Based on theoretical analysis and practical experience in filtration, the primary factors influencing the filtration process are as follows:
Temperature
During filtration, if the temperature of the suspension is low, its viscosity is high, resulting in a slow filtration rate. The viscosity of a liquid is an exponential function of temperature; it decreases significantly as the temperature rises. Heating is the simplest and most effective measure for reducing viscosity; however, excessively high temperatures can easily lead to the oxidation of oils and fats. Consequently, the filtration temperature for crude oil generally does not exceed 70°C.
Operating Pressure
Most of the impurities present in crude oil are incompressible solids; therefore, a higher operating pressure generally results in a higher filtration rate. However, if the solid matter suspended in the oil consists primarily of colloidal substances, increasing the pressure causes the voids within the filter cake to shrink. This leads to a gradual increase in filtration resistance and a rapid decline in the filtration rate; thus, the operating pressure must be determined based on the specific circumstances of each application.
Suspension Concentration
A higher concentration of solids in the suspension results in a larger volume of filter cake. In batch filtration operations, this leads to a corresponding reduction in the effective filtration time within each operating cycle, thereby negatively impacting overall yield. Conversely, in the operation of continuous vacuum filters, a higher suspension concentration tends to produce a more uniform filter cake that is easier to discharge promptly; consequently, the impact on yield is minimal.
Filtration Media and Filter Aids
The selection of the filtration medium influences... The clarity of the oil after filtration, as well as the ease with which the filter cake is discharged, are critical factors. In the oil processing industry, filtration media typically consist of woven fabrics. Commonly used cotton textiles include canvas, twill, and fine white cloth, typically woven from 20-count yarn with 5 to 8 plies. Synthetic fiber media-such as #130, #240, #260, and #261 polyester filter cloths, as well as #828 Vinylon filter cloth-are also widely employed. Fabrics with a higher density of warp and weft threads are utilized for filtering crude oil, while those with a lower density are reserved for filtering refined oil; for filtration processes with particularly stringent requirements, a layer of fine white cloth or flannel is often placed over the coarse filter cloth. Furthermore, woven stainless steel wire mesh is frequently used; mesh with a density of 30–40 pores per centimeter is applied to the filtration of crude oil, whereas mesh with 140–180 pores per centimeter is used for filtering decolorized oil.
For suspensions containing a significant concentration of colloidal particles, the addition of filter aids can effectively accelerate the filtration rate. The application of filter aids is of particular importance for suspension systems where process constraints preclude the use of elevated temperatures to facilitate filtration. For instance, when producing edible phospholipids from solvent-extracted oil, diatomaceous earth must be added to serve as a filter aid; similarly, during dewaxing and fractionation processes-which necessitate filtration at lower temperatures-the beneficial effects of filter aids become even more pronounced.
