WO1992022680A1

WO1992022680A1 - Improved process for removal of sulfur dioxide from waste gases

Info

Publication number: WO1992022680A1
Application number: PCT/US1992/004841
Authority: WO
Inventors: John E. Stauffer
Original assignee: Stauffer John E
Priority date: 1991-06-17
Filing date: 1992-06-08
Publication date: 1992-12-23
Also published as: AU2226692A

Abstract

A process for removing sulfur dioxide (SO2) from a waste gas stream electrolytically and recovering the sulfur values as sulfuric acid. The sources of said waste gas stream include effluent vent, flue or exhaust gases from power plants, sulfuric acid plants, or roasters, and solid waste incinerators. The purpose of the process is to achieve economies of operation, provide a useful byproduct, and minimize environmental pollution. The electrolytic process is carried out in a column (10) comprising a scrubbing zone containing packing material (20) which also serves as an anode (20). The scrubbing zone also contains a cathode (30) which is insulated (35) from the packing material but is in intimate contact with the aqueous acid stream.

Description

IMPROVED PROCESS FOR REMOVAL OF SULFUR DIOXIDE FROM WASTE GASES

Field of the Invention The present invention concerns an improved electrolytic process for removing sulfur dioxide (S0₂) from a waste gas stream and recovering the sulfur values as sulfuric acid. The sources of said waste gas stream include effluent vent, flue or exhaust gases from power plants, sulfuric acid plants, ore roasters, and solid waste incinerators. The purpose of the process is to achieve economies of operation, provide a useful byproduct, and minimize environmental pollution.

Background of the Invention An electrolytic process for the removal of sulfur dioxide from waste gases and recovery as sulfuric acid is described in U.S. Patent 4,830,718. This patent discloses 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 converting 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 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 concentrations by means of make-up water or acid. The scrubbing zone conveniently is a 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 favored embodiment of the process, the packing material is electrically conductive, and it serves both as the gas-liquid contact surface for scrubbing and as the electrochemically active surface of the anode of the electrolytic cell. With this configuration, the cathode is located in the electrolytic cell compartment external to the scrubbing column. Electrical contact between the anode and cathode is maintained by the aqueous acid stream flowing through the scrubbing column and the connecting piping leading from the column to the electrolytic cell. The above configuration has obvious advantages.

Hydrogen gas produced at the cathode is kept isolated form the flue gases by means of the aqueous acid seal in the connecting piping. Thus, hydrogen gas cannot intermix with the flue gases forming potentially explosive mixtures. Furthermore, the hydrogen gas formed in the process can be recovered as a useful byproduct for such applications as the production of ammonia. Alternatively, the hydrogen can be burned as a fuel in such general uses as steam generation, or it can be used specifically to reheat the flue gases thereby providing buoyancy to the gases for better dispersibility in the atmosphere. In practice, however, the methods described for removing sulfur dioxide have serious disadvantages. When both electrodes are installed in the electrolytic cell compartment, large quantities of acid must be recycled between the electrolytic cell and the scrubbing column. Large liquid flow rates are necessitated because of the limited solubility of sulfur dioxide in aqueous acid solutions. This drawback is overcome when the column packing material serves not only as the gas-liquid interface but also as the anode of the electrolytic cell. The absorption equilibrium is shifted to the right by the electrolysis of sulfurous acid to the sulfuric acid.

New problems arise, however, when the column packing material is used for the anode. With this modification, the electrical resistance of the aqueous acid solution between the electrodes is increased significantly. In addition, there results troublesome variations in the voltage potential at different column heights. In large installations, no electrochemical - A -

reaction may take place at certain points in the scrubbing tower for lack of sufficient potential. If the voltage is increased, unwanted side reactions can occur at other locations thereby consuming excessive amounts of power. Another concern is the danger of grounding out the electrical current passing through the interconnecting piping. Providing electrical insulation for the lines circulating the aqueous acid solution can prove to be difficult. It is therefore, an object of the present invention to provide a process that overcomes the disadvantages of the conventional methods for removing sulf r dioxide rom waste gases.

It is also an object to provide a process that is completely safe to operate and which will be acceptable under the most stringent regulations.

A further object is to provide a process requiring the minimum investment and offering the lowest operating costs. These and other objects, features and advantages of the invention will be apparent from the accompanying drawings, Figures 1 and 2, and the following description.

Summary of the Invention In one preferred embodiment, the invention concerns a process for removing sulfur dioxide from effluent vent or flue gas by scrubbing the gas with an aqueous acid stream in a column 10 comprising a confined scrubbing zone and simultaneously subjecting this acid stream, which contains dissolved sulfur dioxide, to electrolysis. The confined scrubbing zone contains packing material 20a that is electrically conductive. This packing material both provides gas-liquid contact surface for scrubbing and serves as the anode 20 for the electrolysis reaction. The confined scrubbing zone also contains a cathode 30 that is electrically insulated from the packing material but is in intimate contact with the aqueous acid stream.

By conducting the absorption and electrolysis steps in the scrubbing zone, the need for a separate electrolysis cell is eliminated thereby simplifying the process and reducing capital investment. A circulating pump supplies the aqueous acid stream to the scrubbing zone in order to provide effective contact between the gas stream and the liquid phase. The concentration of the aqueous acid stream is maintained within set limits by means of makeup water or acid. Byproduct acid produced by the process is withdrawn from the system.

In a preferred embodiment of the process, the scrubbing zone consists of a column 10 whose inner walls are manufactured from an electrically conductive material. The walls of the column thus serve the dual purposes of confining the process streams and acting as the cathode 30 for the electrolysis reaction. In this configuration, the inner column walls are insulated from the packing material by insulation means 35 such as webbing, perforated sheet or a porous membrane made from such non-conductive materials as plastics and ceramics. The gaps or holes in the insulation allow effective contact between the aqueous acid stream and the inner column walls 30.

In another configuration the cathode 30 consists of an electrically conductive cylinder that is centered along the vertical axis of the column. This cylinder electrode is insulated from the packing material by insulation means 35 such as described above. It alone may serve as the cathode, or a plurality of spaced apart vertical cylinders may be used. Optionally, such a centrally located electrode may be used together as in Figure 1 with the inner column walls for the cathode. In this configuration, the packing material fills an annular space so that the maximum distance between cathode and anode is one-quarter the diameter of the column.

Brief Description of the Drawings The invention will be better understood by reference to the preferred embodiments illustrated in the accompanying drawings.

FIGURE 1 is a diagrammatic view of the scrubber/ electrolyzer in which the packing material serves as the anode and the inner column walls as the cathode. A pump is provided to circulate acid through the scrubber. FIGURE 2 is a similar view in which a centrally located electrode serves as the cathode.

Detailed Description of the Process Electrolysis is an effective and efficient way to convert sulfurous acid to sulfuric acid when sulfurous acid is formed by stripping sulfur dioxide from waste gases that contain relatively low concentrations of sulfur dioxide. The theoretical potential required for this reaction is 0.2 volts, but because of electrode polarization, the applied voltage must be increased to about 0.6 volts before significant reaction takes place. In practice, additional voltage is required in order to overcome the electrical resistance of the acid in the electrolysis cell. At higher potentials, 1.7 volts and above, electrolysis of water commences to form oxygen and hydrogen. Under normal operations, the applied voltage can be adjusted to permit the electrolysis of sulfurous acid but to avoid the electrolysis of water.

In the electrolysis of sulfurous acid, the following reactions occur: At the anode,

H₂S0₃ + H₂0 → 4H⁺ + SO„⁼ + 2e^"

and at the cathode,

2H⁺ + 2e^~ → H, The net reaction, therefore, is

H₂S0₃ + H₂0 → 2H⁺ + SO.," + H₂

Because the present invention contemplates operating the electrolysis step completely within the confines of the scrubbing zone, the hydrogen gas released at the cathode will enter the flue gas stream. The concentration of hydrogen gas resulting from this source, however, will be small. The hydrogen gas concentration in the existing flue gas is limited by the level of sulfur dioxide initially present in the gas. When the process is used to scrub flue gases generated by the burning of high sulfur coal or vent gases released from contact sulfuric acid plants, the resulting hydrogen concentration will typically be in the order of 0.2 percent. This value is substantially below the explosive limit for hydrogen in dry air, namely 18 percent.

Notwithstanding the wide margin of safety afforded by the low hydrogen concentrations, certain precautionary features designed into the process ensure trouble-free operation. These features include the installation of an exit gas analyzer (not shown) to measure the concentrations of sulfur dioxide and of hydrogen. A gas flowmeter (not shown) will warn of any upsets in the system. In addition, electric current and voltage limiting devices (not shown) prevent the occurrence of side reactions, e.g., the electrolysis of water.

Because of the corrosive nature of sulfuric acid solutions, the selection of materials of construction is critical. Those parts of the equipment in contact with acid that must be electrically conductive, e.g., the electrodes, are suitably fabricated from graphite, lead or the noble metals such as platinum. In addition, certain newer metals like zirconium show promise in this application. Other parts that are non- conductive can be made using a wide choice of materials^' that have been tested with good results in such applications.

One of the preferred embodiments of the invention is shown in Figure 1. Flue gas enters at the top of the scrubbing column 10 and flows concurrently with the acid down the column. In this manner, column loadings can be increased over the expected gas flow rates for counter-current operations. A circulating pump supplies sufficient acid to the column 10 to wet the column packing 20a. Standard packing material is designed to prevent channeling in the column. This attribute assures good wetting of the packing so that a good electrical circuit is achieved between the column walls and the packing. In Figure 2, a central electrode 30 replaces the column walls at the cathode.

Byproduct sulfuric acid is removed from the system during operation of the process. Make-up water or acid is supplied to the acid stream as required to maintain the acid concentration within certain limits. The selected concentration of the acid is a compromise between competing requirements. The electrical conductivities of aqueous sulfuric acid are greatest in the range of 20 weight percent to 40 weight percent. At lower and higher concentrations, the conductivities are reduced but still appreciable within the range of 5 weight percent to 93 weight percent. Balancing the need to maximize the electrical conductivity of the acid is the desire to produce byproduct acid of maximum strength. Concentrated acid has greater utility than weak acid and also is cheaper to ship on an equivalent acid basis. Because the present invention minimizes the spacing between electrodes, acid with higher concentrations can be used in the process for scrubbing without increasing the electrical resistance excessively.

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows:

Claims

1. A process for the mitigation and control of air pollution caused by the emission of sulfur dioxide in effluent vent, flue or exhaust gas, said process comprising the removal of sulfur dioxide from such effluent vent, flue or exhaust gas in a single step by scrubbing the gas in a column with an acid stream comprising aqueous sulfuric acid to solubilize and remove sulfur dioxide from the gas, said column comprising electrically conductive packing material, an electrode and insulation means for insulating the packing material from the electrode while allowing effective contact between the acid stream and the electrode; applying an electrical potential across the packing material and the electrode in the range of about 0.6 volts and 1.7 volts to effect electrolysis; and maintaining the concentration of the aqueous sulfuric acid by means of make-up water or acid.

2. A process according to claim 1, wherein the range of concentration of the aqueous sulfuric is between about 5 weight percent and 93 weight percent.

3. A process according to claim 1, wherein the electrode comprises the inner wall of the column.

4. A process according to claim 1, wherein the electrode comprises a cylinder centered on the vertical axis of the column.

5. A process according to claim 1 comprising withdrawing byproduct aqueous sulfuric acid from the acid stream.

6. A process according to claim 1, wherein the electrolysis comprises the reaction

H₂S0₃ + H₂0 → 2H⁺ + S0₄ ⁼ + H₂

7. A process according to claim 1 wherein the insulation means comprises webbing made from electrically non-conductive material.

8. A process according to claim 1 wherein the insulation means comprises perforated sheet made from electrically non-conductive material.

9. A process according to claim 1 wherein the insulation means comprises a porous membrane made from electrically non-conductive material.

10. A process according to claim 1 wherein the electrode comprises the inner wall of the column in conjunction with a cylinder centered on the vertical axis of the column, such that the maximum distance between the packing^• material and said electrode is one-quarter the diameter of the column.