Batch Process or Continuous Process? Miri Kesner1 In general terms, we can relate to a production process in the chemical industry (as we can to any other production process) as the processing of inputs to outputs in a defined series of actions. The inputs include reactants, auxiliary materials, energy etc. The outputs include products, by-products, energy, etc. The series of actions is actually the conditions of reaction and activation of the chemical reaction. Production processes in the chemical industry can be carried out in several ways: batch, continuous or semi-continuous (in which certain parts are done continuously and some in batch form). The batch process is a single- or multi-stage process in which a certain quantity of inputs (raw materials, auxiliary materials, energy, etc.) are fed into the chemical reaction unit (of the entire reaction) under conditions suitable for obtaining the desired reaction (temperature, pressure, required time, etc.). In the batch process, in the reactor and at any given period of time, various actions take place in the wake of which a concentration of reactants and products varies so long as the reaction progresses. At the conclusion of the process the mixture is removed from the reactor and it then undergoes the appropriate separation and processing stages (either physical or chemical) at the required level of cleanliness. This is generally dictated by the customers for whom the specific product is manufactured. In the batch process, so long as the batch has not undergone the entire series of actions, there is no possibility of preparing a further batch. The batch process can be undertaken in one reactor in which all the actions are carried out one after the other, or in a series of reactors in each of which a different stage of the process is carried out. The quality of the end product can also be controlled by the addition of appropriate separation stages between the various other stages as required. Reactants that do not react and which are separated from the reaction mixture can be returned for a further reaction (usually after 1 I am grateful to Mr. Yigal Antonir and Dr. Haim Strage of “Bromium Compounds” for their constructive remarks and help in the writing of this article. they have undergone treatment or cleaning for by-product contamination), thus fully exploiting the process. A good example of an “installation” working in batches is the washing machine, into which a certain quantity of dirty washing is put. The required inputs are water, electrical energy, washing powder, etc. A “batch” of laundry goes through various stages that are programmed as required: soaking, washing, various rinses, and extraction. All the actions take place in one receptacle and at the conclusion of the process we obtain wet laundry that is clean and ready for drying. From the washing machine the wet laundry is transferred to the drier, in which a “batch” process of separation/purification of the “end product” from water is also carried out, until dry laundry is obtained. This process can be illustrated with a flow chart showing the series of actions: Batch of dirty laundry Soaking Washing Rinse Extraction Washing machine Batch of clean, wet laundry Drying Drier Batch of clean, dry laundry There are washing machines in which the drying process is also carried out and then the entire process takes place in a single receptacle. The continuous process is one in which inputs are fed into the system at a constant rate and at preset ratios (raw materials, auxiliary materials, energy, etc.), and at the same time a constant extraction of outputs is done (products, by-products, energy, etc.). This process is characterized by a constant process taking place in each section of the facility and during the time of its action a constant process takes place. Thus, the concentration of reactants and products at every location in the system is in a durable state and control of the process is done by maintaining these concentrations. The batch and continuous processes can be compared to the production of the cocoa drink, known as “chocolate milk” and various other trade names, using two methods. When the drink is produced in “batch” form, measured amounts of cocoa, sugar and milk are added to the reactor. The materials are mixed and heated, and after a suitable delay, chocolate milk, whose color and taste are constant in accordance with the program, is obtained. Only after the conclusion of the process is the drink put into storage tanks and then the process can be restarted and a further batch of chocolate milk prepared. If no faults occur and the instructions are strictly observed, the second batch will be identical to the first. When chocolate milk is produced using the continuous method, cocoa, sugar and milk are injected into the reactor at the desired and constant ratios through the entry points. These concentrations will, in the end, determine the quality of the product obtained. At the same time, the finished product is extracted at a constant rate through the exit points. The process is planned so that while the ingredients are flowing through the reactor, they are heated and mixed. Because of the different way in which the process is carried out, chocolate milk that is not completely homogeneous will be obtained, and this is liable to contain a certain amount of undissolved sugar and cocoa that came out with the chocolate milk. In addition, there is a great probability that the color and taste of the drink will be subject to certain variations between the place at which they entered the dissolving and mixing unit, and the exit point of the chocolate milk at the conclusion of the process. These variations can, of course, be minimized to an agreed point (according to the prearranged permitted amount of cocoa and sugar that did not react), by setting the ratios of feeding, mixing rate, flow conditions, extraction, etc. In order to obtain chocolate milk identical to that obtained in the batch process, a further action on the product of the continuous process must be carried out, like, for example, filtration of the product and separation of the cocoa and sugar that did not dissolve in the milk. Their reintroduction to the process will not be carried out directly into the reactor (mixing and dissolving), but they will be combined with the flow of raw material entering the process. Their return will improve the process’s efficiency without harming product quality. Each of the production methods (batch/continuous) has its own characteristics. In the batch process, the shift from one stage to the next is carried out in series and so the overall time of the process is, in fact, the sum of the times required for the various stages, and it is relatively extensive. In the continuous process, all the stages are carried out simultaneously (although possibly in different parts of the system), and so the overall time required for the process is shortened. In contrast, the required volume of the tanks for a specific batch process is greater than that required for a parallel continuous process. For example, if a batch process takes one hour and the required output is one cubic meter per hour, a reactor whose volume is one cubic meter will be required. If we carry out a continuous process instead of a batch process, it will be shorter. Thus, if the continuous process takes half an hour and the required output is still one cubic meter per hour, a reactor whose volume is half a cubic meter will be required. Hence, the volume of the reaction systems in the batch process will be greater, and in consequence the investment in equipment will also be greater. In general terms, a batch installation requires more manpower while a continuous installation requires greater computerized and automated control. Experience shows us that over time, an initial investment in control is more feasible that the high day-to-day costs of manpower. Each method has its advantages and disadvantages, and so the decision on whether to build a specific installation using either the batch or continuous method depends on a complex system of considerations that is governed mainly by technological and economic considerations. The economic factors usually tilt the balance. The initial financial investment in the establishment of continuous installations is generally higher than that of batch installations. This is due to the automated control systems and despite the fact that the reactors in the batch installation are bigger. As a result of economic and technological considerations, products required in large quantities and for which there is a year-round demand, are usually manufactured in continuous installations. On the other hand, in the case of materials required in small quantities, or alternatively, for which the annual demand varies, it is unfeasible to build a continuous system. Production processes in chemical industry plants are often planned so that certain sections are continuous while others use the batch method. Planning of this kind is also derived from technological and economic considerations. The following table of comparisons between batch and continuous processes will be of assistance: Batch Process Types of materials Can be used with all types of materials (with non-flow materials, it is easier to use the batch process). Installation size Relatively large installations. Very big investment in land and installations. Reactor Changes occur in the concentrations of materials over time. Feeding raw materials Raw materials are fed before the start of the reaction. Control of the set of actions in the system Simple control. It is easier to control reaction conditions (pH, pressure, temperature). Manual control can also be done. Product(s) Extraction of materials only after all the actions are finished with the conclusion of the reaction. A fault or dealing with a batch requiring “repair” does not cause problems in the other stages. Appropriate tests are conducted after each stage. Trouble shooting Quantities produced Variety of products in the plant Product development stage Preferable when production of small quantities of a specific material are planned. Preferable when the plant produces a wide variety of materials and when the product is likely to be changed now and again, while using the same reactor. Preferable when the process is relatively new and still unfamiliar. In this case the initial investment is in a smaller batch reactor, and thus the economic risk is smaller. Continuous Process Easier for use with flowing materials (today, almost any material can be produced with the continuous process; investment cost is the decisive factor). Relatively small installations. Significant savings in land and installations. At all locations, conditions are constant over time (durable conditions). Constant feeding of raw materials during the entire reaction process. Complex control. Automatic control must be used. Control of reactor conditions is more difficult. Control must be exercised over the rate of flow of the materials. Continuous extraction of products at all times during the reaction. The installations are interconnected, so a fault in one causes a stoppage in all the others. Material that has been damaged cannot be repaired under the same working conditions. It must be isolated and the process restarted. Preferable for large scale production. Preferable for a central and permanent product. Preferable after the conclusion of all the stages of grossing-up and economic feasibility tests. Note! In both methods: Recycling of raw materials according to economic feasibility can be carried out. Required product quality can be attained in accordance with customer requirements and economic feasibility. Optimal efficiency can be attained according to reaction conditions and economic feasibility. Typical students’ errors: A number of difficulties have come to light with students with regard to their grasp of concepts linked to the batch and continuous processes. One of the most typical errors is the tendency to link the flow chart to the continuous and not the batch process. The error originates in the fact that the flow chart is perceived only as a succession of installations and not as a succession of actions that may take place in a batch installation, and also in one reactor. Another mistake is to link the possibility of recycling materials only to the continuous process. This is also apparently related to an erroneous perception of the flow chart. These students view the flow chart as describing only the continuous process, and therefore recycling can only take place in the continuous process. Reactant(s) Reaction Separation Product(s) Recycling (reactants that did not react) In addition, there are students who link product quality (its level of cleanliness) or the process efficiency with the method used. The students generally link better quality with the product manufactured using the batch process, and better efficiency with the continuous process. Apparently, the batch process, because it is non-automatic, is perceived by these students as a process that can be easily controlled. On the other hand, the continuous process has a higher efficiency because the reactants that did not react can be recycled. While teaching this subject it is important to note these errors and emphasize the characteristics of each of the methods and the differences between them. The table on the previous page will be of assistance in this.