1. Field of the Invention
The present invention relates to a distillation apparatus that carries out a distillation operation widely applied to many industrial processes, and more particularly to a heat integrated distillation apparatus.
2. Description of the Related Art
Distillation separation is a unit operation widely applied to industrial processes in general, but consumes a large amount of energy. In the industrial field, therefore, studies have been conducted on an energy saving distillation systems. Such studies have brought about development of a heat integrated distillation column (hereinafter, HIDiC) as a distillation apparatus that save much energy.
As shown in FIG. 1, a basic system of the HIDiC has a structure where a rectifying section (high-pressure unit) and a stripping section (low-pressure unit) are provided such that they are separate from each other. Operation pressure of the rectifying section is set higher than that of the stripping section so that the operation temperature of the rectifying section can be higher than that of the stripping section. This enables a reduction in the amount of heat that is supplied to a reboiler because heat transfer occurs from the rectifying section to the stripping section when there is a heat-exchange surface therebetween. Heat of the rectifying section moves to the stripping section, and hence the amount of heat that is supplied at a reboiler can be reduced. As a result, high energy saving distillation apparatus can be achieved.
In order to put the concept of HIDiC to practical use, a number of distillation apparatuses having double-pipe structures, that is, double-pipe structures constituted of inner pipes forming rectifying sections and outer pipes forming stripping sections (refer to JP2004-16928A) have been proposed. These configurations are described as being capable of reducing the amounts of heat that are supplied to the reboilers and the amounts of heat that are removed at the condensers, since heat transfer occurs from the rectifying sections (inner pipes) to the stripping sections (outer pipes).
However, the heat integrated distillation apparatus having the rectifying section and the stripping section formed into the double-pipe structures as discussed in Patent Literature 1 had the following problems 1) to 6).
1) The product cannot be obtained with side-cut stream. The side-cutting means that a product is withdrawn as an intermediate distillate product, during a distillation process until an end distillate is acquired from top of column.
In the distillation apparatus described in JP2004-16928A, the tube units of the double-pipe structures are arranged to come into contact with each other. Moreover, the outer pipes and the inner pipes are equipped with the structured packing. As a result, no pipe arrangement can be formed to withdraw any intermediate distillate product from the inner pipe of each tube unit. Consequently, the structure disables side-cutting.
2) The feed stage where feed stream is provided cannot be optimized. This is because in the rectifying section and the stripping section formed into the double-pipe structures, packing heights thereof are equal, disabling free setting of the number of stages of the rectifying section and the stripping section.
3) The feed stage cannot be changed so as to meet the feed stream composition. This is because of the structure where free setting of the feeding stage position is disabled as described in 2).
4) Multi-feed stream (reception of a plurality of feed streams) cannot be dealt with. This is because of the structure where no feed stream can be supplied in the midway of the double-pipes as described in 1).
5) Maintenance of the apparatus is difficult. The tube units that use the structured packing are densely arranged to be adjacent to each other as described in 1). This disables complete access to the desired tube unit, and maintenance thereof cannot be carried out.
6) The heat exchanged rate between the rectifying section and the stripping section that uses double-pipes and in which there is no a degree of freedom in design for designing the heat transfer area, depends only on the temperature profile of the distillation column. Hence, in apparatus design, a degree of freedom in design of heat exchanged rate is small.
Q, the heat exchanged rate between the rectifying section and the stripping section, is represented by Q=U×A×ΔT, where U is an overall heat-transfer coefficient, A is a heat transfer area, and ΔT is a temperature difference between the rectifying section and the stripping section. In the HIDiC using the double-pipe structure, an inner pipe wall surface becomes a heat transfer area. This heat transfer area has a fixed value determined by a structure of the double-pipes. The overall heat-transfer coefficient also has a fixed value determined by the heat transfer structure and fluid physical properties involved in heat exchange. Thus, as can be understood from the heat exchanged rate formula, a heat exchanged rate on design specification can be changed based only on the temperature difference between the rectifying section and the stripping section, which is changed by the operating pressure of the rectifying section and the stripping section.
As the heat integrated distillation apparatus that can solve the problem as described above, the present applicant has proposed the apparatus of JP4803470B.
FIG. 2 shows a first example of the distillation apparatus disclosed in JP4803470B. The distillation apparatus includes rectifying column 1, stripping column 2 located higher than rectifying column 1, first pipe 23 for communicating column top 2c of the stripping column with column bottom 1a of the rectifying column, and compressor 4 configured to compress vapor from column top 2c of the stripping column to feed the compressed vapor to column bottom 1a of the rectifying column. The distillation apparatus further includes tube-bundle-type heat exchanger 8 located at a predetermined stage of rectifying column 1, liquid withdrawal unit 2d located at a predetermined stage of stripping column 2 and configured to withdraw a part of liquid from the predetermined stage to the outside of the column, second pipe 24 for introducing the liquid from liquid withdrawal unit 2d to heat exchanger 8, and third pipe 25 for introducing fluids introduced through second pipe 24 to heat exchanger 8 and then discharged out of heat exchanger 8 to a stage directly below liquid withdrawal unit 2d. 
With the above described configuration in FIG. 2, the fluids flow from stripping column 2 to heat exchanger 8 of rectifying column 1 through second pipe 24. Heat is removed from the vapor of rectifying column 1 in heat exchanger 8. Then, the heat can be transferred from rectifying column 1 to stripping column 2 through third pipe 25. The fluids flow from stripping column 2 to rectifying column 1 by gravity. The fluids in heat exchanger 8 are accordingly pushed to flow from rectifying column 1 to stripping column 2. In other words, this heat integrated distillation apparatus employs a thermo-siphon system, and hence no pressure-feeding means such as a pump is necessary for supplying the liquid from rectifying column 1 to stripping column 2 located above in a vertical direction.
FIG. 3 shows a second example of the distillation apparatus disclosed in JP4803470B. The distillation apparatus includes rectifying column 1, stripping column 2 located higher than rectifying column 1, first pipe 23 for connecting column top 2c of the stripping column with column bottom 1a of the rectifying column, and compressor 4 that compresses vapor from column top 2c of the stripping column to feed the compressed vapor to column bottom 1a of the rectifying column. The distillation apparatus further includes liquid sump unit 2e located at a predetermined stage of stripping column 2 and configured to hold liquid that has flowed downward, heat exchanger 8 located in liquid sump unit 2e, partition plate 16 that is set in a predetermined position of rectifying column 1 and configured to apart upper stages and lower stages completely, second pipe 29 for introducing vapor below partition plate 16 to heat exchanger 8, and third pipe 30 for introducing fluids introduced through second pipe 29 to heat exchanger 8 and then discharged out of heat exchanger 8 to an upper side of partition plate 16.
With the above described configuration in FIG. 3, the vapor is withdrawn from rectifying column 1 through second pipe 29. The vapor is introduced to heat exchanger 8 in stripping column 2. Then, heat can be transferred from rectifying column 1 to stripping column 2. High-pressure vapor in rectifying column 1 ascends through second pipe 29 to heat exchanger 8 in stripping column 2. A fluid partially or totally condensed from the vapor in heat exchanger 8 is accordingly pushed out from stripping column 2 to third pipe 30 outside the column. Thus, this configuration also necessitates no pressure-feeding means such as a pump in supplying liquid from stripping column 2 to rectifying column 1 located at a lower side in a vertical direction.
The apparatus configurations of FIGS. 2 and 3 described above are each capable of reducing the amount of heat that is removed at condenser 7 which is attached to a column top of rectifying column 1, and reducing the amount of heat of reboiler 3 attached to a column bottom of stripping column 2 more, as compared with an ordinary distillation apparatus which has a column in which an upper side is a rectifying section and a lower side is a stripping section with a feed location as a boundary thereof, and which is not of a heat integrated type. As a result, it is possible to provide an energy-efficient distillation apparatus.
Rectifying column 1 and stripping column 2 can be configured by using trayed sections or packed bed sections similar to those of a general distillation apparatus. Hence, the apparatus can deal with side cutting or multi-feed stream without the need for any improvement, and it is possible to easily perform maintenance of the apparatus. For the same reason, the number of stages of the rectifying column or the stripping column can be freely set, and a feed stage can be optimized.
A heat transfer area can be freely set, and hence the heat exchanged rate can be determined without any dependence on the temperature difference between the columns.
As described above, according to the apparatus example described in JP4803470B (FIGS. 2 and 3), energy efficiency is high, side-cutting and setting of a feed stage position can be easily dealt with, and maintenance of the apparatus is easy. Further, the apparatus of the present invention has a structure in which a degree of freedom in design is high, and hence can be easily accepted by the user side.
Incidentally, concerning the distillation apparatuses shown in FIGS. 2 and 3, the present inventors aim at further enhancement in energy efficiency, and the respective distillation apparatus examples still have the following room to be improved.
In other words, in the distillation apparatus shown in FIG. 2, the following method is adopted. Part or all of a liquid in an arbitrary stage of stripping column 2 is removed through pipe 24 outside the column, and is supplied to tube-bundle-type heat exchanger 8 located at an arbitrary stage of rectifying column 1, where heat exchange is performed. Thereafter, a part or all of the amount of the liquid which is removed from stripping column 2 is vaporized by vapor in rectifying column 1 at a higher temperature, and returns to directly below the above described liquid removal position of stripping column 2 via pipe 25 outside the column by a thermo-siphon effect, without energy given from outside by a pump or the like. Such circulation of the fluids is performed.
In such a method, a liquid head is needed at the supply side of tube-bundle-type heat exchanger 8 (pipe 24 outside the column) in order to perform circulation of the fluids by the thermo-siphon effect. In other words, as the portions extending in the vertical direction, of pipes 24 and 25 become long correspondingly to the distance (height) between liquid withdrawal position X from stripping column 2 and heat exchanger installation position Y of rectifying column 1, pressure loss through pipe 25 increases. Hence, in order to circulate the fluids by surpassing this, the liquid head based on the inlet position of heat exchanger 8 (end portion of pipe 24 connected with heat exchanger 8) also becomes large. In the tube of heat exchanger 8, however, the pressure becomes high and the boiling point increases due to the increase in the liquid head. Therefore, the temperature difference between the inside of the tube and the outside (shell) of the tube in heat exchanger 8 becomes small correspondingly to the increase of the boiling point. In order to compensate this, a necessity arises to increase the pressure of rectifying column 1, that is, to increase the temperature in rectifying column 1 by increasing the compression ratio of compressor 4. Thus, there is a room to be improved from the viewpoint of energy saving performance.
In other words, in the distillation apparatus shown in FIG. 3, the following method is adopted. Partition plate 16 that completely partitions the inside of the column to an upper side and a lower side is installed in an arbitrary stage of rectifying column 1, all of vapor ascending from below partition plate 16 is withdrawn from the column through pipe 29, and is supplied to tube-bundle-type heat exchanger 8 installed at an arbitrary stage of stripping column 2, where heat exchange is performed. Thereafter, a fluid partially or totally condensed in heat exchanger 8 flows through pipe 30 outside the column to the upper side of partition plate 16 in rectifying column 1 by gravity, and the condensed liquid flows through another pipe 31 to be movable to below partition plate 16. Such circulation of the fluids is performed.
Such a method intend to withdraw all of the vapor in rectifying column 1 to the outside of the column, and hence adopts a complicated structure in which partition plate 16 is installed in rectifying column 1, and the condensed liquid fed onto partition plate 16 from stripping column 2 is further transferred to a lower side space of partition plate 16 through pipe 31 and control valve 17 outside the column. Thus, there is a room to be improved from the viewpoint of the structure and manufacturing cost.
Further, drive force for the fluids passing through the tube of heat exchanger 8 is obtained by giving pressure loss at the upper and lower sides of partition plate 16, and hence pressure of column bottom 1a needs to be made larger than pressure of column top 1c of rectifying section 1 correspondingly to the pressure loss at the upper and lower sides of partition plate 16. Thus, there arises a need for setting pressure to be higher at an outlet side of compressor 4 (namely, increase a compression ratio) correspondingly to increase in the pressure at column bottom 1a side. Therefore, there is also a room to be improved from the viewpoint of energy saving performance.
In order to further improve both of the apparatus configuration examples of FIGS. 2 and 3 as above, the present inventors pay attention to the circumstances as follows.
When the stages for performing side heat exchange between the stripping section and the rectifying section are optimally selected, in order to put the concept of HIDiC to practical use, the lowermost stage of the rectifying section is not used for heat exchange with the stripping section in some cases. FIG. 4 shows a conceptual configuration of HIDiC in this case, and FIGS. 5 and 6 show examples of the mode of carrying out HIDiC in this case. In particular, FIG. 5 shows application of the heat transfer system of the apparatus of FIG. 2 to the conceptual configuration of FIG. 4, and FIG. 6 shows application of the heat transfer system of the apparatus of FIG. 3 to the conceptual configuration of FIG. 4.
As is understandable with reference to the configurations of FIGS. 4 to 6, a single stage or a plurality of stages in rectifying section lower part 1d is not or are not involved in heat exchange at all in many cases. This not only applies to the tray, but also to the packed bed layer. In the configurations as above, rectifying section lower part 1d which has the highest temperature is not effectively used for heat exchange.
In the configurations in which rectifying section lower part 1d is not used for heat exchange with the stripping section as in FIGS. 4 to 6, there is no need to connect the outlet pipe of compressor 4 to column bottom 1a (the lowermost portion of the rectifying section) of the rectifying column. Rather, connecting outlet pipe 4a of compressor 4 to the heat exchanging section (heat exchanging section directly above rectifying section lower part 1d) which is located at the lowest position of the rectifying section does not waste heat. However, if outlet pipe 4a of compressor 4 is directly connected to the heat exchanging section located at the lowermost position of the rectifying section without being directly connected to the lowermost portion of the rectifying section, there will not be any gas in lower part 1d, which is located below this, of the rectifying section, and therefore, distillation operation which has liquid-vapor equilibrium as the principle of separation is not established. Therefore, the apparatus which uses this method of connection cannot be realized.
Thus, the present inventors have decided to move and dispose region 2g (the region shown by the dotted line in FIG. 4), that corresponds to lower part 1d of the rectifying section, to a location that is above the upper part of the stripping column (in other words, above feed stage 2f), as shown in FIG. 4, for example. The disposition like this does not change the flow itself of the fluids at all, and if the stage of lower part 1d of the rectifying section is moved and disposed above feed stage 2f, region 2g, that corresponds to the lower part of the rectifying section, can be operated under the pressure of stripping column 2 which is lower than the pressure of rectifying column 1. As a result, the relative volatility in region 2g becomes large, and the energy (heat amount) itself which is originally necessary for a separating process can be reduced.
Further, in the case of the configuration in which outlet pipe 4a of compressor 4 is connected to the position directly above lower part 1d of the rectifying section, as shown by the dotted lines in FIGS. 4 to 6, the vapor from outlet pipe 4a of compressor 4 is supplied into rectifying column 1, and from rectifying column 1, heat is transferred to heat exchanging section 2h in the lower part of stripping column 2, and returns to the position of rectifying column 1 to which outlet pipe 4a is connected, again. The present inventors have considered that, if that is the case, adoption of such an apparatus configuration is more preferable, that outlet pipe 4a of compressor 4 be directly connected to heat exchanging section 2h at the low part of stripping column 2 without being connected all the way to rectifying column 1, and that the fluids which are subjected to heat exchange at heat exchanging section 2h be introduced into rectifying column 1.
In the case of the apparatus configuration based on the above concept, the apparatus configuration of FIG. 5 becomes an apparatus configuration of FIG. 7, and the apparatus configuration of FIG. 6 becomes an apparatus configuration of FIG. 8, though the details thereof will be described later. According to the apparatus configurations of FIGS. 7 and 8, among a plurality of heat transfer systems which perform heat exchange between several stages of respective low-pressure column and a high-pressure column, in one heat transfer system provided between the lower part of a low-pressure column and the lower part of a high-pressure column, high-pressure vapor from compressor 4 is directly fed to heat exchanger 8 at the lower part of the low-pressure column, and the high-pressure vapor which gives heat to the lower part of the low-pressure column through heat exchanger 8 is introduced into rectifying column 1, as shown in FIGS. 7 and 8. In this heat transfer system, both the apparatus configuration of FIG. 2 and the apparatus configuration of FIG. 3 are improved. The reason for this is as follows.
That is to say, in the apparatus examples of FIGS. 2 and 5, circulation of the fluids by the thermo-siphon effect is used in order to perform heat exchange between the rectifying section and the stripping section, and in order to cause the fluid to circulate, a liquid head is necessary at the supply side (pipe 24 outside the column) of tube-bundle-type heat exchanger 8. However, the apparatus configuration of FIG. 7 does not need a liquid head in some heat transfer systems, and therefore, improvement in energy saving performance is expected.
Meanwhile, in the apparatus example of FIG. 3, the high-pressure vapor which is supplied to rectifying column 1 moves into stripping column 2 through pipe 29 outside the column, and the fluids that partially or totally condense in heat exchanger 8 of liquid sump unit 2e in stripping column 2 return to rectifying column 1 through another pipe 30. For this purpose, it is necessary to partition the inside of rectifying column 1 completely with partition plate 16, connect pipe 29 to a lower space of partition plate 16, and connect pipe 30 to an upper space of partition plate 16 to set the pressure of the lower space of partition plate 16 to be higher. But, the apparatus configuration of FIG. 8 does not require a pressure difference in some heat transfer systems, and therefore, improvement in energy saving performance is expected.