Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-03-04
2002-02-19
Doerrler, William C. (Department: 2744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S643000, C062S631000, C062S905000
Reexamination Certificate
active
06347533
ABSTRACT:
BACKGROUND—FIELD OF THE INVENTION
This invention relates to a fractionation column and method of operation thereof. In a specific respect, the invention relates to separating at least one feed stream containing at least two components, into at least one overhead distillate stream, at least one side draw stream and at least one bottom stream, each such product stream containing different averaged volatility than the other product streams. The fractionation column is equipped with at least one condenser and at least one reboiler. Further, the fractionation apparatus includes innovative designs to enable hydraulically balanced and energy efficient operation at various feed rates, compositions and product specifications.
BACKGROUND—DISCUSSION OF PRIOR ART
In fractionation, it is sometimes desirable to separate a multi-component feed stream into a number of streams containing various fractions of desirable components in the product streams. For the case of one feed stream and two product streams, the separation can be accomplished by distillate and bottoms product draw. Further separation can be accomplished by repeating the two-product stream process to either the distillate or the bottoms streams. However, the introduction of additional columns will require a corresponding number of reboilers and condensers. That requirement, in turn, requires additional operating costs as the condensing and the reboiling process is being repeated. Numerous references can be found in prior art documenting efforts to lower both capital and operating costs in the above-mentioned separation. The benchmark of the lowest energy consumption has been set by the well-known PETLYUK system as shown in FIG.
1
.
In this configuration, a prefractionation column separates the feed into two streams using a split vapor stream from the main column's stripping section and a split liquid stream from the main column's rectifying section. The resulting vapor and liquid streams exiting from the prefractionation column is richer in light and heavy components respectively. These two semi-processed streams are then fed back to the main column. This configuration provides an advantage allowing the main fractionation column to enhance the purity of the side stream draw. In turn, the main fractionation column also provides the stripping section and the rectifying section with better quality feeds. The combined effect is a very efficient use of vapor/liquid traffic to yield three product streams. The drawback from such an ingenious design is that the vapor and liquid streams at the communicating crossovers are almost at the same operating pressures, making the column next to impossible to operate. Consequently, not one such design has ever been operated commercially.
Wright in U.S. Pat No. 2,471,134 proposed to combine the prefractionation and main columns into one fractionation unit by erecting a partition along the center part of a column. The column is equipped with one overhead condenser and one bottom reboiler. This prior art is shown in FIG.
2
. This setup is effective in overcoming the hydraulic limitations in the PETLYUK system. At the same time, it reduces capital costs by having only one common shell. However, the fixed internals offers no adjustments in vapor/liquid traffic in either side of the partition. As feed quality and rate changes, the desired operating range could very easily fall outside of the original design envelope.
In U.S. Pat No. 3,053,521 Plaster and Dixon taught the use of introducing a tray in a column having a large cross sectional area. The level of the stored liquid in the tray affects the rate of internal liquid flow through a perforated weir. Resetting a side draw flow controller controls the level in the tray. That effectively controls the internal liquid traffic in the column.
Graven taught the use of a partitioned stripping section as shown in
FIG. 3
using two reboilers on either side of the partition in U.S. Pat No. 3,412,016. This configuration handles two feeds with different quality of higher boiler contents and economizes by sharing a common rectifying section. Two bottoms product streams and one distillate product stream result from such a configuration.
In U.S. Pat No. 4,230,533, as shown in
FIG. 4
, Giroux taught the use of a method to adjust vapor and liquid traffic in a divided wall column that was described by Wright earlier. The proposed column internals and additional control loops were designed to provide control of liquid and vapor traffic in the two sides of the internal partition. Unlike Wright's design, which has fixed internal arrangement, Giroux's proposal claimed to be able to handle changes in feed rate, quality and product specifications. The very concept of a divided wall column configuration has one obvious intrinsic limitation. By sharing a partition, the stages available for prefractionation and main fractionation is tied. In other words, the ratio of stages between feed side and product side cannot deviate too far away from unity. Some design scenarios, however, may be more effective if that ratio is allowed to be higher or lower than unity.
Two other U.S. Patents also made reference to the PETYLUK benchmark. They are described as follows:
U.S. Pat No. 5,755,933 assigned to M.W. Kellogg Company of Houston Tex. taught the use of a partitioned rectification section, each having its condenser in the overhead as shown in FIG.
5
. The lightest product stream is to be collected as distillate on the feed side of the partition. The intermediate product stream is to be collected as the distillate from the other overhead condenser. The heaviest product is to be recovered as the bottoms. This design offers a unique benefit as the vapor flow is directly affected by the corresponding condenser duty. Increasing the condensing duty in either one of the overhead condensers causes a local vapor inventory decrease as vapor is condensed to form liquid. The corresponding lower pressure at the top will cause higher flow of vapor flowing upwards. As well, the reflux can be increased to effect higher liquid/vapor traffic. However, there are a few drawbacks in this proposed configuration.
1) A vaporized feed is highly recommended in order to prevent the light products from passing under the partition. Should that be allowed to happen, the intermediate component will be irreversibly contaminated with the light products.
2) The partition gives the two rectifying sections roughly the same number of stages. That requirement, again, may not be the most desirable design.
3) An additional condenser is required.
U.S. Pat No. 5,970,742 assigned to Air Products and Chemicals Inc. of Allentown Penn. taught the use of a “modified” PETLYUK system. In this design, liquid and vapor is being generated in the prefractionation column by adding a condenser or a reboiler respectively as shown in FIG.
6
. The Patent claims to be almost as energy efficient as the PETLYUK system. A simple analysis would arrive at quite a different conclusion. Let us consider the prefractionation condenser in this configuration. Since this secondary reflux is generated by the secondary condenser, which is situated at a point below the main condenser, the secondary reflux is effectively bypassing the top section of the main column. Similarly, the reboiler duty used in the prefractionation column is again effectively bypassing the bottom stripping section of the main column. Therefore, thermodynamically, this configuration can never be as efficient as the PETLYUK system. The departure from the benchmark in the condenser configuration depends on the relative volatility between the lightest product and the intermediate product as well as the stages above the vapor return to the main column. In effect, how much fractionation the internal reflux bypassing is giving up. Similarly, the departure from the benchmark in the reboiler configuration depends on the relative volatility between the intermediate product and the heavy product and the stages below the liquid return to the main column. In effect, how
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