Questions For Thinking And Discussing

1. It is recognized that pore blockage in a filter medium occurs because: (1) Pores become blocked by the lodging of a single particle in the pore passage, (2) Gradual blockage can occur due to the accumulation of many particles in pore passages; and (3) Blockage may occur during intermittent filtration practices. Consider aim2 surface of filter medium containing Np number of pores. The average pore radius and length are rp and 0p, respectively. Assuming laminar flow, develop an expression to calculate the volume of filtrate through a single pore per unit of time. (Hint - Are you familiar with the Hagen-Poiseuille expression?).

2. Continuing with question 1, develop an expression for the initial filtration rate per unit area of filtration.

3. Consider 1 m3 of very dilute suspension containing n number of suspended particles. Develop an expression describing the rate of filtration through the pores, taking into consideration the number of blocked pores.

4. Explain how the term reservoir description is applied to characterizing a homogeneous floe system.

5. Go to a standard handbook like Perry's Chemical Engineer's Handbook and obtain several hydraulic conductivities (say for sand, crushed stone, gravel, diatomaceous earth, other). Assume a constant head of fluid over a bed of each material, and apply Darcy's law to calculate flowrates and compare the results. Try several hydraulic head calculations and plot the results. Which of the materials studied shows the highest hydraulic resistance?

6. What parameters influence permeability? Can you list them in terms of first-order, second-order and lower order effects?

7. Explain the term connectivity and its relevance to the filtration process.

8. Here's some library or Internet work for you. Compile a table of values on the porosity of common materials (e.g., soils, clay, glass beads, crushed stone, charcoal, other materials). Or if you are really ambitious, apply the equations provided in this chapter along with physical properties data obtained from your search and estimate the porosities.

9. What does the term backflushing mearil How do you think this os performed on a filtering machine.?

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