WO1981003034A1

WO1981003034A1 - Matched stage odor control system

Info

Publication number: WO1981003034A1
Application number: PCT/US1981/000115
Authority: WO
Inventors: R Rhees
Original assignee: Pacific Eng & Prod Co Nevada
Priority date: 1980-04-25
Filing date: 1981-01-26
Publication date: 1981-10-29
Also published as: NO814429L; SE8107718L

Abstract

Scrubbing system for treating malodorous air comprised of two or more stages. In the first stage, the air is passed through a first packed scrubber tower (12) and is scrubbed with a countercurrent flow of a scrubbing solution containing the oxidant sodium hypochlorite (NaOCl). The sodium hypochlorite is produced by passing a salt brine (NaCl) through electrochemical cells (14). In the second stage, the air passes through a scrubbing chamber (26) and then a second scrubbing tower (24). A second scrubbing solution contacts the air in the scrubbing chamber (26) and is collected in the bottom duct (30) connecting the scrubbing chamber (26) and the second scrubbing tower (24). The second scrubbing solution is then recirculated to the top of the second scrubber tower (24). Caustic is added, continuously or intermittently, depending on the make-up of the air, to the second scrubbing solution before it is used in the second scrubbing tower (24).

Description

Matched Stage Odor Control System

Technical Field

This invention relates to scrubber systems for the removal and/or destruction of malodorous compounds in gaseous discharges that are to be released to the atmosphere. More particularly, this invention relates to multiple stage wet scrubbers using an oxidant therein to partially remove or destroy malodorous compounds from the air.

Background Art

The processing of organic wastes and by-products, and the manufacturing of many products, frequently results in the generation of malodorous substances (e.g., reduced inorganic and/or organic sulfur compounds, nitrogen compounds, and other low molecular weight volatile substances) , which are sometimes released in the atmosphere. These malodorous substances are commonly released to the atmosphere from sewage systems, sewage treatment plants, food processing plants, tanneries, pulp mills, etc. It is, of course, desirable, and now frequently required by State or Federal pollution control standards, that some or all of these substances be removed in part or in whole from the air so that the plant is not a nuisance to the population in the vicinity of the plant and does not damage the environment. Aqueous scrubbing systems have long been used to reduce the concentration of malodorous substances in the air that is going to be released into the atmosphere at such a plant. When the concentration of malodorous substances is rather low, or when the air basically has only one malodorous compound or a few very similar compounds, simple one stage scrubbers can be used to effectively reduce the concentration of the malodorous- substance (s) in the to-be-released air. However, if the concentration of odorous compounds is relatively high, or, if the to-be-released air contains several different malo¬ dorous compounds, then the single stage scrubber is relatively ineffective and inadequate.

There are several types of single stage scrubbers now in use. The most basic of these is the single stage scrubber which comprises a simple configuration designed to bring odor- free water into intimate contact with the malodorous air that is being released. This may be done in a variety of ways, for example by sprays, packed towers, etc.

In this simple single stage scrubber, the malodorous sub¬ stances will be dissolved in the odor-free water in accordance with Henry's law. If the odor threshold of the malodorous substances is not too low and' if the volatility of the malo¬ dorous substances is not too high, this scrubbing may decrease the concentration of the malodorous substances below their odor threshold such that the gas discharge will meet State or Federal pollution standards, and/or the objectionable odors cannot be detected by humans in the vicinity where the air is released. Unfortunately, such single stage scrubbers have the disadvantage of being effective only in very limited

OMPI situations when the air to be discharged has only one malodorous substance therein or several similar malodorous substances of low concentration. This system also has the disadvantage of requiring a relatively large volume of odor-free water for effective operation.

There are several known variations of this single stage scrubber that are employed in certain situations depending on the makeup of the malodorous substances in the air. For example, if the malodorous compounds are acidic, the addition of caustic to the scrubbing solution of a single stage scrubber will be of great benefit in reducing the malodorous substances in the air. The caustic neutralizes the acidic odorous com¬ pounds, usually by converting such compounds into relatively soluble salts taken up by the scrubbing water. A single scrubber system having caustic added thereto,- however, has the disad¬ vantage of being useful only when the air-to-be-released has a high relative concentration of acidic odorous substances and a relatively low concentration of alkaline odorous substances. If the malodorous substances are alkaline in nature, the addition of acid to the scrubbing solution of a single stage scrubber will increase the efficiency of the scrubber. The acid neutralizes the alkaline odorous substances and converts these substances into salts having a relatively high solubility. This higher solubility enables the malodorous compounds to be more readily absorbed and removed from the air by the scrubb¬ ing solution.

In addition to the above variations, the following technique has been known to improve a single stage scrubber if the cir¬ cumstances so warrant it: Several of the more common malo¬ dorous substances, both acidic and alkaline, are rapidly oxidized by a variety of chemical oxidants (i.e., sodium hypo- chlorite, potassium permanganate, ozone) . Thus, the addition of an oxidant to the scrubbing solution to treat discharge air containing such substances increases the efficiency of the odor removal system and also may decrease the size of the scrubber required. It is well known in the art that for gaseous discharges having a relatively high concentration of malodorous substances or for air having a complex mixture of malodorous substances, the single stage scrubbers described above are usually inade¬ quate. Thus, it has been necessary to use multiple stage scrubbers employing some combination of the several different variations of single stage scrubbers as described above.

In addition to the above prior art, Pacific Engineering and Production Company of Nevada (PEPCON) discovered and used effectively the finding that the effectiveness of a single stage scrubber system could be increased by using a salt brine (e.g., NaCl) as the scrubbing solution if all or part of the brine is passed through electrolytic cells before the brine entered the scrubbing chamber. The passing of the brine throug the electrolytic cells converts a portion of the salt into sodium hypochlorite (NaOCl) which is a strong oxidant. The sodium hypochlorite more rapidly oxidizes the more common

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Λ, W WII alodorous substances, especially the malodorous compounds commonly released by sewage systems, sewage treatment plants, and some food processing plants, than other oxidants now being used. Furthermore, the sodium hypochlorite brine, after pass- ing through the electrolytic cells, assumes a pH of about

8.5-9.0 which is nearly the ideal pH for removing common malo¬ dorous acidic and basic volatile compounds. A pH of 8.5-9.0 is ideal because it is alkaline to some acidic gases commonly found in discharge gases, such as H„S, while also being "acidic" to such commonly found gases as ammonia and organic amines.

Thus, by passing the salt brine through the electrolytic cell and converting some of the salt to sodium hypochlorite, the single stage scrubbing system becomes more proficient in remov¬ ing malodorous substances. Despite the success of the aforesaid PEPCON system, there are a significant number of sewage plants, manufacturing plants, etc., from which gases are emitted that have such a high con¬ centration of malodorous substances and/or such a wide variety of malodorous substances that neither the PEPCON system nor any of the single stage scrubber systems or compact multiple stage scrubbers now in use can achieve the desired degree of odor control. In such situations large, expensive and complex multiple stage scrubber systems must be used to adequately control the odors emitted at these sites.

Disclosure of Invention

It is apparent from the above that there exists a need

OMPI in the art for a less complex multiple stage scrubber system capable of removing, from air to be discharged into the atmos¬ phere, malodorous substances, particularly where such substanc occur in too high a concentration or are composed of two many different compounds for the simple type of known scrubber syst now in use. It is the purpose of this invention to fulfill this and other needs more apparent to the skilled artisan once given the following disclosure:

Generally speaking, this invention fulfills these needs in the art by providing a method of purifying air having there malodorous constituents, the steps comprising: providing a first scrubbing solution comprised of a salt brine containing an oxidant capable of reacting with at least a portion of the malodorous constituents in said air, conducting a first scrubb ing of said air, collecting the first scrubbing solution after said first scrubbing of said air, electrolyzing said first scrubbing solution so collected to thereby provide the desired oxidant content and using said electrolyzed solution to conduc said first scrubbing of said air, conducting a second scrubbin of the air scrubbed in said first scrubbing using an aqueous scrubbing medium, conducting a third scrubbing of the air scrubbed in said second scrubbing using an alkaline scrubbing medium, thereby to remove a substantial portion of all malo¬ dorous constituents in said air. This invention combines air-purification processes known in the art in a new configuration and format to derive a new

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/,, W W compact multiple stage scrubber. This new compact multiple stage scrubber is smaller than existing multiple stage scrubbers built to handle malodorous air of the same composition, yet it efficiently cleanses these malodorous gas discharges. Use of this invention will result in great savings, in both capital and operating costs, to the owner of such a plant.

This invention will now be described with respect to certain embodiments thereof as illustrated in the accompanying drawing, wherein:

Brief Description of the Drawings

Figure 1 is a schematic flow plan of one embodiment of this invention.

Figure 2 is a schematic showing also a prescrubber usable with the system of Fig. 1 in accordance with the practice of this invention.

Best Mode for Carrying Out the Invention

Referring to Figure 1, the air to be treated is collected from the site producing malodorous air by suitable ventilation ductwork (not shown) , and is drawn into the multiple stage scrubber by blower 10. Blower 10 draws the air thru duct 74 into the bottom of the first scrubber tower 12 where the first stage of the air purification occurs. In scrubber tower 12, which is a conventional packed scrubbing tower in this embodi¬ ment having a conventional packing material 80 therein (e.g. , intalox polypropylene saddles, pall rings, or the like) , the air rises against_. the downwardly flowing scrubbing solution being sprayed into the top of the tower. This first scrubbing solution is a salt brine (e.g., NaCl) having a salt concentra¬ tion of about 2 wt.% to 20 wt.%.

The contaminated brine scrubbing solution is continuously recirculated between first scrubber tower 12 and electrochemic cells 14 by taking the brine from basin 16 of first scrubber tower 12 and pumping it through electrochemical cells 14 using pump 18. The brine passes through pipe 42, pump 18, pipe 44, electrochemical cells 14 and pipe 46 to the top of first scrub tower 12. The brine enters the top of scrubber tower 12 throu pipe 48 to spray nozzle 50 which sprays the brine down first scrubber tower 12. The solution then collects in basin 16 whereupon the solution is recycled as described above. In the alternative, part of the scrubbing solution can be pumped directly from basin 16 into the top of first scrubber tower 12, bypassing electrochemical cells 14.

Electrochemical cells 14 are powered by rectifier 20. Figure 1 shows two electrochemical cells but it will be under¬ stood that any number or size of cells may be used and a wide variety of cell configurations may be used. It is essential only that the cells be capable of generating enough oxidant to satisfy the oxidant demand of the contaminants in the malo¬ dorous gases to the system.

Various "fail safe" devices can be incorporated into the system to shut the rectifier and interlocked components down in case of loss of flow, pump or blower failure, or high temper ture in the system. These "fail safe" devices are well known

-£U O to those knowledgeable in the state of the art.

The brine passes through cells 14. A part of the salt in the brine scrubbing solution converts into sodium hypo¬ chlorite when it passes through electrochemical cells 14 in accordance with the following reaction: ^~

NaCl and H₂0 + electricity = NaOCl + H, . For most systems contemplated by this invention, the hypo¬ chlorite solution emerging from the cells will have a concen¬ tration of about 0.02 wt.% to 0.7 wt.%. ^{^}The concentration can be varied, of course, depending on the composition and concentration of the malodorous substances being removed. The first scrubbing solution, containing the sodium hypochlorite, flows downwardly through the packed scrubbing tower contacting, oxidizing and removing malodorous substances that are in the air that is being treated.

The air, after passing through first scrubber tower 12, and still containing a reduced amount of the malodorous sub¬ stances, passes through duct 22 to the second stage 23. Second stage 23 is comprised of two principal^' sections, scrubbing chamber 26 and second scrubber tower 24. Duct 22 connects the top of first scrubber tower 12 to the top of scrubber chamber 26. The air in second stage 23 first passes downward through scrubbing section 26. Scrubbing section 26 has baffle 28 therein under which the air must pass. Scrubbing section 26 has therein spray nozzle 34 spraying a second scrubbing solution into contact with the air as the air passes through scrubbing section 26. Scrubber chamber 26 may or may not contain conventional packing material under spray nozzle 34. The second scrubbing solution is comprised of either fresh water or final effluent (from a waste water treatment plant) which enters the system through pipe 78. If final treated effluent is used its quality must approach that specified by the appropriate governmental environmental agencies and must not contain high concentrations of malodorous volatile sub¬ stances. This water, further removes malodorous, substances from the air. When the air flow reaches the bottom of scrubb- ing section 26, its direction is reversed by end duct 30, whic connects the bottom of scrubbing chamber 26 to the bottom of second scrubbing tower 24, so that the air flows upwardly throu second scrubber tower 24.

The air passes up through second scrubber tower 24 which has conventional packing material 82 therein (e.g. intalox polypropylene saddles) . It is brought into contact with the second scrubbing solution being sprayed into second scrubber tower 24 from the top of second scrubber tower 24 by spray nozzle 32. The second scrubbing solution is circulated within the system as follows. First,' the second scrubbing solution is collected in end duct 30 after being used in scrubbing chamber 26. It is then drawn from end duct 30 by pump 36 and . passed through pipe 58, pump 36, pipe 60, pipe intersection 38, and pipe 62 to the top of second scrubbing tower 24. The second scrubbing solution then passes through pipe 62 within second scrubber tower 24 to spray nozzle 32. Caustic is added to the second scrubbing solution through pipe 76 at pipe

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Λ, WIP intersection 38 as required to maintain alkaline pH in the scrubbing solution entering second scrubber tower 24. The caustic (e.g., NaOH, Na^CO., or the like) is added either con¬ tinuously or intermittently, as required, to neutralize the acidic substances in the air which enters second stage 23.

By varying the flow of caustic into the second scrubbing solu¬ tion, the pH of the system can be adjusted to optimum value. The optimum value is the pH level which will best remove the malodorous substances from the air. This will, of course, vary depending on the composition of the air being treated. Generally speaking, the caustic is added in an amount suf¬ ficient to bring the pH of the spray to about 8.0-9.0.

After the air passes through second scrubber tower 24, it passes through demister section 40 and out into the atmos- phere thru vent 84.

Sight glasses 70 and 72 are attached to end duct 30 and basin 16, respectively, in order for the plant operator to be able to determine the level of liquids therein.

The -volume of scrubbing solution in second scrubbing tower 24 and tower basin 30 is maintained by allowing the used scrubb¬ ing solution to overflow and discharge to drain from pipe 56 at the same rate that fresh water is entering scrubbing section 26 through pipe 78 and spray 34. The volume of water entering and leaving the second scrubber tower can be varied as required to maintain the concentration of malodorous substances below an acceptable level in the recycling scrubbing solution.

The level of scrubbing solution in the first scrubbing tower 12 is maintained by a level control device, not shown, which adds fresh water when it is needed. Periodically every 4 to 6 weeks the scrubbing solution in the first scrubber towe 12 and basin 16 is removed and replaced by fresh brine solu- tion. This replacement is done to prevent accumulation of - oxidation products (sulfate in the case of H„S) from exceeding their solubility in the scrubbing brine solution. Additional concentrated NaCl brine solution is added as required to main¬ tain the NaCl concentration in the scrubbing solution generall between 40 and 70 g/1 but no lower than 15 g/1. Concentrations greater than 70 g/1 up to 200 g/1 may be used.

In another embodiment of this invention shown in Figure 2, prescrubber tower 54 is added to the system before scrubber tower 12. This scrubber tower may be designed to cool down the air being treated and can remove some of the malodorous substances.

In this embodiment, blower 10 for forcing malodorous air through the scrubber system may be located before the prescrubb as shown in Figure 2 or at any appropriate place in the ducts of the multiple scrubber system. The malodorous air from vario process tanks or vessels is forced by blower 10 into basin 52 and then passes upward through the tower packing 60 where it is contacted by waste water treatment plant effluent or fresh water entering the tower through pipe 64, pipe 66, and spray nozzle 68. Duct 92 connects the top of prescrubber tower 54 with the bottom of first scrubber tower 12. Malodorous substances in the air are removed up to the extent of their

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A, W WII solubility in the scrubbing water and the hot malodorous gases are cooled to near ambient temperatures. The scrubbing solu¬ tion is permitted to accumulate in basin 52 of the prescrubber 54 and then to overflow through pipe 86 to drain. Sight glass 94 is attached to basin 52 for the purpose of determining the level of liquids in basin 52.

In a further modification of this process a portion -of the solution in basin 52 may be recycled through pipe 88 to a pump (not shown) and back to the top of the prescrubber 54 - through pipe 90.

In a still further modification, the oxidative scrubber 12 may be constructed with the same configuration as the post oxidative scrubber (second stage 23) so that the downcomer section of duct 92 is eliminated and duct 92 is connected directly to a section identical to section 26 of stage 23.

The air passing out of the top of the prescrubber passes through duct 92 into the oxidative scrubber 12 where it is contacted by the oxidizing sodium hypochlorite brine solution and thence through scrubber section 26 and the post alkaline packed scrubber tower 24 before ^'discharge to the atmosphere as previously described (Fig. 1) .

As alluded to above, the compact multiple stage scrubb¬ ing systems disclosed by this invention have many advantages over the prior art single and multiple stage systems. One advantage of this system is that the oxidant (e.g., sodium hypochlorite) concentrations can be higher and the pH lower in the first scrubber tower, if such is necessary to remove the alkaline malodorous substances, than in the known single stage scrubber system. In a single stage system, and in some multiple stage systems, the hypochlorite concentra¬ tion and the pH level must be strictly controlled to prevent chlorine or hypochlorous acids from being released into the atmosphere. Such strict control is not necessary in the dis-* closed system since the air, when discharged from the first scrubber tower, enters the second stage of the system, and is not released directly into the atmosphere. The higher the hypochlorite concentration the system can handle, without re¬ leasing harmful substances into the atmosphere, the greater the rate the system destroys oxidizable malodorous substances. This system's capability to vary the pH at its different stages increases the effectiveness of the whole system as the first stage can be operated at a lower pH than other systems, thus increasing its ability to remove alkaline malodorous sub¬ stances, while the second stage can be operated at a higher pH than other systems now in use, thus increasing its ability to remove acidic malodorous substances from the air. Another advantage of this-invention is that since the hypo¬ chlorite concentration can be controlled at a relatively high level and the pH level at. a relatively low value, a portion of the oxidant is in the vapor phase, and is carried with the air being treated into the adjoining duct and the second stage. This actually acts as an extension of the first stage since the oxidant, in the vapor phase, continually reacts with the air throughout the system, removing the alkaline and oxidizable

OMP malodorous substances in a very efficient way. This also in¬ creases the detention time of the air in the reactive areas of the scrubber system without increasing the overall height of the system. A further advantage of this invention is that the fresh or plant water feed rate to the second scrubber tower can be varied between wide limits. When most of the malodorous sub¬ stances are destroyed by oxidation or are acidic, the feed rate can be kept low. Then an appreciable amount of the malo- dorous substances are alkaline and not oxidizable, the feed rate can be increased up to a maximum value [approximately equal to the recycle rate of the pump circulation the scrubb¬ ing solution from the basin of the second (post oxidative) stage to the top of that stage] so that the scrubbing solution is well below saturation with malodorous substances and thus is best able to remove- odors by Henry's law effects. This will result in using no more water than required with attendant operating cost savings, and yet will result in an increase in the system's efficiency. Yet another advantage of^' this invention is that the scrubb¬ ing solution added at the second stage does not have to be as pure water as in other multiple stage systems. Since the scrubbing solution will be mixed with the oxidant vapor from the first stage and deodorized by this vapor, it is not neces- sary for the plant operator to go to the expense of supplying sufficiently pure water to the beginning of the second stage. A further advantage of this invention is that the

OMPI . ^" WIPO turbulence created in the air by reversing the direction of the air flow at the bottom of the second stage assures that the oxidant vapor is thoroughly mixed with the air. This, of course, means that the use of oxidants to remove malodorous substances is rendered more efficiently.

Another advantage of this invention is that the duct top entry feature to the second scrubber stage eliminates much of the external duct work in a multiple stage system. The downward passage of air through the first scrubbing chamber of stage 2 not only utilizes this volume for additional scrubb¬ ing but greatly reduces the probability in apparatus built according to this invention, of condensation and corrosion of the duct which connects the two stages. The top entry feature also decreases the amount of ductwork required at groun level, thus providing better utilization of floor space in the plant containing this apparatus.

It is also an advantage of this invention that the first and second stages can be connected by a relatively short duct since it does not matter, in this invention, if the reactions of the oxidant in the first stage are completed before the air can enter the second stage. The use of a short duct reduces the total floor area required for this scrubber which in turn reduces the construction and operating costs of the system. Yet another advantage of this invention is the use of the electrochemical cells to produce the oxidant hypochlorite solution. The use of these cells enables the production rate of the oxidant to be easily varied simply by changing the

OMPI . WIPO rectifier setting. The production rate of the oxidant is pro¬ portional to the ammeter reading of the rectifier. This feature enables the oxidant level to be strictly controlled and widely varied. This allows the scrubbers built according to this invention to be used in a wide^~~variety of applications in which the malodorous substance_s:.±_o_be_.treated are different.

Another advantage of this system is that the oxidant hypo¬ chlorite may be produced from a salt brine comprised of sodium chloride (NaCl) . Sodium chloride is rather inexpensive when compared to commercial oxidants, and since it is also the end product of the reaction within the first scrubber tower, it can be reused over and over, further reducing the operation costs of the system.

A further advantage of this invention is that it can effec- tively operate with little human operator assistance. Most of the hypochlorite produced accumulates in^" he first stage until it is needed to remove oxidizable malodorous substances from the air. Thus, the operator can set the amperage of the rectifier at a slightly higher value than is necessary to re- move the oxidizable substances and the excess will accumulate in the first stage. This excess will be available for periods of use when the oxidant may be in high demand. Also, excesses - from periods of low oxidant demand will build up to provide an oxidant supply for periods of high oxidant demand. Another advantage of this invention is that the volume of relatively pure water necessary to operate the system is significantly reduced relative to the water demanded by the prior art systems due to the extensive recycling of the scrubbe solutions and the small water requirements in the first stage. Also, high quality potable water is not needed. Both of these features greatly reduce the operating costs of the system. A further advantage of this system is described with re¬ spect to Figure 2. In severe situations in which the malodoro content of the air to be treated is very high, and/or if the air coming into the system is at a very high temperature, the system of Figure 1 is compatible with the prescrubber of Figure 2 and may be used to cool, condense, and partially remov odorous substances from the air. This prescrubber can be oper¬ ated at low cost by using the plant effluent on a once-through basis. If such a prescrubber were used in conjunction with this system, the first stage of the subject invention could be redesigned to have the same configuration as the second stage, that is, a top entry scrubbing chamber beside a bottom entry scrubber tower. Furthermore, if this three-stage system was used, it would be very versatile in times of equipment failure. Even if one of the stages was out of service, the other two stages could operate to adequately remove the malo¬ dorous substances. If either of the two stages of this inven¬ tion were out of service, the prescrubber could be operated as the oxidant stage.

Yet another advantage of this invention is that, due to the fact that the concentrations of the scrubbing solutions can be so varied within wide ranges, odor control can be achiev in many diverse situations by one system.

The following examples illustrate a typical operation of the above-described embodiment in the practice of this inven¬ tion:

An odor control test unit was constructed in accordance with the disclosed invention. The oxidative first stage packed section consisted of a PVC pipe 17.5 inches i.d. and 60.5 inches high. The internal packing was intalox polypropylene saddles, and the packed section was mounted on a tower basin, 17.5 inches inside diameter and 31 inches high. A Hartzell duct axial fan was connected to the gas inlet of the tower basin to force the foul air through the scrubbing tower. Air flow rates to the tower varied from 400 scfm to 750 scfm depending on the connecting duct work and whether one or both pre- and post- oxidative scrubbers were used during test sequences.

The hypochlorite generator consisted of two 100 ampere PEPCON cells in series (or parallel) operated by a 200 ampere, 18 volt rectifier. The scrubbing tower was connected to the electrolytic cells with the necessary pump and associated piping so that salt brine solution could be recirculated from the basin of the tower, through the pump, through the cells and to the top of the packed tower where it was then sprayed with a uniform pattern into the tower over the packed section. The sodium hypochlorite reacting with oxidizable malodorous com¬ pounds in the air.

The oxidant generating capability of the unit was de on- strated as follows:

Eighty seven liters of salt brine (30.6 grams of NaCl/1) were placed in the tower basin. The blower and recycle pump were turned on and the rectifier was activated and set to generate 100 amperes D.C. (volts 12.7). Air flow through the reduced tower opening was 400 cfm. The brine solution temperature was 15°C. The electrolytic cells operating electrically in series at 100 amperes produced sodium hypochlorite at the rate of 234 grams/hr. Example 1 The oxidative first stage scrubber described above was operated with the two electrolytic cells connected elec¬ trically in parallel. The rectifier was set at 100 amperes (5.0 volts, brine solution temperature 17°C, air flow 550 scfm The electrolytic cells produced sodium hypochlorite at the rate of 121 grams NaOCl/hr.

Hydrogen sulfide was then introduced into the intake air stream at various concentrations during periods of various sodium hypochlorite concentrations with the following results:

NaOCl concentration H₂S concentration cι₂ g/i ppm w/w in air ppm w/w m air

In Out Out

1.2 5 N.D* 0.5 1.2 9 ^' N.D* 0.5 1.2 20 N.D* 0.4 0.1 20 4 N.D*

*None detected.

It was also observed during this series of tests that the concentration of chlorine in the discharged air in¬ creased as the pH decreased as would be expected from the know equilibrium of chlorine, hypochlorous acid and sodium hypo¬ chlorite (0C1~)

0C1~+ 2H⁺+C1~^=^ HOC1 + HC1 =ϊ=^i: Cl₂ + H₂0

Example 2 The oxidative test scribber described above was tested at a sewage treatment plant treating domestic and industrial wastes. The odor control unit was connected to a discharge duct carrying a mixture of air and gaseous discharges from a sludge thickener and a heat treat sludge dewatering process. This foul air was then passed through the single stage oxida¬ tive scrubber. The foul air was known to contain hydrogen sulfide from the sludge thickener and a combination of low molecular weight volatile organic compounds (organic acids', aldehydes, ketones, esters and mercaptans) in high concentra- tion from the heat treat process.

The hypochlorite production efficiency of the electro¬ lytic system was checked (no foul air through the unit) and was found to be producing NaOCl at better than 85% current efficiency. Foul process air was then passed through the operat¬ ing unit (735 scfm) . The concentration of sulfur containing compounds (H₂S and mercaptans) in the inlet foul air was 3 ppm. The concentration of sulfur containing compounds in the outlet air was below the limit of detection. The organic con- taminants:,in the foul air were significantly decreased but were not decreased below their net odor threshold. The exit gases from the oxidative scrubber were found to contain (a) 1.7% C0₂ by volume (present in the foul air to the. unit plus CO- produced by oxidation in the scrubber) , (b) small amounts of Cl₂ and/or HOCl from the oxidant in the scrubber (0.05 ppm when scrubbing solution had pH 9.5, increasing ppm as pH was lowered; 10 ppm at pH 8), (c) organic acids from incoming air and the oxidation of alcohols and aldehydes in the oxidative scrubber. Oxidant demand (calculated as NaOCl) was 0.60 lbs. NaOCl per hour per 1000 cfm.

A post oxidative scrubber was then connected to the oxidative stage so that the foul air passed through the oxi¬ dative stage and then through the post oxidative 2nd stage. The pH of the 2nd stage scrubber solution was maintained in the 8.0-8.5 range. An estimation of odor removal by the two stage system was made by collecting samples of treated air from the second stage and subjecting a test panel to the air samples. It was found that odor could not be detected when the samples were diluted with small amounts of odor free air. Less than 50 volumes of odor free air was required to lower the concentration of contaminate in the air below odor thresho Such dilution naturally occurs rapidly on discharging treated air to atmosphere and thus such discharges are considered deodorized.

Example 3

A prescrubber was inserted into the test system of Example 2 so that the foul air was first passed through the prescrubber, then through the oxidative scrubber and the post

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Λ, WIP scrubber. Water was used as the scrubbing solution for the prescrubber. It was determined that the prescrubber removed sufficient contaminants from the foul air to (1) decrease the oxidant demand in the oxidative stage by 20%; (2) decrease the aldehyde contaminants to the oxidative stage by about 50%; and (3) lower the sulfur containing compounds to the oxidative stage from 3 to 2 ppm.

Once given the above disclosure, many other features, modifications, and improvements will become apparent to the skilled artisan. Such other features, modifications, and improvements are, therefore, considered to be part of this invention, the scope of which is to be determined by the following claims:

OMPI WIPO

Claims

1. A method of purifying air having therein malorodus con¬ stituents, the steps comprising: providing a first scrubbing solution comprised of a salt brine containing an oxidant capable of reacting with at least a portion of the malodorous constituents in said air, conducting a first scrubbing of said air, collecting the first scrubbing solution after said first scrubbing of said air, electrolyzing said first scrubbing solution so collected to thereby provide the desired oxidant content and using said electrolyzed solution to conduct said first scrubbing of said air, conducting a second scrubbing of the air scrubbed in said first scrubbing using an aqueous scrubbing medium, conducting a third scrubbing of the air scrubbed in said second scrubbing using an alkaline scrubbing medium, thereby to remove a substantial portion of all of said malodorous constituents in said air.

2. A method according to claim 1 wherein said malodorous constitutents are of a type and in a concentration suf¬ ficient such that a one-stage aqueous scrubbing is insufficient to remove a substantial amount of said constituents.

3. A method according to claim 1 or 2 wherein said salt brine

JR£ OMPI comprises in part sodium hypochlorite.

4. The method according to claim 1 or 2 wherein the step of conducting a first scrubbing of said air comprises spraying of said first scrubbing solution countercurrent to the flow of the air in a first scrubbing apparatus.

5. The method according to claim 4 wherein the spraying of said first scrubbing solution is in a downward direction and the flow of the air is in an upward direction.

6. The method according to claim 1 or 2 wherein the step of ° collecting the first scrubbing solution comprises providing a basin in the bottom of said first scrubbing apparatus into- which said first scrubbing solution runs after being sprayed in said first scrubbing apparatus.

7. The method according to claim 1 or 2 wherein the step of 5 electrolyzing said first scrubbing solution so collected comprises circulating said first scrubbing solution after it has been collected thru an electrochemical cell, and then circulating said first scrubbing solution, now electrolyzed, back to said first scrubbing apparatus.

0 8. The method according to claim 5 wherein the step of con¬ ducting a second scrubbing of the air comprises spraying said aqueous spraying medium in an upward direction as said air is flowing in a downward direction in a second scrubbing apparatus.

5 9. The method according to claim 8, wherein the step of conducting a third scrubbing comprises spraying said alkal scrubbing medium in a downward direction as said air is flowing in an upward direction in a third scrubbing appara

10. A method according to claim 1 or 2, further comprising the steps of: collecting said aqueous scrubbing medium, and circulating said aqueous spray medium so collected to a point where an alkaline composition is added thereto, thus converting said aqueous scrubbing medium into said alkaline scrubbing medium, for use in said third scrubbing

11. A method according to claim 10 wherein the alkaline composition is caustic.

12. A method according to claim 1 or 2, further comprising the step of passing the air through a demister after the step conducting a third scrubbing of the air.

13. A method according to claim 9 further comprising the step of: reversing the direction of the flow of the air after it passes thru said first scrubbing apparatus, reversing the direction of the flow of the air after the air leaves the second scrubbing apparatus.

14. A method of purifying malodorous air according to claim 4 further comprising the step of passing the air under a baffle in said second scrubbing apparatus during said

O second scrubbing.

15. A method of purifying malodorous air according to claim 4 further comprising the steps of: collecting the air from an area thru ductwork, and blowing the air into said first scrubbing apparatus.

16. A method of purifying malodorous air according to claim 9 wherein said first scrubbing apparatus, and said third scrubbing apparatus are scrubbing towers with conventional packing material therein.

17. A method of purifying malodorous air according to claim 7 wherein the circulating of the salt brine thru said electrochemical cell produces sodium hypochlorite.

18. A method of purifying malodorous air according to claim 1 wherein the concentration of the salt in the first scrubbing solution is about 2 wt.% to 20 wt.%, wherein the concentration of sodium hypochlorite in the first scrubbing solution varies between 0.02 wt.% to 0.7 wt.%, and wherein the concentration of caustic in the third scrubbing solution is sufficient to give the solution a pH between 8 and 10.