WO1988007023A1

WO1988007023A1 - A method for the catalytic oxidation in aqueous medium of sulphite and/or bisulphite to sulphate

Info

Publication number: WO1988007023A1
Application number: PCT/DK1988/000042
Authority: WO
Inventors: Birger Borregaard; Anders Søren JOHANSEN; Johannes Jacobus Mallan; Mette Tingleff Skaanild
Original assignee: DANSK BIOLOGISK PRODUKTION AMBA
Current assignee: DANSK BIOLOGISK PRODUKTION AMBA
Priority date: 1987-03-13
Filing date: 1988-03-11
Publication date: 1988-09-22
Anticipated expiration: 1989-09-13
Also published as: DK130587D0; DK130587A

Abstract

The catalytic oxidation in aqueous medium of sulphite or bisulphite takes place at pH 6.5-9.5, preferably at about 8.6 and at a temperature below 50°C, preferably 25-30°C, by catalysis with a sulphite oxidizing enzyme. A sulphite oxidase originating from liver from oxen, calves or chicks is particularly suitable. It may be used in form of cell fragments or organelles, especially mitochondria, or in an isolated, more or less purified form. Especially it may be immobilized, e.g. in microcapsules having a semipermeable wall and together with buffer substances, together with ion exchanger particles for collecting sulphate ions. The method is especially useful for the oxidation of sulphite obtained in purification of flue and roast gases with basic compounds of alkali metals or alkaline earth metals or ammonium, e.g. in the double alkali process.

Description

A METHOD FOR THE CATALYTIC OXIDATION IN AQUEOUS MEDIUM OF SULPHITE AND/OR BISULPHITE TO SULPHATE

Field of the Invention The present invention relates to a method for the catalytic oxidation in aqueous medium of sulphite and/or bisulphite to form sulphate, which is removed for use or depositing.

The method is especially suitable for use in connection with the purification of effluent gases, such as flue gases or roast gases, for sulphur dioxide, in which method the sulphur dioxide is absorbed by one or more basic compounds of alkali metals, alkaline earth metals or ammonium in aqueous solution or suspension while forming sulphite or bisulphite, which is then oxidized in aqueous medium to form sulphate which is removed for use or precipitated for depositing. However, the method according to the invention is also suitable for the oxidation of sulphite in other sulphite containing waste materials obtained in other chemical reactions, e.g. for rendering harmless sulphite lye obtained in the cellulose and paper industries or desulphurization of other types of effluent gases.

Background of the Invention

The invention is explained in the following in relation to use of the method for the desulphurization of S0₂-containing flue and roast gases. Several methods of this type are known. It is common to these methods that they are difficult to control and carry out optimally, and especially the oxidation of sulhite (SO~~) or bisulphite (HS0₃) to sulphate is causing difficulties. The most important of the processes known are to be briefly mentioned. The most commonly used method is the limestone process. According to this process the flue gases are usually scrubbed in power plants after the removal of, i.a., fly ash, using calcium carbonate (limestone) for removing SO„ in accordance with the reactions:

CaCO ₃ + S0₂ + 1/1 H₂0 <==t CaS0₃.1/2 H₂0 + C0₂ (5)

where (5) is the overall reaction provided an excess amount of CaC0₃ is present, whereby a sparingly soluble calcium sulphite is formed which precipitates as the hemihydrate forming soft deposits at various sites in the plant.

In case of deficit of calcium carbonate there is formed soluble calcium hydrogensulphite according to the reaction

CaC0₃ + 2S0₂ + H₂0 » Ca⁺⁺ + 2HS0₃~ + C0₂ (6)

In the lime process the scrubbing of the flue gas_r usually also after the removal of, i.a., fly ash, is carried out with calcium oxide (lime) or calcium hydroxide (hydrated lime) for the removal of S0₂ according to the reactions

S0₂ + H₂0 <= H₂S0₃- =--± H⁺ + H30₃~ (1)

Overall: CaO + S0₂ + 1/2 H₂0 *= CaS0₃,1/2 H₂0 (10) A side reaction to this can be the absorption of the carbon dioxide of the flue gas:

CaO + C0₂ > CaC0₃ (11)

in which case the calcium carbonate reacts further according to reaction (2).

The formation of Ca in the limestone process depends on the concentration of H and CaCO-, and the process has its pH optimum typically about 6. In the lime process the formation of Ca depends only on the concentration of CaO, and its optimum pH is higher, typically about 8. For thermodynamic reasons the equilibrium for the overall reactions (5) and (10) is strongly shifted to the right; the free energy G is -13.4 kcal/mole and -44.5 kcal/mole for (5) and (10), respectively, while the equilibrium constant log K at

25°C is 10.9 and 32.-8, respectively.

When oxygen is present in the scrubber liquid, sulphite is oxidized to sulphate according to the equation

HS0 ^~ + Ca⁺⁺ + 1/2 0₂ -z→ CaS0₄ + H⁺ (12)

The calcium sulphate formed is sparingly soluble and crystallizes at the conditions normally used as the dihydrate, i.e. gypsum. The likewise sparingly soluble calcium sulphite gives soft and spongy deposits while the gypsum deposits are hard.

The oxidation of sulphite to sulphate is increasing with decreasing pH (which increases the solubility of CaSO- , and the solubility of the gypsum is decreasing with decreasing pH; both facts tend to shifting equilibrium (12) to the right.

In sodium processes the absorption of sulphur dioxide is carried out with sodium hydroxide or sodium carbonate depending on the amount of base used according to the overall reactions S0₂ + NaOH ^=- NaHS0₃ (13)

2S0₂ + NaC0₃ + H₂0 ^= 2NaHS0₃ + C0₂ (14) or

S0₂ + 2NaOH ^—* a₂S0₃ + H₂0 (15)

S0₂ + Na₂C0₃ ^=± a₂S0₃ + C0₂ (16)

The pH of the scrubber solution determines the nature of the final product. Below pH about 6 there is mainly produced bisulphite HS0₃ , above pH about 6.5 mainly sulphite SO-.

The bisulphite ions produced are oxidized at a relatively low pH to sulphate ions:

HS0₃ ^" + 1/2 0₂ > S0₄ ^" + H⁺ (17)

i.e. in principle the same reaction as in the limestone and lime process. However, the conversion to sulphate ceases at pH 3 to 2. The sodium base absorption of sulphur dioxide is more expensive than the limestone and lime process and is most suitable for smaller plants, and the sodium sulphate produced is substantially useless. This process has the advantage that all sodium salts are easily soluble in water and therefore no problems concerning deposition and clogging will occur. The process can be carried out analogously with potassium compounds, but in this case will be still more expensive. In addition it may be effected analogously-with certain ammonium compounds.

Recently the so-called double alkali process has turned up, in which it is attempted to combine the advantages of the lime/limestone process with those of the sodium process, but at the same time to reduce the drawbacks of both of them. However, the double alkali process is accompanied by extremely difficult regulation problems. As in the sodium scrubbing the actual absorbent in this process is sulphite according to the equation

S0₂ + SO ~~ + H₂0 ^=^ 2HS0₃ ^~ (18)

which best proceeds at about pH 6. An important side reaction will be reaction (17) shown hereinbefore.

It is important that oxidation of sulphite to sulphate in the scrubber is kept at the lowest possible

10 level as sodium is to be reused in the absorption and the sodium sulphate is a waste product.

The used scrubber liquid is pumped to a regeneration vessel in which the active sulphite is re-formed by the addition of, e.g., calcium hydroxide (e.g. as milk of lime) :

15

Ca(OH)₂ + 2HSO ~ —» S0₃ + CaS0₃.1/2 H₂0 + 3/2 H₂0 (19)

or by the use of a double stoichiometric amount and hence stronger basic conditions:

20

Ca(OH)₂ + HS0₃ — CaS0₃.1/2 H₂0 + OH + 1/2 H₂0 (20)

If calcium carbonate is used instead of milk of lime the overall reaction is:

25

CaC0₃ + 2HS0₃ → CaS0₃.1/2H₂0 + S0₃ + C0₂ + 1/2H₂0 (21)

The sodium sulphite recovered in this regeneration

30 is recycled to the absorption vessel.

It is a main advantage of the double alkali process that no substantial deposition (scaling) takes place in the absorber but that precipitation of sparingly soluble compounds takes place under control in a separate vessel, -jj. An essential drawback is a complicated process control where various chemical reactions proceed and must be controlled simultaneously. It is a special drawback that a substantial part of the solid waste products is a soft and thixotropic sludge containing a substantial amount of calcium sulphite, whereas no or only a small amount of waste product is obtained as gypsum. The sludge contains a certain amount of notably sodium sulphate that must be replaced by supplying new sodium carbonate to the scrubber.

In the scrubber some oxidation of sodium sulphite to sodium sulphate may occur, and it is conveyed to the regeneration vessel and precipitated there as sulphate:

Ca(0H)₂ + S0₄ J. CaS0₄ + 2 OH

which most frequently is precipitated as double salts with CaSO-₃ or as gypsum.

In the lime/limestone process a number of conditions such as temperature, pH and the relative concentrations of the various substances present, influence the conversion of sulphite to sulphate and the process is difficult to control. There is a tendency to deposition of either calcium sulphite or calcium sulphate at various sites in the plant. It is desirable but difficult to ensure conversion of all of the sulphite to sulphate since it is difficult to remove water from calcium sulphite, which is an environmental hazard in deposition or draining off to the recipient. Gypsum is considered to be relatively harmless and could be a commercial product when produced in a fairly pure state. However, it is difficult to obtain gypsum in such a pure state in the described known processes. From US patent specification No. 4,587,112 it is known that oxidation of sulphite in the lime or limestone process proceeds slowly once crystals of calcium sulphite have been formed, so that subsequently only small crystals of gypsum will be produced. Therefore it is suggested according to the publication to carry out the absorption in an aqueous suspension of calcium oxide, calcium hydroxide or calcium carbonate in the presence of oxygen or hydrogen peroxide as oxidizing agent, the concentration of oxygen preferably being controlled to a partial pressure of 1-50 torr. This known method can onlky be used in relation to the lime and limestone process and only if • large crystals of gypsum are desired as the final product. From US patent specification No. 4,080,428 it is known in the purification of flue gases and other waste gases with respect to sulphur dioxide, likewise in the lime or limestone process, to promote the oxidation of sulphite by catalysis with elemental iron. It is undesirable to obtain a final product, in this case gypsum, contaminated with heavy metal, whether it is to be used or deposited. The publication is aware of this risk and advises the use of iron in the form of refined steel or a copper-iron alloy, which will render the process more expensive. On the other hand it is also stated that the oxidation of sulphite may be promoted by alkaline salts such as ferric chloride or ferric sulphate. In this case it must be unavoidable that iron passes over into the final product.

In addition to the two known processes for optimizing of formation of gypsum using the lime or lime¬ stone process, it is known more generally from US patent specification No. 3,710,548 to free gases containing excess of oxygen from sulphur dioxide and other gaseous contaminants, e.g. NO , by homogeneous or heterogeneous catalysis in an aqueous liquid. Although it is intimated that activated carbon may be used as a catalyst, it appears that in homogeneous catalysis (i.e. using a dissolved catalyst) there is used Mn, Cu, Ti, Fe, Zn, Ni, Co, Cr, V or Mo and in heterogeneous catalysis, the same ones or possibly Sn. These are all heavy metals which it is undesirable to pass on to the sulphur compound ultimately formed from sulphur dioxide, whether it is to be used for an industrial purpose or to be deposited.

Brief Summary of the Invention

It is the object of the invention to provide a general method in which the oxidation process in the described processes is more easy to control than in the known processes, whether using the lime/limestone method, the Na, K, or ammonium method or the double alkali method, and in which there is a high degree of certainty that all of the sulphite or bisulphite is converted to sulphate, preferably calcium sulphate and preferably in a form sufficiently pure for commercial use, at the same time substantially avoiding problems with regard to depositions related especially to the lime/limestone method and at the same time avoiding contamination of the final product with heavy metals originating from inorganic catalysts.

This according to the invention is obtained by oxidation in aqueous medium of sulphite or bisulphite to sulphate at pH 6.5-9.5, preferably about 8.6 and at a temperature below 50 C, preferably 25-30 C by catalysis with a sulphite-oxidizing enzyme.

Advantageously, according to the invention a bacterial sulphite oxidase is used, e.g. originating from species of the genus Thiobacillus, e.g. Thiobacillus thioparus or Thiobacillus novellus. Such a sulphite oxidase has its optimum pH at about 8.0 and to a high degree looses its activity at temperatures above 55 C (cf. Lyric and Suzuki, Can. J. Biochem. _48_, pp. 334-343 (1970)). According to the invention there is preferably used a sulphite oxidase originating from liver of vertebrates, especially mammals or birds.

Especially, in the method according to the invention a sulphite oxidase originating from liver from oxen, calves or chicks can be used advantageously.

Such an enzyme has been known for a long time, cf. Cohen and Fridovich, J. Biological Chemistry, no. 2, January 1972, pp. 359-366 and 367-373. It has its optimum pH and hence its highest activity at pH 8.6 and is irreversibly inactivated at about pH 5. It is irreversibly inactivated at temperatures above 50 C and has the highest activity at 25-30°C. S0₄ ions have a protecting effect on the enzyme. It is commercially available in a purified form.

It is not necessary to use the hepatic sulphite oxidase in a highly purified form; it can according to the invention as well be used in an isolated, more or less purified form. It is not necessary for the sulphite oxidase to have been isolated from cells and organelles; when used it may be bonded in cells or parts of cells. According to the invention it can be preferably used as mitochondria from liver cells.

Detailed Description of Embodiments of the Invention

It is possible to use the sulphite oxidase dissolved in water in a reactor wherein the conversion of sulphite to sulphate is to take place. However, this is not appropiate since it will cause an unacceptably high consumption of the enzyme and thereby unacceptable costs. According to the invention it is therefore used in an immobilized form irrespectively of being used in a form liberated from cells or organelles, or in a form bonded in cells, cell fragments or organelles.

The immobilization of the sulphite oxidase can be conducted in various ways well-known in the technology using enzymes on an industrial scale. Bonding to an ion exchanger matrix or covalently to a carrier material, e.g. using glutaraldehyde, are examples thereof. If the enzyme in its bonded state has a relatively high molecular weight, it can be suitable to have it present in a gel matrix, in particular if it is desired to use the enzyme in the form of mitochondria or while present in cell fragments. It is important to ensure that the sulphate produced in the oxidation does not precipitate in the immediate surroundings of the enzyme because thereby its activity could be reduced. On the other hand the sulphate must be removed currently to ensure a complete oxidation of SO.. and HS0-. to sulphate. An appropriate method for ensuring this according to the invention is to use the enzyme in a state in which it is bonded to or in particles (e.g. of a gel matrix of a hydrocolloid such as carrageenan, an alginate or polyacrylamide) which is suspended in the oxidation reactor for^* sulphite/bisulphite together with particles of an ion exchanger for the exchange of sulphate ions with hydroxyl ions, the ion exchanger particles being removed continuously from the reaction as they are loaded with sulphate, whereupon the loaded ion exchanger particles are regenerated and recycled to the oxidation reactor.

Thus the sulphite is oxidized enzymatically to sulphate and the sulphate ion is ion exchanged with another ion, e.g. OH on a strongly basic anion exchanger, e.g. one of those commercially availabe under the name "Amberlite^,Λ^ being quaternary ammonium compounds.

It is most appropiate to carry out the continuous removal of the ion exchanger particles from the reaction by using their different densities in the sulphate loaded and the sulohate unloaded state and/or their density differing from the density of the ircitiobilized enzyme particles.

Another advantageous method for the immobilization of the sulphite oxidase is to encapsulate it, either in in the form of free enzyme or, e.g., mitochondria, into microcapsules the walls of which are semipermeable and which are produced by polymerization of a suitable organic material. This method can be advantageously combined with the former such that the microcapsules are suspended in the water in the oxidation reactor together with ion exchanger particles. It is particularly advantageous if the enzyme according to the invention is immobilized in microcapsules having a semipermeable wall and together with buffer substances, e.g. ampholines having such a molecular weight that they cannot pass the semipermeable wall and which keep pH in the water in the microcapsules on or close to the optimum value of 8.6.

Brief Description of the Drawings

Fig. 1 schematically as an example shows a plant - for desulphurization of flue gas by the double alkali method and utilizing the principles of the invention.

Fig. 2 schematically and in larger scale shows an oxidation reactor being a part of a preferred embodiment of the plant according to fig. 1, and Fig. 3 schematically in very large scale shows a microcapsule containing the sulphite oxidizing enzyme, usable in the reactor shown in Fig. 2.

Detailed Description of the Drawings In the plant shown in Fig. 1 the flue gas, which has been cooled (e.g. by being coupled to a district heating system) and freed from floating dust particles and other possible contents but still contains largely the whole amount of S0„, is passed through a conduit 10 to a scrubber tower 12 (absorption vessel) which at the top has a demister 14 and at the bottom water containing a basic substance, in the present case sodium hydroxide and/or sodium carbonate, the plant being adapted to utilize the double alkali method. In the scrubber tower 12 the reactions explained above take place, from the top of the tower the gaseous components are led to the atmosphere through a conduit 15, and from the lower portion of the tower an aqueous solution containing sodium sulphite and sulphite and bisulphite ions are extracted through a conduit 16 via a filter 18 to an oxidation reactor 20 containing the sulphite oxidizing enzyme in immobilized form together with an ion exchanger, both of which in the suspended state. In the oxidation reactor the ^"liquid is kept at about pH 8.6, at least in the immediate vicinity of the enzyme or the particles in which the enzyme is immobilized.

An inlet 19 from a buffer tank 22 is connected to conduit 16, the buffer tank being fed from a conduit 24 that via a filter 25 is fed from the upper portion of the oxidation vessel 20 and is adjusted to increase the pH value to about 9 with sodium hydroxide, so that the sulphite solution in conduit 16 enters the oxidation reactor 20 at a pH of about 8-9. From the buffer tank 22 a return pipe 26 leads to scrubber tower 12 for the pH adjustment of the scrubber water.

Oxygen, usually in the form of air, is passed to the oxidation tank through a pipe 28. As explained in more detail in connection with the discussion of Fig. 2, two types of particles are present in suspension in reactor 20, viz. firstly particles with immobilized enzyme and secondly ion exchanger particles on which the sulphate is precipitated which has been produced by the oxidation of sulphite and bisulphite under the influence of the enzyme. This causes an increase of the density of these particles and thus, as indicated in the figure, they settle in the lower portion of the oxiation reactor 20. From there they are passed through a pipe 30 to a tank 32, in which the loaded ion exchanger particles are regenerated as the sulphate ions are eluted from the ion exchanger particles. Regenerated ion exchanger particles are passed through a pipe 34 from the bottom of tank 32 via a washing system 35 back to reactor 20. The elution of sulphate ions in the tank 32 is effected, e.g., with strong sodium hydroxide so as to form sodium sulphate. The sodium sulphate is passed as a solution in water through a pipe 36 to a vessel 38.

If it is desired to produce gypsum, milk of lime is passed to vessel 38 through a pipe 39. In this way Ca(0H)₂ will take the place of a₂S0₄:

Ca(0H)₂ + Na₂S0₄ > 2Na0H + CaS0₄.2H₂0

The sodium hydroxide is passed through a pipe 37 back to vessel 32 and the gypsum precipitated in vessel 38 is passed through a pipe 40 to a centrifuge in which it is dewatered and is removed for deposition or use, the separated water being returned to vessel 38. The above conduit 37 extends from the precipitation tank 38 so that sodium ions are returned to tank 32, in which there is optionally supplemented with fresh sodium hydroxide.

Fig. 2 in a larger scale shows the oxidation reactor 20. It is indicated that suspended herein there are present particles (e) (shown in highly exaggerated enlargement) with immobilized_^enzyme and particles © with ion exchanger particles binding SO. and hence obtaining a higher density and sinking to the bottom of reactor 20, from where they are removed through conduit 30. The water containing dissolved sulphite enters through the pipe 16; its pH is if required adjusted to about 8 with, e.g., NaOH or KOH. The scrubber water freed from sulphite and sulphate as mentioned is drained off through pipe 24 to the buffer tank. At the inlet from oxidation tank 20 to pipe 24 a separator 42 is arranged to withhold the enzyme containing particles and ion exchanger particles.

Fig. 3 in extremely large scale schematically shows how enzyme can be encapsulated in microcapsules together with a buffer substance - in this case one or more ampholines - keeping pH near the optimum ^'pH - value of 8.6. The microcapsules have semipermeable walls that can¬ not be passed by the ampholine and the enzyme, but can be passed by sulphite and sulphate ions as intimated. in Fig. 3 E stands for enzyme and A for ampholine, but it is to be understood that there is aimed at molecules or minor aggregations thereof, not a further immobilization within the microcapsules which themselves constitute the immobilization.

Use in Practice of the Invention

The invention is notably expected to be of importance in the desulphurization of flue gases and roast gases.

Claims

Patent Claims

1. A method for the catalytic oxidation in aqueous medium of sulphite and/or bisulphite to form sulphate, which is removed for use or depositing, characterized in that the oxidation takes place in the aqueous medium at pH 6.5-9.5 and at a temperature below 50 C under catalysis by a sulphite oxidizing enzyme.

2. A method according to claim 1, characterized in using a sulphite oxidase originating from vertebrates, especially mammals or birds.

3. A method according to claim 2, characterized in using a sulphite oxidase originating from liver from oxen, calves or chicks.

4. A method according to claim 2 or 3, characterized in using the sulphite oxidase in the form of mitochondria from liver cells.

5. A method according to claim 1, characterized in using a micro-organism of bacterial origin, especially from strains of the genus Thiobacillus.

6. A method according to claim 2, 3 or 5, characterized in using the sulphite oxidase in at least a partly purified form.

7. A method according to claim 1 , characterized in that the enzyme is immobilized in or on particles which are suspended in an oxidation reactor together with ion exchanger particles for collecting sulphate ions, the ion exchanger particles being continuously removed from the reactor as they are loaded with sulphate, whereupon the ion exchanger particles are regenerated and recycled to the oxidation vessel.

8. A method according to claim 7, characterized in that the ion exchanger particles are continuously removed from the reactor while utilizing their different density in sulphate loaded and sulphate unloaded state or/and the density differences with respect to density of the immobilized enzyme particles.

9. A method according to claim 7, characterized in that the enzyme is immobilized in microcapsules having semipermeable walls and together with buffer substances having a molecular weight such that they cannot pass the semipermeable wall and which keeps the pH in the liquid of the microcapsules at or near the optimum value of 8.6.

10. A method according to anyone of claims 1-5 in which the sulphite and/or bisulphite originates from absorption of sulphur dioxide from flue or roast gases in an aqueous suspension containing calcium carbonate, calcium hydroxide and/or calcium oxide, characterized in that the calcium sulphate formed in the enzymatic oxidation is removed as gypsum and that possible other sulphates formed are converted to calcium sulphate by ion exchange and removed as gypsum.