Batch heating and cooling of fluids

Heating or cooling of process fluids in a batch-operated vessel is common in the chemical process industries. The process is unsteady state in nature because the heat flow and/or the temperature vary with time at a fixed point. The time required for the heat transfer can be modified, by increasing the agitation of the batch fluid, the rate of circulation of the heat transfer medium in a jacket and/or coil, or the heat transfer area. Bondy and Lippa [45] and Dream [46] have compiled a collection of correlations of heat transfer coefficients in agitated vessels. Batch processes are sometimes disadvantageous because  

Cleaning or regeneration is an integral part of the total operating period. 

The following diseusses various heating or eooling proeess eon-ditions in a bateh vessel and the proeessing time relationships. 

BATCH HEATING INTERNAL COIL, ISOTHERMAL HEATING MEDIUM 

Integration of Equation 7-100 from tj to tj while the bateh pro-eessing time passes from 0 to 0 yields  

The variables in batch heating or cooling processes are surface requirement, time, and temperature. Heating a batch may be by external means (e.g., a jacket or coil) or by withdrawing and recirculating process liquid through an external heat exchanger. In either case, assumptions are made to facilitate calculation, namely, 

The following discusses various heating or cooling process conditions in a batch vessel and the processing time relationships. 

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We equip batch and semi-batch reactors with temperature control that actuates heating and cooling mechanisms per need. In addition to controlling heating and cooling rates, we control reactant addition rate to semi-batch reactors with temperature. We most often achieve fluid level control via weigh cells, either under the feed vessel or under the reactor. We also use metering pumps to control the fluid level in the reactor. Basically, we do not over-burden a batch or semi-batch reactor or process with instrumentation. Because of their simple construction and simple control schemes, batch and semi-batch reactors are relatively inexpensive to obtain and to install. 

Pasteurization may be carried out by batch- or continuous-flow processes. In the batch process, each particle of milk must be heated to at least 63°C and held continuously at this temperature for at least 30 min. In the continuous process, milk is heated to at least 72°C for at least 15 s ia what is known as high temperature—short time (HTST) pasteurization, the primary method used for fluid milk. For milk products having a fat content above that of milk or that contain added sweeteners, 66°C is requited for the batch process and 75°C for the HTST process. For either method, foUowiag pasteurization the product should be cooled quickly to <7.2° C. Time—temperature relationships have been estabHshed for other products including ice cream mix, which is heated to 78°C for 15 s, and eggnog, which must be pasteurized at 69°C for 30 min or 80°C for 25 s. 

Then, the quantity of heat that could be removed in batch reactors whose volume varies from 11 to 1 m is calculated. In order to compare with experimental results, the temperature gradient is fixed at 45 °C (beyond which water in the utility stream would freeze and another cooling fluid should be used). The maximum global heat-transfer coefficient is estimated at an optimistic value of 500 W m K h The calculated value of the global heat transfer area of each batch reactor. A, is in the same range as the one given by the Schweich relation [35] ... 

Start up of a jacketed batch reactor requires control of the heat-up and cool-down rates. This involves determining and setting the jacket heat transfer fluid temperatures. An alternative is to make a trial heat-up and incorporate the results into a time-dependent heat transfer equation ... 

Compared with batch reactors, tubular reactors have the advantage of easier heat removal or supply Heat release or consumption at the entry of a tube is as great as in a batch reactor at start, but the surface-to-volume ratio is more favorable, and the entering fluid can help to cool or heat. A disadvantage compared with a batch reactor is that a tube at steady state, like a CSTR, gives information only on the conversion achieved at the conditions of the respective experiment, whereas one single batch experiments with samples taken at frequent intervals scans the entire conversion range. 

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