments were, therefore, carried out in a jet stirred 100 cc backmixed
reactor at between 300° C and 400° C, for residence times of 5 seconds to
15 seconds. An isothermal backmixed reactor model, set up in GBHE-
PEG-RXT-818, gave a very good description of this reactor. This model
was based on the assumption that, for chain reactions, the relative rates of
reaction are independent of temperature. This model was used in all
further reactor optimization work.

6 INTERPRETATION OF KINETICS INFORMATION
The most convenient method for comparing the model prediction of
product distributions in different reactor environments is in terms of moles
of chlorine added per mole of b-picoline. Then, for example, application of
the model at different temperatures for a backmixed reactor with a given
inlet feed ratio and residence time will generate concentration plots of
each species over the entire range of chlorine addition (see Figure 2).
The model predicts that the maximum yield of CCMP was 40% in an
isothermal backmixed reactor and 52% in an isothermal plug flow reactor.
Although the achievement of the maximum CCMP yield is desirable,
reactor stability, i.e. temperature control for this fast, very exothermic
reaction, has also be considered.
In the laboratory, stable reaction conditions were achieved in the 10 cm
inside diameter tubular reactor (plug flow with some backmixing) and in
the jet stirred reactor (backmixed flow). Unstable conditions i.e. reaction
runaway were encountered in the 2.5 cm inside diameter tubular reactor
(plug flow). The 10 cm inside diameter tubular reactor and the jet stirred
reactors were stable because they operated at essentially isothermal
conditions i.e. reaction exothermicity was damped by heat losses and/or
backmixing.
The reaction strike temperature is about 300° C and above 450° C
demethylation of the b-picoline occurs and CCMP begins to over-
chlorinate rapidly. Operation of a plug flow reactor with a CTC diluent feed
rate sufficient to maintain approximately isothermal conditions (say inlet of
350° C and outlet of 425° C) is uneconomic. Also, the heat transfer
through the walls of a plug flow reactor will be limited by fouling. Thus,
with this reaction it is not possible to control the temperature (and hence
conversion) in a plug flow reactor. Reaction runaway will occur.
However, it is possible to maintain isothermal conditions in a backmixed
reactor with an economic feed rate of CTC diluent by using "cold" inlet
feed gases. Thus, the design strategy is to combine a backmixed reactor
(to take the reaction through its fast initial stages) with an adiabatic plug
flow reactor (to finish off the reaction, and to increase the final CCMP
yield).

If a large amount of the reaction is carried out in the backmixed stage,
high stability is obtained at the cost of a low yield. If a large amount of the
reaction is carried out in the plug flow stage, then the yield is higher but
the stability is poorer. The optimum was determined using the kinetics
model. It predicted that the optimum conversion in the backmixed stage
was about 2.8 moles of chlorine per mole of b-picoline, i.e. about 70% of
the total reaction. The remaining 1.2 moles of chlorine are added in the
plug flow stage. The model predicted that the overall yield would then be
just over 50% i.e. very similar to the yield in a one stage plug flow reactor.
FIGURE 2 CHLORINATION OF b-PICOLINE : MODEL PREDICTIONS
OF PRODUCT DISTRIBUTION IN FULLY-MIXED
REACTOR

7 OPERATING CONDITIONS AND REACTOR DESIGN
The CTC diluent feed ratio was set at 20:1 (CTC: b-picoline) from enthalpy
and heat of reaction considerations. CTC is essentially inert in high
temperature chlorine rich systems, is easily vaporized, has a high molar
heat capacity and is a convenient solvent for the reaction products in the
downstream recovery system. CTC also decreases the flammability
hazards (mixtures of b-picoline and chlorine are flammable at some
concentrations).
The chlorine feed ratio was set at 10:1 (chlorine : b-picoline) both to
ensure appreciable excess for minimizing under-chlorination reactions and
to maintain a reasonable reaction rate for the slow (final) ring chlorination
step.
The maximum permitted plug flow reactor temperature was assumed to be
450° C (to minimize CCMP over-chlorination). In this reactor 1.2 moles of
chlorine are added to each mole of b-picoline. With a mix composition of
1:10:20 (b-picoline : chlorine : CTC), if operation is with an outlet
temperature of 430° C, then the inlet temperature has to be 375° C. The
isothermal operating temperature of the backmixed reactor is equal to the
inlet temperature of the plug flow reactor, i.e. 375° C.
In the backmixed reactor, for a given reactor temperature and inlet feed
ratio, the inlet feed gas temperature required to absorb the heat of
reaction can be obtained by a heat balance, if the heat capacities and
heats of formation of the reactants and products are known. For this
reactor, operating at 375°C, and a feed ratio of 1 : 10 : 20, the inlet gases
have to be at 240°C.
At 375° C in a backmixed reactor, the kinetic model predicts that a
residence time of 6.0 seconds is required to achieve 2.8 moles of chlorine
per mole of b-picoline. The corresponding reactor volume is 0.28m3. The
residence time in the plug flow reactor to effect conversion to 4.0 moles of
chlorine per mole of b-picoline was predicted to be 8.5 seconds under
diabatic conditions. The corresponding reactor volume is 0.41m3. The
conversion/time plot is shown in Figure 3.

FIGURE 3 TWO-STAGE REACTOR: RATE OF CHLORINATION
OFPICOLINE
Thus the required reactors are:
(a) Primary chlorinator: Backmixed: isothermal: volume = 0.28 m3
(b) Secondary chlorinator: Plugflow: adiabatic: volume = 0.41 m3
In the case of the primary chlorinator analogies were drawn with the
Stauffer chloromethanes process and the Kellogg 'Kelchor' process in that
both processes involve fast, highly exothermic reactions in 'draft-tube'
backmixed reactors. The reactor takes the form of a vertical vessel with
a centrally-mounted internal draft-tube into which the feed gases are
injected. Momentum effects at the entrance to the draft-tube induce flow
back down to annulus thus effecting mixing and entrainment of hot
reaction gases with cold incoming gases. The high degree of mixing
enables effective control of the temperature rise in the reactor by altering
the inlet temperature or the flow of diluent vapor. An added advantage is
the low energy requirement of this type of reactor. The primary chlorinator
was, therefore, designed in the form of a draft-tube reactor based on the
Kellogg principles.

The pertinent dimensions of the reactor are:
(1) Vessel inside diameter 400 mm
(2) Draft tube NB 250 mm
(3) Draft tube Length 1500 mm
(4) Inlet nozzle NB 38 mm
(5) Bottom inlet, top outlet NB 100 mm
(6) Vessel volume 0.282 m3
The combined carbon tetrachloride/chlorine stream is jet-mixed with the b-
picoline stream in the inlet nozzle prior to injection into the draft-tube. The
predicted recirculation rate at design rates is 4 times the inlet flow. This
rate is important in raising the temperature of the incoming gases quickly
up to reaction temperature.
The secondary chlorinator comprises 50 m of 100 mm NB tubing. This
reactor will be lagged and trace-heated to offset ambient heat losses.
The material of construction for both reactors is Inconel 600. The nominal
operating pressure of the primary chlorinator has been set at 3.0 bar a: the
design pressure has been set 21.7 bar a on the grounds that should a
flammable mixture ignite in the reactor the explosion pressure rise is not
expected to be greater than an order of magnitude.
8 REACTOR STABILITY AND CONTROL
The backmixed reactor temperature was chosen as the control parameter.
It can be set by varying the feed ratio of b-picoline to CTC according to the
inlet temperature and rate of reaction. The kinetic model was run using
different feed ratios for a constant reactor temperature of 375 °C. The
model predicted that, within reason, conversion was independent
of feed ratio (this seems to be due to the CTC flow being the major flow
and hence residence time) concentration effects complemented each
other. The model was then used to check whether a fixed backmixed
reactor temperature stabilized final conversion in the plugflow reactor: the
backmixed temperature was fixed at 375 °C and the flows varied over a
wide range. The model predicted that CCMP yields in the range 40 to 50%
could be achieved for flow variations of 100% above and 30% below
design rate. This represents a significant improvement in reactor stability.
Hence, a control strategy based on fixing the backmixed reaction
temperature will be used.

The control strategy of the reactors is thus to set up a predetermined CTC
flow dependent on the desired CCMP production rate. Ratioed flows of b-
picoline and chlorine (1:10) are bled into the backmix reactor until the
desired reaction temperature is established. Changes in process
conditions are compensated for by automatic increase or decrease of b-
picoline flow.
The degree of chlorination is altered by altering the first stage reaction
temperature. The advantages of this control strategy are that CTC is the
major flow which largely sets the system pressure drop. Hence, start-up of
the chlorinators can be affected prior to reaction, and an increase in
reaction temperature will tend to cause a reduction in b-picoline flow
thereby throttling the "fuel" source and lessening the hazard potential.
The kinetics model was also used to predict reactor response to changes
in feed conditions in order to gauge stability. For a 30% increase in b-
picoline flow, the predicted reaction temperature in the backmixed reaction
rises by more than 100° C. It follows that an increase in b-picoline flow
without a corresponding change in inlet temperature will lead to a
significant loss of product. Thus, an operating strategy based on control of
inlet flow is unacceptable. For a feed temperature 10° C above the design
inlet temperature the reaction temperature increased by 25° C whilst a fall
of 10° C results in a 50° C drop in reaction temperature. The former
change results in little product loss whilst the latter change results in highly
under-chlorinated products. Wide fluctuations in final conversion from 50%
to less than 10% were obtained. Hence, a control strategy based on inlet
temperature is also unacceptable.
Note:
The above considerations are dependent to a large extent on a stable
reaction zone. This assumes that the reaction mixture is all vapor, is well
mixed and diluent flow is in excess of 2 moles of CTC per mole of b-
picoline. Process management and instrument protection will be of the
utmost importance in ensuring stable reaction conditions at all times.

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following document:
GBHE-PEG-RXT-818 Tools for Reactor Modeling

Reactor Arrangement for Continuous Vapor Phase Chlorination

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