Removal of monocyclic aromatic compounds (BTEX) from a gas

Abstract

The present invention relates to an improved process and system for purifying a gas, preferably an energy gas, containing aromatic compounds and isolating a fraction of aromatic compounds from said gas. In the process according to the invention, the gas is contacted with a washing liquid in step (a), at a temperature of 15-250° C., to obtain a purified gas, which is depleted in aromatic compounds, and a spent washing liquid wherein the aromatic compounds are dissolved. The spent washing liquid is stripped in step (b) with a stripping gas comprising at least 50 vol. % steam, to obtain a stripped washing liquid which is advantageously reused in step (a) and a loaded stripping gas comprising the aromatic compounds. The aromatic compounds are separated from the loaded stripping gas in step (c) by condensation of the steam and/or the aromatic compounds comprised in the loaded stripping gas to obtain an immiscible composition and isolating the aromatic compounds therefrom. The cleared stripping gas which is advantageously reused in step (b).

Claims

1. A process for isolating a fraction of aromatic compounds from a gas, comprising: (a) contacting the gas with a washing liquid, at a temperature of 15-250° C., to obtain the purified gas and a spent washing liquid comprising aromatic compounds; (b) contacting the spent washing liquid with a stripping gas comprising at least 50 vol. % steam, to obtain a loaded stripping gas comprising aromatic compounds and a stripped washing liquid; and (c) separating the aromatic compounds from the loaded stripping gas obtained in step (b) by condensation of the steam and/or the aromatic compounds comprised in the loaded stripping gas to obtain an immiscible composition and isolating the aromatic compounds therefrom.

2. The process according to claim 1, wherein the contacting of step (a) occurs at a temperature of 60-250° C. and the aromatic compounds include tar-like components preferably having a boiling point above 150° C.

3. The process according to claim 2, wherein the purified gas obtained in step (a) is further purified by: (a′) contacting the gas with a post-washing liquid, at a temperature of 15-60° C., to obtain the further purified gas and a spent post-washing liquid comprising monocyclic aromatic compounds; (b′) contacting the spent post-washing liquid with a post-stripping gas, to obtain a loaded post-stripping gas comprising aromatic the monocyclic aromatic compounds and a stripped post-washing liquid; and (c′) optionally separating the monocyclic aromatic compounds from the loaded post-stripping gas.

4. The process according to claim 1, wherein the contacting of step (a) occurs at a temperature of 15-60° C. and the aromatic compounds include monocyclic aromatic compounds preferably having a boiling point below 100° C.

5. The process according to claim 4, wherein the gas is subjected to a pre-washing step prior to being subjected to step (a), wherein the pre-washing comprises: (a″) contacting the gas with a pre-washing liquid, at a temperature of 60-250° C., to obtain pre-purified gas which is subjected to step (a) and a spent pre-washing liquid comprising aromatic compounds including tar-like components; (b″) contacting the spent pre-washing liquid with a pre-stripping gas, to obtain a loaded pre-stripping gas comprising the tar-like components and a stripped pre-washing liquid; and (c″) optionally separating the tar-like components from the loaded pre-stripping gas.

6. The process according to claim 3, wherein the post-washing liquid separating or the pre-washing liquid comprises at least 50 vol. % steam and the separating of step (c′) or (c″) is performed by condensation of the steam and/or the aromatic compounds comprised in the loaded post-stripping gas or the loaded pre-stripping gas to obtain an immiscible composition and isolating the aromatic compounds therefrom.

7. The process according to claim 1, wherein in step (c) and optionally (c′) or (c″) the steam and the aromatic compounds are condensed to obtain an immiscible liquid composition and isolating the aromatic compounds by oil/water separation.

8. The process according to claim 1, wherein condensation in step (c) and optionally (c′) or (c″) is accomplished by lowering the temperature and/or increasing the pressure of the loaded stripping gas.

9. The process according to claim 1, wherein the gas comprises the aromatic compounds and one or more components selected from H.sub.2, CO and C.sub.1-4 alkanes, preferably wherein the gas is an energy gas, which preferably originates from the gasification of biomass, waste, coal or a combination thereof.

10. The process according to claim 1, wherein the washing liquid comprises an organic polysiloxane.

11. The process according to claim 1, wherein the stripping gas comprises at least 95 vol. % steam.

12. The process according to claim 1, wherein step (b) is performed at a temperature of 120-220° C.

13. The process according to claim 1, further comprising removing sulphur-containing components from the gas by hydrodesulphurisation or adsorption, preferably by adsorption.

14. A modular system for performing the process according to claim 1, comprising: (a) an absorbing unit comprising a gas inlet (a1) for receiving an gas comprising aromatic compounds, a liquid inlet (a2) for receiving a washing liquid, a gas outlet (a4) for discharging a purified gas and a liquid outlet (a5) for discharging a spent washing liquid; (b) a stripping unit, comprising a liquid inlet (b1) for receiving the spent washing liquid, a gas inlet (b2) for receiving a stripping gas comprising steam, a gas outlet (b3) for discharging a loaded stripping gas and a liquid outlet (b4) for discharging a stripped washing liquid; and (c) a separating module, comprising a gas inlet (c) for receiving a loaded stripping gas, means (c2) for condensing steam and/or aromatic compounds from the loaded stripping gas, an outlet (c3) for discharging a cleared stripping gas, and an outlet (c4) for discharging the aromatic compounds, wherein at least one of outlets (c3) and (c4) is a liquid outlet. wherein outlet (a5) is in fluid connection with inlet (b1), outlet (b3) is in fluid connection with inlet (c1) and preferably outlet (b4) is in fluid connection with inlet (a2) and outlet (c3) is in fluid connection with inlet (b2).

15. The modular system according to claim 14, further comprising: (a′) a post-absorbing unit comprising a gas inlet (a′1) for receiving the purified gas, a liquid inlet (a′2) for receiving a post-washing liquid, a gas outlet (a′4) for discharging a purified gas and a liquid outlet (a′5) for discharging a spent post-washing liquid; (b′) a post-stripping unit, comprising a liquid inlet (b′1) for receiving the spent post-washing liquid, a gas inlet (b′2) for receiving a post-stripping gas, a gas outlet (b′3) for discharging a loaded post-stripping gas and a liquid outlet (b′4) for discharging a stripped post-washing liquid; and preferably (c′) a separating module, comprising a gas inlet (c′1) for receiving a loaded post-stripping gas, means (c′2) for separating the post-stripping gas from the monocyclic aromatic compounds, an outlet (c′3) for discharging a cleared post-stripping gas, and an outlet (c′4) for discharging the monocyclic aromatic compounds. wherein outlet (a4) is in fluid connection with inlet (a′1), outlet (a′5) is in fluid connection with inlet (b′1), outlet (b′3) is preferably in fluid connection with inlet (c′1) outlet (b′4) is preferably in fluid connection with inlet (a′2) and outlet (c′3) is preferably in fluid connection with inlet (b′2); or: (a″) a pre-absorbing unit comprising a gas inlet (a″1) for receiving an gas comprising tar-like components, a liquid inlet (a″2) for receiving a pre-washing liquid, a gas outlet (a″4) for discharging a pre-purified gas comprising monocyclic aromatic compounds and a liquid outlet (a″5) for discharging a spent pre-washing liquid; (b″) a pre-stripping unit, comprising a liquid inlet (b″1) for receiving the spent pre-washing liquid, a gas inlet (b″2) for receiving a pre-stripping gas, a gas outlet (b″3) for discharging a loaded pre-stripping gas and a liquid outlet (b″4) for discharging a stripped pre-washing liquid; and preferably (c″) a separating module, comprising a gas inlet (c″1) for receiving a loaded pre-stripping gas, means (c″2) for separating the pre-stripping gas from the tar-like components, an outlet (c″3) for discharging a cleared pre-stripping gas, and an outlet (c″4) for discharging the monocyclic aromatic compounds. wherein outlet (a″4) is in fluid connection with inlet (a1), outlet (a″5) is in fluid connection with inlet (b″1), outlet (b″3) is preferably in fluid connection with inlet (c″1) outlet (b″4) is preferably in fluid connection with inlet (a″2) and outlet (c″3) is preferably in fluid connection with inlet (b″2).

Description

FIGURES

[0116] FIG. 1 depicts a preferred embodiment of the process and system according to the invention, with reference to the description of the system above and accompanying reference numbers.

[0117] FIG. 2 depicts a preferred embodiment of separation module (c), with reference to the description of the system above and accompanying reference numbers.

EXAMPLES

Example 1

[0118] Beech wood (5 kg/h) was subjected to gasification with 1 kg steam per h in an indirect allothermal biomass gasifier (MILENA), which is coupled to a pre-washing unit (OLGA absorber). A trace amount of argon, which was used as tracker, was added to the gasifier gas, which was fed to the pre-washing unit with an average gas flow of about 15 NI/min, based on dry gas. The partly cleaned gas was led via a cooler (T=5° C.), a safety filter (soxhlet filter) and glass beads to an absorber (BTEX scrubber). The absorber of the BTEX scrubber operated at 35° C. and ambient pressure with polysiloxane as a washing liquid. The loaded absorbent was stripped at 120° C. using 205-820 g/h steam. Loaded stripping gas was led to condensers via tubes heated at 120-160° C. The first condenser operated at 25-27° C. and the second condenser at 4-5° C. Liquids were collected from the first condenser, while remaining gases were led to the second condenser. Liquids were collected from the second condenser. The scrubbed gas was subjected to HDS and subsequently steam reforming to obtain a bio-SNG. The experiment was ran continuously for 75 h at ambient pressure.

[0119] Collection of liquids occurred by first collecting the aqueous layer by opening a tap at the bottom of the collection flask. Collection of the aqueous layer was stopped just prior to the meniscus reached the tap. An as small as possible mixed fraction was collected and discarded, after which the organic layer was completely drained in a separate flask. A total of 1.17 kg of organic layer was collected (885 g from the first condenser at 26° C. and 285 g from the second condenser at 5° C.) over the complete duration of the experiment, of which 86.6 wt % benzene, 6.5 wt % toluene and 0.20 wt % xylene. A detailed compositional analysis of the combined organic layers is given in Table 1. The average compositions of the gas flows over the complete duration of the experiment, as determined by micro-G, are given in Table 2.

TABLE-US-00001 TABLE 1 Composition of the combined organic layers First condenser Second condenser Compound (26° C.) (4-5° C.) Total total (g) 884.5   284.7   1169.2   total BTEX (g) 826.2   266.9   1093.0   benzene (g, wt %) 753.6 (85.2 wt %) 258.9 (90.9 wt %) 1012.5 (86.6 wt %) toluene (g, wt %) 68.7 (7.77 wt %) 7.77 (2.73 wt %) 76.50 (6.54 wt %) xylene (g, wt %) 2.26 (0.26 wt %) 0.06 (0.02 wt %) 2.32 (0.20 wt %) ethylbenzene (g, wt %) 1.66 (0.19 wt %) 0.07 (0.02 wt %) 1.74 (0.15 wt %) styrene (wt %) 1.48 0.07 1.14 cresol (wt %) 0.37 0.00 0.28 naphthalene (wt %) 0.75 0.00 0.57 further aromatic 0.82 0.39 0.72 compounds (wt %) thiophene (wt %) 0.12 0.13 0.12 water (wt %) 0.07 0.67 0.22

TABLE-US-00002 TABLE 2 Composition of the gas flows (based on dry weight) Gasifier pre-washed purified Component gas .sup.[a] gas .sup.[b] gas .sup.[c] inert (vol %) .sup.[d] 3.7 4.8 5.4 CH.sub.4 (vol %) 11.3 10.9 10.5 CO (vol %) 32.0 28.9 23.7 CO.sub.2 (vol %) 25.3 25.0 27.1 C.sub.2 (vol %) .sup.[e] 3.9 3.9 3.0 H.sub.2S (ppmV) 100 151 88 COS (ppmV) 3 5 0 benzene (ppmV) 8131 4680 271 toluene (ppmV) 596 261 0 thiophene (ppmV) 20 17 0.9 tar (mg/Nm.sup.3) 17565 680 61 H.sub.2 (vol %) .sup.[f] 22.9 26.0 30.0 .sup.[a] gas emerging from the gasifier, prior to being subjected to pre-washing; .sup.[b] gas emerging from the OLGA absorber, prior to being subjected to the BTEX scrubber; .sup.[c] gas emerging from the BTEX scrubber; .sup.[d] Ar + N.sub.2; .sup.[e] ethane + ethylene + acetylene; .sup.[f] H.sub.2 content estimated, based on total volume of 100 vol %.

[0120] Tar-like components were mainly removed in the pre-washing step, while monocyclic aromatic compounds, such as benzene, toluene, ethylbenzene and even thiophene, were largely maintained in the permanent gas stream. The BTEX scrubber effectively removed the monocyclic aromatic components. The benzene concentration in the partly cleaned gas emerging from the OLGA absorber was between 4000 and 7000 ppm (vol.), which was lowered to 300 ppm (vol.) in the gas stream emerging from the BTEX-scrubber. The average removal of benzene amounted to 95% using a steam flow of 820 g/h, which reduced to 89% and 87% at a gas flow of 410 g/h and 205 g/h respectively. 100% of the toluene was removed at all gas flows. Only trace amounts of tar-like components were obtained in the organic layers obtained in the first and second condensers.

[0121] The composition of the gas was analyzed prior to and after the condensers, and the removal percentages obtained during the BTEX scrub of a variety of compounds are given in Table 3. The aromatic compounds benzene, toluene, xylene and thiophene (and its derivatives) were effectively removed during the BTEX scrub, wherein generally the highest steam flow provided the highest removal. At the same time, permanent gases such as C.sub.1-C.sub.3 hydrocarbons (alkanes and alkenes) are effectively retained in the gas stream. Especially methane, CO and CO.sub.2 are completely retained. Any transport thereof to the stripping gas is cancelled when the permanent gases are recycled to the entrance of the BTEX scrubber. The content of the permanent gases in the fuel gas (at the entrance of the BTEX scrubber) and in the permanent gas stream after stripping (downstream of the second condenser) are given in Table 4. In view of its very small volume, nitrogen gas was added to the permanent gas stream to enable measure of its contents. The amount of these permanent gases that were transported to the stripping gas is also included in Table 4. The permanent gas stream further contained 4.6 vol % benzene (based on the permanent gas stream without added nitrogen).

TABLE-US-00003 TABLE 3 Removal percentages for certain compounds steam flow during stripping Compound 205 g/h 410 g/h 820 g/h Aromatic compounds benzene 87% 89% 95% toluene 93% 92% 100%  xylene 100%  100%  100%  Sulphur components thiophene 94% 91% 96% 2-methyl-thiophene .sup.[b] 100%  100%  100%  3-methyl-thiophene .sup.[b] 100%  100%  100%  COS  6%  0% 14% methyl mercaptan 67% 68% 60% ethyl mercaptan 76% 78% 77% [a] nd = not determined .sup.[b] no methyl-thiophenes were detected after the BTEX-scrub.

TABLE-US-00004 TABLE 4 Experimentally determined transport values of permanent gases Permanent gases In fuel gas In strip gas transported methane (vol %) 11.00 0.13 1.18 ethane (vol %) 0.20 0.0094 4.70 ethene (vol %) 3.10 0.168 5.43 ethyne (vol %) 0.155 0.0185 11.94 CO.sub.2 (vol %) 25.00 0.72 2.88 CO (vol %) 28.00 0.07 0.25 H.sub.2 (vol %) 26.00 0.00 0.00 H.sub.2S (ppmV) 150.00 63 42.00 COS (ppmV) 5.00 0.00 0.00

[0122] The results in Table 3 show that the BTEX-scrub, i.e. step (a) of the process according to the invention, effectively removes a fraction of aromatic compounds, that contains almost exclusively BTEX. The BTEX fraction can be used as deemed fit, e.g. marketed as bio-based BTEX or the like and the energy gas is sufficiently purified from tar-like components and aromatic components by virtue of the combined pre-washing and BTEX-scrub such that conversion into bio-SNG is readily accomplished.

Example 2

[0123] Refuse-derived fuel (RDF) (3.9 kg/h) was subjected to gasification with 2 kg steam per h in an indirect allothermal biomass gasifier (MILENA), which is coupled to a pre-washing unit (OLGA absorber). Nitrogen gas was added to the steam flow to maintain sufficient gas flow throughout the system. The gasifier gas, which was fed to the pre-washing unit with an average gas flow of about 15 NI/min, based on dry gas. The partly cleaned gas was led via a cooler (T=5° C.), a safety filter (glass beads) and a pre-washing unit to remove tars (washing liquid=polymethylphenylsiloxane; T=80° C., ambient pressure) to an absorber (BTEX scrubber). The absorber of the BTEX scrubber operated at 35° C. and ambient pressure with polymethylphenylsiloxane as a washing liquid. The loaded absorbent was stripped at 160° C. using 0.25 m.sup.3/h steam. Loaded stripping gas was led to condensers via tubes heated at 160° C. The first condenser operated at 25-27° C. and the second condenser at 4-5° C. Liquids were collected from the first condenser, while remaining gases were led to the second condenser. Liquids were collected from the second condenser. A fraction of monocyclic aromatic compounds comprising benzene, toluene and xylene was collected. The experiment was ran continuously for 3.5 h at ambient pressure. The average compositions of the gas flows over the complete duration of the experiment, as determined by micro-GC are given in Table 5.

TABLE-US-00005 TABLE 5 Composition of the gas flows Gasifier pre-washed purified Component gas .sup.[a] gas .sup.[b] gas .sup.[c] inert (vol %) .sup.[d] 51.7 53.8 53.4 CH.sub.4 (vol %) 7.8 7.7 7.5 CO (vol %) 9.0 9.0 9.0 CO.sub.2 (vol %) 11.4 12.2 11.2 C.sub.2 (vol %) .sup.[e] 7.4 7.8 7.2 H.sub.2S (ppmV) 524 633 449 COS (ppmV) 9 16 0 benzene (ppmV) 9853 9135 302 toluene (ppmV) 1490 1269 35 thiophene (ppmV) 43 29 nd .sup.[f] tar (mg/Nm.sup.3) 38373 809 61 H.sub.2 (vol %) .sup.[g] 8.4 8.4 8.4 .sup.[a] gas emerging from the gasifier, prior to being subjected to pre-washing; .sup.[b] gas emerging from the OLGA absorber, prior to being subjected to the BTEX scrubber; .sup.[c] gas emerging from the BTEX scrubber; .sup.[d] Ar + N.sub.2; .sup.[e] ethane + ethylene + acetylene; .sup.[f] not determined; .sup.[g] H.sub.2 content estimated, based on total volume of 100 vol %.