The first-order processes are characterized by:
(a) their capacity to store material, energy or momentum, and
(b) the resistance associated with the flow of mass, energy or momentum in reaching the capacity.
Thus, the dynamic response of tanks which have the capacity to store liquids or gases can be modeled as
first-order. The resistance is associated with the pumps, valves, weirs, pipes which are attached to the
inflowing or outflowing liquids or gases. Similarly, the response of solid, liquid, or gaseous systems
which can store thermal energy (thermal capacity, cp) is modeled as first-order.
For such systems the resistance is associated with the transfer of heat through walls, liquids or gases. In
other words, a process which possesses a capacity to store mass or energy and thus act as a buffer
between inflowing and outflowing streams will be modeled as a first-order system.
The stirred tank heater and the mixing processes , are typical examples of first-order processes.
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