Temperature control for semi-batch reactor

SEMIEX - Temperature Control for Semi-Batch Reactor... 

Temperature Control for Semi-Batch Reactor 430 Parallel Reactions in a Semi-Continuous Reactor 347 Sequential-Parallel Reactions in a Semi-Continous Reactor 350... 

In semi-batch or continuous operation, the feed rate allows control of the reaction course. Flence it plays an important role concerning the safety of the process. With an exothermal reaction, it is important to be able to limit the feed rate by technical means. One possibility is feed by portions, a method that is only applicable for semi-batch reactors. This mode of addition is the traditional way of limiting accumulation. In this case, the addition must be controlled by the conversion that is, the next portion is added only if the previous portion has been consumed by the reaction. Different criteria can be used to follow the reaction the temperature, the appearance of the reaction mass, chemical analysis, and so on. For a well designed process, the additions can also be performed on a time basis. 

Generally, the temperature changes with time or, equivalently, with distance from the reactor inlet (for flow reactors). This change is usually controlled well in reaction calorimeters but can become uncontrolled in other conventional laboratory flow or (semi)batch reactors. The balance equations of a batch reactor for a single reaction of a-th order kinetics are given by ... 

A process is described [224] in which an exothermic reaction takes place in a semi-batch reactor at elevated temperatures and under pressure. The solid and liquid raw materials are both toxic and flammable. Spontaneous ignition is possible when the reaction mass is exposed to air. Therefore, the system must be totally enclosed and confined in order to contain safely any emissions arising from the loss of reactor control, and to prevent secondary combustion reactions upon discharge of the materials to the atmosphere. Further, procedures and equipment are necessary for the safe collection and disposal of solid, liquid, and gaseous emission products. 

In addition to these three principal types of reactor, there is also the semi-batch reactor in which one reactant is added gradually to the others. This is a convenient manner of operation for some highly exothermic reactions since the temperature can be controlled by adjusting the rate of addition. 

This is the simplest way of temperature control for a semi-batch reactor only the temperature of the cooling medium is controlled. The temperature of the reaction... 

Additionally the semi-batch reactor with constant cooling medium temperature, also in cases where a stationary temperature can be achieved, shows a high sensitivity to its control parameters, that is, initial temperature and coolant temperature. This means that even for small changes in these temperatures, the behavior of the reactor may suddenly change from a stable situation into a runaway course. 

This is the most common mode of addition. For safety or selectivity critical reactions, it is important to guarantee the feed rate by a control system. Here instruments such as orifice, volumetric pumps, control valves, and more sophisticated systems based on weight (of the reactor and/or of the feed tank) are commonly used. The feed rate is an essential parameter in the design of a semi-batch reactor. It may affect the chemical selectivity, and certainly affects the temperature control, the safety, and of course the economy of the process. The effect of feed rate on heat release rate and accumulation is shown in the example of an irreversible second-order reaction in Figure 7.8. The measurements made in a reaction calorimeter show the effect of three different feed rates on the heat release rate and on the accumulation of non-converted reactant computed on the basis of the thermal conversion. For such a case, the feed rate may be adapted to both safety constraints the maximum heat release rate must be lower than the cooling capacity of the industrial reactor and the maximum accumulation should remain below the maximum allowed accumulation with respect to MTSR. Thus, reaction calorimetry is a powerful tool for optimizing the feed rate for scale-up purposes [3, 11]. 

The experiments were conducted in a glass semi-batch reactor allowing for temperature control, reagent addition via peristaltic feed pumps and mechanical agitation. Furthermore, the 2L reaction vessel had baffles to improve mixing conditions. 

Two semi-batch reactors were used in this study. The slurry reactor was a 0.5-liter jacketed glass reactor. The slurry reactor was operated at atmospheric pressure. The gas-phase reactor was a one-liter stainless steel reactor which was immersed in an oil bath for temperature control. The solids in the gas-reactor were kept in suspension by an AE MagneDrive stirrer. 

This temperature dependency is exploited in optimal control problems of batch reactor where optimal temperature profile is obtained by either maximizing conversion, yield, profit, or minimizing batch time for the reaction. One of the earliest works on optimal control of batch reactor was presented by Denbigh[25] where he maximized the yield. The review paper by Srinivasan et al.[26] describes various optimization and optimal control problems in batch processing and provides examples of semi-batch and fed-batch reactor optimal control. 

The condition for the practical implementation of such a feed control is the availability of a computer controlled feed system and of an on-line measurement of the accumulation. The later condition can be achieved either by an on-line measurement of the reactant concentration, using analytical methods or indirectly, by using a heat balance of the reactor. The amount of reactant fed to the reactor corresponds to a certain energy of reaction and can be compared to the heat removed from the reaction mass by the heat exchange system. For such a measurement, the required data are the mass flow rate of the cooling medium, its inlet temperature, and its outlet temperature. The feed profile can also be simplified into three constant feed rates, which approximate the ideal profile. This kind of semi-batch process shortens the time-cycle of the process and maintains safe conditions during the whole process time. This procedure was shown to work with different reaction schemes [16, 19, 20], as long as the fed compound B does not enter parallel reactions. 

Feed by portions this method, presented in Section 7.8.1, is obviously only applicable to discontinuous processes as semi-batch. It reduces the amount of reactant present in the reactor, that is, the accumulation and therefore the energy that may be released by the reaction in case of loss of control. The amount allowed in one portion can be determined in such a way that the maximum temperature of the synthesis reaction (MTSR) does not reach a critical level as the maximum temperature for technical reasons (MTT) or the temperature at which secondary reactions become critical (TD24). The difficulty is to ensure that an added portion has reacted away, before adding the next portion. Generally, the feed control is performed by the operator, but can also be automated. 

In semi-batch operation, the SCISR is first filled with a solution of sodium silicate with certain concentration, and then a sulfuric acid solution of a given concentration is dripped at a certain rate into the reactor to react with the sodium silicate at a controlled temperature. The reaction continues for a certain interval of time after the dripping has finished. Stirring is then stopped for ageing of the precipitate for a term, and then the precipitate is sampled and the sample is measured with a laser particle-measuring instrument of FAM type to obtain the sizes and size distribution of the particles in the wet product. 

For fast exothermic reactions, temperature control can be a problem. This is often solved by external circulation of part of Ihe reactor content through a heat exchanger, or by adding an internal heat exchange area. Alternatively, semi-batch operation can be applied, i.e., part of a reactant can be fed steadily over time or at certain intervals. Ihis also minimizes the occurrence of unwanted side reactions. 

The experiments were carried out in a semi-batch stirred reactor with continuous oxygen feed. The reactor is a one liter Pyrex flask with flattened bottom and baffles which is fitted with five standard 24/40 necks to accomodate the gas inlet, gas vent, sampling tube, and pH electrode. The reactor is immersed in a standard water bath for temperature control. Gas is fed at a flow rate of 3.0 + 0.1 1/min through a rotameter. The solution is stirred at 1620 rpm. 

Continuous processes are preferred for large-scale production of commodities or intermediates, as in basic organic, inorganic, petrochemical and polymer industries. The boundary might be placed between 5000 to 10000 tonne/year. Batch reactors are more difficult to control. Therefore, continuous processes might be suitable even for small rates of dangerous products, which could be at best produced and consumed on site, and not stored and transported. Semi-batch processes may also be suitable for temperature sensitive reactions. 

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