Screw presses are continuous-feed expression devices that involve the application of gravity drainage and mechanically applied pressure to deliquor suspensions and form compressible filter cakes. A typical screw press comprises of a rotating helical screw fitting closely inside a perforated frame. Both screw and frame are tapered toward the discharge to increase the pressure on the material. The pressure can also be increased by varying the pitch of the screw. As the feed moves through the unit, pressure is progressively increased to continuously express the liquid phase and discharge cake through a variable orifice nozzle. Typical uses include fruit juice and oil extraction and sludge dewatering.
● Screw Press Filtration Procedure
If the “Perform Reaction Prior To Separation” option is checked, then you can specify one or more reactions through the “Reactions” tab to be carried out before separation takes place.
There are two main options for doing the material balances for the separation step:
● Set Component Removal To Filtrate
● Set Component Retention in Cake
If the “Set Component Removal To Filtrate” option is selected for the material balances for the separation step, you must specify the removal percentages of one or more components from the feed stream to the filtrate stream. Based on the specified component data, the program will do the component mass balances to determine the component flows of the cake and filtrate streams.
If the “Set Component Retention in Cake” option is selected for the material balances for the separation step, then the component data table is used to specify the retained percentages of particulate solid components only (i.e., undissolved solids, suspended or precipitated) in the cake. Only pure components that are set as solid through the Pure Component Properties dialog can be set as particulate solids in the components table. Based on the specified retained component percentages, the program can solve the mass balances for the particulate solid components in order to calculate the flows of those components in the cake and filtrate streams. In addition to setting one or more solid components as particulate solids and specifying the retained percentages of those components in the cake, you must also specify either the total (particulate and dissolved) or particulate solids percentage of the cake. Based on this information, the program will calculate the total retained percentage of all other components besides particulate solids (i.e., dissolved solids, liquids and gases), assume that the retained percentage of each of those components is the same as the total retained percentage of those components, and solve the component mass balances for those components to calculate the flows of those components in the cake and filtrate streams.
The overall particulate solids recovery is calculated by dividing the total flow of particulate solids in the cake by the total flow of particulate solids in the feed stream.
In Design Mode, the program calculates the operating mass throughput based on the feed stream. If the operating throughput exceeds the specified maximum equipment throughput through the equipment data dialog, then the program assumes multiple units operating in parallel with a total equipment throughput equal to the total operating throughput. In Rating Mode, the user specifies the number of units and the rated equipment throughput. If the operating throughput per unit exceeds the rated equipment throughput, a warning message is displayed advising the user to increase the number of units or use larger equipment.
1. Tarleton S. and Wakeman R. (2007). Solid/Liquid Separation:.Equipment Selection and Process Design. First edition, Elsevier.
2. “Saravacos G. D. and Maroulis Z. B. (2011). Food Process Engineering Operations, CRC Press.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Screw Pressing: Oper. Conds 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