This operation models washing of impurities from discrete entities using a washer. One type of washer consists of a belt conveyor feeder with a spraying system on top. As the belt conveyor transports the feed, an aqueous detergent is sprayed to wash away impurities from the discrete entities.
The washing operation model solves the mass and energy balances based on specification of the bulk amount per entity and composition of the discrete entities in the feed stream. The following component mass balances are solved based on the assumption that no mass transfer occurs from the wash-in stream to the feed stream or to the washer:
|
|
|
|
where:
● mF is the component mass flow rate in the feed stream (kg/s),
● mP is the component mass flow rate in the product stream (kg/s),
● mwi is the component mass flow rate in the wash-in stream (kg/s),
● mwo is the component mass flow rate in the wash-out stream (kg/s), and
● x is the component mass fraction that is removed from the feed stream by the wash stream.
Energy balance calculations are performed by treating the flow of discrete entities as bulk flow and assuming that the feed and wash streams are mixed adiabatically and reach a certain level of thermal equilibrium. The approach to thermal equilibrium factor is used as a measure of heat transfer between the feed and wash streams. It is defined as the ratio of the true temperature difference between the feed stream and the product stream to the temperature difference that the two streams would have if the product and wash-out streams reached thermal equilibrium. The equilibrium temperature is calculated based on the VLE data specifications for the corresponding procedure state stream. Based on the above definition, the product temperature is calculated by the following expression:
|
|
where:
● f is the approach to thermal equilibrium factor,
● TF is the temperature of the feed stream (K),
● TP is the temperature of the product stream (K), and
● Teq is the equilibrium temperature of the product and wash-out streams (K).
Alternatively, the product temperature can be set directly by the user. In both cases, the temperature of the wash-out stream is calculated based on the overall energy balance in the washer:
|
|
where:
● hF is the enthalpy of the feed stream (J),
● hP is the enthalpy of the product stream (J),
● hwi is the enthalpy of the wash-in stream (J), and
● hwo is the enthalpy of the wash-out stream (J).
The operating throughput is calculated based on feed rate of discrete entities.
If the equipment size option is in Design Mode, the user specifies the maximum equipment throughput per unit. If the calculated operating throughput exceeds the maximum throughput per unit, then, the program assumes multiple, identical units operating in parallel with an operating throughput per unit of less than the maximum and a total operating throughput equal to the feed rate of discrete entities.
If the equipment size option is in Rating Mode, the user specifies the rated throughput and the number of units. If the calculated operating throughput per unit exceeds the rated throughput, a warning message is displayed advising the user to increase the number of units or reduce the feed rate of discrete entities.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Discrete Washing: Oper. Conds Tab
● Power, see Discrete Throughput Operations: Utilities Tab
● Labor, etc, see Operations Dialog: Labor etc. Tab
● Description, see Operations Dialog: Description Tab
● Batch Sheet, see Operations Dialog: Batch Sheet Tab
● Scheduling, see Operations Dialog: Scheduling Tab
