edited by
real world of
W. C. FERNELIUS Kent State Univemity Kent. OH 44242 HAROLD WITTCOFF K W Chemicals Ltd. Beer-Sheva, Israel P.O.B. 60
Chemical Processing-Batch or Continuous Part l S. M. Englund 800 Building, Dow Chemical, Michigan Division, Midland, MI 48640 For years many chemical engineers have accepted the guiding principle that chemical processes should he continuous if ~ossihle.In continuous processes reactants and other components are added continumsly and the product is continuously removed from the reactor. The continuous reactor may consist of pipe coils or combinations of pipes and tanks in series, or even a single tank provided with a means for continuous withdrawal of the product. Tanka may he agitated (stirred) or nonagitated. Batch processes, in which the ingredients are brought together and reacted in an agitated tank, had the stigma of "pots and pans" chemistry-unsophisticated processes that hadn't been "engineered." We all knew that as soon as a good chemical engineer became involved the batch process would give way to a continuous one. Those familiar with the petroleum and petrochemical industries know that batch processing in those industries is rare. ' In hoth, giant continuous plants with enormous capacities are almost the rule. However, there is a large segment of the chemical industry in which hatch processing is still "king" and probably will be for a long time. Poly(viny1 chloride), for example, popularly known as PVC or "vinyl," is manufactured in billion-pound quantities, almost all by batch processes known as suspension polymerization. Suspension polymerization as well as emulsion polymerization are used in the hatch production of other lesser volume plastics. In emulsion polymerization, very small droplets of a material are held dispersed in water by use of an emulsifying agent, such as soap, which keeps the particles from coalescing. The product is called a latex. Suspension polymerization is similar, hut the drodets are laraer and are stahilized with protective colloids such as meth;lcellulose. Although most synthetic rubber is made hv continuous emulsion polvmeriiation, a large amount of emulsion polymerized latex is manufactured by hatch processing. Continuous Processing
The obvious advantage of continuous chemical processing is elimination of the "dead time" lost in the repeated starting and stopping of most hatch processes. Continuous processes are also usually easier to control and often facilitate the manufacture of uniform products. I t is often more difficult than it might first appear to convert a batch process to a continuous process that will yield the same end product. Figures IA, 1B, and 1C are three methods of continuous processing which are sometimes considered as equivalent replacements for a batch process. However, these continuous processes are not equivalent to batch processes because new reactants are continuously being mixed into the reaction 766
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Reactants added contlnu~usly
Healing w cooling
C Figure 1. Continuous processes that attempt to duplicate batch processes. A, stirred tank: B, pipe reactor; C, m i l reactor.
products. The reactor therefore contains a mixture of unreacted ingredients and of product with varying reactor time history. This history is not the same as that produced in a batch reactor. "Back-mixing," the mixing of reactants with product, is the culprit that causes the continuous processes shown in Figure 1not to be equivalent to batch processes. I t is obvious that the process in 1A and 1C have hack-mixing. The process shown in 1B also has back-mixing because wall drag produces a nonuniform flow. If one could develop a "plug flow" reactor, such as is shown in an idealized way in Figure 2, there would
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Figure 2. Idealized "plug flow" continuous reactor.
k Figure 4. Conventional batch process-manual
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20 Time IMm)
Figure 5. Reactor and jacket temperature in conventional batch Processmaximum rate of reaction with goad temperature control. Figure 3. Continuous processes that are nearly equivalent to a batch process. A. stirred tanks in series; B, baffled continuous stined tank. (Mixing Equipmew Co., Rochester. New York).
he no hack-mixing, and the product ohtained with this hypothetical reactor would he the same as one produced in a batch reactor. 'I'hese results can he arhieved, at least approximately, by such methods as those shown in Figure 3. Several stages are required, sometimes as many as 10 or 20. The total reactor volume of an "ideal" continuous reactor process with "plug flow" is the same as the volume of a hatch reactor for the same process. A "real" continuous process will not be perfectly "plug flow" and will have somewhat larger total reactor volume than an "ideal" continuous reactor. Continuous processes of the type shown in Figure 3 are best suited for making products at fairly high rates where one or at most a few different products are required. Usually, for small-volume products, a single batch reactor will he simpler and less expensive than a series-type continuous reaction system. In continuous nrocesses changing is time-con. products . suming and ran produce large amounts of off-gradematerial. For t h ~ reason. s . lona- .i~rodurtionruns of one r~roducrare desirahle. On the other hand, development andscale-up of new products may he more time-consuming with continuous processes than with batch processes. This is the result of the sienificant time required in both lahoratory-scale and prodiction-scale contikous process to reach a steady state where reliable operating.data and product samples can he ohtained. For example, a.liquid-phase continuous process with an average residence time of 4 hr may require 10-16 hr to reach steady state where reliable operating data may be ohtained. In comparison, in a hatch process with a reaction time of 4 hr reaction data and the final product will he available in 4 hr. Continuous nrocesses are not well suited for products that tend to build u i on reactor surfaces, since it may be necessary to shut the reactor down periodicdlv and drain it for cleanout. In batch processes the reactor is emptied after each run and the reactor can he cleaned each time to avoid excessive buildup. The cleaning process may he fully automated. ~
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Conventional Batch Technology In conventional hatch processes the ingredients are usually added manually to an agitated vessel and the temperature is raised to the required level by heating the reactor jacket or coils in the reactor. When the exothermic reaction begins, the vessel is cooled by circulating coolant in the jacket or coils as shown in Figure 4. Automatic temperature control to a fixed temperature set point is usually provided. This process creates many problems which are discussed in the sections below. Safety
Chemical reactions are usually exothermic (heat producing) and the reaction rate increases exponentially with temperature. Loss of temoerature control of the reactor can result from lass of cooling or agitation, or from errors in loading which can easilv occur when loadine" is done manuallv. This can result in high pressure in the reactor which can cause some or most of the contents to he lost through the emergency pressure relief system. In extreme cases the reactor can blow up. I t is usuallv more difficult to design a suitable emergencv - - pressure relief system for a conventional batch process than a continuous process. ~~
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Heat Transfer Heat transfer means the removal or addition of heat, usually by conduction through the metal wall of a tank or pipe. The term "heat transfer coefficient" is a measure of the resistance to the flow of heat across the tank or pipe wall. If the heat transfer coefficient hetween the reactor contents and the jacket fluid is constant, the maximum rate of heat transfer availahle -~~~~~~is ohtained when the temoerature difference hetween the reactor contents and the jacket fluid is maximum. However, there can be a significant change in the characteristics of the reactants during reaction, such as an increase in apparent viscosity that can greatly affect heat transfer characteristics. Assuming that the heat transfer coefficient does not vary significantly while the hatch is in the reactor, the maximum capability for transferring heat occurs when the cooling fluid in the jacket is at the temperature of the available cooling fluid. In many cases, particularly in polymerization reactions, the temperature profile shown in Figure 5 results. ~~
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Volume 59
Number 9
September 1982
767
ratio of monomer to water in a suspension or emulsion polymerization), temperature control may be lost, as shown in Figure 6. In this case when the "hot spot" is reached the reactor temperature rises instead of staying constant. This can result in poor quality products or a runaway reaction which can exert too much pressure on the vessel. Productivity
The process described in Figure 4 has poor productivity for two reasons: nm I h l
Reactor and jacket temperature in conventional batch reaction rate too fast-poor temperature conbol. Figure 6.
(1) Manual addition and process control is slow and is dependent on process-
This is a simplified temperature chart of an actual hatch reaction. The maximum reaction rate consistent with good temperature control is that rate which will cause the jacket temperature to fall to the available cooling fluid temperature (in this case 20°C). We usually call the time when this rate occurs the "hot spot" of the reaction, since this is the time in the reaction when heat is produced at the highest rate. I t is also the time when the product is being produced at the highest rate possible in this equipment. At no other time is the equipment producing its theoretical maximum. If we alter the chemistry of the process by adding more initiator, for example, or by increasing the ratio of reactants to nonreactants (such as the
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the performance of each task at the right time by the operator. In other words, the process can be "person limited," not equipment limited. (2) The maximum heat transfer capacity of the reactor is used for onlv a short time durine each batch and oroduct is oroduced at the reactor's theoretical maximum rate for only that short period. Energy
Even though the process shown in Figure 4 is exothermic, energy is required for heating the reactor to reaction temperature, and electrical energy is needed for a long time during each hatch for pumping cooling fluid. None of the reaction heat is used or recovered. Part I1 in the next issue will discuss recent developments in improved technology that may make batch processhg more attractive than continuous processing for certain applications.
