Improving batch process productivity

Methods have been developed for improving batch process productivity in the manufacture of styrene-butadiene latex by the continuous addition of reactants so the reaction occurs as the reactor is being filled. These are not continuous processes even though the reactants are added continuously during most of a batch cycle. The net result is that reactants can be added almost as fast as heat can be removed. There is relatively little hazardous material in the reactor at any time because the reactants, which are flammable or combustible, are converted to non-hazardous and non-volatile polymer very quickly. 

The above batch process has undergone numerous refinements to improve yields, processing characteristics, purity, and storage stabiUty, but it remains the standard method of manufacture for these products. Recentiy a continuous process has been reported by Bayer AG (6) wherein the condensation is carried out in an extmder. The by-products are removed in a degassing zone, and the molten polymer, mixed with stabilizers, is subsequendy cracked to yield raw monomer. 

The majority of thermal polymerizations are carried out as a batch process, which requires a heat-up and a cool down stage. Typical conditions are 250—300°C for 0.5—4 h in an oxygen-free atmosphere (typically nitrogen) at approximately 1.4 MPa (200 psi). A continuous thermal polymerization has been reported which utilizes a tubular flow reactor having three temperature zones and recycle capabiHty (62). The advantages of this process are reduced residence time, increased production, and improved molecular weight control. Molecular weight may be controlled with temperature, residence time, feed composition, and polymerizate recycle. 

Initially, all of the SBR polymer known as GR-S produced during World War II was by the batch process. Later, it was thought that a higher volume of polymer would be needed for the war effort. The answer was found in switching from batchwise to continuous production. This was demonstrated in 1944 at the Houston, Texas, synthetic mbber plant operated by The Goodyear Tire Rubber Company. One line, consisting of 12 reactors, was lined up in a continuous mode, producing GR-S that was mote consistent than the batch-produced polymer (25). In addition to increased productivity, improved operation of the recovery of monomers resulted because of increased (20%) reactor capacity as well as consistent operation instead of up and down, as by batchwise polymerisation. 

In this case, the co-solvent dosage rate is programmed in order to control the transient level of supersaturation in an effort to improve on the product crystal size distribution from simply dumping in all the solvent at the start of the batch. An experimental crystallizer within which a programmed microcomputer determines the set point of a variable speed-dosing pump is shown in Figure 7.7. Controlled co-solvent dosing improves the product crystal size, with a consequent increase in the filterability of the product. These process concepts are developed further in Chapter 9. 

OS 51] [R 17] [R 19] [R 26] [P 37] Methane and hexane nitration were successfully performed under safe conditions in micro reactors [37]. The very preliminary character of these experiments did not allow any detailed process information to be derived apart from demonstrating feasibility and the fact that selectivity was low in the first runs. No significant improvement over the batch processing was obtained here. The formation of many imdesired products was particularly high for nitration of hexane [37]. 

In comparison to batch processing, this process operates at the final concentration with associated low fluxes. The use of multiple stages where the retentate outlet from one stage feeds the next allows successive systems to operate at different fluxes and will improve membrane productivities. 

Baking is currently performed by continuous operation. Modern variations involve using heated crushing or milling equipment, such as kneader dispersers or oscillating mills at approximately 200°C [9]. This technique significantly improves the reaction control over a batch process. If baking is performed by continuous process, phthalonitrile only remains within the reaction vessel for a very short period of time (between 3 and 20 minutes). It is important to remember that the temperature may not exceed 250°C. The product which evolves from this process is usually purified by acid treatment. 

Cost reductions usually arise out of improvements to the process control for both continuous and batch processes. Process analyzers enable chemical composition to be monitored essentially in real time. This in turn allows control of the process to be improved by shortening start-up and transition times (for continuous processes) or batch cycle times (for batch processes). This is accomplished by improving the ability to respond to process disturbances, by enabling process oscillations to be detected and corrected, and by reducing product variability. Real-time monitoring of chemical composition in a process allows a manufacturing plant to ... 

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