Controlling the concentration of impurities in a gas stream

• US 3,897,226 A
• Filed: 04/19/1973
• Issued: 07/29/1975
• Est. Priority Date: 04/19/1972
• Status: Expired due to Term

• Current Assignee: Costain Petrocarbon Limited
• Original Assignee: Keith Stanley Doherty
• Inventors: Doherty, Keith Stanley
• Primary Examiner(s): Hart, Charles N.
• Application Number:
  US05/352,748
• Time in Patent Office: 831 Days
• Field of Search: 55/33,62,68,74,75,179 73/23
• US Class Current: 95/12
• CPC Class Codes: B01D 2256/24 Hydrocarbons
• CPC Class Codes: B01D 2257/504 Carbon dioxide
• CPC Class Codes: B01D 2257/80 Water
• CPC Class Codes: B01D 2259/402 using two beds
• CPC Class Codes: B01D 2259/404 using four beds
• CPC Class Codes: B01D 2259/41 using plural beds of the same adsorbent in series
• CPC Class Codes: B01D 53/04 with stationary adsorbents B01D53/025 takes precedence
• CPC Class Codes: B01D 53/0454 Controlling adsorption controlling temperature swing adsorption B01D53/0462, controlling pressure swing adsorption B01D53/047
• CPC Class Codes: Y02C 20/40 of CO2

Abstract

FLUCTUATING CONCENTRATIONS OF IMPURITY in a gas stream such as a regenerating stream recovered from an adsorber unit, are smoothed by diverting at least a portion of the gas stream through a bed of adsorbent for the impurity while the concentration of the impurity exceeds a predetermined value, and continuing the diversion of the stream for a period after the impurity concentration has dropped back below that level so as to strip the adsorbed impurity from the bed to regenerate it for the next adsorption cycle. The invention is particularly applicable to the treatment of natural gas regenerating streams containing fluctuating amounts of an impurity such as carbon dioxide, and also water vapor as a second impurity, and further includes a method of reducing the water dew point of such streams.

Claims

1. SENSING THE LEVEL OF CONCENTRATION OF IMPURITY IN THE GAS STREAM,

2. A method as claimed in claim 1 in which the gas stream to be upgraded is a regenerating stream containing fluctuating amounts of an impurity desorbed from an adsorber unit used to purify a gaseous process stream by removal of said impurity from it.

3. A method as claimed in claim 2 in which the regenerating stream is a hot regenerating stream and the proportion which is diverted through the adsorption zone is cooled prior to passage through said zone.

4. A method as claimed in claim 2 in which the gaseous process stream contains a second impurity which is removed therefrom by passing said process stream through a second adsorber unit, the two adsorber units being arranged in series with respect to the flow of the process stream therethrough with each adsorber being supplied with a separate regenerating stream and wherein the upgraded regenerating stream is combined with the regenerating stream from said second adsorber for subsequent utilization.

5. A method as claimed in claim 4 in which the gaseous process stream is a hydrocarbon feedstock containing water vapor as said second impurity and the regenerating stream supplied to the second adsorber unit is passed through said unit at elevated temperature to desorb water vapor from said second adsorber unit, and thereafter cooled to condense water from the stream prior to combination with said upgraded regenerating stream for subsequent utilization.

6. A method as claimed in claim 4 in which each regenerating stream is derived from the gaseous process stream which has been purified by passage through the two adsorber units.

7. A method as claimed in claim 2 in which the gaseous process stream contains water vapor as a second impurity which is removed from the stream by passing the stream through a second adsorber unit, the two adsorber units being arranged in series with respect to the flow of the stream therethrough, and in which at least a portion of the upgraded regenerating stream is passed through said second adsorber unit at elevated temperature as a regenerating stream therefor to desorb water vapor therefrom, subsequently cooled to condense out water and recombined with the remainder, if any, of said upgraded regenerating stream for subsequent utilization.

8. A method as claimed in claim 7 in which the regenerating stream which is upgraded is derived from the gaseous process stream which has been purified by passage through the two adsorbers.

9. A method as claimed in claim 1 in which the impurity present in fluctuating amounts is carbon dioxide.

10. A method as claimed in claim 2 in which the gaseous process stream is natural gas.

11. WHEN THE SENSED LEVEL OF CONCENTRATION OF IMPURITY IN THE GAS STREAM RISES TO SAID PREDETERMINED VALUE, WITHDRAWING AT LEAST A PART OF SAID GAS STREAM, PURIFYING SAID WITHDRAWN PART BY PASSING SAID PART THROUGH AN ADSORPTION ZONE CONTAINING ADSORBENT FOR SAID IMPURITY WHEREBY TO ADSORB SAID IMPURITY FROM SAID WITHDRAWN PART, AND THEREAFTER RE-COMBINING THE PURIFIED PART WITH ANY REMAINDER OF SAID GAS STREAM TO FORM SAID PRODUCT STREAM, III.. CONTINUING STEPS (II) WHILE THE CONCENTRATION OF IMPURITY IN THE GAS STREAM EXCEES SAID PREDETERMINED VALUE, IV. CONTROLLING THE PROPORTION OF GAS STREAM THAT IS WITHDRAWN AND PURIFIED IN ACCORDANCE WITH STEPS (II) AND (III) SO AS TO MAINTAIN THE OVERALL CONCENTRATION OF IMPURITY IN SAID PRODUCT STREAM SUBSTANTIALLY AT OR BELOW SAID PREDETERMINED VALUE, V. AFTER THE CONCENTRATION OF THE IMPURITY IN SAID GAS STREAM HAS RETURNED BELOW SAID PREDETERMINED VALUE, REGERATING SAID ADSORBENT FOR SUBSEQUENT RE-USE BY WITHDRAWING AT LEAST A PART OF SAID GAS STREAM AND PASSING SAID PART THROUGH SAID ADSORPTION ZONE WHEREBY TO DESORB FROM SAID ADSORBENT IMPURITY ADSORBED THEREBY DURING STEPS (II) AND (III), VI. TERMINATING STEP (V) AFTER THE IMPURITY ADSORBED BY SAID ADSORBENT DURING STEPS (II) AND (III) HAS BEEN SUBSTANTIALLY REMOVED, AND VIII. REPEATING STEPS (II) THROUGH (VI) EACH TIME THE SENSED CONCENTRATION OF IMPURITY IN THE GAS STREAM RISES TO SAID PREDETERMINED LEVEL.

12. A method as claimed in claim 8 in which the gaseous process stream is derived from a stream of natural gas and is liquefied for storage after purification by passage through the two adsorbers and in which the product of combining the regenerating stream from the second adsorber with the remainder if any of the upgraded regenerating stream is returned to the stream of natural gas downstream from the point of derivation.

13. A method of obtaining a regenerating stream having a low water dew point from adsorber units used in the purification of a gas stream containing water vapor and at least one other impurity which method comprises passing said gas stream in series through at least two adsorber units one of which adsorbs the water vapor and the other of which adsorbs said other impurity, passing a first regenerating stream at elevated temperature through the water vapor adsorber unit to desorb the water vapor therefrom, passing another regenerating sTream through said other adsorber unit to desorb said other impurity therefrom, cooling the regenerating stream leaving the water vapor adsorber unit to condense water therefrom, separating out the condensed water and combining the residual gas stream with the regenerating stream from said other adsorber unit.

14. A method as claimed in claim 1 in which during steps (ii) and (iii) the proportion of said gas stream that is withdrawn is increased with increase in the level of concentration of impurity in the gas stream and decreased with decrease in said level, and during step (v) the proportion of said gas stream that is withdrawn is increased with decrease in the level of concentration of impurity in the gas stream.

15. Apparatus for upgrading an impurity-containing gas stream in which the concentration of the impurity varies with time, to provide a product stream in which the concentration of the impurity substantially does not exceed a predetermined value which is between the maximum and minimum levels of concentration of impurity found in the gas stream, said apparatus comprising:

a. pipeline means for conveying said gas stream from an inlet to an outlet of the apparatus;

b. vessel means for containing an adsorbent for said impurity, said vessel means having a gas inlet and a gas outlet;

c. conduit means for diverting at least a portion of the gas stream from said pipeline means through said vessel means and thereafter returning it to the pipeline means downstream of the point from which it was diverted;

d. valve means for controlling the proportion of the gas stream that is diverted from said pipeline means through said vessel means;

e. measuring means for continuously measuring the level of the concentration of the impurity in the product stream at a point downstream of where that portion of the gas stream which is diverted through said vessel means is returned into said pipeline means, said measuring means being adapted to provide a signal corresponding to the measured level of impurity concentration;

f. first automatic valve control means for opening said valve means in response to said signal when the measured level of concentration of the impurity in said gas stream rises above a predetermined level and for controlling the degree of opening of said valve means while said concentration exceeds said predetermined level to prevent the measured level of concentration of impurity in said product stream from rising substantially above said predetermined level; and

g. second automatic valve control means for controlling the valve means to allow the diversion of at least a portion of the gas stream through said vessel means after the level of impurity in said gas stream has fallen below said predetermined level, and to close the valve means after the adsorbent in said vessel means has substantially been regenerated.

16. Apparatus as claimed in claim 15 further including two adsorber units arranged upstream of and connected to said pipeline means for passage therethrough in series of a gaseous process stream to be purified, each adsorber unit being piped and valved to permit regeneration of the adsorbent by passage therethrough of a separate regenerating gas stream, and wherein the outlet for the regenerating gas leaving one of said adsorber units is connected to the inlet to said pipeline means and the other of said adsorber units is a water vapor adsorber unit, said apparatus also including means for heating the regenerating gas stream to be fed to each adsorber unit, means for cooling, the regenerating gas stream from said water vapor adsorber unit to condense the water vapor in it, separator means for separating the condensed water from the cooled regenerating gas stream and means for combining the gas stream from said separator means with the gas leaving said pipeline means.

17. Apparatus as claimed in claim 15 further including two adsorber units arranged upstream of and connected to said pipeline means for passage therethRough in series of a gaseous process stream to be purified, each adsorber unit being piped and valved to permit regeneration of the adsorbent by passage therethrough of a regenerating gas stream and wherein the outlet for the regenerating gas leaving one of said adsorber units is connected to the inlet of said pipeline means and the other adsorber unit is a water vapor adsorber unit, said apparatus also including means for heating the regenerating stream to be fed to said one adsorber unit, means for withdrawing at least part of the gas stream recovered from the outlet of said pipeline means, and feeding it as a regenerating means for cooling the regenerating stream recovered from said water vapor adsorber unit, separator means for separating condensate from the cooled regenerating stream and means for recombining the gas stream recovered from the separator with the remainder if any of the regenerating stream leaving said pipeline means.

18. Apparatus as claimed in claim 15 in which said first automatic valve control means is adapted in an initial mode of operation to control the valve means in the sense of increasing the degree of valve opening with increase in the level of concentration of impurity in the gas stream and reducing the degree of valve opening with decrease in said level, and said second automatic valve control means comprises timing means and reversing means responsive to said timing means for reversing the mode of operation of said first valve control means, said timing means being adapted to activate said reversing means after a first time interval to reverse the mode of operation of said first valve control means, whereby decrease in the level of impurity in the gas stream increases the degree of valve opening, and to activate said reversing means again after a second time interval to restore said first automatic valve control means to said initial mode of operation.

19. Apparatus as claimed in claim 18 further including principal adsorber means of the kind including at least two adsorption zones, an inlet and outlet for gas to be treated in the principal adsorber means, an inlet and outlet for regenerating gas, means connecting at least one of the adsorption zones, between said inlet and outlet for the gas to be treated, for passage of said gas through said zone for adsorption of adsorbable impurity from said gas, means connecting at least another of said adsorption zones between said inlet and outlet for regeneration gas for passage of said gas through said other zone for regeneration of the adsorber in said zone, and automatic means for periodically disconnecting said one zone from between first mentioned inlet and outlet and subsequently connecting it between said second mentioned inlet and outlet and for periodically disconnecting said other zone from between said second mentioned inlet and outlet and subsequently connecting it between said first mentioned inlet and outlet;

the regeneration gas outlet of said principal adsorber means communicating with said pipeline means upstream of said conduit means; and

wherein said automatic means comprises a timing means which also provides the timing means of said second automatic valve control means.