Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-09-03
2001-09-11
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S623000
Reexamination Certificate
active
06286335
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH FOR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The present invention pertains to the field of distillation of multicomponent fluid mixtures, and in particular to distillation processes for separating a ternary mixture having components, A, B and C (mixture ABC) into three product streams each enriched in one of the components, where A is the most volatile component and C is the least volatile component.
To separate a ternary mixture ABC into almost pure components a distillation process must use at least two distillation columns. Five such distillation processes are well known in the prior art: direct sequence, indirect sequence, side rectifier, side stripper and fully thermally coupled columns. (For example, see page 711 of the book entitled “Separation Processes” by C. J. King, McGraw-Hill, 1981). Of all the prior art ternary distillation processes, the fully thermally coupled column system requires the least amount of heat duty (“Minimum Energy Requirements of Thermally Coupled Distillation Systems”, Z. Fidkowski and L. Królikowski, AICHE Journal, pages 643-653, volume 33, 1987). In spite of this attractive performance, the fully thermally coupled column system has not been used widely in commercial applications, in part because of operating problems.
FIG. 1
shows a fully thermally coupled column (FC) process. A feed mixture containing components A, B and C (stream
100
) is fed to a first distillation column
110
, where the feed stream is separated into two streams (
122
,
132
) that are primarily binary mixtures. The liquid (stream
122
) from the bottom of this distillation column is primarily a binary mixture composed of components B and C. Similarly, the vapor (stream
132
) from the top of this distillation column is primarily a binary mixture composed of components A and B. Both of these primarily binary mixture streams
122
and
132
are fed to different locations of a second distillation column
120
. A portion of the liquid from the bottom of a second distillation column
120
is recovered as C-enriched product stream
190
, and another portion of this liquid is boiled in reboiler
182
and returned as vapor stream
192
to provide boilup for the second distillation column
120
. The vapor from the top of second distillation column
120
is condensed in condenser
112
, and a portion is recovered as A-enriched product stream
170
while the other portion (stream
172
) is returned to provide the needed liquid reflux for this distillation column. The B-enriched product stream
180
is produced from an intermediate location of second distillation column
120
. (A location of a distillation column is an “intermediate location” when there is at least one separation stage above and one separation stage below that location. A “separation stage” is a mass transfer contact device between liquid and vapor phases, such as a suitable mass transfer tray or a packed height of a suitable packing.) This withdrawal location (intermediate location) is in between the feed locations of the two primarily binary feed streams
122
and
132
.
The first distillation column
110
does not use a reboiler or a condenser. The boilup at the bottom of this column is provided by feeding a vapor stream
127
from the second distillation column
120
. It is important to note that the withdrawal location of vapor stream
127
is from the same location of the second distillation column
120
as the feed location of the primarily binary liquid stream
122
. This leads to a two-way communication between the two distillation columns. (In a two-way communication mode, when a vapor stream is sent from one column to another column, then a return liquid stream is implemented between the same locations of the two columns.) Similarly, the liquid reflux stream
137
to the top of the first distillation column
110
forms another two-way communication between the two distillation columns since stream
137
is withdrawn from the second distillation column
120
at the same location as the feed location of vapor stream
132
. It is taught in the prior art that two two-way communications are needed to achieve the lowest heat demand for ternary distillation.
While the heat demand for the process in
FIG. 1
with two two-way communications is lowest, it rarely has been used. The lack of use has often been attributed to fear of control problems (“Thermal Coupling for Energy Efficiency”, H. Rudd, Supplement to the Chemical Engineer, pages S14-S15, Aug. 27, 1992; “The Design and Optimization of Fully Thermally Coupled Distillation Columns”, C. Triantafyllou and R. Smith, Trans. IChemE, pages 118-132, Volume 70(A), 1992). One of the often cited concerns is the flexibility to control the flows over a wide range both at the top and bottom ends of the first distillation column
110
. For the vapor AB in stream
132
to flow from first distillation column
110
to the second distillation column
120
, it is required that the pressure at the top of the first distillation column
110
be greater than the pressure at the feed point of stream
132
in the second distillation column
120
. At the same time, for the vapor BC in stream
127
to flow from the second distillation column
120
to the first distillation column
110
it is necessary that the pressure at the bottom of first distillation column
110
be lower than the pressure at the originating point of stream
127
in the second distillation column. This leads to an unique restriction that the pressure at the bottom of the first distillation column
110
be lower than the pressure at a point in the bottom section of the second distillation column
120
, and at the same time, the pressure at the top of the first distillation column must be higher than the pressure at a point in the top section of the second distillation column. This requires careful adjustment of pressures in both of the columns and presents operating concerns for plants requiring wide ranges of variation in flow rates and other operating parameters. C1 early, there is a need for alternative column arrangements with higher operating flexibility while maintaining lower heat demand for distillation.
It is worth noting that both of the liquid transfer streams
137
and
122
at the top and bottom of the first distillation column
110
flow in a direction opposite of the flow of the vapor streams (
122
,
132
) at each end. This requires that either a pump be used on each of the liquid streams or that the relative heights of the two columns be adjusted to allow each of the liquid streams to be transferred through gravity.
Recently, Agrawal and Fidkowski introduced the process shown in
FIG. 2
(U.S. Pat. No. 5,970,742) In this figure, the bottom end of first distillation column
110
has a two-way communication with the bottom section of second distillation column
120
and the top end of the first distillation column
110
has only one-way communication with the top section of the second distillation column
120
. Thus, liquid stream
122
from the bottom end of the first distillation column
110
is sent to the bottom section of second distillation column
120
. A vapor stream
127
is withdrawn from the second distillation column
120
and sent to the bottom of the first distillation column
110
. A portion of the vapor stream exiting from the top end of the first distillation column
110
is sent as stream
132
to the second distillation column
120
. Unlike
FIG. 1
, there is no liquid return stream to the top of the first distillation column from the second distillation column. Instead, a portion of the vapor stream (stream
234
) from the top of the first distillation column
110
is condensed in condenser
215
and returned as liquid reflux in line
236
. Once again, a vapor stream (stream
132
) is transferred from the first distillation column to the second distillation column and a second vapor stream (stream
127
) is transferred in the reverse direction. This leads to s
Air Products and Chemicals Inc.
Capossela Ronald
Jones II Willard
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