Semibatch method

Removing a reaction by-product efficiently can be necessary for good product yields and purities. In the semibatch method of preparing pentenoic acid ethyl ester (see Figure 13.9), efficient reaction of allyl alcohol and removal of the byproduct EtOH were necessary [15].The allyl alcohol was introduced subsurface, to prevent loss by flash volatilization in the head space over the reaction surface. EtOH was displaced from the reaction by operating at a temperature significantly above EtOH s boiling point. A low reflux ratio of about 4 1 (moles of liquid returned to the system moles of EtOH collected as distillate [16]) was used to ensure that the EtOH was not returned to the reaction. 

The earliest polymerization processes were either batch mode or semibatch. The semibatch method was used for products, where the two monomers differed greatly in reactivity, as in Union Carbide s early Dynel, acrylonitrile-vinyl chloride, process. Bulk, solution, and emulsion polymerization processes have also been developed for acrylonitrile and its copolymers. However, in recent years nearly every major acrylic fiber producer has used a continuous aqueous suspension process, employing a redox catalyst, followed by a series of steps, which includes slurry filtration and polymer drying. 

Adsorption of liquids percolation The dehydration of liquids by stationary beds of adsorbent has already been mentioned. In addition, the colors of petroleum products, such as lubricating oils and transformer oils, and of vegetable oils are commonly reduced by percolation through beds of decolorizing clays sugar solutions are deashed and decolorized by percolation through bone char and many other liquid-treating operations use these semibatch methods. 

The unsteady-state operations include those where the solids and liquids are contacted in purely batchwise fashion and also those where a batch of the solid is contacted with a continually flowing stream of the liquid (semibatch method). Coarse solid particles are usually treated in fixed beds by percolation methods, whereas finely divided solids, which can be kept in suspension more readily, can be dispersed throughout the liquid with the help of some sort of agitator. 

The semibatch method of producing copolymers of uniform composition (Figure 11.4a) can be used in conjunction with all modes of polymerization but suspension. Why ... 

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

Font, R. and Laveda, M.L., 1996. Design method of continuous thickeners from semibatch tests of sedimentation. Chemical Engineering Science, 51, 5007-5015. 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

For semibatch or semiflow reactors all four of the terms in the basic material and energy balance relations (equations 8.0.1 and 8.0.3) can be significant. The feed and effluent streams may enter and leave at different rates so as to cause changes in both the composition and volume of the reaction mixture through their interaction with the chemical changes brought about by the reaction. Even in the case where the reactor operates isothermally, numerical methods must often be employed to solve the differential performance equations. 

The pulse reactor method is similar to semibatch in that all the ingredients except HCN are placed in a small, well-mixed vessel in a thermostated bath. Very small amounts of HCN are then rapidly injected into the reaction mixture with vigorous mixing and the exotherm is monitored. Repeated pulses are made only after the reaction mixture has come back to temperature equilibrium with the bath. In this manner, kinetic information may be obtained. 

Whereas much mechanistic information can be obtained by one of the above methods, any practical applications must be demonstrated under conditions similar to process operation, i.e., continuous flow. Small glass reactors which allow controlled addition of reagents by syringe pump and continuous removal and monitoring (IR spectroscopy) of product mixture have been developed. Much of the information obtained from semibatch operation has been reproduced under these continuous flow conditions. 

Process. Commercial processes manufacturing lalex can he div ided into batch, semibatch, and continuous methods... 

In Chapter 3, the analytical method of solving kinetic schemes in a batch system was considered. Generally, industrial realistic schemes are complex and obtaining analytical solutions can be very difficult. Because this is often the case for such systems as isothermal, constant volume batch reactors and semibatch systems, the designer must review an alternative to the analytical technique, namely a numerical method, to obtain a solution. For systems such as the batch, semibatch, and plug flow reactors, sets of simultaneous, first order ordinary differential equations are often necessary to obtain the required solutions. Transient situations often arise in the case of continuous flow stirred tank reactors, and the use of numerical techniques is the most convenient and appropriate method. 

A method of measuring kya which has gained wide acceptance in recent years is the dynamic method using fast dissolved oxygen probes. The rate of change of oxygen tension in a semibatch... 

Finally, some remarks on the operation of mechanically agitated gas-liquid reactors are worth mentioning. The mode of operation (i.e., batch, semibatch, continuous, periodic, etc.) depends on the specific need of the system. For example, the level of liquid-phase backmixing can be controlled to any desired level by operating the gas-liquid reactor in a periodic or semibatch manner. This provides an alternative to the tanks in series or plug flow with recycle system and provides a potential method of increasing the yield of the desired intermediate in complex reaction schemes. In some cases of industrial importance, the mode of operation needs to be such that the concentration of the solute gas (such as Cl2, H2S, etc.) as the reactor outlet is kept at a specific value. As shown by Joshi et al. (1982), this can be achieved by a number of different operational and control strategies. 

Compositional control for other than azeotropic compositions can be achieved with both batch and semibatch emulsion processes. Continuous addition of the faster reacting monomer, styrene, can be practiced for batch systems, with the feed rate adjusted by computer through gas chromatographic monitoring during the course of the reaction (54). A calorimetric method to control the monomer feed rate has also been described (8). For semibatch processes, adding the monomers at a rate that is slower than copolymerization can achieve equilibrium. It has been found that constant composition in the emulsion can be achieved after ca 20% of the monomers have been charged (55). 

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