Pressure or vacuum filtration. The first optimization is the filtration rate. A premeasured amount of slurry is added from the top. Pressure or vacuum filtration begins, and the amount of filtrate vs. time is recorded.
Parameters that are varied sequentially in this step include cake depth, filtration pressure or vacuum and filter media. For thin-cake filtration technologies, cake depths can vary between 5 millimeters (mm) and 25 mm. Maximum cake thickness for the PLF unit is 150 mm.
Displacement washing. Displacement washing is performed after the filtration step is completed. A measured amount of wash liquid is added carefully in a predetermined wash ratio so the cake is not disturbed. Once again, pressure and time are measured. One or more wash tests can be conducted with the same or different wash liquids.
Cake pressing. Several thin-cake technologies can perform cake pressing or squeezing. The PLF can simulate this pressing procedure with a "pressing plug." The pressing plug is actuated by nitrogen pressure and squeezes the cake onto filter media. This pressing can be conducted before, during or after the filtration, washing and drying steps.
Drying. Product drying in the PLF is tested by blowing ambient-temperature or hot gas through the cake or via vacuum. In addition, both the vessel jacket and base jacket are heated to simulate a production unit. The pressure is kept constant, and gas throughput is measured vs. time. After a preselected drying time, the cake is removed, and the cake depth and weight are determined. The cake then is analyzed for moisture content. Several iterations are required.
Results and analysis. Once testing is completed, the vendor's process engineers analyze the data to recommend one or more filtration technologies. The test report includes an executive summary, test objectives, test methods and facilities, test data (in table form), test results (in written and graphical form, including filtration and drying curves), recommendation of production equipment and scale-up and any other recommendations and "path-forward" action steps.
Based on the PLF tests and recommendations, pilot-scale tests can be conducted. These tests should most often be conducted at the plant site using actual feed material from the reactor, as well as the actual washing and drying media, operating conditions, etc.
It is also important for the plant to ensure the same vendor engineer who conducted the PLF tests conducts the pilot tests. The testing procedure and testing "tricks" employed on the bench-top, therefore, also will be employed in the pilot testing.
Currently, the most efficient approach to selecting and/or optimizing a pressure or vacuum filtration system is to use a PLF unit. With assistance and process support from the vendor and accurate data from the testing ," combined with filtration theory and experience ," proper selection, scale-up, optimization and process guarantees can be realized.
Perlmutter is president and managing director of BHS-Filtration Inc., Charlotte, N.C. Contact him at email@example.com.