Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-01-12
2002-02-26
Bennett, Henry (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06349567
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a packing with a cross channel structure for a material exchange column with a high specific separation performance, to a column with a packing of this kind and to a method for the operation of a column of this kind.
2. Description of the Prior Art
Packings with cross channel structure have already been known for many years (see e.g. DE-A 26 01 890). They are assembled as a rule from a plurality of packing elements which are arranged one above the other, with each packing element being constructed of a large number of parallel layers. The layers make contact and form the cross channel structure with mutually open flow passages which are inclined with respect to the vertical (column axis). A material and/or heat exchange can be carried out with a column packing of this kind, namely between a ripple film on the packing surface and a gas flow which flows through the passages.
In a standard treatise on distillation (H. Z. Kister, “Distillation Design”, McGraw-Hill, Inc. 1992; pages 441-458) packings with cross channel structure for carrying out material separation processes are discussed. These packings are optimally utilizable when the specific separation performances which are required are not too high. The specific separation performance can be quantified through a parameter NTSM which specifies the number n of theoretical stages per meter (or in Kister the parameter HETP, “height equivalent of a theoretical plate”, which is measured in inches and is substantially the reciprocal value of the parameter NTSM). The specific separation performance is considered here to be high when NTSM is greater than 4.5 m
−1
(i.e. n>4.5).
In known uses it has turned out in practice that a class of packings in which the angle of inclination of the passages is in each case equally large is optimally utilizable. In the named treatise 45° is given as the value for this angle of inclination (see Kister, Table 8.1, in particular the class of packings “Mellapak®” 125.Y, 250.Y, 350.Y and 500.Y). In the packing class of “Mellapak®” (hereinafter, packings 125.Y, . . . ) this angle does not amount to 45° but rather to 42.5°; this angle has proven to be more favorable.
The inventor was confronted with the problem of making packings available for separation columns for which the specific separation performance is high and which is advantageous in particular in methods for air decomposition. In this he recognized that it is recommendable to carry out further basic experiments with respect to the manner of action of the packings with cross channel structure in order thus to be able to propose criteria for more economical packings as a result of newly won results.
SUMMARY OF THE INVENTION
The object of the invention is to create a packing with cross channel structure which is distinguished by a high specific separation performance and which enables as economical a separation process as possible. This object is satisfied by packings such as a packing with cross channel structure for a material exchange column having a high specific separation performance. The packing is specifiable by a specific surface area, a, and an inclination angle of the passages, A. It belongs to a class of packings with a =a
cl
and &phgr;=&phgr;
cl
in accordance with the following assertions:
for fluids which flow in the packing, equally large fluid flows are in each case provided for all packings;
a number n of theoretical separation stages per meter which can be empirically determined for the packing, namely the parameter NTSM, can be represented by a first function f
1
(a, &phgr;) of the variables a and &phgr;;
a flow resistance which results for a gas flow which flows through the packing can be characterized by a pressure loss per meter, &Dgr;p, as a second function f
2
(a, &Dgr;);
under the auxiliary condition n=const, &Dgr;p assumes a relative minimum for which the variables a and &phgr;—in dependence on the parameter n—assume the values a
m
(n) and &phgr;
m
(n) respectively;
it is true that n>4.5, a
cl
<a
m
(n), &phgr;
cl
>&phgr;
m
(n) and &phgr;
cl
>45°.
The invention will be illustrated with reference to two examples:
The packing 750.Y (specific surface area a=750m
−1
, angle of inclination of the passages &phgr;=42.5°) has 5.5 separation stages per meter (n=5.5). If the angle is increased to 50°, then the packing surface area which is required for the material separation can be reduced to 500 m
2
/m
3
, through which the cost and complexity for the packing material is reduced to 66%. In this the separation performance, at a practically unchanged flow resistance of the packing (pressure loss &Dgr;p=2 mbar/m at F=v
G
&rgr;
G
=1.5 Pa
0.5
), likewise remains unchanged. Additional material could be saved for the same separation performance, at the price, however, of an increased flow resistance (for a=450 m
−1
and &phgr;=57°, &Dgr;p=2.7 mbar/m).
For the packing 500.Y (a=500 m
−1
, &phgr;=42.5°), n=4.5. For &phgr;=46°, a=450 m
−1
, i.e. the material cost and complexity is reduced to 90%, with the flow resistance being practically unchanged. For &phgr;=50°, a=400 m
−1
, (material cost and complexity 80%), but the flow resistance increases to 123% of the value to be expected for the packing 500.Y.
REFERENCES:
patent: 4929399 (1990-05-01), Lockett et al.
patent: 5644932 (1997-07-01), Dunbobin et al.
Dunbobbin, Hanley B., et al.: “A Unified Model for Countercurrent Vapor/Liquid Packed Columns. 1. Pressure Drop”, in:Ind. Eng. Chem. Res.1994, 33, 1208-1221.
Hanley, Brian, et al.: “A Unified Model for Countercurrent Vapor/Liquid Packed Columns. 2. Equations for the Mass-Transfer Coefficients, Mass-Transfer Area, the HETP, and the Dynamic Liquid Holdup” in:Ind. Eng. Chem. Res.1994, 33, 1222-1230.
Bennett Henry
Drake Malik N
Sulzer Chemtech AG
Townsend and Townsend / and Crew LLP
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