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