NL8800525A - Desulphurisation of oxygen-contg. gas streams - by hydrogenating oxygen and sulphur di:oxide, converting organic sulphur cpds. into hydrogen sulphide and converting to sulphur - Google Patents

Abstract

Process for purifying a gas stream contg. S-contg. cpds. as well as O2 and opt. SO2, comprises carrying out at least the following stages: (1) hydrogenating the O2 and opt. SO2 in the gas stream; (2) converting the organic S cpds. in the gas stream into H2S and (3) converting the H2S into elementary sulphur. The gas stream may be washed with water prior to stages (1), (2) and (3) to condition the gas stream to pref. moisture content and to suppress the undesired CO-shift side reaction.

Description

* ï N.0. 34989 1 i

Short designation: Method for purifying a gas stream.

The invention relates to a process for purifying a gas stream containing sulfur-containing compounds as well as oxygen and optionally sulfur dioxide.

In the art, desulfurization of gases is used to remove SO 2, H 2 S or organic sulfur compounds from waste gases (product gases) or starting gases (synthesis gases). In the case of waste gas, the removal of sulfur compounds is generally carried out for environmental reasons, in the case of synthesis gases the protection of used catalysts is often important.

The well-known Claus process is used for the desulfurization of waste gases. The Claus reaction proceeds as follows: H2S + 1.5 02 H20 + S02 2 H2S + S02 —- 3 S * 2. H20 20 The gross reaction equation is: 2 H2S + 02 ~ 7 2 S + 2 H20.

The other sulfur-containing components present in the waste gas can be converted into H2S according to known methods.

Sulfur can then be recovered from this product according to the Claus reaction mentioned above.

For example, "The Oil and Gas Journal" March 12, 197S, pp. 76-80 describes a process for recovering sulfur from gas containing sulfur-containing compounds. In this process, two sections are used: in a first section, the starting gas is hydrogenated to convert SO 2 and elemental sulfur to H 2 S and then hydrolysed to convert COS and CS 2 to H 2 S. A single catalyst is used for both hydrogenation and hydrolysis, namely a cobalt molybdenum catalyst from Union Oil designated N-25. In a second section, the gas from the first section, in which the organic sulfur compounds are substantially converted to H 2 S, is oxidized with oxygen from the air to form elemental sulfur, according to the Claus reaction.

The so-called Selectox-32 catalyst, 40 also from Union Oil, is used for the latter conversion.

.8800525 * 2 *

An improvement of the above-described process is achieved by using an additional catalyst referred to as G41P in the hydrogenation / hydrolysis step in addition to the cobalt-molybdenum catalyst (cf. Gil & Gas Journal, 4 July 5 1986, pp. 54-57 According to a brochure from the company Süd-Chemie AG, the catalyst G41P consists of 10 wt.% Cr 2 O 3 on AI 2 O 3, which also contains a promoter. In this brochure, G41P is particularly recommended for the selective hydrolysis reaction of COS. The catalyst also appears to play a role in the conversion of CS2, for example, according to an example given in this brochure, where a gas consisting of 97% by volume of CO and furthermore of minor amounts of CO2, H2, N2, H2S, COS and CS2s treated with the catalyst at a pressure of 8 bar and a temperature of 150-200 ° C achieve a significant removal of COS and CS2.

15 Hydrocarbon Processing, April 1978, pp. 99-103 discusses the influence of some catalysts on the following conversions: H2S + SO2 3 S + 2 H20 20 2 COS + S02 3 S + 2 C02

The catalysts are referred to as S-201 and S-501. In this publication, it is stated that the presence of significant amounts of oxygen in a raw material stream subjected to a Claus reaction is undesirable. The oxygen causes deactivation of the catalyst by sulfate formation at a relatively low temperature of 200 ° C. It is also reported that even low oxygen concentrations of only a few ppm after a long period will poison the catalyst.

The influence of oxygen on catalytic conversions of the above type is also reported in Hydrocarbon Processing, November 1982, pp. 189-19J. As in the aforementioned publication, the solution of the oxygen problem is sought in the use of a catalyst which is less sensitive to oxygen.

A process as mentioned in the preamble has now been found, wherein at least the following steps are carried out: 1) hydrogenation of the oxygen present in the gas stream and optionally sulfur dioxide, 2) conversion of the organic sulfur compounds present in the gas stream in hydrogen sulfide and .8800525 3 * 3) conversion of hydrogen sulfide to elemental sulfur.

The method according to the invention has the advantage that oxygen and sulfur dioxide are removed from a gas stream in a relatively simple manner (step 1), which gases, in particular oxygen, can be disturbing in the conversions according to steps 2 and 3. If at The conversion of steps 2 and 3 to a catalyst is found to have a much longer service life in the process according to the invention than in the known processes.

It is advantageous in the process according to the invention if the gas stream is washed with water prior to steps] - 3. The gas is hereby conditioned, that is to say that the water content in the gas can be regulated in the desired manner. A consequence of this is also that an undesired side reaction (the so-called CO-shift reaction) can be suppressed or at least controlled. Moreover, in the case of a hot gas flow, a cooling thereof is effected.

The various steps of the process according to the invention are preferably carried out in the presence of a catalyst. For example, in step 1, a cobalt-molybdenum or platinum-containing catalyst with aluminum oxide as support is preferably used. In step 2, chromium trioxide is preferably used on alumina or titanium dioxide.

The conversion of step 3 is preferably carried out according to a direct oxidation process, for instance according to the per se known LoCat, Stretford, Takahax, Sulferox or Sulfolin process.

In the process according to the invention, an output gas stream can be desulphurized almost quantitatively. In any case, a removal efficiency of at least 95% can be achieved.

The method according to the invention can be carried out with favorable results with gas flows, the oxygen content of which is small, that is to say no greater than a few percent by volume, for instance a maximum of 1.5 vol. %. The limiting factor here is the temperature rise which occurs as a result of the hydrogenation of oxygen during the first step of the process according to the invention. In connection with the equipment to be used (mechanical construction) and in order to prevent undesired side reactions, for example methanation, it is desirable to keep the temperature during the first step at least below 450 ° C.

Also the water content in the gas flow may not be too high because of the occurrence of the CO-shift reaction. It has been found that .8800525 β 4 * reduces the efficiency of the process according to the invention if the water content in the gas flow rises above a certain value. The hydrolysis reaction in step 2 can be controlled by adding water or steam after step 1.

The process according to the invention is eminently suitable for desulphurizing a fuel gas, which is produced in the preparation of silicon carbide, for example according to the so-called Acheson process. The known desulfurization techniques are generally not readily applicable to the fuel gas which is released during the production of silicon carbide. This gas has a specific composition, which requires a special approach. As already mentioned, the oxygen content in this gas is an important disturbing factor. It is an important aspect of the method according to the present invention that gas with a relatively low temperature and pressure can also be treated efficiently.

The temperature in the preparation of silicon carbide from silicon dioxide and carbon, often in the form of petroleum coke, is high, for example about 2500 ° C. The fuel gas, which initially has a corresponding temperature, is cooled to a value below 100 ° C, for example 30 - 70 ° C.

As already mentioned above, the cooling of the fuel gas can be carried out by washing with water. As a result, the cooled fuel gas will be saturated with water. The fuel gas then generally contains the following components:

25 H2 20 - 50 vol. Z.

CO2 5-30 vol. % GO 20 - 60 vol. % N2 0-4 vol. % 02 0.1 - 1.5 vol. %

30 SO2 0 - 0.3 vol. Z.

CH4 3 - 10 vol. Z.

H2S 0.5-3 vol. Z.

COS 0 - 0.3 vol. Z.

CS2 0 - 0.2 vol. Z.

NH3 0-700 ppm HCN traces

Higher hydrocarbons traces

Polycyclic aromatics Traces of dust (very fine particles) Traces 40 .8800525 * * 5 *

It is noted that it is also possible that the fuel gas (temporarily) does not contain any oxygen at all. .

The pressure of the gas stream, which can be treated by the method of the invention, is not critical and is, for example, slightly higher than atmospheric pressure up to about 1.4 bar.

The invention is further elucidated on the basis of an embodiment, as shown in the accompanying figure.

According to this embodiment, a gas stream containing sulfur compounds and originating from the preparation of silicon carbide is supplied via line 1 to a washing device 2 in which the gas is washed by spraying with water. The washing water is discharged by means of a pump 3 at the bottom of the washing column 2 and cooled in an air cooler 4 and returned to the washing device 2.

A valve 6 is operated by means of a control device 5 and washing water 15 can be discharged at 7. For unsaturated gas, water may need to be added.

The gas saturated with water vapor is introduced via line 8 into a water separator 9. The water droplets still present are removed therein and water is discharged at 10. Subsequently, the gas is brought via line 11 into device 12 and pressurized there, and then brought via line 33 into device 14 in which it is heated with a steam, supplied via line 15, at a temperature of 150-220 ° C. The hot gas stream is then introduced via line 36 into a reactor 17, which contains a CoMo catalyst. Here, the oxygen present is converted into water and the water present is converted into hydrogen and carbon dioxide via the so-called CO2 shift. In addition, any SO2 present is converted into H2S. The conversions that take place in the reactor 17 cause a temperature rise of about 75-300 ° C, depending on the composition of the gas flow.

The gas stream leaving the reactor 17 via line 38 is supplied with steam or water at 19. The water can also serve to cool the gas flow. The entering temperature at the hydrolysis reactor 20 is generally about 225-450 ° C. In the reactor 20, COS and CS2 are hydrolysed, i.e. converted into H2S. Use is made of a per se known catalyst such as G41P or CRS31. After the hydrolysis, the gas stream is cooled via line 21 in an air cooler 22 to a temperature of 40 - 50 ° C. After this cooling, the gas is introduced via line 23 into a water separator 24, in which the excess water is removed via outlet 25. The (partially) 40 dewatered gas stream is led via 26 to a direct oxidation process, which is known per se and is indicated in the figure by reference numeral 27. The gas stream is thereby desulphurised substantially quantitatively. clean gas, indicated by reference numeral 28 and elemental sulfur, indicated by reference numeral 29.

The invention is further illustrated by the tests described below.

Test with a hydrolyzing Ti09 catalyst and a gas stream from the SiC production.

A TiO2 catalyst was tested for approximately 250 hours at the following process conditions:

Gas flow rate:: approx. 6.5 mm / h 35 Quantity of catalyst: 5 kg

Ttop I: 225-350 ° C

02 “content: approx. 0.4%

Water content: 4 7 vol. % 20 During this test, the process variables Tbottom, Ttop I, Ttop II, flow, gauge level and (^ percentage) were kept every hour (T = temperature).

The incoming and outgoing gas flows from the hydrolysis column were automatically sampled continuously every hour. An analysis of the concentration of COS, CS2, and H2S was performed on a gas chromatograph

in the incoming and outgoing flow, respectively. The measured values are the concentrations on a dry basis.

Measuring conditions: Column: Porapak Q 80/300 mesh (treated) 30 ------------- 2 m x 1.4 ”glass

Carrier gas: He 20 ml / min

Oven temp. : 100 ° C (5 min.) 30 ° C / min 200eC

(10 min)

Detector temp. (TPD): 250 ° C 35 Detector temp. (TCD): 200 ° C

COS conversion results

The COS conversion across the column varies between 70% and 92%. The average input concentration is about 3800 ppm (full). The variation in the 40 conversion is directly proportional to the temperature in the .8800525 catalyst bed. At 225 ° C, the conversion is about 70%. At a catalyst temperature above 300 ° C, the conversion varies between 88% and 92%.

An increase in the 0 £ concentration in the process gas causes a decrease in the conversion of COS. This in spite of the associated temperature increase in the catalyst bed as a result of the reaction: 2 H + O2 H2O. At an O2 concentration of 0.4%, the conversion is about 90% (catalyst temperature about 300 eC). An increase of the O2 concentration to about 1.2 X gives a conversion of 80% (catalyst temperature 400 ° C). A subsequent decrease in the O2 concentration in the process gas increases the COS conversion again to roughly the original level of 90% (catalyst temperature 320 ° C).

The conversion of CS2 into the process gas is directly proportional to the catalyst temperature. At a catalyst temperature of 275 ° C, the conversion is about 85% and at 350 ° C it is about 95% (initial concentration of CS2 avg: 1000 ppm). Here too, a negative influence of an increase in the O2 concentration on the conversion CS2 is noticeable. When the O2 concentration increases to approx.

1.2%, the conversion CSj is about 75%. The conversion increases again (92 X) when the O2 concentration is reduced to 0.4%. This conversion roughly corresponds to the initial situation.

Sulfur formation

A sulfur balance has been used to determine whether elemental sulfur forms. It appears that roughly 0 - 1500 ppm vol 2 wavel is created at an O2 content of approx. 0.4%. When the oxygen content in the process gas is increased to about 1.2%, sulfur formation appears to increase to 2500 ppm. It has been found that sulfur formation has no deactivating effect on the TiO2 catalyst.

30

Analysis catalyst

The carbon dioxide and sulfur content of the TiO2 catalyst was analyzed after a run time of 250 hours.

C: 2.5 wt. %

35 S: 2.0 wt. X

The color of the catalyst changed from off-white to black. Conclusions

No deterioration in the activity of the catalyst can be observed after a run time of 250 hours. Oxygen has a negative 880 05 2.5 * 8 * but reversible influence on activity. A minimum catalyst temperature of 300 ° C is required for a conversion of 90% COS. It turns out that the relationship between COS conversion and the catalyst temperature does not correspond to that reported in the literature. It speaks of an inverse proportional relationship.

Test with a hydrogenating CoMo catalyst and a gas stream from the SiC production.

10 Trial 1

A CoMo catalyst was tested for 350 hours under the following conditions:

Gas flow rate: 8 m ^ / h 15 Quantity of catalyst: approx. 7 kg.

Bed height: 0.8 m O2 content: approx. 0.2%

During this test, the process variables such as Tbottom, 20 Tmiddle, Ttop I, Ttop II, flow, gauge level and O2 content are monitored every hour.

Every hour the incoming and outgoing process gas flows were automatically sampled and analyzed for the concentrations of H2, CO2, H2S, O2, N2> ch4 and CO using e.g. the gas chromatograph.

25

Measuring conditions:

Column: Porapak QS 80/100 mesh 9 'x 1/8 "x 2 mm SS + molsive 5A, 60/80 mesh 6 x 1/8" SS 30 Oven temp. : 70 ° C isotherm Detector: TCD 150 ° C Carrier gas: He, 20 ml / min.

Results of trial I

The temperature increase over the catalyst bed due to the shift reaction is about 85 ° C at an average inlet temperature of 215 ° C. The water temperature (Tbottom) is approx. 40 ° C. The corresponding concentration of H2O (g) is about 7% (40 ° C; 1.05 bar).

There is a conversion of CO for approx. 15%, 40 based on an average input concentration of CO of 32.4 vo. %.

. 8800525 9.

The catalyst is able to remove all O2 present from the process gas. The temperature gradient over the catalyst bed increases. A Z * T of 160 ° C was measured at an O2 concentration of 1.5. Under normal conditions the O2 content in the process gas is approximately 0.2%.

After a run time of 75 hours, sudden violent steam formation caused the catalyst to emit via the flare. It is estimated that about half of the CoMo catalyst was lost as a result. The conversion of CO via the shift reaction decreased to less than 5 P, with the temperature gradient over the catalyst bed not exceeding 60 ° C.

In this test, partial conversion of COS to H2S was also found to take place (conversion + _ 15%). The conversion of CS2 could not be determined in this test.

15

Analysis catalyst

The COM0 catalyst has been analyzed for the occurrence of carbon and sulfur 1: C: 2.0 wt. % 20 S: 3.0 wt. %

The color of the catalyst changed from blue to black.

Results of trial 2

The catalyst of Run I was reused for Run 2 and supplemented carbon black "fresh" catalyst.

Catalyst quantity: approx. 5 kg Gas flow rate: approx. 8 m ^ / h

The recording of the process variables and the analysis of the process gas 30 is the same as for test I.

Tested for 108 hours at an average input temperature of 181 ° C. The temperature rise of the gas over the catalyst bed is about 80 ° C at an O2 concentration of 0.6 vol. On average. %. The conversion CO is on average 14% at an average input-35 CO concentration of 32.6 vol. % and a water content of about 5% by volume (33 ° C; 1.05 bar). The temperature rise over the catalyst bed also appears to depend on the O2 content in this test.

It could not be demonstrated that there may still be O2 in the column effluent.

40 .8800525

Claims (6)

1. Process for purifying a gas stream containing sulfur-containing compounds as well as oxygen and optionally sulfur dioxide, wherein at least the following steps are carried out:]) hydrogenation of the oxygen present in the gas stream and optionally sulfur dioxide, 2. conversion of the organic sulfur compounds present in the gas stream in hydrogen sulfide and 3) conversion of hydrogen sulfide into elemental sulfur. 2. Process according to claim], characterized in that the gas stream is washed with water prior to steps] -3. 3. Process according to claim J or 2, characterized in that step i is carried out using a hydrogenating catalyst such as cobalt / molybdenum on alumina, 4. Process according to any one of claims 1 to 3, characterized in that step 2 is carried out with a hydrolysing catalyst such as chromium trioxide on alumina or titanium dioxide. 5. Process according to any one of claims J - 4, characterized in that the heat generated in step I is used in the conversion of step 2 and / or 3 or in step 1 itself. 6. Method or device, as indicated in the description. ****** .8000525