Batch and Continuous Operations

For batch operations, the physical properties such as temperature, concentration, pressure, and reaction rate change at any point within the reactor as the reaction proceeds. In continuous operations, these properties are subject to only small, if any, local fluctuations. Two key differences on waste regeneration between batch and continuous processes are:

Waste streams from batch processes are generally intermittent, whereas those from continuous processes are continuous.

The composition and flow rates of waste streams leaving batch processes typically vary, whereas those of continuous processes are fairly constant.

The variability of waste from batch processes creates more difficult waste management problems. For example, if the total volume of waste must be handled, the instantaneous maximum flow is higher in a batch plant, and larger equipment is required to handle this waste. Also, waste generation rates are often high during start up and shutdown periods, and these periods occur most frequently in batch units. Therefore, waste reduction factors generally favor continuous rather than batch processing (Rossiter, Spriggs, and Klee 1993).

The main factors and heuristics usually considered in a decision between batch and continuous operations are (Rossiter, Spriggs, and Klee 1993):

Production rate: Under 500,000 lb/yr, batch processing is invariably used; between 500,000 and 5,000,000 lb/yr, batch processing is common; at higher rates, continuous processing is preferred. Product life: Batch plants are better suited to products with short life spans where a rapid response to the market is required.

Multiproduct capabilities: If the unit must make several similar products using the same equipment, batch processing is usually preferred. Process reasons: A number of process-related factors can lead to batch processing being preferred; for example, cleaning requirements that need frequent shutdowns, difficulties in scaling up laboratory data, or complicated process recipes. If potentially serious environmental problems are anticipated with a process, the selection of a continuous unit is favored. Batch operations are preferred for reactions where rapid fouling occurs or contamination is feared.

In practice, some of the other factors mentioned previously can dictate that a batch operation is preferred. Then, the chemical engineer should consider smoothing intermittent or variable flow streams (for example, by adding buffer storage capacity) to simplify processing and recovery of waste material.

The capital cost for a batch operation is often less than for a corresponding continuous process. Therefore, it is frequently favored for new and untried processes which will be changed to a continuous operation at a more advanced stage of development.

As a final observation on the use of heuristics, Haseltine (1992) notes that the inherent flexibility of batch plants often makes raw material and product substitution simpler in these processes.

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