Unit operations, control product quality

The field of chemical process miniaturization is growing at a rapid pace with promising improvements in process control, product quality, and safety, (1,2). Microreactors typically have fluidic conduits or channels on the order of tens to hundreds of micrometers. With large surface area-to-volume ratios, rapid heat and mass transfer can be accomplished with accompanying improvements in yield and selectivity in reactive systems. Microscale devices are also being examined as a platform for traditional unit operations such as membrane reactors in which a rapid removal of reaction-inhibiting products can significantly boost product yields (3-6). 

In the case of the studied FCC unit, it is found that several rules are already used for control purposes. It can be also seen from the results that the temperature of the catalyst is an important attribute because if it increases in the range of possible operation, the product quality, MON also increases. The temperature of the catalyst is fixed in the regenerator where the coke being on the catalyst s surface is burned away. The coke is formed in the reactor part of the FCC during the cracking of heavy hydrocarbons. 

Then design the composition control loops for each unit operation. Buckley called these the product-quality loops. Determine the closedloop time constants of these product-quality loops. 

Tests have been conducted on wet and dry cement processes in the United States, Canada, Sweden, and Puerto Rico for wastes containing a wide range of chlorinated chemicals, including PCBs (13-16). Generally, the DKEs have been found to be in the range of 99.99%, with no adverse impact on product quality or plant operation if chlorine addition is restricted to less than 1% of the net fuel/waste feed. DKEs of less than 99.99% have been observed, however, where poor control of combustion air exists and waste is inadequately atomized, even when an acceptable cement product is being produced. 

Most operations scaled up in the pharmaceutical industry use semibatch (and batch) processing in the general-purpose equipment. Such operations allow for fine control of slow unit operations, for example, reactions needing hours to complete, fermentation, and crystallization, and such fine control may be necessary to ensure high quality and productivity. 

Computer Control. The use of computer systems to control the operation of submerged arc furnaces, including calcium carbide, has been successfully demonstrated in the United States (see Expert systems Process control). Operations direcdy under control are mix batching, electrode position and slip control, carbide gas yield, power control, and cooling water systems. Improvements in energy usage, operating time, and product quality are obtained. 

Process characterization represents the methods used to determine the critical unit operations or processing steps and their process variables, that usually affect the quality and consistency of die product outcomes or product attributes. Process ranging represents studies that are used to identify critical process or test parameters and their respective control limits, which normally affect the quality and consistency of the product outcomes of their attributes. The following process characterization techniques may be used to designate critical unit operations in a given manufacturing process. 

Control objectives related to the operation of the process units and the process itself (production rate, product quality, unit-level, and total inventory) should be addressed in the fast time scale. For instance, when a multi-loop linear control strategy is considered, the reset time for the controllers should be of the order of magnitude of the time constants of the individual process units. 

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