Gas and liquid contact apparatus – Contact devices – Porous mass
Reexamination Certificate
2001-11-21
2004-07-06
Bushey, Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Porous mass
C261S110000, C062S121000
Reexamination Certificate
active
06758463
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and a method for distributing a liquid from a liquid distributor to a packing in an exchange column for heat and/or mass transfer processes. The apparatus and method have particular application in cryogenic air separation processes utilizing distillation, although the apparatus and method may be used in other heat and/or mass transfer processes that use liquid distributors and packing (e.g., random or structured packing).
The term, “column”, as used herein, means a distillation or fractionation column or zone, i.e., a column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements.
The term “packing” means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are “random” and “structured”.
“Random packing” means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column. “Structured packing” means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of expanded metal or woven wire screen stacked in layers or as spiral windings; however, other materials of construction, such as plain sheet metal, may be used.
The terms “orifice,” “hole,” and “aperture” are used interchangeably herein to mean an opening through which a fluid may pass. Although circular orifices are shown in the drawings, the orifices may have other shapes, including irregular as well as regular shapes.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen).
Various packings may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases. The use of packing for distillation is standard practice and has many advantages where pressure drop is important. However, packed column performance is very dependent on creating and mantaining a balance between the downward flow of liquid and the upward flow of vapor locally in the packing. The distribution of the liquid and the vapor within the packing as influenced by the initial presentation of these fluids to the packing.
Initial presentation of liquid and vapor to the packing is usually made by means of distributors. A liquid distributor, the role of which is to irrigate the packing uniformly with liquid, is located above the packing, while a vapor distributor, the role of which is to create uniform vapor flow below the packing, is located below the packing.
There are three main types of typical liquid distributors—pipe, pan, and trough distributors. Each type is discussed briefly below.
Pipe distributors are comprised of an interconnecting network of closed pipes or ducts, typically comprising a central pipe or manifold and a number of arms or branches radiating from the central pipe. The arms are perforated to allow the liquid passing from the central pipe and into the arms to be dripped or sprayed onto a packed bed below the pipe distributor. Upwardly flowing vapor passes easily in-between each arm. Pipe distributors receive liquid from a separate liquid collector or an external source piped through the wall of the column. While simple and inexpensive to construct, pipe distributors may distribute liquid poorly when vapor gets trapped in the arms.
Pan distributors are comprised of a pan or pot having holes in the bottom for feeding liquid to the packing below and tubes or risers for the vapor to pass upwardly through the distributor. Pan distributors often make a complete seal with the wall of a column. Thus, pan distributors can act as liquid collectors as well as distributors. However, since large pan distributors are costly to build, pan distributors usually are used in smaller columns, i.e., columns with diameters less than 1.5 meters.
Trough distributors comprise a collection of interconnecting open troughs or channels having irrigation holes in the base to feed liquid to the packing below. At least one upper collection trough, or a simple pot on top of the lower troughs, feeds liquid to the lower troughs through a series of holes or overflowing notches. Vapor from the packing below passes upward between the liquid-containing troughs.
FIG. 1
shows a typical liquid distributor
10
of the trough type. Liquid from feed assembly
12
enters a pre-distributor
14
, which distributes the liquid to the distributor. The distributor is mounted on a combined hold-down/support grate (not shown) above the packing (not shown).
After entering the distributor
10
, the liquid flows in a plurality of channels or troughs
18
spaced apart by vapor risers
16
throughout the distributor. A typical main channel
17
and multiple troughs or channels
18
on each side of the main channel
17
are shown in FIG.
2
. Liquid from the main channel enters each channel at the inlet end
20
of the channel and flows in a direction
22
away from the inlet end. Streams of liquid
24
then exit each channel through orifices or holes
26
in the bottom
28
of the channel. If the liquid does not flow from the holes in uniform directions, some areas of the packing below the distributor are under irrigated areas
30
while other areas of the packing are over irrigated areas
32
, as shown in FIG.
2
. Also, some of the liquid may impact internal structures between the bottom of the distributor and the packing, such as distributor supports/hold-down grates
34
, as shown in FIG.
2
. (These internal structures may support the distributor and/or hold down the packing.)
Some liquid distributors used in distillation processes are disclosed in U.S. Pat. No. 5,752,538 (Billingham, et al.); U.S. Pat. No. 5,240,652 (Taylor, et al.); U.S. Pat. No. 6,086,055 (Armstrong, et al.); U.S. Pat. No. 4,729,857 (Lee, et al.); U.S. Pat. No. 5,192,465 (Petrich, et al.); and U.S. Pat. No. 5,645,770 (McNulty, et al.).
The prior art distributors generally use three types of distribution regulation mechanisms: the weir type, where liquid flows horizontally through a gap; the orifice type, where liquid flows vertically, or horizontally, usually through a circular hole; and the pressure type, where feed under pressure is distributed through a series of spray nozzles. The orifice type is distinguished by the fact that the flow rate of liquid through the hole is proportional to the square root of the height of liquid above the orifice. For a narrow weir, the flow can be taken as being proportional to the height of liquid raised to the power 1.5. Use of orifices is often preferred because the effects of minor changes in the liquid level or the levelness of the distributor are reduced by having the flow rate being proportional to the height squared if a reasonable liquid depth is used. However, this comes at the expense of reducing the operating range of a simple distributor because the height available for the distributor is often limited. Weir type distribution is often preferred when a large amount of liquid must be distributed, high rangeability is required, or in the pre-distribution section of the distributor.
In the case of orifice type distribution, the thickness of the orifice material plays an important part in regulating the flow and the direction of the liquid stream. Orifices generally may be divided into two classes—those in thick material and those in thi
Auvil Steven Ray
Kalbassi Mohammad Ali
Kovak Kenneth William
Sacks Raymond Elliott
Zone Ian Robert
Air Products and Chemicals Inc.
Bushey Scott
Jones II Willard
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