TWO STAGE QUENCH SYSTEM FOR GAS COOLING

The present invention relates to a method of cooling a gaseous stream, such as a flue gas stream or a reduced recycle stream, and an apparatus for the same.

It is known to cool hot gaseous streams by the use of a quench column. In a quench column, a hot gaseous stream is contacted with water to provide a cooled gaseous stream. The water used to cool the hot gaseous stream is normally withdrawn from the quench column, cooled in an air cooler and then recycled back to the quench column by a pump.

In hot climates having a higher ambient air temperature, a chiller may be required downstream of the air cooler in order to reduce the water temperature to a lower temperature than would be possible with ambient air cooling alone. However, the addition of a chiller downstream of the cooler produces a significant cooling requirement, particularly in terms of chilling load.

It is an object of the invention to provide an improved method of, and apparatus for, cooling a gaseous stream.

It is a further object of the present invention to provide a method of, and apparatus for, cooling a gaseous stream having a reduced cooling requirement.

In one aspect, the present invention provides a method of cooling a gaseous stream, such as a flue gas stream, the method comprising at least the steps of:

(a) passing a gaseous stream at an inlet of a first cooling stage;

(b) first cooling the gaseous stream against a first cooled liquid stream in a first cooling stage to provide

a first liquid stream and a first cooled gaseous stream; and

(c) second cooling the first cooled gaseous stream against a second cooled liquid stream in a second cooling stage to provide a second liquid stream and a second cooled gaseous stream; wherein the first cooled liquid stream has a first temperature and the second cooled liquid stream has a second temperature, the first temperature being higher than the second temperature.

The method of cooling described herein carries out the cooling in first and second cooling stages . By providing two cooling stages, a reduced cooling requirement is achieved compared to the known method in which cooling is carried out in a single stage.

Typically, the present invention allows reductions in the required chilling load of approximately 20 to 40%, compared to the known single stage method discussed above . In a further aspect, the present invention provides an apparatus for the cooling of a gaseous stream, such as flue gas, the apparatus at least comprising: a first cooling stage and a second cooling stage, the second cooling stage connected to the first cooling stage, the first cooling stage comprising a first inlet for a gaseous stream, a second inlet for a first cooled liquid stream and a second outlet for a first liquid stream, the second outlet being below the second inlet, and the second cooling stage comprising a first outlet for a second cooled gaseous stream, a third inlet for a second cooled liquid stream and a third outlet for a

second liquid stream, the third outlet being below the third inlet .

By "connected to" is meant that the first cooling stage is linked to the second cooling stage such that the first cooled gaseous stream provided by the first cooling stage can be passed to the second cooling stage. For instance, the first and second cooling stages may be present in the same cooling tower. Alternatively, the first and second cooling stages may be present in separate cooling towers connected by a pipeline to transmit the first cooled gaseous stream from the first cooling stage to the second cooling stage .

Hereinafter the invention will be further illustrated by the following non-limiting drawings. Figure IA schematically shows a process scheme in accordance with one embodiment of the present invention. Figure IB schematically shows a process scheme in accordance with another embodiment of the present invention . Figure 2 schematically shows a process scheme in accordance with a further embodiment of the present invention .

Figure 3 schematically shows a process scheme in accordance with a another embodiment of the invention. For the purposes of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components .

The method of cooling the gaseous stream described herein can be used with any gaseous stream. The gaseous stream can be obtained from the treatment of a natural gas stream, for example a flue gas upstream of acid gas removal, for instance for the capture of carbon dioxide.

Alternatively the gaseous stream may be a reduced recycle stream from the reactor of a tail gas treating unit, especially a Shell Claus Off-gas Treatment (SCOT) unit. These tail gas treating units are for example described in the well-known textbook by Kohl and Riesenfeld, Gas

Purification, 3 ^rd ed. Gulf Publishing Co, Houston, 1979. Suitably, in a tail gas treating unit removal of residual contaminants takes place. In a SCOT unit, catalytic reduction of residual sulphur dioxide to hydrogen sulphide takes place. A typical reduced recycle stream from a SCOT unit comprises hydrogen sulphide, water vapour, carbon dioxide, nitrogen and argon.

In one embodiment of the apparatus and method described herein, the first cooling stage and the second cooling stage are present in a single cooling column. This construction provides the improved reductions in cooling load and is most economical because only a single cooling column is required. However, the apparatus and method described herein also relates to an embodiment in which the first cooling stage is provided in a first cooling column and the second cooling stage is provided in a second cooling column.

In the embodiment in which the first and second cooling stages are provided in the same cooling column, it is preferred that the first cooling stage and second cooling stage are separated by a draw-off tray. The draw- off tray may be a partial draw-off tray or a total draw- off tray.

In a further embodiment, one or both of the first cooling stage and second cooling stage may comprise one or more packed beds, trays or spray sections. The packed bed may comprise ceramic, metal or carbon shapes. The packed shapes may be selected from the group consisting

of: raschig rings, intalox saddles, berl saddles and pall rings. The tray may be one or more selected from the group consisting of: bubble cap trays, valve tray, sieve trays, turbo girds, side to side pans, disc and doughnut trays, uniflux trays, ripple trays, Linde trays and the like.

In another embodiment of the apparatus described herein, an air cooler having an outlet connected to the second inlet of the first cooling stage and an inlet connected to the second outlet of the first cooling stage is provided. Thus, in a further embodiment of the method described herein, the first cooled liquid stream can be provided by air cooling the first liquid stream. The temperature of the first cooled liquid stream (the first temperature as used herein) may be in the range of 30 to 80 ¹ C.

In another embodiment of the apparatus described herein, a heat exchanger having a first refrigerant circuit and an outlet connected to the third inlet of the second cooling stage and an inlet connected to the third outlet of the second cooling stage is provided. Thus, in another embodiment of the method described herein, the second cooled liquid stream is provided by heat exchanging the second liquid stream against a first refrigerant stream. Reference herein to first refrigerant stream is to a stream comprising a cooling medium colder than air. The first refrigerant stream may be selected from the group consisting of: cooling water, chilled water and propane. The temperature of the second refrigerant stream (the second temperature as used herein) may be in the range of 0-60 °C.

In a further embodiment of the method described herein, the flow of one or more or the first and second

cooled liquid streams with one or more of the gaseous stream or the first cooled gaseous stream is selected from the group consisting of: counter-current, co-current and cross-current flow. Thus, in a further embodiment of the apparatus described herein the second cooling stage can be situated above the first cooling stage. Such an embodiment may provide a counter-current flow of the first and second cooled liquid streams with the gaseous stream and first cooled gaseous stream. Alternatively, the second cooling stage can be situated adjacent to the first cooling stage. Such an embodiment may provide a cross-current flow of the first and second cooled liquid streams with the gaseous stream and first cooled gaseous stream. In a preferred embodiment of the method described herein, the first and second cooled liquid streams comprise water.

Figure IA schematically shows a process scheme and apparatus (generally indicated with reference numeral 1) for cooling a gaseous stream 10, such as a flue gas upstream of carbon dioxide capture in the processing of natural gas, or a reduced recycle stream in a tail gas treating process.

When gaseous stream 10 is a reduced recycle stream in a tail gas treating process, it generally comprises hydrogen sulphide, water vapour, carbon dioxide, nitrogen and argon.

Gaseous stream 10 is passed to a first cooling stage 46, which may be in a cooling column. As shown in Figure IA, gaseous stream 10 may be introduced into a lower region of first cooling stage 46. The gaseous stream passes generally upwards and comes into intimate contact a first cooled liquid stream 60. First cooled

liquid stream 60 may be introduced into an upper region of first cooling stage 46 and provides a descending flow of liquid.

First cooled liquid stream 60 has a first temperature, which when lower than the temperature of gaseous stream 10, provides a first cooling of the gaseous stream 10 to produce a first cooled gaseous stream 30 upon leaving first cooling stage 46. The heat exchange between the first cooled liquid stream 60 and the gaseous stream 10 raises the temperature of first cooled liquid stream 60 to produce first liquid stream 62. It is preferred that the first cooled liquid stream 60 is a water stream.

The first cooled gaseous stream 30 is then passed to a second cooling stage 48, which may be in a cooling column. As shown in Figure IA, first cooled gaseous stream 30 may be introduced into a lower region of second cooling stage 48. In a similar manner to first cooling stage 46, first cooled gaseous stream 30 passes generally upwards and comes into intimate contact a second cooled liquid stream 70. Second cooled liquid stream 70 may be introduced into an upper region of second cooling stage 48 and provides a descending flow of liquid.

Second cooled liquid stream 70 has a second temperature, which is lower than the first temperature of the first cooled liquid stream 60. When the second temperature is lower than the temperature of first cooled gaseous stream 30, further cooling of first cooled gaseous stream 30 is achieved to provide a second cooled gaseous stream 50 upon leaving second cooling stage 48. The heat exchange between second cooled liquid stream 70 and first cooled gaseous stream 30 heats second cooled liquid stream 70 to produce second liquid stream 64. It

is preferred that second cooled liquid stream 70 is a water stream.

The first and second cooling stages may each be present in separate cooling columns. Alternatively, the first and second cooling stages may be carried out in the same cooling column, represented by optional numeral 20 in Figure IA. In the latter embodiment, the second cooling stage may be placed vertically above the first cooling stage. The gaseous stream 10 (and first cooled gaseous stream 30) in Figure IA flow in the opposite direction to the first and second cooled liquid streams in the first and second cooling stages 46 and 48 respectively, providing a counter-current flow in each case . In an alternative embodiment schematically shown in

Figure IB, a process scheme and apparatus utilising a cross-current flow is provided. In cross-current operation, the flow of gaseous steam 10 is perpendicular to the direction of flow of the first cooled liquid stream 60 in first cooling stage 46. Similarly, the flow of first cooled gaseous stream 30 in second cooling stage 48 is perpendicular to the direction of flow of second cooled liquid stream 70 in second cooling stage 48. This can be achieved by providing a horizontal flow of gaseous stream 10 and first cooled gaseous stream 30, in combination with a vertical flow of descending first and second cooled liquid streams 60 and 70 in first and second cooling stages 46 and 48 respectively.

In a similar manner to the embodiment of Figure IA, the first and second cooled streams 60 and 70 are introduced to an upper region of first and second cooling stages 46, 48 and intimately contact gaseous stream 10

and first cooled gaseous stream 30 respectively in order to provide cooling.

In the embodiment illustrated in Figure IB, first and second cooling stages 46, 48 can be present in the same cooling column, which is represented by optional numeral 20. In an alternative embodiment, first and second cooling stages 46, 48 may each be present in separate cooling columns.

Figures 2 and 3 represent preferred embodiments of the method and apparatus described herein in which the first and second cooling stages are present in a single cooling column. In the proceeding discussion, the inlets and outlets of the first and second cooling stages discussed correspond to identically named inlets and outlets of the cooling column in which the first and second cooling stages are provided.

Figure 2 shows a further embodiment of the method and apparatus described herein in which the first and second cooling stages are carried out in a single cooling column under counter-current flow. Gaseous stream 10, which may be at a temperature in the range of 30 to 80 °C, is fed to a first inlet 22 of a first cooling stage 46.

First cooling stage 46 can be provided in a cooling column 20. First inlet 22 may be in a lower region of column 20. Cooling column 20 further comprises second cooling stage 48 arranged generally vertically above first cooling stage 46. After entering column 20 via first inlet 22 the gaseous stream passes generally upwards through the first and second cooling stages in column 20 while being cooled. The cooled gaseous stream exits the second cooling stage 48 via first outlet 24 as second cooled gaseous stream 50. First outlet 24 is

generally situated in an upper region of cooling column 20. The cooled gaseous stream 50 may then be further processed, such as in a carbon dioxide capture unit or passed to the burner of a Claus off-gas treating unit. First cooling stage 46 comprises a second inlet 34 for first cooled liquid stream 60. Second inlet 34 can be situated in an upper region of first cooling stage 46. First cooled liquid stream 60 provides a descending flow of first cooling liquid in the first cooling stage, which intimately contacts the ascending gaseous stream to effect cooling of the gaseous stream to provide a first cooled gaseous stream which then ascends to second cooling stage 48. The direction of the descending flow of first cooling liquid is approximately 180° to the direction of the gaseous stream to be cooled. The first cooling liquid is preferably water.

First cooling stage 46 may comprise one or more packed beds 26 or one or more trays. Packed beds comprising ceramic, metal or carbon shapes are known. The packed shapes may be selected from the group consisting of: raschig rings, intalox saddles, berl saddles and pall rings. The one or more trays may be one or more selected from the group consisting of: bubble cap trays, valve tray, sieve trays, turbo girds, side to side pans, disc and doughnut trays, uniflux trays, ripple trays, Linde trays and the like.

First cooled liquid stream 60 may be passed through one or more nozzles (not shown) after entry through second inlet 34 in order to provide a spray of first cooling liquid. In an alternative arrangement, multiple flows of cooling liquid can be provided by passage of the first cooled liquid stream through a mesh screen 52 situated below second inlet 34 in first cooling stage 46.

Upon reaching the lower portion of the first cooling stage, the first cooling liquid exits column 20 via second outlet 36 as first liquid stream 62. Second outlet 20 is generally situated at the bottom of first cooling stage 46, for instance at the bottom of column 20. Second outlet 36 is connected to inlet 64 of an air cooler 66, optionally via a pump 86. Air cooler 66 cools the first cooling liquid to a temperature in the range of 30-80 °C. The first cooling liquid exits air cooler 66 via outlet 68 as first cooled liquid stream 60, which is passed to second inlet 34 of first cooling stage 46 via splitter 94. A portion of the first cooled liquid exiting splitter 94 can be passed to valve 82 as stream 98.

The first cooled gaseous stream exiting the top of first cooling stage 46 enters the bottom of second cooling stage 48. Second cooling stage 48 comprises a third inlet 38 for second cooled liquid stream 70. Third inlet 38 can be situated in an upper region of second cooling stage 48. Second cooled liquid stream 70 provides a descending flow of second cooling liquid in the second cooling stage, which intimately contacts the ascending first cooled gaseous stream to effect cooling of the first gaseous stream to provide a second cooled gaseous stream which exits cooling column 20 via first outlet 24. The direction of the descending flow of second cooling liquid is approximately 180° to the direction of the ascending flow of the first cooled gaseous stream. The second cooling liquid is preferably water.

Second cooling stage 48 may comprise one or more packed beds 28 or one or more trays. The packed beds or trays can be the same as those discussed above for first cooling stage 46.

In a similar manner to first cooled liquid stream 60, second cooled liquid stream 70 may be passed through one or more nozzles (not shown) after entry through third inlet 38 in order to provide a spray of second cooling liquid. In an alternative arrangement, multiple flows of second cooling liquid can be provided by passage of the second cooled liquid stream through a mesh screen 54 situated below third inlet 38 in second cooling stage 48. Upon reaching the bottom of the second cooling stage, the second cooling liquid is collected in partial or full draw-off tray 32 and exits column 20 via third outlet 42 as second liquid stream 62. Third outlet 36 is connected to inlet 74 of heat exchanger 76, optionally via a pump 88. Heat exchanger 76 cools the second cooling liquid to a temperature in the range of 0-60 °C. Heat exchanger 76 can have a refrigerant circuit. If heat exchanger 76 is a chiller, the refrigerant in the refrigerant circuit may be water. Alternatively, the refrigerant may be propane . The second cooling liquid exits heat exchanger 76 via outlet 78 as second cooled liquid stream 70, which is passed to third inlet 38 of second cooling stage 48.

Fourth outlet 44 of cooling column 20 is connected 84. Stream 96 exiting LC 84 is connected to a valve 82, where it can be combined with stream 98 of the first cooled liquid from splitter 94, and withdrawn from the apparatus as stream 92.

Figure 3 shows a further embodiment in which the first and second cooling stages are carried out in a single cooling column under cross-current flow. Gaseous stream 10, which may be at a temperature in the range of 30-80 °C, is fed to a first inlet 22 of first cooling stage 46. First cooling stage 46 can be situated in a

cooling column 20. In this embodiment, cooling column 20 is in a horizontal orientation compared to the apparatus of Figure 2.

Cooling column 20 further comprises a second cooling stage 48 arranged generally horizontally adjacent to first cooling stage 46. Gaseous stream 10 enters first cooling stage 46 via inlet 22, which can be situated at one end of the cooling column 20, and can pass generally horizontally through column 20 while being cooled. The cooled gaseous stream exits second cooling stage 48 via first outlet 24 as second cooled gaseous stream 50. Fist outlet 24 can be situated at the opposite end of cooling column 20 from first inlet 22, as shown in Figure 3. The cooled gaseous stream 50 may then be further processed, as discussed above for the embodiment of Figure 2.

First cooling stage 46 comprises a second inlet 34 for first cooled liquid stream 60. Second inlet 34 is situated in an upper region of first cooling stage 46, typically on the upper surface of column 20. First cooled liquid stream 60 provides a descending flow of first cooling liquid in the first cooling stage, which intimately contacts the gaseous stream traversing cooling stage 46 to effect cooling of the gaseous stream to provide a first cooled gaseous stream which then enters the second cooling stage 48. First cooling liquid is preferably water .

In contrast to the embodiment of Figure 2, the cross-current operation of Figure 3 provides a generally horizontal flow of gaseous stream which is at approximately 90° to the generally vertical flow of first cooled liquid stream 60 in first cooling stage 46.

First cooling stage 46 may comprise one or more packed beds 26 as discussed above in relation to Figure 2.

First cooled liquid stream 60 may be passed through one or more nozzles (not shown) after entry through inlet 34 in order to provide a spray of first cooling liquid. In an alternative arrangement, multiple flows of cooling liquid can be provided by passage of the first cooled liquid stream through a mesh screen 52 situated below inlet 34 in first cooling stage 46.

Upon reaching the bottom of first cooling stage 46, the first cooling liquid exits column 20 via second outlet 36 as first liquid stream 62. Second outlet 36 is generally situated at the bottom of first cooling stage 46, typically on the bottom surface of cooling column 20 below second inlet 34. Second outlet 36 is connected to inlet 64 of an air cooler 66, optionally via a pump 86. Air cooler 66 cools the first cooling liquid to a temperature in the range of 30-80 °C. The first cooling liquid exits air cooler 66 via outlet 68 as first cooled liquid stream 60, which is passed to second inlet 34 of first cooling stage 46.

The first cooled gaseous stream exiting first cooling stage 46 then enters second cooling stage 48. Second cooling stage 48 comprises a third inlet 38 for second cooled liquid stream 70. Inlet 38 is situated in an upper region of second cooling stage 48, typically on the upper surface of column 20. Second cooled liquid stream 70 provides a descending flow of second cooling liquid in the second cooling stage, which intimately contacts the first cooled gaseous stream to effect cooling of the first cooled gaseous stream to provide a second cooled gaseous stream which exits second cooling

stage 48 via first outlet 24. The second cooling liquid is preferably water.

The cross-current operation of Figure 3 provides a generally horizontal flow of first cooled gaseous stream, which is at approximately 90° to the generally vertical flow of second cooled liquid stream 70 in second cooling stage 48.

Second cooling stage 48 may comprise one or more packed beds 28. as discussed previously in relation to Figure 2.

In a similar manner to first cooled liquid stream 60, second cooled liquid stream 70 may be passed through one or more nozzles (not shown) after entry through third inlet 38 in order to provide a spray of second cooling liquid. In an alternative arrangement, multiple flows of second cooling liquid can be provided by passage of the second cooled liquid stream through a mesh screen 54 situated below third inlet 38 in second cooling stage 48. Upon reaching the bottom of second cooling stage 48, the second cooling liquid exits column 20 via third outlet 42 as second liquid stream 62. Third outlet 42 is generally situated at the bottom of second cooling stage 48, typically on the bottom surface of cooling column 20 below third inlet 38. Third outlet 42 is connected to inlet 74 of heat exchanger 76, optionally via a pump 88. Heat exchanger 76 cools the second cooling liquid to a temperature in the range of 0-60 "C. Heat exchanger 76 can have a refrigerant circuit. If heat exchanger 76 is a chiller, the refrigerant in the refrigerant circuit may be water. Alternatively, the refrigerant may be propane. The second cooling liquid exits heat exchanger 76 via outlet 78 as second cooled liquid stream 70, which is passed to third inlet 38 of second cooling stage 48.

The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention. For example, a combination of counter-current and cross-current flow could be provided, in which the first cooling stage utilises cross-current flow of the gaseous stream and first cooled liquid stream and the second cooling stage utilises counter-current flow of the first cooled gaseous stream and the second cooled liquid stream, or vice versa.