Rethinking Gas, Liquid and Solids Operations

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Many chemical processes can benefit from techniques or methods that are either relatively new or unique approaches. Many of these techniques and methods become standard practice if they are successful and can substantially improve the bottom line.

Some helpful approaches to gas, liquid and solid operations are reviewed here. Are you using any of them? Have you developed any unique methods yourself?

Diaphragm distributors. Distributors often are used in gas-liquid-solids contacting such as slurry reactor applications. The goal of a distributor is to attain a uniform distribution of gas bubbles across the process area. One of the better ways to accomplish this task is through the use of a flexible diaphragm distributor in gas-liquid systems. If the flexible diaphragm can survive under processing conditions, then it should be considered for use.

Independent control of distributor holes. For gas-liquid-solid systems, common pipe-style distributors often prove to be unsatisfactory. Solids "sand in" the distributor until only a few holes are operating. To overcome this problem, you must control each distributor hole independently to ensure its operation in a system with solids.

Column tilt. Bubble columns often are used in gas-liquid contacting, and column tilt is an important process variable quantity. A bubble column should be as vertical as possible. If not, the gas flow will go up one side of the column, and a downward liquid flow will occur on the other side of the column. Contacting is reduced. The key is to ensure vertical columns are in place.

Hollow blade impeller. For gas-liquid contacting, a disk-style hollow-blade impeller, often called the Smith impeller, should be used in agitated tanks. The hollow blades essentially are pipe sections cut in half. These are mounted symmetrically on a disk so they are cupped in the direction of rotation. The disk is a baffle that prevents gas from rising along the shaft. The impeller also should have as many blades as reasonably possible. The resulting geometry prevents streamlining of the blade by the gas and the formation of large gas cavities behind the blade. High input power levels and excellent gas dispersion are attained. Make certain you are using this new type of impeller.

Surfactant placement. Flotation, as the name implies, is the process by which material is collected and floated to the surface by bubbles. In the biotech area, high-molecular-weight molecules are floated. In the mining industry, metal particles in low-grade ore such as gold are floated. The worthless material is left behind.

Surfactants on the bubble surface accomplish the flotation by preferentially picking up the desired material. As the bubbles are formed in equipment, the surfactant is added as a mist or spray directly onto the bubble surface as the bubbles are being formed. Surfactant is not added to the entire quantity of liquid. This approach reduces the amount of surfactant used and increases the efficiency of the flotation. Flotation is accomplished in bubble columns, aerated agitated tanks or static mixers.

Proper trough size. Harvesting the floated material in flotation can be a problem. Once the floated material is on the surface, it should overflow into a trough. Unfortunately, if a trough is not close by, the material falls back, returns to the liquid and is not harvested. The key is to determine whether the flow to the trough is sufficient to prevent the material's return. A dye can be spread over the floated surface to determine whether an adequate flow exists. Intuitively, if the trough length is "L" and the harvest area is "L squared," then their ratio is "1/L." For large units, the trough length tends to be insufficient ," i.e., undersized ," to harvest the floated material.

This last approach addresses a common industrial equipment problem: process equipment that has not been sized properly. Engineering and cost analysis should be able to solve the problem and determine if the solution is cost effective and justified. When the equipment is sized properly, efficiency increases and the bottom line is improved.

Do you have an interesting approach to gas, liquid and solid operations that you would like to share? I am always interested in learning new things. Write to me at gbt@ncat.edu, and your "trick" might be included in my next column. CP

Tatterson is a technical editor for

Chemical Processing. He is a professor at North Carolina A&T State University in Greensboro. Contact him at gbt@ncat.edu. He also teaches short courses for the Center for Professional Advancement, www.cfpa.com.
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