METHOD FOR SYNTHESIS GAS PURIFICATION

Abstract

The present invention relates to an integrated method and apparatus for providing a synthesis gas to a cryogenic separation unit installed for separating synthesis gas into products selected from carbon monoxide, crude hydrogen, methane-rich fuel and syngas with a particular H.sub.2:CO ratio. More specifically, the invention relates to the purification of synthesis gas routed to a downstream cryogenic separation unit and minimizing temperature disturbances in the separation unit.

Claims

1. A continuous purification method of a synthesis gas stream obtained from a pre-purification unit to remove substantially all H.sub.2O and CO.sub.2 prior to routing the synthesis gas product stream to a downstream cryogenic separation unit comprising: supplying a synthesis gas feed stream to a synthesis gas purification unit comprised of at least two adsorbent beds undergoing a temperature swing adsorption (TSA) cycle where each bed undergoes at least two phases: (1) a feed phase for producing a synthesis gas product stream substantially free of H.sub.2O and CO.sub.2 by adsorbing these components on the adsorbent bed and (2) a regeneration phase to desorb H.sub.2O and CO.sub.2 from the adsorbent bed using a regeneration gas and routing the H.sub.2O and CO.sub.2-laden regeneration gas to upstream of the pre-purification unit, where said regeneration gas is formed by routing a regeneration portion of the synthesis gas product stream through a compressor, and the regeneration phase of the TSA cycle comprising multiple steps including: a pressurization step to increase the pressure of the adsorbent bed to be regenerated in a controlled manner using the regeneration gas; a heating step to heat the regeneration gas in a heater and supplying it to the adsorbent bed to remove H.sub.2O and CO.sub.2 from the adsorbent bed; a first cooling step in which heat addition to the heater stops while continuing the flow of the regeneration gas through the heater and the adsorbent bed; a second cooling step to cool the adsorbent bed further with the regeneration gas while by-passing the heater; a depressurization step in which the flow of regeneration gas to the adsorbent bed is stopped and the adsorbent bed is depressurized to the pressure of the product synthesis gas product stream in a controlled manner from a product end of the adsorbent bed; and a final cooling step to cool the adsorbent bed to a temperature that is substantially the same as that of the synthesis gas feed stream by flowing a portion of the synthesis gas feed stream through the adsorbent bed; wherein: during the depressurization and final cooling steps, the gas stream exiting the adsorbent bed from the product end is combined with the regeneration gas stream portion of the synthesis gas product stream, and the combined mixture is compressed in the compressor to form a regeneration gas and the compressed mixture is routed to up-stream of the pre-purification unit thus bypassing the adsorbent bed.

2. The continuous purification method of claim 1, wherein the synthesis gas feed stream obtained from a pre-purification unit has a pressure of about 10 bar(a) to about 50 bar(a).

3. The continuous purification method of claim 1, wherein the synthesis gas feed stream obtained from a pre-purification unit has a temperature of about 35 F. to about 125 F.

4. The continuous purification method of claim 1, wherein the regeneration portion used for regeneration is between 0% and 25% of the synthesis gas product stream.

5. The continuous purification method of claim 1, wherein the portion used for final cooling is between 5% and 25% of the synthesis gas feed stream.

6. The continuous purification method of claim 1, where the synthesis gas purification unit is a two-bed system where one bed is under a feed phase and other bed is under a regeneration phase.

7. The continuous purification method of claim 1, further comprising an additional stand-by phase where each adsorbent bed undergoes a feed phase, a regeneration phase and a stand-by phase in that order.

8. The continuous purification method of claim 1, wherein the regeneration gas stream is heated to about 225-500 F. in the heater.

9. The continuous purification method of claim 8, wherein the regeneration gas stream is heated to about 300-450 F. in the heater.

10. The continuous purification method of claim 7, wherein the synthesis gas purification unit is a three-bed system where one bed is in a feed phase, one bed is in a regeneration phase and one bed is in a stand-by phase.

11. The continuous purification method of claim 1, wherein the cryogenic separation unit produces at least one product selected from high-purity CO, synthesis gas with a specified H.sub.2:CO ratio, crude hydrogen, and methane-rich fuel.

12. A continuous purification method of a synthesis gas to remove substantially all H.sub.2O and CO.sub.2 prior to routing said synthesis gas to a cryogenic separation unit, comprising: supplying a synthesis gas feed stream obtained from a pre-purification unit to a synthesis gas purification unit comprised of at least two adsorbent beds undergoing a temperature swing adsorption (TSA) cycle where each bed undergoes at least two phases: (1) a feed phase for producing a synthesis gas product stream substantially free of H.sub.2O and CO.sub.2 by adsorbing these components on the adsorbent bed and (2) a regeneration phase to desorb H.sub.2O and CO.sub.2 from the adsorbent bed using a regeneration gas; forming a regeneration gas stream by routing a regeneration portion of the synthesis gas product stream through a compressor where the regeneration gas stream is used to regenerate the adsorbent bed in the regeneration phase; routing the regeneration gas leaving the adsorbent bed in the regeneration phase to upstream of the pre-purification unit; stopping the flow of regeneration gas to the adsorbent bed after it is regenerated, depressurizing the adsorbent bed and introducing a portion of the synthesis gas feed stream to the second adsorbent bed to cool it to substantially the same temperature as the synthesis gas feed stream, wherein, during depressurization and subsequent cooling, the gas stream exiting the product end of the adsorbent bed is combined with the regeneration portion of the synthesis gas product stream and the combined gas mixture is compressed in the compressor forming the regeneration gas which is routed upstream of the pre-purification unit thus bypassing the regenerated bed.

13. The continuous purification method of claim 12, wherein the regeneration phase of the TSA cycle comprises at least a heating step, a cooling step, and a final cooling step.

14. The continuous purification method of claim 13, further comprising heating the regeneration gas in a heater during a heating step of the TSA cycle and sending said regeneration gas to the adsorbent bed undergoing the regeneration phase.

15. The continuous purification method of claim 13, further comprising stopping the addition of heat to the regeneration gas heater during a cooling step of the TSA cycle to cool the heater.

16. The continuous purification method of claim 13, further comprising by-passing the regeneration gas heater during a cooling step of the TSA cycle and sending the adsorbent bed undergoing the regeneration phase.

17. An integrated apparatus for continuous purification of a synthesis gas to remove substantially all H.sub.2O and CO.sub.2 prior to routing the synthesis gas product stream to a downstream cryogenic separation unit, comprising: a synthesis gas purification unit comprised of at least two adsorbent beds undergoing a temperature swing adsorption (TSA) cycle wherein the adsorbent beds alternately undergo a feed phase during which an adsorbent bed purifies a synthesis gas feed stream and produces a synthesis gas product stream substantially free of H.sub.2O and CO.sub.2 and a regeneration phase during which an adsorbent bed is regenerated using a regeneration portion of the synthesis gas product stream; a conduit arrangement and valves for routing the synthesis gas product stream to a cryogenic separation unit; a compressor and heater disposed in series; a conduit for routing a regeneration portion of the synthesis gas product stream to the low-pressure side of the compressor to form a regeneration gas; a conduit arrangement and valves for routing the regeneration gas through a heater, for by-passing the heater, and for routing the regeneration gas upstream of the pre-purification unit; a conduit arrangement and valves for routing the regeneration gas to the product end of the adsorbent beds; a conduit arrangement and valves for withdrawing gas from the feed end of the adsorbent beds and routing gas to upstream of the pre-purification unit; and a conduit arrangement and valves for withdrawing a synthesis gas stream from the product end of the adsorbent beds and routing the gas stream to the conduit for routing a regeneration portion of the synthesis gas product stream to the low-pressure side of the compressor.

18. The continuous purification method of claim 17, wherein materials of construction for the heater and piping is made of austenitic steels to reduce the rate of contamination formation.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0047] The objects and advantages of the invention will be better understood from the following detailed description of the preferred embodiments thereof in connection with the accompanying figure wherein like numbers denote same features throughout and wherein:

[0048] FIG. 1 is an illustrative flow sheet of a related art apparatus for syngas purification utilizing a syngas dryer product gas for regeneration.

[0049] FIG. 2 is an illustrative flow sheet of an integrated process and apparatus in accordance with an exemplary embodiment of the present invention for syngas purification using syngas dyer product gas for regeneration.

[0050] FIG. 3 is an illustrative flow sheet of an integrated process and apparatus in accordance with an exemplary embodiment of the present invention for a three-bed syngas purification using syngas dryer product gas for regeneration.

DETAILED DESCRIPTION OF THE INVENTION

[0051] The present invention provides for a method and apparatus for improving the operation of a cryogenic separation unit producing at least a purified CO product stream by eliminating in-situ produced contaminants and temperature disturbances in the cryogenic separation unit feed stream that are the result of regenerating a temperature swing adsorption (TSA) unit. It will be understood by those skilled in the art, that the terms TSA, syngas dryer, and synthesis gas purification unit are utilized interchangeably. The improved process provides a means to cool the adsorbent bed without introducing warm gas to the downstream cryogenic separation unit, thus improving its performance. Further, it results in substantially less contaminants in the cryogenic separation unit feed stream and therefore eliminates the possibility of freeze up events.

[0052] An exemplary embodiment of the present invention is that of a syngas dryer process unit with product gas regeneration for the production of a purified syngas stream to be fed to a cryogenic separation unit, as illustrated in FIG. 2. Similar to the related art syngas dryer process described above, the process shown in FIG. 2 utilizes a typical TSA cycle. The TSA cycle chart describing the valve positions for each step for the exemplary process shown in FIG. 2 is provided in Table 2, below. The inventive process flow sheet shown in FIG. 2 is similar to the related art flow sheet described in FIG. 1 with the exception of the conduit arrangement at the outlet of adsorption beds (100, 200), where valves (163, 263), which are opened during the Depress and Final Cooling steps, are connected with the regeneration gas portion of the TSA system through control valve (410). Similarly, the TSA cycles used for both processes are essentially the same except for the addition of the control valve (410), and the conduit system of FIG. 2, where the valves are operated as shown in Table 2. The syngas dryer processes shown in FIGS. 1 and 2 operate in essentially the same manner except for the beneficial modifications to the process particularly pertaining to the Depressurization (Depress) and Final Cooling steps that eliminate the introduction of contaminants into the cryogenic separation unit and minimize temperature disturbances therein during the Depressurization (Depress) and Final Cooling steps. For purposes of brevity, only the inventive aspects and the associated benefits of the novel process will be described in detail.

TABLE-US-00002 TABLE 2 Adsorbent Adsorbent Bed Bed 100 200 151 152 161 162 163 251 252 261 262 263 401 402 403 404 410 520 Feed Press O X O X X X X X O X X X C/O C X X Feed Heat O X O X X X O X O X O X X C X C Feed Cool-1 O X O X X X O X O X O X X C X X Feed Cool-2 O X O X X X O X O X X O X C X X Feed Depress O X O X X X X X X O X X X C C X Feed Final Cooling O X O X X O X X X O X X X C C X Blend Blend O X O X X O X O X X X X X C X X Press Feed X X X O X O X O X X X X C/O C X X Heat Feed X O X O X O X O X X O X X C X C Cool-1 Feed X O X O X O X O X X O X X C X X Cool-2 Feed X O X O X O X O X X X O X C X X Depress Feed X X X X O O X O X X X X X C C X Final Cooling Feed O X X X O O X O X X X X X C C X Blend Blend O X O X X O X O X X X X X C X X O = Open X = Closed C = Controlled C/O = Controlled or Open

[0053] In the preferred embodiment of this invention, the outlet of valve (410), stream (40), is beneficially introduced to the low-pressure side of the regeneration compressor (400) by routing this stream (40) and mixing same with a regeneration portion (10) of the synthesis gas product stream (6) utilized subsequently for regeneration. Naturally, stream (40) may have its particulates removed by routing through a filter (not shown). Thus, stream (40) is not combined with the product gas (5) as is practiced in the related art. This modification eliminates temperature disturbances in the downstream cryogenic separation unit by ensuring that the warm product gas exiting from the product end of the freshly regenerated bed during the Depress and Final Cooling steps is not combined with the cryogenic separation unit feed stream (7). Specifically, the gas exiting the adsorbent bed (200) from the product end is combined with the regeneration gas stream portion (10) of the synthesis gas product stream and the mixture is compressed in compressor (400) to form regeneration gas (11), wherein the compressed mixture is routed to upstream of the pre-purification unit (not shown), and bypassing the adsorbent bed. As such, the flow through the warm adsorbent bed can be increased to about 100% of the capacity of the regeneration gas compressor (400) during the final cooling step. This maximizes cooling rate of the warm adsorbent bed without affecting the temperature of the cryogenic separation unit feed stream. Further, this configuration effectively ensures that contaminants produced during regeneration and accumulated in the product end of the adsorbent bed and/or in the product end overhead space and piping are not introduced to the downstream cryogenic separation unit. The process and apparatus described herein provides a method for ensuring that the gases containing the said in-situ produced contaminants, particularly those produced by the freshly regenerated adsorbent bed during the Depressurization and Final Cooling steps, are not combined with the syngas dryer product gas and fed to the downstream cryogenic separation unit. Instead, the product gas from the Depressurization and Final Cooling steps is introduced to the low-pressure side of the regeneration gas compressor (400) and returned upstream of the pre-purification process where they can be removed/rejected from the process. This provides a means of rejecting the undesirable products from the process without degrading the performance of the downstream cryogenic separation unit.

[0054] In another exemplary embodiment of the invention, a syngas dryer process consisting of three (3) adsorbent beds with product syngas regeneration for the production of a purified syngas stream to be fed to a cryogenic separation unit is illustrated in FIG. 3. The three (3) bed system is very similar to the two (2) bed system described above with the notable additions of the adsorbent bed (300) and the associated valves (351, 352, 361, 362, 363) with no changes to the rest of the system. The 3-bed system operates a standard TSA cycle, which is very similar to that of the 2-bed systems described in relation to FIG. 2, with the addition of a Standby phase. An exemplary TSA cycle chart describing the valve position for each step for the 3-bed syngas dryer process unit shown in FIG. 3 is provided in Table 3, below. In this example, the TSA cycle phases follow the order: FeedRegenerationStandby. The Feed and Regeneration phases of the cycle remain the same as for the two-bed process described above. As such, for brevity, only the transition of adsorbent bed (300) in to and out of the Standby phase is described. With the completion of the Final Cooling step, the TSA cycle proceeds and the freshly regenerated bed (300) enters the Standby phase and the adsorbent beds in the Standby (200) and Feed (100) phases enter the Blend step as the transition between Feed and Regeneration phases. The corresponding valve actions are depicted in Table 3, below. In this embodiment, a portion of the feed syngas is passed through the adsorbent bed (300) during the Standby phase to continue cooling the bed to the temperature of the feed stream (1) and/or to ensure that the bed remains at or near the temperature of the feed stream during the Standby phase and as such the valves associate with adsorbent bed (300) remain unchanged. At the same time, adsorbent bed (200) transitions from the Standby phase to the Feed phase by transitioning from the Standby step to the Blend step. The corresponding valve actions are depicted in Table 3. Valve (261) is opened and valve (263) is closed so that the feed stream (1) flows through both adsorbent beds (100) and (200). The Blend step ensures a smooth transition, in terms of temperature and compositional variations, as adsorbent bed (200) comes on-stream to treat the feed syngas (1). One notable difference between the 2-bed and 3-bed systems is that gas flows continuously through valve (410) and as such it is in control mode throughout the entire cycle. In addition to the benefits described above for the 2-bed system, the 3-bed system described herein has the benefit of a stable cryogenic separation unit feed temperature particularly during the Depressurization, Final Cooling, and Blend steps.

TABLE-US-00003 TABLE 3 Ad- Ad- Ad- sorber sorber sorber 100 200 300 151 152 161 162 163 251 252 261 262 263 351 352 361 362 363 401 402 403 404 410 520 Feed Standby Press O X O X X O X X X O X X X X O X X C/O C C X Feed Standby Heat O X O X X O X X X O X O X O X O X X C C C Feed Standby Cool-1 O X O X X O X X X O X O X O X O X X C C X Feed Standby Cool-2 O X O X X O X X X O X O X O X X O X C C X Feed Standby Depress O X O X X O X X X O X X X X O X X X C C X Feed Standby Final O X O X X O X X X O O X X X O X X X C C X Cooling Blend Blend Standby O X O X X O X O X X O X X X O X X X C C X Press Feed Standby X X X X O O X O X X O X X X O X X C/O C C X Heat Feed Standby X O X O X O X O X X O X X X O O X X C C C Cool-1 Feed Standby X O X O X O X O X X O X X X O O X X C C X Cool-2 Feed Standby X O X O X O X O X X O X X X O X O X C C X Depress Feed Standby X X X X O O X O X X O X X X O X X X C C X Final Feed Standby O X X X O O X O X X O X X X O X X X C C X Cooling Standby Blend Blend O X X X O O X O X X O X O X X X X X C C X Standby Press Feed O X X X O X X X X O O X O X X X X C/O C C X Standby Heat Feed O X X X O X O X O X O X O X X O X X C C C Standby Cool-1 Feed O X X X O X O X O X O X O X X O X X C C X Standby Cool-2 Feed O X X X O X O X O X O X O X X X O X C C X Standby Depress Feed O X X X O X X X X O O X O X X X X X C C X Standby Final Feed O X X X O O X X X O O X O X X X X X C C X Cooling Blend Standby Blend O = Open X = Closed C = Controlled C/O = Controlled or Open

[0055] The invention is further explained through the following example, which compare the related TSA process with the one based on exemplary embodiments of the invention, which are not to be construed as limiting the present invention.

Comparative Example

[0056] A raw syngas stream exiting a pre-purification unit consisting an aqueous-amine CO.sub.2 removal system and a chiller/separator unit is fed to a syngas dryer process unit (i.e., TSA) for the substantial removal of H.sub.2O and CO.sub.2. The raw syngas stream having a composition of 64.8% H.sub.2, 33.5% CO, 1.1% CH.sub.4, 50 ppm CO.sub.2, 554 ppm H.sub.2O, and 0.5% inerts (N2+Ar) enter the syngas dryer process as stream (1) in FIGS. 1 and 2 at a rate of 181 MMscfd and at 50 F. and 392.7 psia. The adsorbent beds (100, 200) contain 128,000 lbs of a 13 adsorbent. Following the TSA cycle described above, the raw syngas stream is fed to adsorbent bed (100) for the substantial removal of CO.sub.2 and H.sub.2O. The purified gas exits the adsorbent bed containing <10 ppb CO.sub.2 and <1 ppb H.sub.2O at a slightly elevated temperature of 52 F. and is fed as stream (7) to the downstream cryogenic separation unit. The small increase in the purified gas temperature is associated with heat released during adsorption of H.sub.2O and CO.sub.2 and heat gained from the environment. Adsorbent bed (200), having just completed the Cool-2 step, has a mean bed temperature of about 90 F. As the cycle proceeds to the Depressurization step, warm product gas is released from adsorbent bed (200). In the related art process described above and shown in FIG. 1, the warm product syngas from adsorbent bed (200) is combined with the purified gas exiting adsorbent bed (100) and fed directly to the downstream cryogenic separation unit. Only a small amount of gas exits the warm adsorbent bed (200) during the Depressurization step causing little variation in the combined feed gas stream. However, as the cycle proceeds to the Final Cooling step, in which for this comparative example 16% of the feed gas is introduced to the warm adsorbent bed (200), the temperature of the syngas dryer product stream (6) to the cryogenic separation unit increases rapidly to 57.0 F. The fraction of feed gas passing through the warm adsorbent bed was set based on the regeneration flow rate for this example. The temperature excursion remains until the adsorbent bed temperature is reduced to the feed temperature, which can last several hours, typically from about 20 to about 60% of the total regeneration phase time. In this comparative example, the temperature excursion lasts for approximately 3.5 hours. This method effectively transfers the residual heat in the adsorbent bed at the end of the Cool-2 step into the downstream cryogenic separation unit.

[0057] In the inventive process shown in FIG. 2, the gas exiting the warm adsorbent bed (200) during the Depressurization and Final Cooling steps is not combined with the cryogenic separation unit feed stream (7) and instead this stream (40) is combined with the regeneration gas stream (10) and directed to the low-pressure side of the regeneration gas compressor (400). As such there is no variation in the temperature of the cryogenic separation unit feed stream. It remains unchanged at 52 F. In this exemplary embodiment of the present invention, an effective means of avoiding the introduction of a temperature disturbance in the downstream cryogenic separation unit caused by the cooling of the freshly regenerated adsorbent bed is provided. Instead of transferring heat from the adsorbent to the downstream cryogenic separation unit as is done in the comparative example of the related art, the described invention provides a means of rejecting the residual heat to the upstream process which is unaffected by the variation in the temperature of the returned regeneration gas (14).

[0058] While the invention has been described in detail with reference to specific embodiments thereof, it will become apparent to one skilled in the art that various changes and modifications can be made, and equivalents employed, without departing from the scope of the appended claims.