DE2702583A1 - Sulphur dioxide removal from residual gases - by absorbing with a buffered aq. soln. of organic acids - Google Patents

Abstract

is removed from residual gases or outgoing air, to concns. 0.1 vol.%, by absorption with a buffered aq. soln. of organic acids, with subsequent regeneration of the soln. by heat treatment. The gases are washed in >=1 scrubbers at =50 C. with aq. dicarboxylic acid soln. buffered to pH 2.8-9, this soln. having a concn. representing 10-95% of the saturation concn. The SO2 dissolved is removed from the washing soln. by heating between is approx. 50 degrees C. and the b.pt. of the soln. The soln. is then cooled to 0-50 degrees C. and reused in a closed circuit for SO2 absorption. Gases contg. not only a low SO2 content but those with high SO2 contents, e.g. >20 vol.% can be purified. Pure SO2 can be obtd. and used in the prodn. of H2SO4.

Description

Process for removing sulfur dioxide from exhaust air

(Addition to patent application P 26 23 963.4) Subject of the present The invention is a further embodiment of the method for removing sulfur dioxide from exhaust air according to patent application P 26 23 963.4.

The patent application P 26 23 963.4 relates to a method for removal of sulfur dioxide from exhaust air or exhaust gas and the recovery of the sulfur dioxide in concentrated form by expelling from an absorbent cycle of aqueous dicarboxylic acid solutions There are known processes in which the removal of sulfur dioxide from exhaust air or exhaust gas flows is described. The 502 falls but at such low concentrations that direct recovery or Further processing into useful chemicals is not possible. The amounts of sulfur-containing Substances that arise as a result of these exhaust air purification processes are getting bigger and bigger and it is not always possible to find an appropriate use for this To find substances, since practically only pure so2, H2So4 or elemental sulfur are good can be recycled. Procedures in which the Sulfur dioxide is converted into no longer usable materials, appear because of the landfill and / or sewage problems that cause such substances are problematic as a permanent solution. The process in which the mentioned useful chemicals from the waste sulfur dioxide are won, therefore win for both economic and environmental reasons ever greater importance. Particularly interesting are those procedures in which only small amounts of additional chemicals are consumed, which means few waste materials deliver and use little energy.

One - in this sense advantageous - process works with aqueous Solutions of organic substances, (see DRP 606 447, 645 879, 660 286, 621 760, 623 018, US-PS 2,295,587, 2,399,013 and GB-PS 371,888) show a strong temperature dependence of the S02 partial pressure above the solution. With such absorption solutions the sulfur dioxide from the exhaust gas to be cleaned, usually in countercurrent, is 0 at low temperatures, for example 20 C, washed out.

The solution loaded with sulfur dioxide is then heated, e.g. to approx. 100 ° C, whereby sulfur dioxide and water vapor escape. By cooling the S02-water vapor mixture one obtains concentrated sulfur dioxide with less than 5 vol% water vapor, that can be further processed, e.g.

on sulfuric acid. The largely sulfur dioxide-free after the abortion Solution is cooled down and then recycled again to wash out the items to be cleaned Exhaust air used. The selection of suitable organic substances for this process is limited by the requirement of a) vanishingly small internal vapor pressure, so that the exhaust gases to be cleaned are not additionally loaded with organic substances and to keep the chemical losses small; b) large absorption capacity the washing solution; c) Sufficiently low SO2 partial pressure of the regenerated solution after cooling to meet the requirement for a low residual load on 502 of the Exhaust air to meet; d) chemical and thermal resistance, as well as lower Formation of undesirable substances during absorption or regeneration; e) economy and availability in large quantities.

The one proposed, for example, in GB patent specification 400 998, with sodium hydroxide solution Buffered aqueous solution has the economic disadvantage of using it of citric acid and a noticeable proportion of the absorption in the meantime The sulfite formed is oxidized to sulfate. Because of this sulphate formation must be constant Sodium hydroxide solution to maintain the pH value required for sufficient S02 absorption must be replenished, in addition, the sulfate must be removed from the solution.

Recently, a modified citrate process (S.Vasan, Chemical Engineering Program, 71 (1975) 5, pp. 61-65) has found interest in which the sulfur dioxide from the absorption solution, an aqueous, buffered citric acid solution, is not heated driven off, but by the subsequent introduction of hydrogen sulfide (xstoff in this solution according to the Claus reaction: is converted to elemental sulfur. This sulfur is then filtered off from the solution, the effort required to separate the solids being significant and subsequently drying.If no hydrogen sulfide is available, it is proposed to add this by reducing part of the sulfur filtered off, e.g. with methane produce. Even with this modified citrate process, the relatively high oxidation rate of 0.5 to 1% sulphate accumulation, based on the sulfur dioxide absorbed, is a problem.

In the method according to the invention, gases containing SO2 are passed through Absorption in a buffered, aqueous solution of organic acids and subsequent Expulsion cleaned by thermal treatment or concentrated on SO2 or worked up to a S02.

The subject of the invention according to application P 26 23 963.4 is a method for the removal of SO, from exhaust air or exhaust gases to concentrations below 0.1% by volume by absorption in a buffered, aqueous solution of organic acids with subsequent Regeneration of the absorption solution through thermal treatment, which thereby is characterized in that a) the gases in one or more scrubbers at temperatures around 0 to 500C with an aqueous to a pH range of about 2.8 to 9, buffered Dicarboxylic acid solution, which has a concentration of about 10 to 95% of the saturation concentration possesses, are washed, b) then the dissolved SO from the washing solution, at Temperatures between about 500C and the boiling point of the washing solution is driven off and c) the washing solution regenerated in this way after cooling to temperatures around 0 up to 500C for renewed sulfur dioxide absorption in the circuit.

In a further embodiment of the method according to application P 26 23 963.4 it was found that not only SO2-containing exhaust gases with low SO2 contents can be cleaned, but also gases that have a high SO2 content. Furthermore, such gases can be processed to pure SO2 with the present process will.

The present invention is therefore a method for Removal of sulfur dioxide from gases containing sulfur dioxide to concentrations less than 0.1% by volume through absorption in a buffered, aqueous solution of organic Acids with subsequent regeneration of the absorption solution by thermal treatment in several stages, with a) the gases in one or more washers at tempera-0 tures to about 0 to 50 C with an aqueous to a pH range of about 2.8 to 9 buffered dicarboxylic acid solution, which has a concentration of about 10 - 95 9 ' the saturation concentration has, be washed, b) then from the washing solution the dissolved sulfur dioxide at temperatures between about 50 C and the boiling point the washing solution is driven off and c) the washing solution regenerated in this way after cooling to temperatures around 0 to 500C for renewed sulfur dioxide absorption in the circuit is returned, according to patent application P 26 23 963.4, which is characterized is that such gases, which contain more than 20 vol. 9 'sulfur dioxide, a treatment are subjected to stages a - c.

As readily water-soluble dicarboxylic acids such. B. phthalic acid, maleic acid, Malonic acid and glutaric acid used individually or in a mixture as an absorption solution.

With the help of this process, the final content of SO2-containing Gases, also of fluctuating composition, continuously between about 0.02 and 0.1% by volume being held. The washing acid in the scrubber, e.g. B. a countercurrent column is on pH values buffered from the acidic to the weakly basic range, usually to a pH range of about 2.8 to 9. The pH range of 3.5 to is preferred 6.5. When buffering to pH values above 7, small amounts are formed of salts, which are removed when part of the absorption solution is discharged can be.

The buffering takes place with alkalis, whereby for this all alkaline reacting Carbonates and hydroxides can be used in dissolved or solid form. the end For reasons of economy, however, sodium and / or potassium hydroxides should preferably be used or carbonates can be used.

The dicarboxylic acid solution has a concentration between about 10 and 95 8 the saturation concentration at the temperatures used, preferably between 40 and 90%, very particularly preferably around 75%. The washing solution has temperatures between 0 and SOOC, preferably between 20 and 250C. As dicarboxylic acids are Phthalic acid, maleic acid, malonic acid, or glutaric acid, individually or in any Mixtures used. So-called undistilled glutaric acid is preferred a mixture of about 40-60% glutaric acid, 15-30% adipic acid and about 20-40 % Succinic acid is used. The dissolved sulfur dioxide is increased at Temperatures, d. H. Above about 500C from the solution. The upper applicable Temperature depends on the composition of the solution, its boiling point should be however do not exceed. Temperatures around 85 to 1200C are preferred applied.

Of course, it is also possible to have the abortion reduced To carry out pressure, which of course the specified temperatures accordingly change.

The solution is to keep the process energy as low as possible preheated in upstream heat exchangers before the sulfur dioxide gas in is driven out by an abortionist. As an abortionist as well as a washer the usual and known apparatus can be used. Of course it is with that Method according to the invention also possible, not just one scrubber and one stripper, but to operate several in parallel as well as one after the other. To the To improve the expulsion, preference is given to steam or inert gas from below fed into a countercurrent apparatus. Water vapor, for example, is used for this in a Amount of approx. 0.01 to 0.4 kg / 1 solution to be desorbed, preferably from 0.1 to 0.3 kg / l, particularly preferably around 0.2 kg / l. The washing solution has after leaving of the desorber has a residual sulfur dioxide content of about 2 to 40 g 502/1, preferably about 10 to 20 g of 502/1 and is used as a regenerated washing solution again in the absorption apparatus fed into the circuit for renewed sulfur dioxide loading. If necessary, this must the pH value can be corrected by adding alkali, accordingly z. B. a possible, slight sulphate formation. This is also where applicable the washing solution can be refreshed with fresh washing solution.

In the meantime, they oxidize in the absorption solution The sulfite formed by absorption is unavoidable even with dicarboxylic acids as detergents small amounts to sulfate. In order to avoid this oxidation as much as possible the concentration of visual metal ions, such as B. iron, copper, cobalt, which the Catalyze oxidation, kept low, preferably around or below minor. For this reason, equipment and lines made of non-metallic materials are also preferred Materials, e.g. B.

Plastic or glass and heat exchange surfaces preferably made of graphite, Titanium or tantalum is used. Furthermore, all known means can be added to the solution z. B. glue or chelates are added, which the free concentration of heavy metal ions humiliate.

Surprisingly, this process is not only suitable for cleaning of SO2-containing exhaust gases, which are usually between 0.1 and a maximum of 20 96 sulfur dioxide but also for cleaning gases with a higher proportion of SO2. The process is also used to concentrate SO2 or to work it up pure SO2.

That obtained in pure form with the aid of the method according to the invention S02 can then be supplied to the intended use, e.g. B. as a starting gas or mixed gas in sulfuric acid production or liquefied in pure form will. Such higher percentage sulfur dioxide-containing gases occur in the most diverse chemical processes in industrial processes and combustion processes.

Thus, according to the method according to the invention, for. B. Roasting gases, the with the burning of sulfur as a heat source, processed to pure SO will. Higher percentage gases also fall with reddening with oxygen-enriched ones Air. The gases produced in desulphurisation systems can also, after a Any existing H2S content transferred to 802 by suitable incineration was concentrated according to the present process or worked up to pure SO will. Other gases with a high concentration of SO2 fall e.g. B. in the case of oxygen-enriched Roasting of calcined sulphates, e.g. B. FeSO4, on or during the decomposition of sulphites, which are obtained from waste gas scrubbing and can, if necessary after gas scrubbing, z. B. with sulfuric acid, worked up.

In general, gases with a high concentration of S02 can have a content of SO, from over 20% by volume to well over 90 9 'according to the present process Pure S02 can be processed, also let gases with fluctuating contents of S02 to work up without further ado.

The concentration of the circulating absorption solution can, such as already mentioned, between about 10 and 95% of the saturation concentration at the corresponding Temperature.

In a preferred variant of the method, this concentration is as possible high, i.e. around 80 to 90 | set and kept constant over time. These Variant has the advantage that a larger load of the detergent with SO, per unit volume is possible #, i.e. the volume of the absorption solution becomes relative kept low and the procedural regulation simplified.

In the regeneration part of the absorbent cycle it occurs because of the inevitable heat losses to a partial condensation of the steam and thus to a dilution of the absorbent. In the preferred process variant this amount of water is removed from the absorption solution again by evaporation in the expulsion part removed so that the acid concentration remains constant over time.

The condensing and evaporating amount of water is due to each other coordinated choice of the total pressure in the absorption part, the entry humidity and temperature of the gas to be cleaned, the specific amount of scrubbing solution / m3 Exhaust gas, the specific amount of regeneration steam for any temperature and the the washing solution supplied to the absorption part is kept constant.

From the demand for the greatest possible cost-effectiveness of the process the greatest absorption and the resulting sO2 load of the gases result in the respective Apparatus dimensions and the specific amounts of washing solution.

The cleaned exhaust air is steamed according to its temperature loaded. By choosing the temperature for the absorption solution such Difference in the water vapor load of the gas between inlet and outlet is set that the amount of water removed by evaporation is equal to the amount of condensate is. The setting of the appropriate temperature of the absorption solution can e.g. done by a heat exchanger.

In the figure, the numbers have the following meaning: 1 exhaust gas inlet 2 scrubber, countercurrent column 3 absorption solution 4 exhaust gas outlet 5 absorption solution running off 6 heat exchanger 7 regenerated absorption solution 8 vapors 9 expeller, regeneration column 10 Heat exchanger 11 Preheated solution 12 Live steam 13 Aftercooler 14 Cooling medium 15 Residual condenser 16 Cooling medium 17 S02 gas 18 Condensate containing S02 19 Infeed 20 Outward transfer The process is further developed using an example (Fig.) explained.

Example (Fig.). Exhaust air <1) with 27 vol-9 'sulfur dioxide is in a countercurrent column (2), e.g. a tray column with 25 trays, at 200C with a 35%, aqueous solution of technical glutaric acid (3) with sodium hydroxide solution is adjusted to pH 5.8, washed out. At a ratio of 6 1 solution / m3 Exhaust air is the sulfur dioxide down to 0.1 vol% Sz at the outlet (4) from the exhaust air removed. The washing solution running off with 145 g of SO2 / l solution from column (2) (5) is in the heat exchanger (6) in countercurrent to the regenerated washing solution (7) 850c and then preheated with the vapors (8) from the regeneration column (9) in the heat exchanger (10) brought almost to boiling temperature. This preheated solution (11) is in countercurrent in the column (9) with live steam (12) in a ratio of 0.2 kg of live steam (12) regenerated to 1 1 solution (fl), whereby the sulfur dioxide again up to 20 g 502/1 solution (7) is driven off. The solution (7) is in the heat exchanger (6) 350C, and then cooled to 200C in the aftercooler (13) with cooling water (14), and runs as regenerated washing solution (3) in the circuit of the absorption column (2).

The sulfur dioxide vapor mixture (8) leaving the column (9) at the top, with 15.3 vol.% SO2, is partially condensed in the heat exchanger (10) and then in the following Residual condenser (15) cooled to 300C with cooling water (16). That still 4.5 vol.% Sulfur dioxide gas (17) containing water vapor reaches further processing. The sulfur dioxide-saturated condensate (18) from the heat exchangers (10) and (15), in an amount of 1.2 kg / m3 exhaust gas (1), is also processed.

The glutaric acid concentration remains the same with the procedure described constant in the detergent circuit. There is also a small proportion in the new process of the sulphite in the washing solution is oxidized to sulphate, a partial flow (20) must be discharged and fresh solution and sodium hydroxide solution to keep the pH constant at (19) However, these quantities are more than an order of magnitude smaller than the known processes working with citrate solutions. About the small rate of oxidation To achieve it, it is also necessary that the concentration of iron and others Oxidation catalysts (e.g. cobalt, copper) in the solution less than 10 6 mol / l is held. Therefore, the columns (2) and (9) and the lines are made of non-metallic Materials, and the heat exchanger surfaces of the devices (6), (10) and (13) made of graphite, Made of titanium or tantalum.

Claims (2)

Claims = 1) Method for removing sulfur dioxide from Gases containing sulfur dioxide to concentrations below 0.1 Vo1.96 by absorption in a buffered, aqueous solution of organic acids with subsequent regeneration the absorption solution by thermal treatment in several stages, wherein a) the Gases in one or more washers at temperatures around 0 to 500C with a aqueous dicarboxylic acid solution buffered to a pH range of about 2.8 to 9, which has a concentration of about 10 - 95 X the saturation concentration, washed be, b) then the dissolved sulfur dioxide from the washing solution at temperatures between about 50 C and the boiling point of the washing solution is driven off and c) the washing solution regenerated in this way after cooling to temperatures around 0 to 500C renewed sulfur dioxide absorption is returned in the cycle, according to patent application P 26 23 963.4, characterized in that gases which are more than 20 vol .- # S02 are subjected to a treatment according to steps a to c. 2. The method according to claim 1, characterized in that as aqueous Solution of organic acids a solution of phthalic acid, maleic acid, malonic acid, Glutaric acids are used in any mixture or individually.