US2863295A - Process for separating a gas mixture into its components - Google Patents

Description

Dec. 9, 1958 c. L. NEWTON 2,863,295

PROCESS FOR SEPARATING A GAS MIXTURE INTO ITS COMPONENTS Filed July 19, 1955 2 Sheets-Sheet 1 IN VEN TOR.

Char/es L. Newfon.

A TTOE/VEYS 9, 1958 c. L. NEWTON PROCESS FOR SEPARATING A GAS MIXTURE INTO ITS COMPONENTS 2 Sheets-Sheet 2 Filed July 19, 1955 O O O O OTm O O O O O O O O O O O O O O O O O O O O U O O O O O O O O O O O O O O O O O O O O O O O O O O O o O O (J O O O O O (J O D O O O O O O O O O O hm wk hm ww ww INVENTOR.

United States Patent O PROCESS FOR SEPARATING A GAS MIXTURE INTO ITS COMPONENTS Charles L. Newton, Columbus, Ohio, assignor to Herrick L.hi Johnston, Inc., Columbus, Ohio, a corporation of Application July 19, 1955, Serial No. 522,924

9 Claims. (Cl. 62-43) This invention relates to the separation of gas mixtures and more particularly to a process for continuously eitecting such separation and for more efficiently eliminating higher boiling point impurities from the gas mixtures. In the separation of gas mixtures, such as air, reversing heat exchangers have been employed to remove the undesirable higher boiling impurities such as water vapor and carbon dioxide. A major problem resulting from the use of such reversing exchangers results from the fact that such higher boiling impurities precipitate from the gas mixture, accumulate in the paths of the exchangers,.

and must therefore be efficiently removed from the paths in order to provide an efficient system. The deposits of higher boiling impurities are removed from the paths of the exchanger by periodically alternating the flow of Warm incoming gas mixture and backward returning cold product in any given path through the exchanger. The reversal of flow is eflected before the accumulation of higher boiling impurities has become great enough to plug the paths of the exchanger. i

In order to achieve efllcient operation of systems of this type it is necessary to effectively maintain conditions influencing complete re-evaporation of the higher boiling impurities from the path in which such impurties are deposited. Hence it is necessary to provide sufiicient volume of backward returning cold product into which the deposited impurities can be evaporated. Moreover, it is necessary to maintain suflicient vapor pressure at the deposited impurities to eflect an eificient evaporation rate of the impurities.

It will be understood that the vapor pressure of the deposited impurities is a function of the temperature and that such impurities are usually removed at a temperature lower than the temperature at which they are deposited. Hence the smaller the temperature difference, the smaller will be the difference between the existing vapor presure of the impurities at the time of their precipitation and re-evaporation.

It is an object of the present invention to provide a novel method which effects such small temperature differences, between the incoming gas mixture and the backward returning cold product, at any region along the paths of the reversing heat-exchanger means utilized in the process.

It is another object of the present invention to provide the above described results in a novel manner whereby a plurality of portions of the stream of backward returning cold product are cyclicaly diverted from the warm ends of a plurality of parallel reversing heat-exchangers. Such diverted portions are then passed through an auxiliary heat-exchanger for effecting temperature control of the diverted portions after which the diverted portions are cyclically united with the backwardly returning cold product entering the cold ends of said plurality of parallel heat-exchangers.

It is another object of the presentinvention to provide a novel process of the type described adapted to con tinuously warm cold substantially saturated gas passing from a fractionating zone subsequent to its passage through the reversing heat-exchangers, and prior to the introduction of such gas to an expansion means whereby said gas is conditioned for isentropic expansion.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred form of embodiment of the invention is clearly shown.

In the drawings:

Figure 1 is a diagram schematically illustrating apparatus for continuously separating a gas mixture into its components, with such apparatus being constructed according to the present invention; and

Figure 2 consists of a table which illustrates the sequence of valve actuation for the valves utilized in the apparatus of Figure 1.

Referring to Figure 1 of the drawing, the incoming atmospheric air first passes through a compressor 20 and then through line 22 to a warm reversing heat-exchanger 24. A valve 25 serves to alternately connect line 22 with two paths 27 and 28 through the exchanger 24. The air leaves the warm exchanger 24 through one of the air check valves 30 and 31 after being cooled and partially dried.

The air flow is then split into two portions. One portion of the air flow passes to a cold reversing heat-exchanger 35, with a reversing valve 36 serving to alternately direct the flow through the paths 37 and 38 of the exchanger 35. The other portion of the air flow from warm exchanger 24 passes to a second cold reversing heat-exchanger 40, with a reversing valve 41 serving to alternately direct the flow through the paths 42 and 43 of the exchanger 40. The cold reversing heat-exchangers 35 and 40 serve to further dry the air and free it of its carbon dioxide content. The dried and carbon dioxide free air leaves the cold exchangers through the air check valves 45 or 46 and 47 or 43 and then passes through the line 50 to a liquifier 51. i

A stream of cold nitrogen waste product leaves the liquifier 51 through a line 53. The nitrogen stream is divided into two portions, with one portion entering the cold end of the cold reversing exchanger 35 and the other portion entering the cold end of cold reversing exchanger 40. Nitrogen check valves 55 and 56 serve to alternately direct a portion of the nitrogen flow through paths 37 and 38 of exchanger 35, and nitrogen check valves 57 and 58 serve to alternately direct a portion of the nitrm gen flow through paths 42 and 43 of exchanger 40.

In passing through the cold exchangers 35 and 40, the nitrogen is passed in counter current heat-exchange with the air whereby the nitrogen is warmed in giving up its refrigeration to the air passing through the adjacent parallel path in the same exchanger. serves to pick up the water and carbon dioxide from the path through which it is flowing, with such water and carbon dioxide having been deposited in the path by incoming air prior to reversal of exchanger paths,

The nitrogen then leaves the warm ends of the cold reversing exchangers 35 and 40, and a portion of the nitrogen is passed directly to the cold end of the warm reversing exchanger 24. A separate portion of the nitrogen leaving the warm ends of the cold reversing exchangers is diverted through a reheat exchanger 61.

Two- way reversing valves 63 and 64 serve to alternately connect the paths 37 and 38 of the cold reversing exchanger 35 with the Warm exchanger 24 whereby the backward flowing nitrogen passes to the exchanger 24. In a like manner, two- way reversing valves 66 and 67 serve to alternately connect the paths 42 and 43 of exchanger 40 with the warm exchanger 24.

At this point it should be pointed out that a portion The nitrogen also 6.9 of the nitrogen flow is continuously diverted through the reheat exchanger 61, but such diverted portion is cyclically taken from the exchanger paths 37, 42, 38 and 43 in the mentioned sequence. Moreover, nitrogen is not diverted from any path of flow until the second half of the period of nitrogen flow. For example, with the nitrogen flowing in path 37 for a period of ten minutes, for the first five minutes of nitrogen flow through path 37, all of the nitrogen from path 37 will pass through valve 63 and one of check valves, 70 or 71, and thence through the warm exchanger 24 and out through one of the reversing valves 73 and 74. After five minutes of nitrogen flow has occurred in path 37, a two-way reversing valve 76 is opened whereby a portion of the nitrogen flow through path 37 is diverted through the reheat exchanger 61.

In reheat exchanger 61 the nitrogen portion is passed in counter current heat-exchange with substantially saturated gas from a line 80 which leads from the base of a fractionating' column 81. Line 80 serves to continuously supply substantially saturated gas from the high pressure side of fractionating column 81 to the reheat exchanger 61 wherein the nitrogen stream is cooled and gives up part of its heat content to the saturated gas from the column.

Continuing with the description of the cyclical operation of the cold reheat exchangers 35 and 40, after nitrogen has flowed through path 37 for an entire ten minute period, check valve 55 is closed and check valve 56 is opened whereby nitrogen flow ceases in path 37 and commences in path 38. In addition, valve 76 is closed Whereby no nitrogen is diverted from path 38 during the first five minutes of nitrogen flow through that path.

At the time valve 76 is closed, valve 77 is opened whereby a portion of the nitrogen flow from path 42 of exchanger 40 is diverted and passed through reheat exchanger 61. It will be understood that due to the valve timing, at the time valve 77 is opened, nitrogen will have been flowing for five minutes in path 42 whereby the path, and nitrogen flow therethrough, will be clear of the Water and carbon dioxide previously deposited by the incoming air.

After the ten minute nitrogen flow terminates for path 42, portions of the nitrogen flow will next be diverted from paths 38 and 43 in that sequence to complete the cyclical diverting of nitrogen from all four paths in sequence. The cycle is then repeated beginning with the diverting of a portion of the nitrogen flowing through path 37.

Reference is next made to Figure 2 which consists of a chart setting forth the sequence of valve'operation for the various time phases of the cycle operation. It will be noted that the cycle is completed in twenty minutes of operation after which the valving sequence is repeated.

Upon leaving the reheat exchanger 61, the above described. diverted portion of the nitrogen, having been cooled, passes to a centrifugal blower 85 wherein the nitrogen is slightly compressed to make up for the pressure drop occurring during the recirculating cycle of the nitrogen. The recirculated nitrogen from blower 85 is then introduced into line 53 to combine with the nitrogen stream from the liquifier 51. The combined nitrogen stream then passes through the two cold reversing heat exchangers 35 and 40.

Referring again to the reheat exchanger 61, the reheated air stream leaving the reheat exchanger through line 88 is united with the portion of the air which bypasses the reheat exchanger by means of a by-pass line 90. In passing through the reheat exchanger 61, the heat content of the air from the base of the fractio-nating column 81 is increased whereby such air is conditioned for isentropic expansion prior to passing the air through an expansion engine 92. Hence it is seen that reheat exchanger 61 serves the dual purpose of cooling the recirculated portion of the nitrogen and reheating the sub- 2!; stantially saturated cold air stream from the base of the fractionating column.

Line 96 leading from liquefier or condenser 51 to the high pressure side of the column 81 carries gaseous a r to provide high pressure condensate for fractionation in the low pressure side of the column 81.

Line 97 carries the high pressure air condensate from condenser 51 into the low pressure side of the column where it is used as reflux.

Line 98 from the high pressure side of column 81 carries high pressure air condensate into the low pressure side of the column for reflux.

Line 80 carries substantially saturated gas from above I the high pressure condensate in the high pressure side of column 81 to reheat exchanger 61 for the purpose previously described.

Line 94 carries low pressure nitrogen-rich gas from the low pressure side of column 81 to condenser 51.

Line 09 carries the liquid oxygen product from the low pressure side of column 31 to storage means or to a point of use for such product.

' As seen in Figure 1, column 81 includes a plurality of fractionating trays 103 and a reboiler condenser unit 104. The reflux liquid entering the column 81 through lines 07 and 98 passes downwardly across each tray 103 to the low pressure side of reboiler condenser 104. The

liquid is vaporized by heat transfer with the condensing gas on the high pressure side of reboiler condenser 104. The boil-off gas from reboiler condenser 104 flows upwardly through each of the trays 103 whereby fractlonation is accomplished and the rising gas becomes rich in nitrogen. The gas then leaves column 81 through line 94 and enters line 95 along with a low pressure gaseous stream from expansion engine 92 via line 93. The combined gas in line 95 enters the low pressure slde of condenser unit 51. The high pressure gas from reversing exchangers 35 and 40 flows to the hlgh pressure side of the condenser unit 51 via line where part of such high pressure air is condensed by means of heat transfer with the above mentioned combined gases The condensed liquid then leaves condenser unit 51 via line 97 and the high pressure gases leave the condenser unlt 51 via line 96 as previously described. The low pressure combined gases leave the condenser unit 51, after being warmed, via line 53 and enter the reversing heat exchangers 35 and 40.

While the form of embodiment of the present 1nvention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow:

I claim:

1. A process for the separation of a gas mlxture 1nto its components comprising passing a first gaseous stream of said mixture through a reversing heat-exchange zone along a cooled path therein progressively decreasing in temperature from a warm end to a'cold end; passing a second gaseous stream of cold separation product through said path in the opposite direction after sald first stream has ceased to flow for a first time interval sufiicient for said second gaseous stream to clear said path of the higher boiling point components deposited in sa1d path by said first gaseous stream; diverting a portion of the second mentioned stream from the warm end of said heat-exchange zone subsequent to said'first time interval while continuing to pass said second gaseous stream through said path; passing said diverted portion through another heat exchange zone for a second time interval to elfect cooling thereof and combining said diverted portion during said second time interval with the cold separation product entering the cold end of said first heat exchange zone, to effect cooling of said cleared path during said second time interval.

2. A process for the separation of a gas mixture into its components comprising passing a first gaseous stream of said mixture through a reversing heat-exchange zone along. a cooled paththerein progressively. decreasing 1n enemas temperature from a warm end to a cold end passing a second gaseous stream of cold separation product through said path in the opposite direction after said first stream has ceased to flow, for a first time interval sutficient for said second gaseous stream to clear said path of the higher boiling point components deposited in said path by said first gaseous stream; diverting a portion of the second mentioned stream from the warm end of said heat-exchange zone subsequent to said first time interval While continuing to pass said second gaseous stream through said path; passing said diverted portion through another heat-exchange zone in counter current heat exchange with a third gaseous stream of cold separation product for a second time interval; and uniting said diverted portion during said second time interval with cold separation product entering the cold end of said first heat exchange zone to effect cooling of said cleared path during said second time interval.

3. A process for the separation of a gas mixture into its components comprising passing a first gaseous stream of said mixture through a reversing heat-exchange zone along a cooled path therein progressively decreasing in temperature from a warm end to a cold end; passing a second gaseous stream of cold separation product through said path in the opposite direction after said first path has ceased to flow for a first time interval sufiicient for said second gaseous stream to clear said path of the higher boiling point components deposited in said path by said first gaseous stream; diverting a portion of the second mentioned stream from the warm end of said heat-exchange zone subsequent to said first time interval while continuing to pass said second gaseous stream through said path; passing said diverted portion through another heat-exchange zone for a second time interval to effect cooling thereof; passing said diverted portion through a compressing zone during said second time interval to effect compression thereof; and uniting said diverted portion during said second time interval with said second stream entering the cold end of said first heat-exchange zone to effect cooling of said cleared path during said second time interval.

4. In a process for the separation of a gas mixture into its components, the steps which comprise passing a first gaseous stream of said mixture in one direction of flow through a first reversing heat-exchange zone alternately along two cooled paths therein to effect cooling of the first stream; passing a second gaseous stream of said mixture in one direction of flow through a second reversing heat-exchange zone alternately along two cooled paths thereinto effect cooling of the second stream; passing a third gaseous stream, free of the higher boiling point components of said mixture, alternately through said two paths in said first heat-exchange zone in the opposite direction of flow in heat exchange relationship with said first stream, passing a fourth gaseous stream, free of the higher boiling point components of said mixture, alternately through said two paths in said second heat-exchange zone in the opposite direction of flow in heat exchange relationship with said second stream; cyclically diverting a portion of said third stream and a portion of said fourth stream from said paths at the warm ends of said heat-exchange zones; passing said cyclically diverted portions through a third heat-exchange zone to effect cooling of said portions; and uniting said cyclically diverted portions with said third and fourth streams entering the cold ends of said heat-exchange zones to effect cooling of said third and fourth streams.

5. In a process for the separation of a gas mixture into its components, the steps which comprise passing a first gaseous stream of said mixture in one direction of flow through a first reversing heat-exchange zone alternately along two cooled paths therein to effect cooling of the first stream; passing a second gaseous stream of said mixture in one direction of flow through a second reversing heat-exchange zone alternately along two cooled paths therein to effect cooling of the second stream; passing a third gaseous stream, free of the higher boiling point components of said mixture, alternately through said two paths in said first heat-exchange zone in the opposite direction of flow in heat-exchange relationship with said first stream; passing a fourth gaseous stream, free of the higher boiling point components of said mixture, alternately through said two paths in said second heat exchange zone in the opposite direction of flow in heat exchange relationship with said second stream; cyclically diverting a portion of said third stream and a portion of said fourth stream from said paths at the warm ends of said heat-exchange zones, each portion being diverted after its respective path has been freed of substantially all of said higher boiling point components; passing said cyclically diverted portions through a third heat-exchange zone to effect cooling of said portions; and uniting said cyclically diverted portions with said third and fourth streams entering the cold ends of said heat-exchange zones to effect cooling of said third and fourth streams.

6. In a process for the separation of a gas mixture into its components, the steps which comprise passing a first gaseous stream of said mixture in one direction of flow through a first reversing heat-exchange zone alternately along two cooled paths therein to effect cooling of the first stream; passing a second gaseous stream of said mixture in one direction of flow through a second reversing heat-exchange zone alternately along two cooled paths therein to effect cooling of the second stream; passing a third gaseous stream, free of the higher boiling point components of said mixture, alternately through said two paths in said first heat-exchange zone in the opposite direction of fioW in heat exchange relationship with said first stream; passing a fourth gaseous stream, free of the higher boiling point components of said mixture, alternately through said two paths in said second heat-exchange zone in the opposite direction of flow in heat-exchange relationship with said second stream; cyclically diverting a portion of said third stream and a portion of said fourth stream from said paths at the warm ends of said heat-exchange zones; cyclically passing each of said diverted portions in heat-exchange relationship with a third gaseous stream of cold output components obtained from at least one of said first and second streams after passage of said first and second streams along said cooled paths to effect cooling of said diverted portions and warming of said third gaseous stream; and uniting said cyclically diverted portions with said third and fourth streams entering the cold ends of said heatexchange zones to effect cooling of said third and fourth streams.

7. The method of freeing higher boiling point components from a gaseous mixture in a parallel combination of first and second reversing heat exchangers having first and second paths and third and fourth paths, respectively, comprising: routing in one direction portions-of a stream of said gaseous mixture in repetitive timed sequence through the second and fourth paths, then the fourth and first paths, then through the first and third paths, and finally through the third and second paths, so that the flow of one portion of said mixture in each path occurs for a predetermined period but the flow of another portion of said mixture begins in another path midway during said period, routing in the opposite direction portions of a cold separation product stream in like repetitive timed sequence through the first and third paths, then through the third and second paths, then through the second and fourth paths, and finally through the fourth and first paths, so that the flow of one portion of said cold separation product in each path occurs for a predetermined period but the flow of another portion of said product begins in a different path midway during such period, diverting separation product successively from the first, third, second and fourth.

paths after scavenging, coolingthe diverted cold separation product, and. finally uniting the diverted cold separation product with. said cold separation product stream entering the cold endsof such exchangers.

8. The method of freeing higher boiling point components from a gaseous mixture in a parallel combination of a plurality of reversing heat exchangers each having two paths comprising: routing portions of a stream of said gaseous mixture in one direction in repetitive timed overlapping sequence first through a first series of paths in said heat exchangers and then through a second series of paths in said heat exchangers, so that the flow of one portion of said mixture in any path occurs for a predetermined period and the flows of said mixture in the next succeeding paths begin at intervals equal to a fraction of said period, said fraction having two as its numerator and the number of paths as its denominator, routing portions of a cold separation product stream in the opposite direction in like repetitive timed overlapping sequence first through said second series of paths and then through said first series of paths so that the flow of one portion of said separation product in any path occurs for a predetermined period and the flows of said product in other paths begin at intervals equal to the same fraction of said period, diverting separation product from at least one of said paths after scavenging, cooling the diverted cold separation product, and uniting the diverted cold separation product with said cold separation product stream entering the cold ends of such exchangers.

9. The method of freeing higher boiling point compo-- nents from a gaseous mixture in a parallel. combination of a plurality of reversing heat exchangers each having two paths comprising: routing portions of a stream of said gaseous mixture in one direction in repetitive timed overlapping sequence first through a first series of paths in said heat exchangers and then through a second series of paths in said heat exchangers, so that the flow of one portion of said mixture in any path occurs for a predetermined period and the flows of said mixture in the next succeeding paths begin at intervals equal to a fraction of said period, said fraction having two as its numerator and the number of paths as its denominator, and routing portions of a cold separation product stream in the opposite direction in like repetitive timed overlapping sequence first through said second series of paths and then through said first series of paths so that the flow of one portion of said separation product in any path occurs for a predetermined period and the fiows of said product inother paths begin at intervals equal to the same fraction of said period.

References Cited in the file of this patent UNITED STATES PATENTS 2,534,478 Roberts Dec. 19, 1950 2,562,812 Ogorzaly July 31, 1951 2,579,498 Jenny Dec. 25, 1951 2,668,425 Skaperdas Feb. 9, 1954 2,753,701 Palmer et al. July 10, 1956

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