Cake Wash

General Description

In the cake wash operation, the filtration cake is washed to remove components that remain in its interstitial space. For a component to be removed, it must be marked particulate and soluble as well as have appropriate solubility data or loss percentage values. Notice that the particulate components are going to be determined by the previous filtration operation (if there is any present) and cannot be changed. On the other hand, if there is no filtration operation present in the procedure, the particulate components must be set by the user.

Furthermore, the displacement of solvent due to the wash liquid is also considered. The amount and type of washing determines the displacement of solvent from the cake to the wash out stream. The user has the option to specify directly the percentage of displaced liquid or have the program calculate it by assuming either a displacement or slurry washing type. For displacement washing type, the solvent in the cake is displaced in an embolic (plug-flow) fashion. For slurry washing type, the washing liquid is mixed with the cake and then the cake is filtered to its original dryness.

Unit Procedure Availability

●      Nutsche Filtration Procedure

●      Plate and Frame Filtration Procedure

●      Centrifugation in a Basket Centrifuge (Top Discharge) Procedure

●      Centrifugation in a Basket Centrifuge (Bottom Discharge) Procedure

Cake Wash: Modeling Calculations

Cake Wash Material Balances

The amount of a component that is carried out by the wash stream can either be specified directly in the form of Loss % or be calculated by SuperPro based on the given values of the solubility concentration and approach to equilibrium percentage. In the latter case, the massflow of component i, in the wash out stream is given by:

where, S_i is the component solubility concentration, A_i is the approach to equilibrium and Q_w is the volumetric flow of the wash inlet stream

In the case of displacement wash type, the displaced fraction of solvent from the cake is calculated by an iterative procedure. Specifically, the wash in stream is assumed to pass from the cake in several passes, each pass having the same volume as the liquid retained by the cake. Starting with an initial displacement fraction f_disp,0 = 0, the final displacement fraction is calculated by:

where α is the approach to theoretical displacement percentage.

Finally, in the case of slurry wash type, the displaced fraction of solvent from the cake is calculated by the following equation:

where V_w is the volume of the wash liquid per batch and V_c,L is the volume of the liquid portion of the cake.

Equipment Sizing and Filtration Time Calculation

In design mode, the process (wash) time (t_p) is specified by the user and the filter area (A) is calculated using the following equation:

where V_w is the volume of the wash liquid per batch, N_cycles is the number of cycles per batch, and J is the average filtrate flux. The wash volume is specified on an absolute (L per cycle) or relative basis (L of wash liquid per L of cake). If any of the retained components are soluble to the wash solvent, their solubility or the amount lost in the wash stream can be set in the ‘Solubility’ Tab, see Cake Wash: Solubility Tab. If the calculated filter area exceeds the maximum that the user specifies, the program automatically assumes multiple units of equal size with a total filter area equal to the calculated.

In rating mode, the user specifies the filter area and the number of units and the program calculates either the average filtrate flux (when the wash time is specified) or the wash time (when the flux is specified).

Energy Balance Calculations

The program will calculate the final temperatures of the wash out stream and the remaining cake based on the user-specified thermal equilibrium approach percentage. If the percentage is set to 100% then the two temperatures are assumed to be the same. Otherwise, the final temperature of the cake is calculated by

:

where T_cake is the temperature of the cake, T_eq is the temperature that corresponds to the full thermal equilibrium and f is the approach to equilibrium percentage. With the final cake temperature known, the wash out stream is calculated by the energy balance.

Cake Wash: Interface

The interface of this operation has the following tabs:

●      Cake Wash, see Cake Wash: Cake Wash Tab

●      Solubility, see Cake Wash: Solubility 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