AU770690B2 - Removal of toxic gases using ozonated water technology - Google Patents

Description

-1-

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant/s: Actual Inventor/s: Address for Service: Invention Title: BOC Gases Australia Limited Ilyas Pervez BALDWIN SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 'REMOVAL OF TOXIC GASES USING OZONATED WATER

TECHNOLOGY'

Details of Associated Provisional Application No. PP9041 dated 05 Mar 1999 The following statement is a full description of this invention, including the best method of performing it known to me/us:- File: 27244AUP00 -la- TECHNICAL FIELD The present invention relates to a process for treatment of gases and particularly, but not only, toxic gases.

BACKGROUND ART In these ever-increasing environmentally aware times, the disposal of various toxic products is becoming increasingly difficult.

The discharge of industrial toxic gases into the environment has caused growing national and international concern. The processes which have been developed to accomplish toxic gas purification vary from simple once-through wash operations to complex multiple-step recycle systems. In many cases, the process complexities arise from •the need for recovery of the impurity or reuse of the material employed to remove it.

Current disposal methods used by the industry include dilution, adsorption, wet scrubbing (caustic), passive bum boxes and combustion chambers. These solutions are unattractive because of either cost, throughput capacity, or maintenance and cleaning requirements.

Accordingly, it can be seen that there is a need for a safe, cheap and reliable process for the disposal of toxic gases. The present invention seeks to overcome at least some of "the disadvantages of the prior art or provide a commercial alternative thereto.

STATEMENT OF INVENTION In a first aspect, the present invention provides a method for treating toxic gases comprising contacting the toxic gas with ozone in an aqueous environment for a period •sufficient to convert the toxic component into a benign product.

The present invention is suitable for treatment of a wide range of toxic gases including H 2 S, SO 2 CO, PH 3

NO.

In one form the inventive process operates as follows. Firstly, an ozonated aqueous solution is provided by dissolving ozone in water. The toxic gas mixture to be treated which includes various concentrations of toxic gases is then contacted with the ozone solution eg in a scrubber, by bubbling therethrough etc. While not wishing to be bound by any particular theory, the applicant has found that such a contacting step causes a reaction between the toxic component of the mixture and the ozone. The reaction product dissolves in the water and the residual gas may be vented to the atmosphere.

The major reactions that occur in the liquid phase with different toxic gases are shown below: -2-

PH

3 203 H 3

PO

4 02

H

2 S 03 H 2 S0 3

SO

2 0 3

H

2 SO4 H 2 0 NO 03 H 2 0 HNO 3 CO 0 3

H

2 0 H 2

CO

3 The resultant non-toxic soluble acids are then easily disposed.

Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

BRIEF DESCRIPTION OF THE DRAWINGS So that the present invention may be more clearly understood it will now be described by way of example only with reference to figure 1 which is a flow diagram of a plant suitable for carrying out the inventive process.

BEST MODE FOR CARRYING OUT THE INVENTION The plant is divided into basically two separate areas. Area 10 is one which firstly provides ozone and then dissolves the ozone in water to provide the aqueous ozone solution.

The second area 20 is where the toxic gas mixture is contacted with the aqueous ozone solution.

Area 10 comprises an ozone generator 11, in this case fed with oxygen 12 although air can also be used. The resultant ozone is then dissolved in water in a series of three interconnected U tubes 14 fed via line 13. The resultant ozonated water then enters phase separator 15 from where the ozonated water is pumped to the top of scrubbing tower 21 via line 16. The scrubbing tower 21 may be a conventional packed column. Line 17 returns ozone which has gone out of solution back to the U tubes 14 to be recycled.

Treatment area 20 comprises the scrubbing tower 21 which, as discussed above, is fed with ozonated water at the top. The toxic gases are fed via line 22 into the bottom of scrubber 21 from where they rise and countercurrently contact the ozonated water which falls through the scrubbing tower 21.

While not wishing to be bound by any particular theory, it is believed that the reaction whereby the toxic component of the gases is rendered harmless is an -3oxidation/reduction reaction. This oxidation reduction can be carried out entirely in the gas phase, alternatively the toxic gas may be dissolved in the water and ozone passed into solution. Lastly, it is possible to dissolve the ozone in water and contact with the toxic gas.

As an example of the effectiveness of the present invention, we offer the following examples.

Several treatments of various toxic gases were performed using the plant of figure 1 under the following conditions: Volume of water in the system 60 L Water pH (after saturation with ozone) 5.6 Water flow rate 12 L/min "Ozone flow rate 3 L/min Concentration of dissolved ozone 1.5 mg/L (at ambient temperature System pressure 40 kPa System temperature ambient (-22'C) The product of the reaction dissolves in water, and the residual gas is monitored at the top of the column before being vented to atmosphere through an air blower. It may be mentioned that ozone is injected continuously and once the pilot plant reaches steady state it can simply continue treating the toxic gases. The process is stopped when residual gas reaches TLV (Threshold Limit Value) of the toxic component. Threshold Limit Values refer to airborne concentrations of substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed day after day without adverse health effects. The Threshold Limit Value of some of the toxic components, set by the EPA (Environmental Protection Agency) are given in Table 1.

TABLE I TOXIC COMPONENT TLV (ppm) Hydrogen Sulphide Phosphine 0.3 Sulphur Dioxide 2 Carbon Monoxide Nitric Oxide -4- The results of this experiment are shown in Table 2.

TABLE 2 TOXIC GAS CONCENTRATION TOXIC GAS FLOW AVERAGE TIME TO RATE, L/min BREAKTHROUGH,

MIN

PH

3 in air 50 ppm 7 43

PH

3 in air 5000 ppm 1 4

PH

3 in CO 2 2% 1 11 H2S in nitrogen 1% 2 21

H

2 S in nitrogen 550 ppm 3 no trace at exit H2S in nitrogen 550 ppm 10 16

SO

2 in nitrogen 1% 2 28

SO

2 in nitrogen 5000 ppm 5 19 NO in nitrogen 5000 ppm 1 12 CO in nitrogen 5000 ppm 5 CO in nitrogen 5000 ppm 1 no trace at exit Time to breakthrough is defined as the time taken for TLV level of toxic gas to be detected at the point directly on top of column.

The ozone generator used during the experiment had the capacity of generating of ozone using oxygen feed at 20 kPa inlet pressure. The toxic gas mixture was introduced into.the system after the maximum dissolved ozone concentration i.e. 1.5 mg/litre was reached in the liquid phase. This took approximately 20 minutes. Dissolved ozone was measured using the Indigo trisulfonate colorimetric method. TLV level of the toxic gas was monitored continuously using a Drager Monitoring Unit. The pH of the liquid phase did not vary significantly during the runs (between 6.00 to 7.30).

While ozonation can be expensive, the applicant believes that the present inventive process can be economically competitive with alternative processes for treatment of toxic gas streams. The dominant expense in capital cost and power requirements is the cost of ozone generation. Substantial reductions in cost may also be obtained by providing better ozone/waste mixing systems etc.

It is clear that dissolved oxygen is a viable method for eliminating low concentrations of toxic gas components in industrial gas mixtures. The resultant products of the inventive process are non-toxic soluble acids which make the disposal of the spent solution easier and less costly in comparison to other existing scrubbing methods.

The person skilled in the art will understand that the inventive process may be embodied in other forms without departing from the spirit or scope of the invention.

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Claims (7)

1. A method of treating toxic gases comprising contacting the toxic gas with ozone in an aqueous environment for a period sufficient to convert the toxic component into a benign product.

2. A method as claimed in claim 1 wherein a toxic gas includes H 2 S, SO 2 CO, PH 3 NO.

3. A method as claimed in claim 1 or claim 2 comprising the steps of dissolving ozone in water to provide an ozonated aqueous solution and passing the toxic gas through the ozonated solution.

4. A method as claimed in any one of the preceding claims arranged as a batch or continuous process.

A method as claimed in any one of the preceding claims wherein the toxic gas is contacted concurrently with the ozonated aqueous solution.

6. A method as claimed in any one of the preceding claims wherein the toxic gas is contacted countercurrently with the ozonated aqueous solution.

7. A method of treating toxic gases substantially as herein described with reference to anyone of the embodiments of the invention illustrated in the accompanying drawings. DATED this 18th Day of December 2003 BALDWIN SHELSTON WATERS Attorneys for: BOC GASES AUSTRALIA LIMITED o•* ooo.o* *°o°O *o