Patent Application
20140165650
The present invention provides a shell and tube heat exchanger having tubes located within a shell and a distillation column arrangement useful for air separation in which the heat exchanger operably associates higher and lower pressure columns in a heat transfer relationship to vaporize an oxygen-rich liquid produced in the lower pressure column through indirect heat exchange with a nitrogen-rich vapor produced in the higher pressure column to condense the nitrogen-rich vapor. More particularly, the present invention relates to such a heat exchanger in which the tubes are arranged in inner and outer arrays and the inner arrays present a greater average open area between the tubes thereof than that of the outer array of tubes to lower pressure drop.
Shell and tube heat exchangers are used in a variety of industrial processes to indirectly exchange heat between a liquid and a vapor. Such heat exchangers have an arrangement of tubes located within a shell. The liquid is introduced into the tubes where the liquid at least partially vaporizes through indirect heat exchange with a vapor that is introduced into the shell. As a result of the heat exchange, the vapor condenses and the condensate is discharged from the shell. Typically, the tubes are supported by tube sheets located at opposite ends of the shell that is of cylindrical configuration. One of the tube sheets has an inlet for the vapor and the other of the tubesheets has an outlet for discharging the resulting condensate.
Shell and tube heat exchangers are used in a variety of locations within an air separation plant. In an air separation plant, air is compressed and then purified of higher boiling impurities such as water vapor and carbon dioxide in a pre-purification unit. The pre-purification unit employs beds of adsorbent that are operated in an out-of-phase cycle to adsorb the water vapor and carbon dioxide and thus, produce a compressed and purified air stream. The pre-purification unit can also be designed to remove carbon monoxide and hydrocarbons that may be present in the air. The resulting compressed and purified air is then cooled to a temperature that is at or near the dew point of the air and then introduced into a distillation column arrangement having higher and lower pressure columns that are arranged in a heat transfer relationship by a shell and tube heat exchanger such as described above. In this regard, the higher pressure column will typically operate at between 5 and 6 bara and the lower pressure column will operate at between 1.1 and 1.5 bara.
The air is separated within the higher pressure column into a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms also known as kettle liquid. A stream of the kettle liquid is further refined in the lower pressure column into a nitrogen-rich vapor column overhead and an oxygen-rich liquid. In an air separation plant the shell and tube heat exchanger is employed as a condenser-reboiler to condense a stream of the nitrogen-rich vapor through indirect heat exchange with the oxygen-rich liquid, thereby to partially vaporize the oxygen-rich liquid and provide boilup to the lower pressure column. Such a condenser reboiler can be situated within a sump of the lower pressure column. The liquid nitrogen is used both as reflux to the higher and lower pressure columns and also, optionally, as a liquid product. It is to be noted that shell and tube heat exchangers are used in other places within the air separation plant, for instance, as an argon condenser to condense argon within an argon column attached to the lower pressure column to produce an argon product.
U.S. Pat. No. 4,436,146 illustrates a shell and tube heat exchanger employed as a condenser reboiler in a double column arrangement of an air separation plant. The condenser reboiler functions to condense nitrogen-rich vapor overhead of the higher pressure distillation column through indirect heat exchange with an oxygen-rich liquid produced in the lower pressure column. The resulting nitrogen-rich vapor will condense at the higher pressure of the higher pressure column and thereby supply the heat of vaporization for the oxygen-rich liquid. In the shell and tube heat exchanger shown in this patent, the tubes are open at opposite ends that the heat exchanger sits in a pool of the oxygen-rich liquid that collects as a column bottoms within the low pressure column. The vaporization of the oxygen-rich liquid within the tubes entrains the liquid which rises in the tubes. The resulting oxygen-rich vapor provides boilup within the lower pressure column and unvaporized or residual oxygen-rich liquid is returned to the pool of the oxygen-rich liquid collected in the bottom of the lower pressure column. Such a heat exchanger is known as a thermosiphon reboiler. The nitrogen-rich vapor condenses on the exterior of the tubes and is discharged to a piping network from which the resulting nitrogen-rich liquid is introduced into both the higher and lower pressure columns as reflux and can be taken as a liquid product.
U.S. Pat. No. 4,436,146 incorporates features that are employed in condenser reboilers and also, shell and tube heat exchangers generally. For instance, a baffle plate is provided opposite to the vapor inlet to help urge the flow of the incoming nitrogen vapor in an outward, radial direction of the shell. Beneath this plate is an elongated cylindrical member that will collect non-condensable substances in the air such as neon and helium. Additionally, the area provided at the exterior of the tubes for heat transfer can be enhanced by provision of length-wise extending fins, also known as fluting. Further, the shell can be provided with a bellows-like expansion joint that will help reduce tensile or compressive loading between the tube sheets and the tubes and the tubesheets and the shell arising from the existence of a temperature gradient between the tubes and shell which would tend to cause an unequal expansion or contraction therebetween.
U.S. Pat. No. 5,699,671 discloses a shell and tube heat exchanger that can be used as a condenser reboiler within a double column arrangement of an air separation plant such as has been described above. The type of heat exchanger shown in this patent is known as a downflow heat exchanger because the condensing liquid flows in a downward direction of the tubes. In this patent a reservoir is provided for collecting the oxygen-rich liquid. The tubes penetrate the tube sheet and extend into the reservoir to receive the oxygen-rich liquid. A central conduit extends downwardly, into the reservoir and is in registry with the inlet for the nitrogen-rich vapor situated within the tubesheet to feed the nitrogen-rich vapor into the shell. The oxygen-rich liquid flows downwardly through the tubes and partially vaporizes. The liquid that is not vaporized is collected as a liquid column bottoms within the lower pressure column and the resulting vapor provides boilup within the lower pressure column. It is to be noted that the internal surface of each of the tubes can be provided with a thin metallic film coating having a high porosity and a large interstitial surface area to increase the surface area for boiling.
In any application of a shell and tube heat exchanger, it is important that the heat transfer area per unit volume available for indirectly exchanging heat between the vapor and the liquid be as large as possible so that the heat exchanger is as compact as possible. In air separation, this heat transfer area will also have a major effect on the costs of operation of the plant as well as the profitability of the sale of the separated components of the plant, for instance, liquid oxygen produced in the lower pressure column. The reason for this relates to the operation of the higher and lower pressure columns. At the lower operational pressure of the lower pressure column, the oxygen-rich liquid is sufficiently cold enough to condense the higher pressure, nitrogen-rich vapor produced in the higher pressure column or in other words create a sufficient temperature difference across the tubes to condense the higher pressure, nitrogen-rich vapor. As this temperature difference is decreased, the saturation temperature of the higher pressure, nitrogen-rich vapor will be lower. As result, the degree to which the air needs be compressed will also be lower. Since a major expense in operating an air separation plant is its electrical power costs incurred in motors used to drive compressors that compress the air, it is desirable that the plant be operated at pressure that is as low as possible.
The heat transfer area will have a direct effect on the temperature difference; namely, the higher the heat transfer area provided by the tubes, the lower the temperature difference. Therefore, in a shell and tube heat exchanger, particularly, for use in an air separation plant, it is desirable to have as many tubes as possible within the shell to maximize the heat transfer area through which heat transfer can occur. However, the problem with simply increasing the number of tubes in the same volume is that pressure drop within the heat exchanger will also increase. As can be appreciated, as the number of tubes is increased, the space between the tubes decreases resulting in the higher pressure drop for the vapor as its proceeds in the outward, radial direction. However, as the pressure drop increases, the advantage of providing more tubes to increase the heat transfer area diminishes given that the vapor to be condensed nevertheless has to be compressed to a sufficient pressure to compensate for the increased pressure drop.
As will be discussed, the present invention provides a shell and tube heat exchanger and a distillation column arrangement for an air separation plant in which, among other advantages, the tubes are in an arrangement that will decrease pressure drop and allow for more tubes to be used to increase heat transfer area.
The present invention provides a shell and tube heat exchanger that comprises two opposed tube sheets, a cylindrical shell connecting the two opposed tube sheets, a central vapor inlet and a central liquid outlet. The central vapor inlet is centrally positioned with respect to a central axis of the shell to introduce a vapor into the shell. The central liquid outlet is centrally positioned with respect to the central axis of the shell for discharging condensate produced by condensing the vapor. In a specific embodiment of the present invention, the central vapor inlet can be located in one of the two opposed tube sheets and the central liquid outlet can be located in the other of the two opposed tube sheets.
Tubes connect the two opposed tube sheets for indirectly exchanging heat between a liquid flowing within the tubes and the vapor, thereby, condensing the vapor and producing the condensate within the cylindrical shell. The condensation of the vapor, at least in part, induces a flow of the vapor in an outward, radial direction toward the shell. The tubes are arranged in an inner array of the tubes spaced apart from one another and surrounding the central vapor inlet and the central liquid outlet and an outer array of the tubes surrounding the inner array of the tubes and having a greater number of tubes than the inner array of the tubes. The inner array of the tubes and the outer array of the tubes are spaced apart from one another to present areas between the tubes. The areas of the inner array of the tubes have an average of the areas greater than that of the areas of the tubes of the outer array of the tubes situated directly adjacent the inner array of the tubes to lower pressure drop of the flow of the vapor in the outward, radial direction.
Since there will be an average open area between tubes greater than that of the tubes of the outer array situated directly adjacent the inner array of tubes, the velocity of the gas will be reduced to a level that is less than that which would otherwise have been obtained had the inner and outer array presented the same average area between tubes. This reduction in velocity will reduce pressure drop of the vapor as it flows in the outward, radial direction towards the shell. Since there will be some condensation of the vapor due to the heat transfer provided at the inner array of tubes, the flow and therefore, the velocity of the vapor will be reduced after passage of the vapor through the inner array of tubes to also reduce pressure drop within the flow of the vapor. Consequently, it is possible to stack more tubes within the heat exchanger than would have been possible if all of the tubes were set at an equal spacing.
The present invention also provides a distillation column arrangement for an air separation plant that comprises, a higher pressure distillation column, a lower pressure distillation column and a condenser reboiler. The higher pressure distillation column is configured to separate nitrogen from the air and thereby to produce a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms. The lower pressure distillation column is configured to further refine the crude liquid oxygen and thereby to produce an oxygen-rich liquid and a lower pressure nitrogen-rich vapor column overhead. The condenser reboiler partially vaporizes an oxygen-rich liquid produced in the lower pressure column and condenses at least part of the nitrogen-rich vapor column overhead produced in the higher pressure column. A means is provided for introducing the oxygen-rich liquid within tubes of the condenser reboiler. The condenser reboiler has the features of the shell and tube heat exchanger discussed above. In this regard, the condenser reboiler is provided with the two opposed tube sheets, a cylindrical shell, a central vapor inlet, a central liquid outlet. The central vapor inlet is centrally positioned with respect to a central axis of the shell and connected to an inlet conduit communicating between the central vapor inlet and the higher pressure column to receive a nitrogen-rich vapor stream composed of the nitrogen-rich vapor column overhead and thereby introduce the nitrogen-rich vapor stream into the shell. The central liquid outlet is centrally positioned with respect to the central axis of the shell and connected to a piping network having a first conduit connected to the higher pressure column for introducing a reflux stream composed of the part of the nitrogen-rich liquid into the higher pressure column and a second conduit connected to the lower pressure column for introducing another reflux stream composed of another part of the nitrogen-rich liquid into the lower pressure column. In a specific embodiment, the central vapor inlet can be located in one of the two opposed tube sheets and the central liquid outlet can be located in the other of the two opposed tube sheets.
The tubes of the condenser reboiler connect the two opposed tube sheets for indirectly exchanging heat between the oxygen-rich liquid flowing within the tubes and the nitrogen-rich vapor, thereby condensing the nitrogen-rich vapor and producing the nitrogen-rich liquid within the cylindrical shell, at least partially vaporizing the oxygen-rich liquid within the tubes and, at least in part, inducing a flow of the nitrogen-rich vapor in an outward, radial direction toward the shell as a result of the condensation of the nitrogen-rich vapor. The tubes in the inner and outer array of the tubes are arranged in the same manner as the shell and tube heat exchanger discussed above to lower pressure drop of the flow of the nitrogen-rich vapor in the outward, radial direction.
The oxygen-rich liquid can be composed of the oxygen-rich liquid column bottoms produced in the lower pressure column. The oxygen-rich liquid circulation means can comprise the inner array of the tubes and the outer array of the tubes open at opposite ends thereof and with the condenser reboiler submerged within the oxygen-rich liquid column bottoms. The oxygen-rich liquid flows within the inner array of the tubes and the outer array of the tubes through a thermosiphon effect.
In either the shell and tube heat exchanger or the condenser reboiler, at least one support can be located within the shell to support the inner array of the tubes in an intermediate location of the inner array of the tubes between the tube sheets to inhibit vibration within the inner array of the tubes. The at least one support can include a plate having openings through which the inner array of the tubes pass and are thereby supported. The plate is supported within the shell and tube heat exchanger at the intermediate location and opposite to the central vapor inlet so that the plate also acts as a baffle to also help in inducing the outward, radial flow of the vapor. The plate can be provided with a star-like outer periphery having indentations located opposite to innermost tubes of the outer array of the tubes. This, as will be discussed, will help in the assembly of the heat exchanger. Further, the plate can be supported by a set of supports connecting the plate to the one of the two tube sheets. A cylindrical member extends from the bottom of the plate towards the other of the two tube sheets to inhibit flow of the vapor around the plate in a direction taken from the one of the two tube sheets to the other of the two tube sheets.
Again, in either the shell and tube heat exchanger or the condenser reboiler, the inner array of the tubes can be arranged in a circular pattern having an equal spacing between the tubes. Alternatively, the inner array of the tubes can be arranged in a hexagonal pattern having an equal spacing between the tubes. Further, in either the circular or hexagonal pattern of tubes, the outer array of the tubes is arranged in a repeating hexagonal pattern.
While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the drawings in which:
With reference to
It is understood, however, that the distillation column arrangement 1 is but one application of the present invention which has more general applicability to any heat exchanger of shell and tube design and in any application thereof. As such, although for exemplary purposes the present invention is discussed below with respect to condenser reboilers, the invention and such discussion would have application to any shell and tube heat exchanger used in condensing a vapor through indirect heat exchange with a fluid.
Distillation column arrangement 1, as well known in the art, is used in the separation of nitrogen from oxygen to produce nitrogen and oxygen enriched products. Although not illustrated, as also well known, in an air separation plant, incoming air is compressed, purified and cooled to a temperature suitable for its rectification. The purified and cooled air is then introduced into the higher pressure distillation column 2 where an ascending vapor phase is contacted with the descending liquid phase by known mass transfer contacting elements, generally indicated by reference number 10, which can be structured packing, random packing or sieve trays or a combination of packing and trays. The ascending vapor phase of the air becomes evermore rich in nitrogen as it ascends and a descending liquid phase becomes evermore rich in oxygen. As a result, a bottoms liquid known as crude liquid oxygen or kettle liquid collects in the bottom of the higher pressure column 2 and a nitrogen-rich vapor collects in a top portion 12 thereof.
A stream of the kettle liquid that collects within the higher pressure column 2 is introduced into the lower pressure column 3 for further refinement. Again, ascending vapor and liquid phases are contacted within mass transfer contacting elements such as generally indicated by reference number 14. The liquid phase becomes evermore rich in oxygen as it descends within the lower pressure column 3 to form an oxygen-rich liquid column bottoms 16. As is also known in the art, as the liquid descends, the concentration of the argon within the liquid phase will increase. Although not illustrated, an argon and oxygen containing vapor stream could be removed from the lower pressure column 3 and then further refined in an argon column to produce an argon-rich product. Further, although not illustrated, a stream of the oxygen-rich liquid column bottoms 16 could be taken as a product directly or vaporized within the main heat exchanger to help cool the air or pumped and then vaporized within a heat exchanger against a boosted pressure stream of air to produce a product at pressure. The resulting liquid air could also be introduced into the lower pressure column 3 and/or the higher pressure column 2. As with the higher pressure column 2, the vapor phase within the lower pressure distillation column 3 will become evermore rich in nitrogen as it ascends.
The descending liquid phase in each of the higher and lower pressure distillation columns 2 and 3 is initiated by refluxing the columns with a nitrogen-rich liquid produced by condensing the nitrogen-rich vapor collected in the top portion 12 of higher pressure column 2 through indirect heat exchange with the oxygen-rich liquid column bottoms 16 of the lower pressure column 3. All of such vapor need not, however, be condensed in that some of it could be removed as a high pressure product. This indirect heat exchange is carried out within condenser reboilers 4A and 4B. Although two of such heat exchangers are shown, as would be known to those skilled in the art, there could be only one or more than two of such heat exchangers in a specific application of the present invention. A stream of the nitrogen-rich vapor “A” is introduced into an inlet conduit 18 that branches out to condenser reboiler 4A and 4B through inlet branches 20 and 22. As will be discussed, the nitrogen-rich vapor indirectly exchanges heat with the oxygen-rich liquid column bottoms 16 ascending in tubes thereof to partially vaporize the liquid and to fully condense the vapor. The liquid ascends within the tubes by the thermosiphon effect, discussed above. The vaporization of the oxygen-rich liquid column bottoms 16 initiates formation of the ascending vapor phase within lower pressure distillation column 3 as shown by arrowheads “B”. Although not illustrated, liquid that is not vaporized is returned to the bottom of the lower pressure distillation column 3.
The resulting condensate that consists of nitrogen-rich liquid is discharged from the condenser reboilers 4A and 4B by a piping network 24 that includes branches 26 and 28 connected to the condenser reboilers 4A and 4B to collect the nitrogen-rich liquid shown by arrowhead “C”. A first conduit 30 is connected to the higher pressure column 2 for introducing a reflux stream “D” composed of the part of the nitrogen-rich liquid into the higher pressure column and a second conduit 32 is connected to the lower pressure column 3 for introducing another reflux stream “E” composed of another part of the nitrogen-rich liquid into the lower pressure column. Preferably, a liquid distributor 34 is provided within the top portion 12 of the higher pressure column 2 to collect the reflux and distribute it to the underlying mass transfer contacting elements 10. Although not illustrated, a similar arrangement would be used in connection with the introduction of liquid reflux stream “E” into the top of the lower pressure column 3.
With reference to
The tubesheets 36 and 38 are connected by tubes 52 which are all of the same design and diameter. It is to be noted that all of the tubes 52 could be provided with an outer fluted surface and the interior of the tubes could be provided with an enhanced boiling surface such as been discussed above. The incoming nitrogen-rich vapor “A” will be condensed through indirect heat exchange with the oxygen-rich liquid column bottoms flowing upwardly through the tubes 52 due to the thermosiphon effect. Since the nitrogen-rich vapor “A” centrally enters the shell 40, through the central vapor inlet 44 and then flows in an outward, radial direction where the nitrogen-rich vapor “A” is successively condensed, a pressure gradient will be created in the flow of the nitrogen-rich vapor due to such condensation. The result of this gradient is that the flow of nitrogen-rich vapor “A” will be displaced from an axial flow, with respect to shell 40, to a flow in an outward, radial direction as shown by arrowheads “A′” and “A″”. As will be discussed, a baffle plate 66 can also be provided that will also have an effect of urging the incoming flow in the outward radial direction “A″”.
As has been discussed above, it is desirable to maximize the surface area provided by the tubes 52 for the indirect heat exchange. However, as the number of tubes 52 increases, the pressure drop within the condenser reboiler 4A with respect to the nitrogen-rich vapor will also increase. The reason for this is that as the number of tubes 52 increases, there will be less area between the tubes for the nitrogen-rich vapor to flow and therefore, the velocity of the nitrogen-rich vapor between tubes 52, on average, will increase to in turn increase the pressure drop. This would be true in any shell and tube heat exchanger and in any application thereof. However, in case of an air separation plant, the end effect would be increased compression requirements for the incoming air to the plant to overcome the increased pressure drop that would negate the advantage of having the increased surface area for the indirect heat exchange.
With reference to
With reference to
Although regular spacing for the inner array of tubes 54 and 54′ is illustrated in
Therefore, in accordance with the present invention, the average area for the inner array of tubes, for instance 54, will always be less than that of the outer array of tubes, for instance 58, that are located directly adjacent the inner array. The average area of the inner array is not always less than that of succeeding tubes of the outer array that are not located directly adjacent the inner array. For clarity of this concept, tubes 54 of the inner array shown by the dashed line 56 and two tubes of the outer array has been labeled as tubes 58a for purposes of showing the tubes 58 that are located adjacent the inner array of tubes which must present a greater average area than the inner array of tubes.
Rather than the hexagonal pattern shown for the outer array of tubes 58 of
In the practice of the present invention, although as mentioned above, positive results can be obtained by simply removing a tube from the inner array, more predicable results can be obtained. In this regard, in a practical application of the present invention, the velocity would first be computed in the spacings “S1” provided in the inner array of tubes 54. This would be done by dividing the mass flow by the product of the minimum flow area between adjacent tubes and the fluid bulk density. From this velocity, the frequency of shedding of vortices from the back of the tubes is computed as described in Heat Exchanger Design Handbook, “Flow induced vibration,” Ch. 4.6.1-4.6.6, Hemisphere Publishing Corporation (1987) and compared with the natural frequency of the tubes to make certain that the computed frequency is not at the natural frequency and in fact is preferably below 80 percent or above 120 percent of such frequency. This frequency is computed by the following formula.
(2)
Next the velocity for the adjacent row of tubes in the outer array, namely tubes 58 situated directly adjacent tubes 54, is calculated. This is done by dividing the mass flow by the product of the minimum flow area and the bulk fluid density. The pressure drop can then be calculated from the first row of tubes 54 to succeeding rows using well known pressure drop correlations for flow across a tube bank such as disclosed in the Heat Exchanger Design Handbook, “Banks of Plain and Finned Tubes,”, Chapters 2.2.4-7 to 11. A spacing is then chosen for the inner array of tubes 54 that will lower pressure drop from the center to the periphery.
With reference again to
With reference to
It is understood that baffle plate 66, while required in the condenser reboiler specifically illustrated in the Figures, is not required in all cases. For example, if a shell and tube heat exchanger were fabricated in accordance with the present invention that has a lesser height “H” than that illustrated, then a baffle plate and intermediate support of the inner array of tubes 54 might not be required.
As mentioned previously, the above discussion would have applicability to the design of any shell and tube heat exchanger. It would also have application to any shell and tube heat exchanger used in connection with a double distillation column for an air separation plant. In this regard, although the condenser-reboiler was illustrated as a thermosiphon type of heat exchanger, a condenser reboiler in accordance with the present invention could also be constructed as a down flow type. In such case, a liquid reservoir would receive oxygen-enriched liquid from the overlying mass transfer contacting element which could be structured packing as shown by reference number 14. The collected liquid would then be distributed to the tubes which would flow in a downward direction where it would be partially vaporized through the indirect heat exchange with the nitrogen-rich vapor. The liquid phase would collect as the oxygen-rich liquid column bottoms and the vapor phase would provide boilup in the low pressure column 3.
While the present invention has been described with reference to preferred embodiments, as will be understood by those skilled in the art, numerous additions and omission can be made without departing from the spirit and scope of the present invention as set forth in the appended claims.
