Batch heating

The heat requirements in batch evaporation are the same as those in continuous evaporation except that the temperature (and sometimes pressure) of the vapor changes during the course of the cycle. Since the enthalpy of water vapor changes but little relative to temperature, the difference between continuous and batch heat requirements is almost always negligible. More important usually is the effect of variation of fluid properties, such as viscosity and boiling-point rise, on heat transfer. These can only be estimated by a step-by-step calculation. 

BATCH HEATING INTERNAL COIL, ISOTHERMAL HEATING MEDIUM... 

Batch heating internal coil isothermal heating... 

Fig ure 7-28. Batch heating through an external heat exchanger, isothermal heating medium. 

Batch heating/cooling of fluids external heat exchanger (counter-current flow) non-isothermal cooling medium... 

A major problem to be solved for multiproduct plants is the occurrence of disparities in the cycle times and size requirements for the different stages. In the following it will be assumed that the size factors as well as the cycle times of all units are independent on equipment size. This assumption is usually relaxed in further stages of the design. In case of batch heating and cooling, or reactors operated in semi-continuous mode, this is necessary in order to adopt the cycle times to the capacity of equipment, which is related to batch size. 

A simple example is the heating of a liquid. If the amount of the fluid is rather small (e.g., 1 kl day ), then batch heating is more economical and practical, with the use of a tank that can hold the entire liquid volume and is equipped with a built-in heater. However, when the amount of the liquid is fairly large (e.g., 1000 kl day ), then continuous heating is more practical, using a heater in which the liquid Hows at a constant rate and is heated to a required constant temperature. Most unit operations can be carried out either batchwise or continuously, depending on the scale of operation. 

Heat (6) to 210F, stir in (7), then pour into the batch. Heat (B) to 210F and add to the batch. Add (9) then (10) slowly to the batch, allowing complete mixing of each. 

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,... 

Figure 12-27. Temperature versus time plot from a semi-batch heat flow experiment. (Source Hazard Evaluation Laboratory Ltd.)...

Alternatively, for batch heating with a non-isothermal heating medium,... 

Related Calculations. This procedure can also be used for batch heating with internal coils and isothermal heating media. The equations in such cases are... 

In addition, we define gasifiable inorganic matter other than water as a fifth component. The following data were obtained by Hubbard and Robinson in batch heating experiments at 400°C with a shale containing 7.9% kerogen. 

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