BHS-Sonthofen Inc.

Thinking like sherlock holmes for Process filtration technology selection


Source: BHS-Sonthofen Inc.

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Sherlock Holmes and Dr. John Watson are fictional characters of Sir Arthur Conan Doyle. Process engineers who live in the real-world can learn many things from the two of them for solving process filtration problems. This paper will intertwine the detective techniques (mindfulness, astute observation, logical deduction and others) of Holmes and Watson with the problem solving skills required to select process filtration systems.
One example that Holmes proves time and again is that there is no benefit to jumping to conclusions. The paper begins with a discussion of the bench-top laboratory tests that are conducted for problem analysis, technology selection and scale-up. The tests include pressure/vacuum/centrifugation, filter media, cake thickness, temperature and viscosity concerns, filter aids and similar process parameters. Testing will avoid 'jumping to conclusions.'

Another technique used by Holmes and Watson is 'recreating events.' Holmes talks through his theories out loud to Watson and only then do gaps and inconsistencies rise to the surface that were not apparent before. The paper continues with four case history examples discussing slurry testing, process analysis and then process filtration selection for pressure filtration, vacuum filtration, centrifugation and clarification. The case histories illustrate the methods followed from testing through decision-making.

Finally, the paper concludes with a general review of the problem-solving skills of Holmes and Watson such as the 'occasional silence', 'employing distancing' and 'learning to tell the crucial from the incidental.' These skills can be utilized by process engineers as a framework for 'idea-generation' when analyzing an operating bottleneck issue or new process development problem. In all cases, by combining Holmes and Watson with accurate lab and pilot testing, the optimum filter selection can be realized.


According to Holmes and Watson, it is important to train yourself to be a better decision maker. For example, using checklists, formulas, structured procedures; those are your best bet. Figure 1 shows a typical Experimental Test Routine.

Overview of Bench Top Testing for Pressure and Vacuum Filtration

The BHS bench top testing is conducted using the BHS Pocket Leaf Filter, as shown in Figure 1. The test device is a BHS pocket leaf filter with a filter area of 20 cm2 and a vacuum and pressure connection. The testing will analyze cake depths, operating pressures, filter media, washing and drying efficiencies and qualitative cake discharge. The data collection sheets are shown in Figure 2. The steps in filtration testing are as follows:

First, it is necessary to clearly state the process description. This includes the slurry characteristics (particle size distribution, particle shape, density, etc.), washing of the cake (i.e. number of washes and wash ratios), drying / pre-drying of the cake (vacuum, pressure blowing, and mechanical pressing) as well as the upstream and downstream equipment. With this definition, the type of samples that need to be collected and analyzed can be determined.

Secondly, it is necessary to know what are the requirements for the operation such as solids/hour and cake quality (percent moisture, percent contaminants, etc.).

Thirdly, with the above in mind, the testing must determine the following objectives: Choice of a suitable filter cloth Vacuum or pressure filtration Wash ratios for the washing of the filter cake Drying techniques Cake thickness

Overview of Bench Top Testing for Centrifugation

Centrifugation lab testing include static settling test, filtration rate test and spin settling rate test.

The static settling test will be able to determine the densities of the solid and liquid phases and if there are different densities then centrifugal forces can be applicable for separation.

The filtration rate tests are conducted with the BHS pocket leaf filter using vacuum. Depending upon the vacuum filtration rates, the type of centrifuge can be determined.

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