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Filtration technology has advanced to meet the demand to remove small particles at low concentrations. When designed correctly, a candle filter or a pressure plate filter may be a more-efficient replacement for a traditional filtration scheme.
Many chemical manufacturing processes, such as those that make chemicals, petrochemicals, pharmaceuticals, biofuels, etc., involve reactions of solid- and liquid-phase reactants to produce a slurry. The slurry typically needs to be separated into its component parts - the liquid (i.e., mother liquor) and the solid.
The valuable material may be the liquid, the solid, or both phases. The location of the valuable component to be recovered determines the type of separation method and equipment used. For example, if the liquid is the desired product, the solids are removed as waste to produce a clean liquid; if the solid is the desired product, removal of small particles increases the product yield.
Generally, the first step after the reaction is a bulk separation process that removes large, coarse solids (larger than 5 pm). This is relatively straightforward, but as processes have become more sophisticated and quality requirements have tightened, it has become necessary to remove residual particle fines from slurries. These particles are small - typically 1-5 pm in diameter, or even smaller - and at low concentrations, on the order of parts per million (ppm).
This article describes clarification technologies used to remove or recover residual particle fines. A brief overview of coarse separation equipment illustrates how these systems fit into the realm of filtration technologies. Examples of installations in chemical, refinery, biofuel, and pharmaceutical plants provide engineers with ideas for evaluating process filtration problems, as well as potential solutions based on laboratory testing.
Filtration system categories
A successful solid-liquid filtration system includes a filter medium that retains the solids while allowing the liquid phase to pass through. Filtration systems are divided into four categories, based on the driving mechanism or force behind the separation: gravitational, vacuum, pressure, and centrifugal.
Gravitational filtration systems use gravity to drive the separation process, in effect draining the liquid from the slurry. They are typically continuous, but can also be operated in batch mode. Because their capital and operating costs are low, they are common throughout the chemical process industries (CPI), but they...