User:Milton Beychok/Sandbox: Difference between revisions

From Citizendium
Jump to navigation Jump to search
imported>Milton Beychok
No edit summary
imported>Milton Beychok
No edit summary
Line 1: Line 1:
[[Image:LC Raschig Rings.jpg|right|thumb|250px|{{#ifexist:Template:LC Raschig Rings.jpg/credit|{{LC Raschig Rings.jpg/credit}}<br/>|}}Figure 1: Randomly dumped packing material used in packed beds.]]
[[Image:LC Structured Packing.jpg|right|thumb|250px|{{#ifexist:Template:LC Structured Packing.jpg/credit|{{LC Structured Packing.jpg/credit}}<br/>|}}Figure 2: Structured packing used in packed beds.]]
In [[chemical engineering]] processes such as [[Continuous distillation|distillation]] and [[absorption]], a '''packed bed''' is most usually a zone within a vertical [[pressure vessel]] that is filled with packing material.<ref name=Seader>{{cite book|author=Seader, J.D. and Henley, Ernest J.|title=Separation Process Principles|edition=2nd Edition|publisher=John Wiley & Sons|year=2006|id=ISBN 0-471-46480-5}}</ref><ref name=Kister>{{cite book|author=Kister, Henry Z.|title=[[Distillation Design]]|edition=1st Edition |publisher=McGraw-Hill|year=1992|id=ISBN 0-07-034909-6}}</ref><ref name=King>{{cite book|author=King, C.J.|2nd Edition|title=Separation Processes|publisher=McGraw Hill|year=1980|id=0-07-034612-7}}</ref><ref name=Perry>{{cite book|author=Perry, Robert H. and Green, Don W.|title=[[Perry's Chemical Engineers' Handbook]]|edition=8th Edition| publisher=McGraw-Hill|year=2007|id=ISBN 0-07-142294-3}}</ref>
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 the downward flowing liquid in [[separation process|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 [[Theoretical trays|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|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.
[[Image:Packed Bed Absorption Column.png|right|thumb|350px|{{#ifexist:Template:Packed Bed Absorption Column.png/credit|{{Packed Bed Absorption Column.png/credit}}<br/>|}} Figure 3: Schematic diagram of a typical packed bed absorption column.]]
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 plate|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. As shown in Figure 3, 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.<ref name=Seader/><ref name=Kister/><ref name=King/> 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.<ref name=Seader/><ref name=Kister/><ref name=King/><ref name=Perry/>
===Other applications===
Packed beds are also used in certain variants of the distillation and absorption processes referred to as ''[[stripper]]s'' and ''[[scrubber]]s''.
Columns used in certain types of [[chromatography]] consisting of a tube filled with packing material are called ''packed columns'' and their structure has similarities to packed beds.
[[Trickle filter]]s used for the [[biochemical oxidation]] of sewage and industrial wastewaters employ packed beds referred to as ''fill'' or ''media''.
==Packed bed process design==
There are numerous equations and correlations that have been published in the technical literature for predicting the pressure drop of the vapor traveling through a packed bed and for predicting the height equivalent to a theoretical plate (HETP).<ref name=Kister/><ref name=Perry/> There are also numerous ''rules of thumb'' that have been published for use in the process design of packed beds and which are simpler to use and probably as accurate as the equations and correlations.<ref name=Kister/> A discussion of all the equations, correlations and rules of thumb would be far beyond the scope of this article.
The major manufacturers of packing material are other sources of reliable process design information.
==References==
{{reflist}}


==External links==
==External links==
*[http://www.cheresources.com/packcolzz.shtml Packed Column Design]
*[http://www.cheresources.com/packcolzz.shtml Packed Column Design]
*[http://www.engin.umich.edu/~cre/asyLearn/bits/pbr/index.htm Packed Bed Reactor Design]
*[http://www.engin.umich.edu/~cre/asyLearn/bits/pbr/index.htm Packed Bed Reactor Design]

Revision as of 13:37, 2 August 2008

External links

Retrieved from "https://citizendium.org/wiki/index.php?title=User:Milton_Beychok/Sandbox&oldid=385154"