FI73603B - FOERFARANDE FOER TILLVARATAGANDE OCH TILLGODOGOERANDE AV SVAVELDIOXIDEN I SVAVELHALTIGA GASER SAOSOM ROEKGASER. - Google Patents

Description

73603

Method for the capture and recovery of sulfur dioxide in sulfur-containing gases such as flue gases

The present invention relates to a method for recovering and utilizing sulfur dioxide in sulfur-containing gases such as flue gases according to the preamble of claim 1.

According to this method, sulfur dioxide is first absorbed into a solution containing sulfite ions, after which the sulfur-enriched solution is treated appropriately to produce the final product.

Sulfur dioxide recovery processes are in many cases based on the neutralization of sulfur dioxide with calcium carbonate or calcium hydroxide. Neutralization can be performed either before the sulfur dioxide stream leaves the combustion or other process or in connection with the sulfur dioxide recovery process. The result is calcium sulfite or calcium sulfate according to the method. An example of an industrial process is A. Ahlström Oy's "Pyroflow®" technology.

The products available have no waste value and, for example, among ashes with other uses, they can even be harmful.

Sulfur dioxide in the flue gases can also be neutralized with sodium, potassium or ammonium bases / e.g. in the McGill process for the production of sodium sulphate or in the process of Walther GmbH for the production of ammonium sulphate.

In the so-called Wellman-Lord process, sulfur dioxide is taken with a Na2SO3 solution, from which it is further driven as a concentrated stream to the required process.

2 73603 Potassium salts can be obtained by using a potassium base.

There are several disadvantages to the above solutions: - the bases are relatively expensive to use, as the products obtained are all high-volume products, - the products obtained, with the exception of potassium sulphate, have little use value, - chlorine can be formed in the base used, partly reducing the environmental benefits of the cleaning process. .

It has previously been known to use sulfur dioxide-acetone-water mixtures, in which case sulfur dioxide is obtained as an acid with properties corresponding to strong mineral acids and its utilization is substantially expanded. Potassium chloride (KCl), a naturally occurring mineral salt, has been used to make potassium sulphate.

An example of such a method is the solution disclosed in CA Patent 832,931 (Wilson and Bejoy).

The latter methods are good and useful in themselves, but can only be used to recover sulfur dioxide already recovered. In addition, the process provides a solution of potassium sulfite that must be further evaporated in order for the process to further render the usable crystalline product to potassium sulfate for processing. Evaporation is a high energy process step.

The object of the present invention is to eliminate the drawbacks associated with the known solutions and to provide a new method for the recovery and utilization of sulfur dioxide.

The invention is based on the use of a K 2 SO 3 solution as an absorbent in the sulfur dioxide recovery process, whereby the sulfur dioxide absorbed in the solution reacts with potassium sulphite and water to form potassium bisulphite, which can be recovered as a crystalline substance and further processed into potassium sulphate.

3 73603 In this context, the term "absorb" is used consistently to indicate that sulfur dioxide is taken up or absorbed (sorbed) into a solution. This is despite the fact that said event is only partially chemical in nature, involving essentially physical adsorption in its first stage.

More specifically, the method according to the invention is mainly characterized by what is set forth in the characterizing part of claim 1.

The invention achieves considerable advantages. Thus, the overall economy of the process becomes very good. The energy costs are considerably lower than in previous processes because the amount of energy used for crystallization and treatment of the crystalline material is less than the thermal energy used for evaporation. The potassium sulphate available as a product is widely used e.g. in the manufacture of fertilizers.

In the following example, the invention will be examined in more detail with the aid of the accompanying drawings.

Figure 1 shows a simplified flow diagram of the method according to the invention.

Figure 2 shows a flow chart of an alternative method of preparing potassium sulfite for use in the invention.

Sulfur dioxide-containing gas, i.e. the air to be purified, which may be e.g. flue gas from the fossil fuel combustion process, is led under pressure through a fan 1 to a gas scrubber 2, i.e. a scrubber, where SO2 is absorbed into an aqueous solution of potassium sulphite. The scrubber 2 used in the method can be of any conventional type, i.e. e.g. an empty tower with spray or spray nozzles at its upper end or filled with filling pieces. The scrubber can also be of the type in which spacers are arranged in the tower, by means of which sudden changes in the direction of the gas flow, which enhance the transfer of matter, are effected.

4 73603

The potassium sulphite solution to be fed to the upper end of the scrubber 2 is sprayed or injected in the scrubber into the gas flowing from the bottom upwards, whereby most of the sulfur dioxide is absorbed into the liquid droplets. Sulfur dioxide forms potassium bisulfite by reacting with the potassium sulfite-water system. Sulfur dioxide free gas exits the upper end of the scrubber tower and can either be discharged directly into the outside air, or used as process air. The operating parameters of the scrubber, i.e. the pressure and temperature prevailing therein, as well as the solution and gas flow rates, are chosen so that the potassium bisulfite-containing I 2 SC 4 solution to be removed from the lower end of the tower is enriched for bisulite but has not yet reached saturation.

The potassium sulphite content of the solution used as absorbent is preferably as high as possible. Thus, it is preferable to use a saturated or almost saturated solution.

From the scrubber 2, the liquid stream is led to the absorber 3, where more sulfur dioxide is introduced into it until a saturation degree of potassium bisulfite is reached. The formed KHSO3 then crystallizes. The absorber 3 to be used may be of a commonly used type, such as a packed column or a bottom column. However, to eliminate the purification problem caused by crystallization, an absorber can also be used in which the gas is allowed to flow in bubbles through the liquid layer. It is also possible to effect crystallization only after the absorber 3 by lowering the temperature.

The impregnation step and crystallization can also be performed in a crystal core to which excess sulfur dioxide is introduced.

The exhaust stream of the absorber is divided into two parts by separating the crystallized KHSO3 from the liquid stream, e.g. by filtration. One effluent (crystallized potassium bisulfite) is passed to calcination 4 and the other (aqueous potassium sulfite solution) to stripper 5. In calcination 4, potassium bisulfite crystals are heated to about 200 ° C, whereupon the bisulfite molecules decompose to form solid potassium sulfate and gaseous sulfur dioxide. The potassium sulphate from the calcination is recovered and the gas mixture of sulfur dioxide and water vapor is divided into two parts, one of which is recycled back to the adsorber 3 and the other is fed to the potassium sulphite production process. Since the temperature required for calcination is as low as 200 ° C, for example, some ordinary solid (continuous) drying devices, such as screw dryers, can be used as the calcination equipment. Of course, various rotary calcination furnaces or the like can also be used if desired.

The potassium sulfite-water solution introduced into the stripper 5 is stripped of any adsorbed but unreacted sulfur dioxide by heating the solution to 80 °. . . 100 ° Cree. The effluents from the stripper 5 are recycled to scrubber 2 by returning sulfur dioxide directly back to scrubber 2, and combining the potassium sulfite with the fresh feed. Potassium sulfite, despite the heat treatment, may contain small amounts of potassium bisulfite, which thus enters the fresh feed through recycling. However, small amounts of potassium bisulfite do not adversely affect the absorption process in the scrubber.

Sulfur dioxide can also be separated from the potassium sulphite obtained from the absorber under reduced pressure, but in practice a rise in temperature has proved to be a more advantageous alternative.

The potassium sulfite used in the process is known to be prepared, for example, by introducing sulfur dioxide gas into a potassium hydroxide or potassium carbonate solution. An alternative method of preparation is shown in Figure 2 (Wilson, W.J., Sulfur dioxide and acetone-water in the preparation of fertilizers and potassium chemicals, DRIB Report No. 3124. May 1984).

The concentrated SO 2 stream from the calcination is run for regeneration where it is absorbed into the water. The resulting saturated aqueous solution is pumped to a regenerator where acetone is added. The strong hydroxysulfonic acid solution thus obtained is pumped into the ion exchange resin in K + form. The resulting potassium hydroxide sulfonate is pumped to the distillation, where excess acetone and SC> 2 are removed from the solution and the resulting solution can be recycled.

Es brand;

To a solution with a K 2 SO 3 content of 780 g / l was introduced sulfur dioxide-containing gas in a scrubber. The starting sulfur dioxide content of the gas was 4800 mg SC 2 / m 2 / and the process temperature was maintained at about 20 ° C. When the SO 2 content of the solution had risen to 105.5 grams per liter (sulfur dioxide was in the form of bisulfite), the solution was removed and subjected to crystallization, where excess sulfur dioxide was introduced until KHSO 3 crystallized.

The amount of KHSC> 3 crystallized per liter of solution was 120 g. After filtration, the mother liquor was run into an SO 2 stripper, where excess sulfur dioxide was removed, after which the resulting solution, which largely contained potassium sulfite, was run back to the scrubber combined with the solution from the preparation of potassium sulfite.

The potassium bisulfite obtained from the crystallization was calcined at 200 ° C and the resulting sulfur dioxide was returned to KHSO 3 solution. Potassium sulphate (K2SO4) from calcination was suitable for use in the manufacture of fertilizers.

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Claims (3)

7 73603 1. A process for recovering and utilizing sulfur dioxide in sulfur-containing gases such as flue gases, which process first absorbs sulfur dioxide into a solution containing sulfite ions, then triturates the sulfur dioxide-enriched solution to obtain the desired final product, a) b) the absorption is continued until the solution is saturated with potassium bisulfite, c) the crystallized potassium bisulfite is separated from the solution, d) it is subjected to a heat treatment to produce potassium sulphate, and e) at least some of the by-products obtained in process steps c) and d) recycled after possible further processing. Process according to Claim 1, characterized in that a saturated or almost saturated potassium sulphite solution is used as absorbent. A method according to claim 2, characterized in that step a) is performed in a scrubber (2) and step b) in an absorber (3), wherein the pressure and temperature in the scrubber and the flow rates of the solution and the sulfur dioxide-containing gas are selected such that the potassium bisulfite-containing K2SC> 3 solution obtained from scrubber (2) has not reached the degree of saturation with respect to potassium bisulfite. Process according to Claims 1 to 3, characterized in that the potassium bisulphite is heated to about 200 ° C in step d). Process according to one of the preceding claims, characterized in that the solution s 73603 obtained after step c) is heated to 80 °. ,. 100 ° C, after which it can be used in process step a). 9 7360.¾ 1. A feedstock for the production and use of sulfur dioxide in Finland and a gas stream in which the gas is absorbed, in which case the sulfur dioxide absorber is used to reduce the emissions of sulfuric acid, (a) a combination of potassium bisulfite and potassium bisulfite, preferably in the form of a potassium bisulfite, (b) a crystalline batch of potassium bisulfite, color processing for the production of potassium sulphate, and (e) a precursor and a by-product of the process and (c) and (d) circulating after eventual recovery. 1 2 3 For a patent according to claim 1, the use of potassium sulphide in the absorption or absorption of potassium sulphide. 2 For the purposes of claim 2, the invention comprises a) for a gas scrubber (2) and a b) and an absorber (3), a color for three cycles and a temperature of the radiator and a scrubber for the operation of a gas-fired and smoke-free gas stream. The potassium bisulfite halide is then added to the potassium bisulfite salt. 3 For the purposes of patents 1-3, the addition of potassium bisulfite up to a maximum of 200 ° C d).