User:Milton Beychok/Sandbox
In chemical engineering processes such as distillation and absorption, a packed bed is most usually a zone within a vertical pressure vessel that is filled with packing material.[1][2][3][4]
The packing material may be randomly dumped objects or it may be specially designed structured packing material such as the examples shown in Figures 1 and 2.
The randomly dumped packing may be steel, ceramic or plastic objects of various geometric designs. The structured packing may be sheet metal, woven wire gauze or plastic of various designs and stacked in various arrangements.
Applications
In most applications, the purpose of a packed bed is to provide intimate contacting of the upward flowing vapor and and the downward flowing liquid in separation processes such as distillation and absorption.
In the packed bed, liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. Packing material can be used instead of trays or plates to improve separation in distillation columns. Packing offers the advantage of a lower pressure drop across the column when compared to trays or plates, which is especially beneficial when used in vacuum distillation columns.
Differently shaped packing materials have different surface areas and different amounts of void space. Both of these factors affect packing performance. In general, the more surface area for a given volume of packing material, the better is the performance of the packing.
Another factor affecting performance, in addition to packing shape and surface area, is the distribution of vapor and liquid as they that enter the packed bed. The number of theoretical stages required to make a given separation is calculated is a function of the vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the packed bed, the liquid to vapor ratio will not be correct and the desired separation will not be achieved. The problem is not the packing itself but the mal-distribution of the fluids entering the packed bed. Columns containing packed beds are designed to include liquid distributors so as to distribute the liquid evenly over the cross-sectional area of the packing in order to optimize the efficiency of the mass transfer.[1][2][3] The design of the liquid distributors is critical to making the packing perform at maximum efficiency.
Packed columns have a continuous vapor-liquid equilibrium curve, unlike conventional tray distillation in which every tray represents a separate point of vapor-liquid equilibrium. However, when designing packed columns it is useful to first determine the number of theoretical equilibrium stages required for the desired separation. Then the packing height needed to constitute a theoretical equilibrium stage, known as the height equivalent to a theoretical plate (HETP), is also determined. The total packing height required is the number theoretical equilibrium stages multiplied by the HETP.[1][2][3]</ref name=Perry/>
Other applications
Columns used in certain types of chromatography consisting of a tube filled with packing material can also be called packed columns and their structure has similarities to packed beds.
Packed bed reactors can be used in chemical reaction. These reactors are tubular and are filled with solid catalyst particles, most often used to catalyze gas reactions. The chemical reaction takes place on the surface of the catalyst. The advantage of using a packed bed reactor is the higher conversion per weight of catalyst than other catalytic reactors. The reaction rate is based on the amount of the solid catalyst rather than the volume of the reactor.
References
- ↑ 1.0 1.1 1.2 Seader, J.D. and Henley, Ernest J. (2006). Separation Process Principles, 2nd Edition. John Wiley & Sons. ISBN 0-471-46480-5.
- ↑ 2.0 2.1 2.2 Kister, Henry Z. (1992). Distillation Design, 1st Edition. McGraw-Hill. ISBN 0-07-034909-6.
- ↑ 3.0 3.1 3.2 King, C.J. (1980). Separation Processes. McGraw Hill. 0-07-034612-7.
- ↑ Perry, Robert H. and Green, Don W. (2007). Perry's Chemical Engineers' Handbook, 8th Edition. McGraw-Hill. ISBN 0-07-142294-3.