Absorber column and process for cleaning crude synthesis gas

Abstract

The invention relates to an absorber column and to the use thereof for separation of unwanted, especially acidic, gas constituents, for example carbon dioxide and hydrogen sulfide, from a crude synthesis gas by absorption with an absorbent, especially under low load states of the absorber column in relation to the synthesis gas velocity. According to the invention, a defined concentration of carbon dioxide in the clean synthesis gas is established by mixing at least a portion of the absorbent regenerated by flash regeneration with the absorbent regenerated by means of hot regeneration prior to the recycling thereof into the absorber column.

Claims

1. An absorber column for production of a clean synthesis gas by at least partial separation of carbon dioxide and sulfur compound from a crude synthesis gas comprising hydrogen, carbon oxides and sulfur compounds, by absorbing with an absorbent, the absorber column comprising: a shell which extends along a longitudinal axis that runs parallel to the vertical and encloses an inner space, wherein the inner space comprises an upper absorption region and a lower absorption region, each of which contains at least one mass transfer zone, wherein the mass transfer zones of the upper absorption region serve predominantly for removal of carbon dioxide and wherein the mass transfer zones of the lower absorption region serve predominantly for removal of sulfur compounds, an inlet disposed below the lowermost mass transfer zone of the lower absorption region for the crude synthesis gas, a clean synthesis gas outlet disposed at the top end of the shell for the clean synthesis gas, a first outlet disposed at the lower end of the upper absorption region for a first fraction of the laden absorbent laden with carbon dioxide, a first regeneration apparatus for the regeneration of the first fraction of the laden absorbent by expansion, a second outlet disposed at the bottom end of the shell for a second fraction of the laden absorbent laden with sulfur compounds, a second regeneration apparatus for the regeneration of the second fraction of the laden absorbent by heating, a first absorbent inlet disposed above the uppermost mass transfer zone of the upper absorption region for the absorbent regenerated by heating, which is in fluid connection with the second regeneration apparatus via a first feed conduit, a second absorbent inlet disposed below the first absorbent inlet and within the upper absorption region for the absorbent regenerated by expansion, which is in fluid connection with the first regeneration apparatus via a second feed conduit, and a connecting conduit between the first feed conduit and the second feed conduit.

2. The absorber column of claim 1, wherein the connecting conduit comprises a controller valve.

3. The absorber column of claim 1, wherein the first feed conduit and/or the second feed conduit comprise(s) a controller valve.

4. The absorber column of claim 1, further comprising a separating tray, which is passable to gases flowing upward but impassable to liquids flowing downward and which is disposed below the upper mass transfer zone and above the lower mass transfer zone.

5. The absorber column of claim 1, in fluid connection to a plant for catalytic methanol synthesis from synthesis gas.

6. A process for producing a clean synthesis gas with a defined carbon dioxide content from a crude synthesis gas, comprising: providing an absorber column according to claim 1, providing and introducing the crude synthesis gas into the absorber column, discharging the clean synthesis gas that has been at least partly freed of carbon dioxide and sulfur compounds and has a defined carbon dioxide content from the absorber column, discharging a first fraction of the laden absorbent laden with carbon dioxide from the first outlet, introducing the first fraction of the laden absorbent into a first regeneration apparatus for the regeneration by expansion (flash regeneration), conducting the regeneration by expansion, discharging the absorbent regenerated by expansion, discharging a second fraction of the laden absorbent laden with sulfur compounds from the second outlet, introducing the second fraction of the laden absorbent into a second regeneration apparatus for regeneration by heating, conducting regeneration by heating, discharging the absorbent regenerated by heating, introducing the absorbent regenerated by heating into the absorber column via the first absorbent inlet, introducing the absorbent regenerated by expansion into the absorber column via the second absorbent inlet, and feeding via the connecting conduit at least a portion of the absorbent regenerated by expansion into the absorbent regenerated by heating prior to introduction thereof into the absorber column.

7. The process of claim 6, wherein the ratio of the flow rate of the regenerated absorbent that flows through the connecting conduit to that which flows through the first feed conduit is between 5% and 60% by weight.

8. The process of claim 6, wherein the ratio of the flow rates of regenerated absorbent that flows through the connecting conduit compared to that which flows through the first feed conduit is adjusted such that the molar proportion of carbon dioxide in the cleaned synthesis gas fed to a methanol synthesis is at least 1 mol %.

9. The process of claim 6, wherein the temperature of the absorbent regenerated by expansion that flows through the connecting conduit, prior to introduction into the first feed conduit, is between −70° C. and −10° C.

10. The process of claim 6, wherein the absorbent regenerated by expansion is taken from a reabsorption apparatus, a medium-pressure flash apparatus or a low-pressure flash apparatus.

11. The process of claim 6, wherein the absorbent comprises one or more components selected from the following group: methanol, N-methylpyrrolidone (NMP), secondary amines, diethanolamine, tertiary amines, methyldiethanolamine, polyethylene glycol dialkyl ethers, and polyethylene glycol dimethyl ether.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Further features, advantages and possible uses of the invention are also apparent from the description of a working example and numerical example which follows and from the drawing. AH the features described and/or visualized, on their own or in any combination, form the subject-matter of the invention, irrespective of their combination in the claims or their dependency references.

(2) The sole FIGURE shows:

(3) FIG. 1 a schematic diagram of an illustrative configuration of the absorber column of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) In the configuration of the process of the invention shown in schematic form in FIG. 1, here in the form of the Rectisol process, or of the absorber column of the invention, crude synthesis gas that has been produced by reforming or gasification of carbonaceous feeds in an upstream synthesis gas production plant which is not visualized is introduced into the absorber column 1 via conduit 111, The crude synthesis gas contains, as well as the desired synthesis gas constituents of hydrogen and carbon monoxide, among other substances, also the unwanted acidic synthesis gas constituents carbon dioxide and hydrogen sulfide, and further organic and inorganic sulfur compounds.

(5) The crude synthesis gas introduced into the absorber column flows upward therein and passes through multiple mass transfer zones 30 and separation trays 40 that are permeable to the gas flow and, in the example shown, are configured as chimney trays. (The reference numerals 20, 30, 40 are introduced in a merely illustrative manner for a column region comprising liquid distributor 20, mass transfer zone 30 and separation trays 40.) The mass transfer zones have been configured either with trays, for example bubble-cap trays, or as structured packings. Combinations of the two configuration forms are also possible. Once the crude synthesis gas has passed through all the mass transfer zones and the unwanted constituents, especially carbon dioxide and hydrogen sulfide, have been separated out therein by mass transfer with the absorbent, methanol in the present example, it leaves the absorber column as clean synthesis gas via conduit 112.

(6) The mass transfer zones present in the absorber column have the following tasks and properties:

(7) Mass transfer zone A: CO.sub.2 fine scrubbing. Fine removal of carbon dioxide down to the residual content required. Contacting with hot-regenerated methanol via conduits 310, 344, 346 (first feed conduit).

(8) Mass transfer zone B: CO.sub.2 main scrubbing. Removal of carbon dioxide. Contacting with methanol regenerated (flash-regenerated) by expansion via conduits 177A, 177 (second feed conduit).

(9) Mass transfer zone C: Cooling stage, Removal of the heat of absorption released in mass transfer zones D and E to increase the absorption capacity of the absorbent.

(10) Mass transfer zone D: H.sub.2S absorption. Removal of carbon dioxide. Contacting with methanol which is discharged from the separation tray of mass transfer zone C via conduit 154, cooled down and recycled to the absorber column via conduit 156, 157.

(11) Mass transfer zone E: Preliminary scrubbing. Removal of HCN, NH.sub.3 and other trace components. Contacting with methanol which has been discharged from the separation tray of mass transfer zone C via conduit 154, optionally cooled down and recycled to the absorber column via conduit 156, 159.

(12) It is also possible to divide the absorber column described into two individual columns between mass transfer zones A-B and D-E, of which the column comprising mass transfer zones A-B serves for CO.sub.2 absorption and the column comprising mass transfer zones D-E for H.sub.2S absorption. Such a divided column shall also be regarded as an absorber column in the context of the present invention.

(13) That fraction of the methanol which is discharged from the separation tray of mass transfer zone C via conduit 154 and has not been recycled to mass transfer zones D and E via conduits 156, 157 or 156, 159 is guided via conduit 161 to the flash regeneration apparatus 600 (first regeneration apparatus) and introduced into it. Via conduit 177A, the absorbent regenerated by flashing is guided to conduit 177, the second feed conduit, and recycled via the latter to mass transfer zone B.

(14) The bottom product from the absorber column, i.e. the methanol stream laden with sulfur components, is discharged from the absorber column via conduits 300 and 160, guided to the hot regeneration apparatus 500 (second regeneration apparatus) and introduced into it. Via conduits 310 and 344, the methanol regenerated by heating is recycled to conduit 346, the first feed conduit, and recycled via the latter to mass transfer zone A.

(15) The conduit 177B provided in accordance with the invention and the controller valve present in the conduit pathway enable the setting of a defined inflow of the methanol regenerated by flashing to the methanol stream regenerated by hot regeneration. This allows fine adjustment of the carbon dioxide concentration in the clean synthesis gas. This is advantageous especially when the absorber column is in a state of operation in which the flow rate of the crude synthesis gas has been reduced to 70% or less, preferably 40% or less, compared to that in standard operation. A reduction in the flow rate of the crude synthesis gas to 40% or less results in a state of operation of the absorber column which, as experience has shown, is at the lower limit of its hydraulic working range. Furthermore, however, even at gas velocities of 70% or less compared to standard operation of the absorber column, a distinct reduction in the CO.sub.2 concentration in the clean synthesis gas below the value required is observed, and so, even here, the inventive supply of partly CO.sub.2-laden absorbent coming from the flash regeneration to the largely CO.sub.2-free absorbent (fine scrubbing agent) coming from the hot regeneration can raise the CO.sub.2 concentration in the clean synthesis gas back to the value required.

(16) The flow rate ratio of the regenerated methanol that flows through the connecting conduit 177B to that which flows through conduit 344 which opens into the first feed conduit 346 is adjusted to values between 5% and 60% by weight, preferably between 10% and 40% by weight. Taking account of these ratios, it is ensured that the CO.sub.2 concentration in the clean synthesis gas can be set within the concentration ranges desired. It is generally the case that the ratio of the flow rates of regenerated absorbent that flows through the connecting conduit to that which flows through the first feed conduit is adjusted such that the molar proportion of carbon dioxide in the cleaned synthesis gas fed to the methanol synthesis is at least 1 mol %, preferably at least 2 mol %. Appropriate setting of these flow rate ratios ensures that the CO.sub.2 concentration in the clean synthesis gas can be set within the concentration ranges desired and the synthesis gas obtained is suitable for use in methanol synthesis.

(17) The temperature of the methanol regenerated by flashing that flows through the connecting conduit 177B, prior to introduction into the first feed conduit 346, is between −70° C. and −10° C., preferably between −60° C. and −30° C. These temperatures are attained by the adiabatic desorption on expansion without additional cooling of the methanol in cooling apparatuses. The methanol cooled in this way is usable directly in the absorber column and has favourable absorption properties.

(18) FIG. 1 also shows, for comparison, the existing noninventive procedure for establishment of the carbon dioxide concentration in the clean synthesis gas by means of the conduit 210 shown as a dotted line and the controller valve present in this conduit pathway. However, a disadvantage is the large dimensions of the pipeline 210 and of the valve that are required owing to the guiding of a gas flow. Experience has shown that the latter is also more prone to faults than the liquid controller valve of the invention in the conduit pathway 177B.

(19) The devices for hot regeneration 500 or flash regeneration 600 shown in FIG. 1 are represented merely schematically as function blocks. Their exact configuration is known per se to those skilled in the art. More particularly, the apparatus for flash regeneration 600 may comprise multiple expansion stages or apparatus components at different pressure levels, including a moderate-pressure flash apparatus.

(20) The tables reproduced below give the most important physical parameters and the compositions (mole fractions) for the entry and exit streams designated in FIG. 1 for each of three operating states of the absorber column according to the above-discussed working example (Rectisol process):

(21) Standard load (comparative example): Operation of the absorber column with standard values (design values) for gas and liquid velocity.

(22) Low load (comparative example): Operation of the absorber column at standard values for liquid velocity, but greatly reduced gas velocity at the lower end of the hydraulic working range.

(23) Invention: Operation of the absorber column at standard values for liquid velocity and greatly reduced gas velocity, but with feeding of the methanol regenerated by flashing via conduit 177B to the methanol stream regenerated by hot regeneration and introduced via conduits 310 and 344. There is a clearly apparent rise in the CO.sub.2 concentration from zero (comparative example) to about 3.9 mol % (invention) in the first feed conduit 346 and from 0.4 to about 2.0 mol % in the clean synthesis gas in conduit 112.

INDUSTRIAL APPLICABILITY

(24) The invention provides an improvement of proven processes for cleaning crude synthesis gas in absorber columns that enables reliable and efficient establishment of a defined CO.sub.2 concentration in the clean synthesis gas, especially also at low gas velocity of the column.

LIST OF REFERENCE NUMERALS

(25) 1 absorber column 20 liquid distributor 30 mass transfer zone 40 separation tray, e.g. chimney tray 111 conduit (crude synthesis gas) 112 conduit (clean synthesis gas) 150 conduit 153-157 conduit 159-161 conduit 177, 177A, 177B conduit 210 conduit (bypass) 300, 310 conduit 344, 346 conduit 500 hot regeneration apparatus 600 flash regeneration apparatus

(26) TABLE-US-00001 Low Standard Low Standard load Invention load load Invention load 112 112 112 111 111 111 Clean Clean Clean Crude Crude Crude syngas syngas syngas Stream No. syngas to syngas to syngas to from from from Description absorber absorber absorber absorber absorber absorber Total flow kmol/hr 6652 6644 24446 3997 4060 16725 rate Temperature ° C. −0.1 −3.7 −20.0 −39.3 −45.6 −34.2 Pressure bar(a) 62.3 62.3 55.6 61.2 61.2 54.5 CO2 0.3618 0.3611 0.3044 0.0041 0.0195 0.0295 CO 0.1885 0.1886 0.2436 0.2919 0.2874 0.3407 H2 0.4256 0.4260 0.4289 0.6951 0.6843 0.6210 N2 0.0043 0.0043 0.0046 0.0068 0.0067 0.0066 AR 0.0010 0.0010 0.0011 0.0015 0.0015 0.0015 H2S 0.0162 0.0163 0.0153 0.0000 0.0000 0.0000 COS 0.0003 0.0003 0.0005 0.0000 0.0000 0.0000 H2O 0.0005 0.0005 0.0004 0.0000 0.0000 0.0000 MEOH 0.0014 0.0015 0.0005 0.0001 0.0000 0.0001

(27) TABLE-US-00002 Low Standard Low Standard load Invention load load Invention load 344 344 344 346 346 346 Fine Fine Fine Fine Fine Fine scrub scrub scrub scrub scrub scrub MeOH MeOH MeOH MeOH MeOH MeOH Stream No. to to to to to to Description absorber absorber absorber absorber absorber absorber Total flow kmol/hr 3629 3629 10712 3623 4869 10703 rate Temperature ° C. −42.2 −42.0 −45.7 −42.2 −46.0 −45.7 Pressure bar(a) 74.7 74.7 67.8 66.9 64.5 60.0 CO2 0.0000 0.0000 0.0000 0.0000 0.0391 0.0000 CO 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 H2 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 N2 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 AR 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 H2S 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 COS 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 H2O 0.0051 0.0051 0.0149 0.0051 0.0049 0.0149 MEOH 0.9949 0.9949 0.9850 0.9949 0.9559 0.9850

(28) TABLE-US-00003 Low Standard Low Standard Low Standard load Invention load load Invention load load Invention load 177B 177B 177B 177 177 177 177A 177A 177A Main Main Main Main Main Main Main Main Main scrub scrub scrub scrub scrub scrub scrub scrub scrub MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH to to to Stream No. to to to to to to fine fine fine Description absorber absorber absorber absorber absorber absorber scrub scrub scrub Total flow kmol/hr 6268 4984 14984 6268 6231 14984 1246 rate Temperature ° C. −57.4 −57.1 −60.2 −57.4 −57.1 −60.2 −57.1 Pressure bar(a) 64.5 64.5 60.3 72.0 72.0 68.0 64.5 CO2 0.1560 0.1531 0.1796 0.1560 0.1531 0.1796 0.1531 CO 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 H2 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 N2 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 AR 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 H2S 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 COS 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 H2O 0.0043 0.0043 0.0122 0.0043 0.0043 0.0122 0.0043 MEOH 0.8397 0.8426 0.8081 0.8397 0.8426 0.8081 0.8426

(29) TABLE-US-00004 Low Standard Low Standard load Invention load load Invention load Low Standard 160 160 160 161 161 161 load Invention load H2S- H2S- H2S- CO2- CO2- CO2- 300 300 300 laden laden laden laden laden laden Prescrub Prescrub Prescrub MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH Stream No. to to to to to to to to to Description hot reg. hot reg hot reg flash reg. flash reg. flash reg. hot reg. hot reg. hot reg. Total flow kmol/hr 181 181 318 9031 8909 23568 191 204 388 rate Temperature ° C. −40.0 −40.0 −40.0 −16.9 −16.8 −23.6 −3.1 −6.6 −20.7 Pressure bar(a) 64.2 64.2 56.6 63.4 63.4 56.7 22.3 22.3 22.3 CO2 0.2538 0.2453 0.2804 0.2538 0.2453 0.2804 0.2275 0.2518 0.3111 CO 0.0070 0.0070 0.0077 0.0070 0.0070 0.0077 0.0054 0.0057 0.0070 H2 0.0041 0.0041 0.0029 0.0041 0.0041 0.0029 0.0041 0.0040 0.0026 N2 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 AR 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 H2S 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0381 0.0421 0.0568 COS 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0006 0.0006 0.0014 H2O 0.0038 0.0038 0.0106 0.0038 0.0038 0.0106 0.0220 0.0207 0.0347 MEOH 0.7310 0.7396 0.6982 0.7310 0.7396 0.6982 0.7018 0.6745 0.5790

(30) It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.