Feed is charged into a system. The system works on the feed until processing is complete. Products are then removed
System is closed, with no material transfer across system boundaries except during charging and product removal.
Examples: Cooking. Brewing. Specialty chemicals.
Feed and products flow continuously through process. System is open, and usually modeled as steady flow.
Examples: Petroleum refining (except coking, blending).
Hybrid of batch and continuous. In one form ("fedbatch") the feed is added continuously, but the product is removed all at once. In other cases (i.e. batch distillation), the feed is charged into the vessel in one step, but the products are removed continuously during the run. A uniform state open system model is one approach to these problems.
(Also called "dynamic" or "unsteady state".) Models are functions of time, so the system can track fluctuations and changes in behavior. These usually require differential equations, and an approach similar to that we used to derive the uniform system equations in Thermo I.
(Also called "stationary" or "static" models.) Models do not change with time. This occurs when a dynamic model achieves balance and all disturbances have attenuated, or at some theoretical "long time" that represents the "normal" behavior of the system. These can usually be modeled with algebraic equations.
Batch and semibatch processes require transient models (they never reach a steady state). Continuous processes may be modeled either as transient or as steady state.
References:
R.M. Price
Original: 6/6/94
Modified: 8/23/96, 12/25/2004
Copyright 1996, 2004 by R.M. Price -- All Rights Reserved