Argon condensation system and method

Abstract

An argon reflux condensation system and method in which a plurality of once-through heat exchangers are connected to an argon column of an air separation plant to condense argon-rich vapor streams for production of reflux to the argon column. Condensation of the argon-rich vapor streams is brought about through indirect heat exchange with crude liquid oxygen streams that partially vaporize and are introduced into a lower pressure column of the plant for further refinement. The flow rate of the crude liquid oxygen streams are sensed and controlled at locations in the plant where the crude liquid oxygen is in a liquid state and in proportion to the size of the once-through heat exchangers. Feed stream flow rate to the argon column is controlled in response to air flow rate to the plant and product flow rate is controlled in response to the feed stream flow rate to the argon column.

Claims

1. A method of condensing argon reflux within an air separation unit having an argon column, a lower pressure column and a higher pressure column, said method comprising the steps of: extracting a crude argon feed stream from the lower pressure column; distilling the crude argon feed stream in the argon column to produce an argon-rich vapor column overhead; directing at least a portion of the argon-rich vapor column overhead to a plurality of once-through heat exchangers; condensing the argon-rich vapor streams within condensation passages of the plurality of once-through heat exchangers such that argon-rich vapor streams composed of argon-rich vapor column overhead are condensed within condensation passages of the once-through heat exchangers to produce an argon-rich liquid product stream and argon-rich liquid reflux stream returning the argon-rich liquid reflux stream to the argon column as reflux; introducing one or more streams of crude liquid oxygen column bottoms from the higher pressure column into vaporization passages of the plurality of once-through heat exchangers to partially vaporize the crude liquid oxygen streams through indirect heat exchange with the argon-rich vapor streams to produce partially vaporized crude liquid oxygen streams introducing the partially vaporized crude liquid oxygen streams into the lower pressure column; sensing liquid flow rates of the crude liquid oxygen streams directed from the bottom of the higher pressure column to the vaporization passages of the plurality of once-through heat exchangers; controlling the flow rates of the crude liquid oxygen streams to the plurality of once-through heat exchangers in response to the sensed liquid flow rates and the level of the crude liquid oxygen bottoms in the higher pressure column; and controlling a feed stream flow rate of the crude argon feed stream in response to an air flow rate into the air separation unit and by controlling the reflux flow rate of the argon-rich liquid reflux by: (i) decreasing the reflux flow rate of the argon-rich liquid reflux stream when the feed stream flow rate is above a feed stream set point to thereby cause the argon-rich liquid reflux to back up into the condensation passages of the once-through heat exchanger causing an increase in pressure of the argon-rich vapor stream and within the argon column; and (ii) increasing the reflux flow rate of the argon-rich liquid reflux stream when the feed stream flow rate is below a feed stream set point to thereby cause a decrease in the pressure of the argon-rich vapor stream and within the argon column.

2. The method of claim 1 further comprising the step of controlling a product flow rate of the argon-rich liquid product stream in response to the feed stream flow rate of the crude argon feed stream.

3. The method of claim 1 wherein the step of controlling the flow rates of the crude liquid oxygen streams further comprises: controlling the flow rates of the crude liquid oxygen streams to the plurality of once-through heat exchangers via one or more flow control valves; and adjusting the flow control valves to attain a flow rate set points that are based on the sensed liquid flow rates and the level of the crude liquid oxygen bottoms in the higher pressure column; wherein the flow rate set points are decreased as the level of the crude liquid oxygen bottoms in the higher pressure column decreases and the flow rate set points are increased as the level of the crude liquid oxygen bottoms in the higher pressure column increase.

4. The method of claim 2 wherein the feed stream set point is a function of the air flow rate into the air separation unit multiplied by a crude fraction.

5. The method of claim 2 further comprising the steps of: measuring temperatures of the partially vaporized crude liquid oxygen streams; and further controlling the feed stream flow rate of the crude argon feed stream and the argon-rich liquid product stream product flow rate in response to the measured temperatures of the partially vaporized crude liquid oxygen streams, wherein the feed stream flow rate of the crude argon feed stream decreases when the temperatures of the partially vaporized crude liquid oxygen streams are above a predetermined temperature indicative of dry out conditions within the vaporization passages.

6. The method of claim 2 further comprising the steps of: measuring temperatures of the partially vaporized crude liquid oxygen streams; and controlling the reflux flow rate of the argon reflux stream in response to the measured temperatures of the partially vaporized crude liquid oxygen streams, wherein the reflux flow rate of the argon reflux stream is decreased causing the argon-rich liquid to back up into the condensation passages and further causing an increase in pressure of the argon-rich vapor stream and within the argon column when the temperatures of the partially vaporized crude liquid oxygen streams are above a predetermined temperature indicative of dry out conditions within the vaporization passages.

7. The method of claim 2 further comprising the steps of: measuring the flow rate of the argon-rich liquid product stream; further controlling the flow rate of the argon-rich liquid product stream via one or more product flow control valves to maintain the flow rate of the argon-rich liquid product stream at a product flow rate set point; wherein the product flow rate set point being a function of the feed stream flow rate of the crude argon feed stream multiplied by a product fraction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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 accompanying drawings in which:

(2) FIG. 1 is a fragmentary, process flow diagram illustrating the physical controls used in a cryogenic air separation plant carrying out a method in accordance with the present invention; and

(3) FIG. 2 is a schematic diagram of a once-through heat exchanger used in FIG. 1.

DETAILED DESCRIPTION

(4) With reference to FIG. 1, a cryogenic air separation plant 1 is illustrated that is designed to rectify air and to produce an argon product stream 10. Although not illustrated, the incoming air is compressed and then purified in purification unit employing beds of adsorbent to adsorb higher boiling impurities such as carbon dioxide and water vapor. The compression and purification produces a compressed and purified air stream 12 that is cooled and then introduced into a distillation column system that, as will be further discussed, has a higher pressure column 18 linked to a lower pressure column 26 in a heat transfer relationship and an argon column 50 that separates oxygen from argon in an oxygen and argon vapor stream discharged from the lower pressure column to produce the argon product stream 10.

(5) Compressed and purified air stream 12 is divided into subsidiary compressed and purified air streams 14 and 16, respectively. Again, although not illustrated, the first subsidiary compressed and purified air stream 14 is cooled to a temperature suitable for its distillation and is then introduced into a higher pressure column 18 and the subsidiary air stream 16 is further compressed and the condensed to form a liquid air stream 20. Such liquid air stream 20 could be formed in connection with heating a pressurized liquid stream to produce a product either as a high pressure vapor or a supercritical fluid. However, this is mentioned for illustration only and cryogenic air separation plants where there is no liquid air is produced are possible. It is further understood that the cooling of the air would take place in a heat exchanger sometimes referred to as a main heat exchanger which could be a series of heat exchangers operated in parallel. In the illustrated embodiment, the liquid air stream is divided into first and second subsidiary air streams 22 and 24 which are introduced into the higher pressure column 18 and a lower pressure column 26, respectively. Expansion valves 28 and 30 are provided to reduce the pressure of the first and second subsidiary air streams 22 and 24 to pressures suitable for their entry into the higher and lower pressure column 18 and 26.

(6) The higher and lower pressure columns 18 and 26 and the argon column 50 to be discussed all have mass transfer contacting elements to contact liquid and vapor phases of the mixture to be distilled in each of the columns. These elements can be sieve trays, structured packing or a combination of such trays and structured packing. The, higher pressure column 18 operates at a pressure of 5.0 to 6.0 bar(a) and serves to separate the incoming air into a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms 32, also known as kettle liquid. The lower pressure column 26 will typically operate at 1.1 to 1.5 bar (a) and is linked to the higher pressure column 18 in a heat transfer relationship by means of a condenser reboiler 34. The lower pressure column serves to further refine the crude liquid oxygen 32 into an oxygen-rich liquid column bottoms 36 and a nitrogen-rich vapor column overhead. A nitrogen-rich vapor stream 38 composed of the nitrogen-rich vapor column overhead produced in the higher pressure column 18 is condensed in the condenser reboiler to produce a liquid nitrogen stream 40 through indirect heat exchange with the oxygen-rich liquid column bottoms 36, thereby partially vaporizing the column bottoms and initiating formation of the ascending vapor phase within such column. The liquid nitrogen stream 40 is divided into liquid nitrogen reflux streams 42 and 44 that are introduced into the higher and lower pressure columns 18 and 26 as reflux and thereby to initial formation of the descending liquid phase of the mixture to be distilled in each of the columns. An expansion valve 46 is provided to let down the pressure of the liquid nitrogen reflux stream 44 to one that is compatible with the operating pressure of the lower pressure column 26. Although not illustrated, liquid nitrogen reflux stream 44 could be subcooled in a subcooling unit also used in subcooling the crude liquid oxygen column bottoms to be further refined in the lower pressure column 26 and thereby inhibit flash of such liquids into vapor fractions. Also not illustrated are various product streams emanating from the lower pressure column. For example, a nitrogen-rich vapor stream and a liquid oxygen stream could be extracted from the lower pressure column 26 and then introduced into the main heat exchanger used in the cooling of the incoming compressed and purified air. Liquid oxygen could be pumped to deliver an oxygen product at pressure after the same was heated through indirect heat exchange with second compressed and purified air stream 20.

(7) In connection with the production of argon, a crude argon feed stream 48 is removed from the lower pressure column 26 and then introduced into the argon column 50 for rectification. Crude argon feed stream 48 is a vapor stream containing oxygen and argon which are separated within the argon column 50. Such rectification produces an oxygen-rich liquid column bottoms, which is returned to the lower pressure column 26 by means of liquid oxygen stream 52 and an argon-rich vapor column overhead. An argon-rich vapor column overhead stream 54 is divided into two subsidiary argon-rich vapor streams 56 and 58 that are condensed in argon reflux condensers 60 and 62, respectively, to form argon-rich reflux streams 64 and 66. Argon-rich reflux streams 64 and 66 combined to form an argon reflux stream 68 that is returned to the argon column 50 as reflux. The argon product stream 10 is withdrawn from the argon column 50. It is understood, however, that such stream could be formed from part of the argon reflux stream 68.

(8) The condensation of the argon-rich vapor streams 56 and 58 within the argon reflux condensers 60 and 62 is brought about through indirect heat exchange with crude liquid oxygen column bottoms 32. A crude liquid oxygen column bottoms stream 70 is withdrawn from the higher pressure column 18 and divided into crude liquid oxygen streams 72 and 74 which are partially vaporized in the argon reflux condensers in indirect heat exchange with the argon-rich vapor streams 56 and 58. This partial vaporization results in the production of partially vaporized crude liquid oxygen streams 76 and 78 that are combined into a combined partially vaporized crude liquid oxygen stream 80 that is introduced into the lower pressure column 26 for further refinement.

(9) The argon reflux condensers 60 and 62 are of the once-through type and although two of such heat exchangers are illustrated, there could be more than two depending upon the condensation requirements. With reference to FIG. 2, argon reflux condenser 60 is provided with an inlet 82 into which argon-rich vapor stream 56 is introduced. The incoming argon-rich vapor flows downwardly, in the direction of arrowhead A, into condensation passages 84 and the resulting argon-rich liquid stream 64 is discharged from outlet 86. The crude liquid oxygen stream 72 is introduced into adjacent vaporization passages 88 through an inlet 90 and flows in an upward direction as indicated by arrowhead B. The indirect heat exchange between the crude liquid oxygen stream 72 and the argon-rich vapor stream 56 results in the partial vaporization thereof and the production of the partially vaporized crude liquid oxygen stream 76 which is discharged from outlet 92. It is understood that argon reflux condenser 62 would be of the same design and function in the same manner with respect to the condensation of the argon-rich vapor stream 58 and the partial vaporization of the crude liquid oxygen stream 74.

(10) With continued reference to FIG. 1, as illustrated, the bottom of the higher pressure column 18 is situated at a sufficient distance below the height of the argon reflux condensers 60 and 62 that the crude liquid oxygen streams 72 and 74 will suffer a loss of head and therefore, pressure by the time the streams reach the argon reflux condensers 60 and 62. As a result of such pressure loss, part of the crude liquid contained in such streams will invariably vaporize. At the same time, since the argon reflux condensers 60 and 62 are identical and have the same heat exchange duty, the crude liquid oxygen bottoms stream 70 has to be divided equally. If this were not done, one of the argon reflux condensers 60 and 62 could suffer dry-out in the vaporization passages 88 leading to the higher boiling hydrocarbons to be deposited within such passages leading to a flammability hazard. It is understood that embodiments of the present invention are possible in which the argon reflux condensers are of different size and the crude liquid oxygen would have to be divided in accordance with the surfaces available for heat exchange provided within vaporization passages 88.

(11) In any case, it becomes highly problematical to accurately divide and control the flow of the crude liquid oxygen streams once vaporization has occurred. In accordance with the present invention, such division and control of the flow occurs where the crude liquid oxygen is in a liquid state rather than one in which the liquid has partially vaporized. This is accomplished by sensing liquid flow rates of the crude liquid oxygen streams 72 and 74 by means of flow transducers 94 and 96, respectively. Flow transducers 94 and 96 are situated within crude liquid oxygen feed conduits at locations thereof where the crude liquid oxygen streams 72 and 74 are in a liquid state to enable the accurate measurement of flow. Flow control valves 98 and 100 are positioned within such crude liquid oxygen feed conduits, downstream of the flow transducers 94 and 96, to control the liquid flow rates. The operation of flow control valves 98 and 100 are controlled by flow controllers 102 and 104, respectively. Flow controllers 102 and 104 are preferably proportional, integral, differential controllers that respond to flow signals generated by the flow transducers 94 and 96 that are referable to the liquid flow rates of the crude liquid oxygen streams 72 and 74 within their associated crude liquid oxygen feed conduits. The flow controllers 102 and 104 respond by controlling the opening of the flow control valves 98 and 100 to maintain flow rate set points which are proportional to vaporization surface areas provided by the vaporization passages 88 of the once through heat exchangers 60 and 62. Thus, if the vaporization surface areas were equal because the once-through heat exchangers 60 and 62 are of equal size, then presumptively, the flow rate set points would be equal to provide equal flows. However, the flows are not exactly equal at all times in that a slight bias may be imparted to the flow rates in a manner that will be discussed. The flow rate set points are preferably generated by a level controller 106 that is responsive to a level transducer 108 that is in turn connected to the higher pressure column 18 to sense the level of the crude liquid oxygen bottoms 32 and generate a level signal referable to the level. The level controller 106 in turn generates the flow rate set points based upon the sensed level. For example, as the level of the crude liquid oxygen column bottoms 32 decreases the set points also have to decrease to allow the level to be maintained at a level set point of constant height for crude liquid oxygen column bottoms 32. The flow rate set points are in turn transmitted to the flow controllers 102 and 104 by means of an electrical connection or a wireless connection shown by line 110.

(12) As can be appreciated, the height separating the once-through heat exchangers 60 and 62 and the bottom of the higher pressure column 18 will result in a loss of head along with pressure of the crude liquid oxygen streams 72 and 74. Also, there will be a pressure drop through the once-through heat exchangers 60 and 62, across valves 98 and 100 and other associated equipment. The result of the loss of pressure will cause vaporization of the liquid within crude liquid oxygen streams 72 and 74. While this loss in pressure is necessary to enable the combined partially vaporized crude liquid oxygen stream 80 to be introduced into the lower pressure column 26 at a compatible pressure that will not result in an evolution of vapor within the lower pressure column 26 that would hurt recovery, the degree of vaporization of the crude liquid oxygen streams just prior to their entry into the once-through heat exchangers 60 and 62 should be limited to less than 20.0 percent, preferably less than 10.0 percent so that dry out can be prevented within the vaporization passages 88 thereof. The degree of vaporization can be controlled somewhat by proper design of piping, valves and etc. and such control may be sufficient form many applications of the present invention. However, such vaporization can also be minimized by subcooling the crude liquid oxygen within a subcooling heat exchanger positioned between the higher pressure column 18 and the branching out of the crude liquid oxygen conduits carrying crude liquid oxygen streams 72 and 74. Typically, such a heat exchanger will accomplish such subcooling through indirect heat exchange with a nitrogen-rich vapor stream produced from column overhead in the lower pressure column 26. It is to be noted here that although the crude liquid oxygen streams 72 and 74 are illustrated as branching from a single line, the associated crude liquid oxygen conduits could be direct connected to the higher pressure column 18 and if a sub-cooling heat exchanger were used, it would need two sets of passages for such purposes.

(13) A feed stream flow rate of the crude argon feed stream 48 to the argon column 50 is preferably controlled, albeit indirectly, by means of an argon reflux control valve 112 that directly controls the flow of argon-rich liquid reflux stream 68 to argon column 50. As a reflux flow rate of the argon-rich liquid reflux stream 68 is successively decreased by closing argon reflux control valve 112, the argon-rich liquid will back up into the condensation passages 84 and thereby cause an increase in pressure of the argon-rich vapor column overhead stream 54 and thus, within the argon column 50. The increase in pressure will thereupon cause a decrease in the feed flow rate of the crude argon feed stream 48. Of course by opening the argon reflux valve 112, the reflux flow rate of the of the argon-rich liquid reflux stream 68, a decrease in pressure within the argon-rich vapor column overhead stream 54 and thus, within the argon column 50 to increase in the feed flow rate of the crude argon feed stream 48. Alternative control systems and methods could be direct control, namely, the control of crude argon feed stream 48 by a valve positioned between the argon column 50 and the lower pressure column 26.

(14) While the control of argon reflux control valve 112 could be through manual intervention by monitoring flow and making remote adjustments, preferably the control of the argon reflux control valve 112 is accomplished with a flow controller 114 that is responsive to the flow rate of the compressed and purified air stream 12. A flow rate of the incoming compressed and purified air stream 12 is sensed by a flow transducer 116 that generates an air stream signal referable to the flow rate of the compressed and purified air stream 12 and transmitted to the flow controller 114 by means of an electrical or wireless connection 118. Additionally, a feed flow transducer 120 is connected to a crude argon feed conduit in which the crude argon feed stream 48 flow to sense the feed stream flow rate and thereby to produce a crude argon signal referable to the feed stream flow rate of the crude argon feed stream 48 which is transmitted to the flow controller 114 by means of an electrical or wireless connection 122. The crude argon flow controller 114 on the basis of the flow rate of the compressed and purified air stream 12 as measured by flow transducer 116 calculates a feed stream set point that is equal to the flow rate multiplied by a crude fraction. The crude fraction is the fraction of argon contained in the crude argon feed stream 48 on a mass basis that is contemplated for the operation of the argon column 50. The feed stream flow rate, as measured by the feed flow transducer 120, is then compared to the feed stream set point and if greater than the set point, the flow controller 114 then reduces the opening of the argon reflux control valve 112. If the feed stream flow rate is less than the set point, the reverse occurs and the flow controller 114 acts to increase the opening of the argon reflux control valve 112.

(15) The flow rate of the argon product stream 10 is controlled by a product flow control valve 124 connected to a product outlet of the argon column 50. Again, although such control valve 124 could be manually controlled, preferably the control is automatic. To such end, a product flow transducer 126 is also connected to the product outlet, upstream of the product flow control valve 124, to sense the product stream flow rate of the argon-rich product stream. The product flow transducer 126 transmits a product signal referable to the product stream flow rate to a product flow controller 128. Product flow controller 128 is connected to the product flow transducer 126 by means of an electrical or wireless connection 130 and also to the feed flow transducer 120 by means of an electrical or wireless connection 132. The product flow controller 128 calculates a product flow set point that is a product of the feed stream flow rate of the crude argon feed stream 48 and a product fraction. The product fraction is the fraction of argon that is calculated to be contained in the argon product stream 10 based upon the flow rate of the crude argon stream 48. The product flow rate as sensed by the product flow transducer 126 is then compared to the product flow set point. If the product flow rate is below the product flow set point, the product flow controller 128 operates to move the product flow control valve 124 to a more open position to increase the flow. If the product flow rate is above the product flow set point, the product flow controller 128 operates to move the product flow control valve 124 towards a closed position to decrease the flow. It is to be noted that the argon product stream 10 is illustrated as being taken from below the top of the argon column 50. The purpose of this is to remove nitrogen from the argon liquid that is drawn off as a product. It is understood that the invention is equally applicable to a system in which the argon liquid is drawn from the condensate that partially serves as reflux to the argon column 50.

(16) As has been mentioned above, the quality of the crude liquid oxygen streams 72 and 74 with respect to their vapor content at their point of entry into the once-though heat exchangers 60 and 62 is important to prevent dry-out operational conditions within the heat exchangers. While the quality of the crude liquid oxygen streams 72 and 74 is largely dependent upon piping and valve design, transient operational conditions of the air separation plant 1 can also possibly have an effect on the quality, or in other words the vapor content of the crude liquid oxygen streams 72 and 74. For example, transient condition occasioned by turning the air separation plant 1 down might produce an increase in such vapor content. In order to further guard against this, temperature transducers 130 and 132 can optionally be provided to sense temperatures of the partially vaporized crude liquid oxygen streams 76 and 78, respectively. These temperatures are indicative of quality of the partially vaporized crude liquid oxygen streams because as the vapor content of such streams rise, the temperature of the streams will rise as well. The temperature transducers 130 and 132 can be connected to a temperature controller 134 by means of electrical or wireless connections. The signals referable to the temperatures can be introduced into programming associated with the temperature controller 134 that will function to average the signals and produce an average temperature. This programming is indicated by reference number 136 and block AVG. The temperature controller is programmed to control valve 112 to move the control valve 112 toward a closed position and reduce the feed stream flow rate of the crude argon feed stream 48 and therefore the product flow rate of the product stream 10 when the average temperature is above a predetermined level indicative of dry out within the vaporization passages. Both the temperature controller 134 and the flow controller 114 are connected to a low select 138 by means of electrical or wireless connections 140 and 142, respectively, so that the lower of the valve openings as computed by the flow controller 114 and the temperature controller 134 are selected to control the position of the control valve 112.

(17) As can be appreciated, simplified systems could be used in which only one temperature were sensed of one of the partially vaporized crude liquid oxygen streams 76 or 78; and such temperature could be used as indicative of the quality of both streams. However, the sensing of the temperatures of both of such streams is advantageous in that is can be used to slightly vary the flow rate of the crude liquid oxygen streams 72 and 74 where the temperatures are unequal and potentially the flow rates of the streams are unequal due to slight differences in piping geometry. This is done through programming associated with one of the flow controllers, for example, flow controller 104. The two temperature signals generated by temperature transducers 130 and 132 are transmitted by means of electrical or wireless connections 144 and 146 to programming designated by reference number 148 as [] that functions to subtract the signals and obtain a difference referable to the difference in temperatures. This difference is fed to other programming indicated by reference number 150 and that will modify the set point sent to flow controller 104 by either decreasing or increasing the set point to thereby increase or decrease the flow of crude liquid oxygen stream 74. For instance, if the temperature of crude liquid oxygen stream 78 is greater than that of crude liquid oxygen stream 76, more vapor is present in the crude liquid oxygen stream indicating that the flow of crude liquid oxygen stream 78 should be biased with a slight increase over the flow of crude liquid oxygen stream 76. And an increase in the set point associated with the flow controller 104 will have such effect in that the total flow of the crude liquid oxygen column bottoms is fixed.

(18) While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes and omissions can be made without departing from the spirit and scope of the present invention as set forth in the appended claims.