AU622372B2 - Removal of sulfur dioxide from waste gases and recovery as sulfuric acid - Google Patents

Description

4 OPI DATE 16/11/90 AOJP DATE 20/12/90 APPLN. ID 38472 89 PCT NUMBER PCT/US89/01597 Ir-"m INTERNATIONAL APPLICATI 1 -LIS D U"ER7HE BENT COOPERATION TREATY (PCT)

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(51) International Patent Classification 4 International Publication Number: WO 90/12753 C01B 17/48, 17/76, C25B 1/22 Al (43) International Publication Date: I November 1990(01.11.90) (21) International Application Number: PCT/US89/01597 Published With international search report.

(22) International Filing Date: 14 April 1989 (14.04.89) (71X72) Applicant and Inventor: STAUFFER, John, E. [US/ US]; 6 Pecksland Road, Greenwich, CT 06830 (US).

(74) Agents: YOUNG, Thomas, N. et al.; Krass Young, 3001 W. Big Beaver, Suite 624, Troy, MI 48084-3109 (US).

(81) Designated States: AT (European patent), AU, BE (European patent), CH (European patent), DE (European patent), FR (European patent), GB (European patent), 'T (European patent), JP, LU (European patent), NL (European patent), SE (European patent).

(54)Title: REMOVAL OF SULFUR DIOXIDE (SO 2 FROM WASTE GASES AND RECOVERY AS SULFURIC ACID (57) Abstract A process is provided by means of acid scrubbing in combination with electrolysis for removing sulfur dioxide from a waste gas stream such as the effluent vent of flue gas, for purposes of economy, for achieving a useful byproduct, and for minimizing environmental pollution.

See back of page JL1 I REMOVAL OF SULFUR DIOXIDE (SO 2

FROM

WASTE GASES AND RECOVERY AS SULFURIC ACID This application is a continuation-in-part of my copending application Serial No. 789,490, filed October 21, 1985, now abandoned.

The present invention concerns a process for removing sulfur dioxide from a waste gas stream such as effluent vent, flue or exhaust gas, for purposes of economy, for achieving a useful by-product, and for minimising environmental pollution.

o. Various methods have been described for the removal OGG: of sulfur dioxide from a waste eras stream. Sulfur dioxide is 0' an environmental pollutant that is implicated as the chief source of acid rain. Among the methods for controlling SO 2 may be mentioned the methods described in U.S. Patent Numbers 3,515,513; 3,523,755; 3,542,511; 3,510,923; 3,475,122; 3,655,547; 4,059,496; 4,306,950; 4,421,726; and, 4,528,173.

A majority of these methods resort to the use of alkaline solutions contain-'.g sodium or lime to remove sulfur dioxide. Others propose to oxidise sulfur dioxide with air or oxygen using catalysts to promote the reaction. Additional processes make use of oxidising reagents such as hypochlorite to convert sulfites to sulfates.

The available emission control methods pre-nit certain technical difficulties and economical disadvantages.

The alkaline scrubbing processes produce waste products that are useless and a problem to dispose of. In the catalytic processes, the catalysts may become fouled and lose their activity. Oxi .ising reagents are costly, require precise metering, and are thermally unstable.

It is therefore an object of the present invention to provide a process for the removal of sulfur dioxide from waste gases that overcomes the disadvantages of the conventional methods.

It is also an object to provide a process for the S ;-HA recovery of sulfur values from vent or flue gas as a valuable

L

1A by-product while avoiding an air pollution problem.

It is a further object to provide a process of the kind mentioned which is relatively economical and does not require the use of catalysts or oxidising reagents.

These and other objects, features and advantages of the invention will be apparent from the accompanying drawing and the following description.

S

S.

K: (e~H ~4 7:'rl 4 7 WO 90/12753 PCT/US89/01597 2-- Summary and Detailed Description In one preferred embodiment the invention concerns a process for removing sulfur dioxide from effluent vent or flue gas, by subjecting the gas cyclically to scrubbing in an acid stream and to electrolysis. The process comprises the steps of scrubbing the gas in a confined scrubbing zone with an aqueous sulfuric acid stream to remove sulfur dioxide from the gas and convert the thus removed sulfur dioxide to sulfurous acid, subjecting the sulfuric acid stream containing the thus produced sulfurous acid to electrolysis in an electrolytic cell compricing an anode -and a cathode, -applying a tential cr i PC4Tr--ccc -n Pn- range aeuF 'do -VO-D e awuL- 1.7 volL~ to oxidize the sulfurous acid to sulfuric acid, recycling the sulfuric acid stream resulting from the electrolysis step to the scrubbing zone, and maintaining the recycled sulfuric acid within a predetermined range of concentration by means of make-up water or acid.

The range of concentration of sulfuric acid in the acid stream is determined largely by electrical conductivities. For dilute aqueous sulfuric acid, from under 5 weight percent acid up to the concentration of oil of vitriol, namely, 93.19 weight percent acid, the SUBSTITUTE SHEET I WO 90/12753 PCT/US89/01597 3conductivities in mho/cm 2 x 104 at 18*C, are greater than 1000. In the range between 5 weight percent and weight percent the values of conductivities exceed 2000, and within the limits of 20 weight percent and 40 weight percent the values are above 6500.

The scrubbing zone conveniently is the chamber contained within an absorber or scrubbing column of conventional design or modified design for passage of a stream of effluent gas therethrough. Preferably, the scrubbing column contains packing material that provides gas-liquid contact surface for the gas stream and the aqueous acid stream. The design of the packing material is critical in order to minimize channeling and thereby achieve greater scrubbing efficiency.

In a preferred embodiment of the process, the packing material is electrically conductive and serves both as a gas-liquid contact surface for scrubbing and as the electrochemically active surface of the electrolytic cell. From time to time, as desired, the by-product sulfuric acid is withdrawn in desired quantity from the acid stream. This is done, for example, while adding or making-up with water to lower the acid concentration to a predetermined level. In a preferred embodiment, the dissolved sulfur dioxide in the acid stream is kept at a minimum concentration by SUBSTITUTE

SHEET

WO 90/12753 90/12753 PCT/US89/01597 4operating at a low current density which in turn is i accomplished by employing an anode with increased area, preferably by employing an anode comprising electrica' conductive packing in the scrubbing column.

Brief Description of the Drawings The invention will be better understood by reference to preferred embodiments illustrated in the accompanying drawing in which: SFiC. 1 is a diagrammatic view of a scrubber and an electrolytic cell in series with a pump for acid-stream-processing of flue gas through -the scrubber and cycling the acid stream via the cell back to the I scrubber; and FI. 2 is a similar view in which the anode of the electrolytic cell is located as gas-liquid contact packing with the scrubber.

Detailed Description of the Preferred Embodiments Electrolysis according to the invention is an effective and efficient method of converting sulfurous acid (as well as SO 2 to sulfuric acid. No additive is I. introduced which might contaminate the by-product acid produced. There are no catalysts which may become fouled. Because the electrochemical potential for the reaction is relatively low, the cost of power is P SUBSTITUTE SHEET L41~, C) 9 WO 90/12753 PCT/US89/01597 moderate. In short, the present electrolytic process has the important advantage that it is direct and simple to operate.

In the electrolytic cell according to the invention, the following reaction take place: At the anode: 3

H

2 0 S0 4 4H 2e EO -0.20v and at the cathode 2H+ 2e---4 H 2

E

0 0 v.

The net reaction therefore is

H

2 S0 3 H20----2H S04= H2 EO -0.20v.

An additional reaction at the anode is as follows: H2SO 3 H20--- HSO4- 3H 2e- E -0.11v The above reaction is favored since the equilibrium constant, K, equals 1.3 X 10- 2 for the following rea:tion: HS04~ Hf S0 4 Therefore the power requirements are less than would be the case if the sulfuric acid were completely disassociated. The above theoretical electrode potentials, however, only indicate the relative power needs. In practice, the theoretical voltaces cannot be SUBSTITUTE SHEET r 6 obtained because of electrode polarisation. To overcome this irreversibility, a significant overvoltage must be applied equal to the minimum potential of about 0.6 volts. Also sufficient voltage must be applied to compensate for the potential drop in the electrolyte. Notwithstanding these less than ideal conditions, power costs are tolerable, especially when the process is applied to the special case of scrubbing of power plant flue gases which power plants ipso facto have wholesale electric power available.

A general expression for the total cost of removing sulfur dioxide pollutant is: S Ec U R where S is the unit cost, E is the cell voltage, C is the cost of electricity, U is the fixed cost, i.e. amortisation, insurance, maintenance, etc., and R is credit given for byproduct acid.

If the applied voltage across the cell is increased sufficiently, an additional reaction takes place at the anode; NO 2H0-4 NO0-+4H +3e- E -0.96v.

The invention contemplates the use of this reaction to strip nitric oxide from flue gases containing this •4 pollutant. Like sulfur dioxide, nitric oxide is only slightly soluble in water but is readily solubilized and removed when oxidised to nitric acid.

The capacity of a pollution control unit to remove S. sulfur dioxide by the present invention is readily adjustable so that upsets can be handled. For example, any surge in the volume of vent gas or increase in sulfur dioxide concentration can adequately be met. This result is accomplished by raising the applied voltage thus causing an increase in electrode current density. To cope with emergencies or irregularities the r i WO 90/12753 WO 90/12753 PC/US89/01597 7potential across the ele trodes can be raised as high as 1.7 volts, which is the potential where nascent oxygei begins to form. The applied cell voltage would thus be equal to the potential across the electrodes plus the potential through the electrolyte. The latter value is dependent on the acid conductivity and cell geometry.

One preferred embodiment of the invention is

FIC,.

shown in Figur 1. Vent or flue gases are contacted at i ambient temperature or higher but below the acid's ii boiling point in an absorber or scrubbing column with a downstream of dilute aqueous sulfuric acid. This acid stream strips the sulfur dioxide from the gas stream.

The acid containing the absorbed sulfur dioxide is passed to an electrolytic cell which converts the SO as sulfurous acid to more sulfuric acid. Hydrogen is vented from the cell. The electrodes must stand up to the corrosive conditions and therefore are fabricated from suitable inert electrode material such as lead, graphite, platinum, palladium, or ruthenium.

By operating the process with dilute sulfuric acid, the electrical resistance is minimized. Acid ML-- between 20% and 40% by weight has the greatest conductance and is preferred. It also rapidly absorbs sulfur dioxide. In order to maintain this con entration, make-up water or concentrated acid must S -A SUBSTITUTE SHEET I W 90/12753 PCT/US89/01597 8be added depending on the moisture content of the flue gas. By-product acid is withdrawn from the system as it is produced.

A preferred modification of the process provides an improvement in its efficiency. In order to remove a maximum amount of sulfur dioxide ferm- the gas stream, the dissolved sulfur dioxide in the acid must be maintained at a minimum concentration. This result, as indicated, can be achieved by increasing the area of the anode. A further advantage of increasing the anode area j is that the overvoltage can thereby be reduced as the j current density drops.

One preferred way of enlarging the anode area i is by using the packing in the scrubbing column for the dual purposes of providing gas-liquid contact surface and also serving as the anode. This design has the further advantage of improving the transport of sulfite ions through the diffusion layer. In this application, the packing must be made of an electrically conductive material such as graphite, graphite treated with a noble metal, or platinum-plated zirconium. Figure 2 shows the J layout of the process in which the column packing functions as the ancde.

One of the principal advantages of the present invention is that a valuable by-product is produced.

SUBSTITUTE SHEET \Qvs o'' ~kp~ I WO 90/12753 PCT/ S89/01597 9-- The weak acid with or without nitrogen values may be used in the production of fertilizer. Alternatively, the acid may be concentrated and purified by procedures well known to the trade. This concentrated acid may be used for many industrial purposes, thus improving the economics of the pollution control process.

The potential applications of the present Ii invention are numerous, but four uses stand out as being ji particularly significant. The process can be applied to the cleaning of flue gases from power plants that burn high sulfur coal or fuel oil. Environmental pollution 'j including acid rain are thereby reduced. Another application is in the control of sulfur dioxide from smelters that roast sulfide ores.

Thirdly, the process can be used to clean the vent gases from contact sulfuric acid plants. The process controls excessive emissions of sulfur dioxide during plant start-up and any upsets in the operation.

It also allows greater utilization of plant capacity without contributing additional emissions of sulfur dioxide to the atmosphere. The weak acid produced by the process is recycled to the acid plant for fortification.

Finally, the process is used to treat flue gases from waste incinerators. The potential sources of SUBSTITUTE SHEET hi- WO 90/12753 PCT/US89/01597 sulfur dioxide include high sulfur fuel used to operate the incinerator, vulcanized rubber goods, and certain plastics. By incorporating an oxidizing agent such as dichromate or permangante into the scrubbing solution in a odor-inhibiting quantity 4 wt. odors form traces of organic substances can be minimized. The oxidizing agent is rejuvenated by the electric current during the electrolysis step.

Ji SUBSTITUTE SHEET

Claims (2)

1. 2. A process according to claim i, where the range of concentration of sulfuric acid in the acid stream is from about 5 to about 70% by weight. Mw-- WO 90/12753 PCT/US89/01597
2. 12-- 3. A process according to claim 1, where the range of concentration of sulfuric acid in the acid stream is from about 20 to about 40% by weight. 4. A process according to claim 1, where the scrubbing zc,ne is contained within an absorber or scrubbing column for passage of a stream of effluent gas therethrough. A process according to claim 4, where the scrubbing column contains packing material that provides gas-liquid contact surface for the gas stream and the aqueous acid stream. 6. A process according to claim 5, where the packing material is electrically conductive and serves both as a gas-liquid contact surface for scrubbing and as the electrochemically active surface of the anode for the electrolytic cell. 7. A process according to claim 1, comprising the step of withdrawing by-product sulfuric acid from the acid stream. 8. A process according to claim 1, where the electrolysis comprises the reaction: H 2 SO 3 H 2 2H+ S04= H 2 E° -0.20 v. SUBSTITUTE SHEET A