Patent Publication Number: US-4256710-A

Title: Process for deodorization

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for deodorization of a bad smell gas. More particularly, it relates to a process for deodorization using a combination of ozone and a wet washing device. 
     2. Description of the Prior Art 
     Heretofore, the system shown in FIG. 1 has been used for the conventional ozone deodorization. 
     In FIG. 1, the reference numeral (1) designates a bad smell gas; (2) designates a wash tower for washing the bad smell gas (1) with water; (3) designates a washing solution stored in the wash tower (2); (4) designates a pump for sucking the washing solution (3) to form a mist (5) in the wash tower (2); (6) designates a demister placed at the upper part of the wash tower (2); (7) designates a duct for feeding the gas (1) which is washed with water through the demister (6), into a detour type ozone reactor (8); (9) designates an ozonizer; (10) designates a diffuser for uniformly mixing the gas (1) in the duct (7) with an ozone-containing gas (11) obtained in the ozonizer (9) and the diffuser is connected to the duct (7) and (12) designates a treated gas which is deodorized in the system. 
     The operation of the system will be described. 
     The gas (1) is fed into the wash tower (2) wherein water soluble bad smell substances contained in the gas (1) are absorbed into the washing solution (3) as the mist (5) to be removed. Then, the bad smell gas (1) is passed through the demister (6) to remove the mist entrained in the gas (1) and it is fed to the duct (7). The washed gas (1) is uniformly mixed, in a moment, with the ozone-containing gas diffused from the diffuser (10) placed in the duct (7) and the mixed gas is fed into the ozone reactor (8). The mixed gas is passed through the detour passage in the ozone reactor (8) for a residence time of from at least several second to 1 minutes and the gas (1) is discharged as the deodorized gas (12). 
     The deodorizing system for the conventional deodorizing process has said structure whereby bad smell substances which can be removed by a water washing such as ammonia, lower fatty acid and lower amines are highly removed depending upon the washing condition. In the ozone reactor (8), the bad smell substrates such as nitrogen oxides methyl sulfide and dimethylamine which are reactable with ozone, are removed. However, bad smell substances such as hydrogen sulfide, methyl mercaptan and aromatic hydrocarbons have their reaction velocity constants of less than 10 5  liter/mol.min. In the concentration of less than the ppm odor, their reactivities to ozone are remarkably low whereby most of the unreacted bad smell substances are discharged out of the system. The ozone-containing gas (11) added to the gas (1) is also discharged as the unreacted gas with the treated gas (12). 
     In the conventional ozone deodorizing process, the deodorizing effect for the other bad smell substances which are different from ozonizable substances such as ammonia, and the unreacted ozone is disadvantageously discharged. Moreover, the washing solution which contains water soluble bad smell substances absorbed by recycling through the wash tower, is directly discharged or is discharged after a dilution. The water containing bad smell substances is drained. This is not a satisfactory deodorization. The discharged water can be further deodorized before the drainage. However, the other apparatus must be connected and this increase the cost for the treatment. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a process for deodorization with ozone in which a small amount of a bromide is added to the washing solution and an ozone bubbling is applied for the washing solution to result high deodorizing effect and to prevent the discharge of the unreacted ozone. 
     The foregoing and other objects of the present invention have been attained by providing a step of feeding an aqueous solution of an alkali bromide and an ozone-containing gas into a bubbling vessel; a step of bubbling the ozone-containing gas into the aqueous solution of an alkali bromide in the bubbling vessel to obtain a solution containing an active bromine oxide compound (hypobromite); and a step of feeding the solution containing an active bromine oxide compound into a wash tower for washing the bad smell gas to carry out the deodorization by contacting them. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a system of for the conventional process for deodorization; 
     FIG. 2 is a diagram of a system of one embodiment of the present invention; and 
     FIG. 3 is a diagram of a system of the other embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, one embodiment of the present invention will be illustrated. 
     In FIG. 2, the reference numeral (1) designates a bad smell gas; (2) designates a wash tower; (13) designates a packing layer packed in the wash tower (2) and a plastic packing of tellerettes, nets or rings made of a plastic such as polyvinyl chloride and polyethylene is packed in a height of 1 to 2 m; (14) designates an ozone bubbling vessel; and a bubbling pipette (16) for forming fine bubbles of the ozone-containing air (11) fed from the ozonizer (9) is placed at the lower part of the ozone-bubbling vessel (14) and a duct (18) through which the bubbled discharged gas (17) is discharged is placed at the upper part of the ozone bubbling vessel (14) and a partition (19) for forming passages is placed in the ozone bubbling vessel (14). The reference numeral (20) designates a make-up solution tank for storing a make-up solution (21) for washing which contains an alkali bromide such as sodium bromide or potassium bromide; (22) designates a pump for feeding the make-up solution (21) into the ozone-bubbling vessel (14); (23) designates a make-up solution which is activated by the gas-liquid contact of the ozone-containing gas (11) with the bubble (15) in the ozone bubbling vessel (14) and the make-up solution (23) is fed through a pipe (24) into the wash tower (2); (25) designates a fresh water fed into the wash tower (2) and (26) designates an overflow solution. 
     The operation of the process will be illustrated. 
     The ozone-containing gas (11) fed into the ozone bubbling vessel (14) is bubbled as fine bubbles (15) having diameters of 2 to 4 mm through the bubbling pipette (16) to result the gas-liquid contact with the make-up solution (21). Thus, ozone in the bubbles (15) reacts with the bromide (MBr) in the make-up solution (21) as the following reaction formula so as to result hypobromite (MBrO) wherein M represents Na, K or Li. 
     
         MBr+O.sub.3 →MBrO+O.sub.2 ps 
    
     The make-up solution (23) containing hypobromite (MBrO) which is formed by the gas-liquid contact with the ozone-containing gas (11) in the ozone bubbling vessel (14) is added, at a predetermined ratio to the washing water (3) at the bottom of the wash tower (2). Thus, the washing solution (3) containing the make-up solution (23) is sprayed from the upper part of the wash tower (2) as sprayed water (5) by the pump (4) and is passed through the packing layer (13) so as to be recycled. On the other hand, the gas (1) fed into the wash tower (2) is brought into contact with the washing solution (3) remained on the packing layer (13) whereby the bad smell substances contained in the gas (1) are immediately oxidized to be decomposed and the treated gas (12) is discharged through the top of the wash tower (2). 
     The deodorization reaction of the gas with the hypobromite (MBrO) has not been confirmed by a quantative analysis. However, according to the result of an analysis of the products and the change of pH of the washing solution (3), it is estimated to result the following deodorization reactions. 
     (a) Ammonia is oxidized to form nitrogen and the hypobromite is converted to the alkali bromide. 
     
         2NH.sub.3 +3MBrO→N.sub.2 +3MBr+3H.sub.2 O 
    
     (b) Hydrogen sulfide is oxidized to form sulfate or sulfur. 
     
         H.sub.2 S+4MBrO→SO.sub.4.sup.-- +4Br.sup.- +2H.sup.+ +4M.sup.+ 
    
     
         H.sub.2 S+MBrO→MBr+S+H.sub.2 O 
    
     (c) Methyl mercaptan and methyl sulfide are oxidized to form methanesulfonic acid. 
     
         CH.sub.3 SH+3MBrO→CH.sub.3 SO.sub.3 H+3MBr 
    
     
         (CH.sub.3).sub.2 S.sub.2 +5MBrO+H.sub.2 O→2CH.sub.3 SO.sub.3 H+5MBr 
    
     (d) Methyl sulfide is oxidized to form dimethylsulfonic acid. 
     
         (CH.sub.3).sub.2 S+MBrO→(CH.sub.3).sub.2 SO+MBr 
    
     (e) Trimethylamine is oxidized to form trimethyloxamine. 
     
         (CH.sub.3).sub.3 N+MBrO→(CH.sub.3).sub.3 NO+MBr 
    
     These reactions are immediately carried out to give the reaction velocity constances of higher than 10 9  liter/mol.min. 
     In the operation for treating the gas (1), a part of the washing solution such as 1 to 3 wt.% (based on the gas (1)) of the washing solution (3) is discharged as the overflow solution (26) out of system in order to prevent excess accumulation of the products formed by the deodorization reaction such as sulfate, sulfur and sulfonic acids in the washing solution (3). 
     The equal amount of the fresh water (25) is fed to balance to the overflow solution (26) and the steam discharged entrained in the treated gas. The ratio of the fresh water is about 1.3 to 3.5 wt.% based on the gas (1). 
     The rate of the ozone fed into the ozone bubbling vessel (14) should be higher than the rate required for maintaining the concentration of BrO -  in the washing solution (3) to at least 0.10 g./liter preferably 0.28 to 0.65 g./liter. When the concentration of BrO -  is lower than 0.10 g./liter, the ability for removing methyl disulfide and methyl mercaptan contained in the gas (1) is too low. On the contrary, when it is higher than 0.65 g./liter, the content of BrO -  discharged in the overflow solution (26) is too much. When the bad smell is removed in a sewage treating factory, the ratio of the ozone can be in a range of about 4 to 15 ppm by volume based on the gas (1). 
     In the packing layer (13) of the wash tower (2), the packing is packed in a height of 1 to 2 m, and the gas (1) is passed through the packing layer (13) at a superficial velocity of 1.0 to 1.8 m/sec. The rate of the washing solution (3) passed through the packing layer (13) in counter-current to the flow of the gas (1) is preferably higher for the increase of the effect of deodorization, but it is in a range of 0.5 to 2.0 times by weight based on the gas (1) in view of the operation cost. 
     The unreacted ozone is included in the discharge gas (17) discharged through the duct (18) at the top of the ozone bubbling vessel (14). Thus, the rate of the discharged gas (12) and the content of the unreacted ozone are small and accordingly, ozone can be easily decomposed by passing through a layer of an ozone-decomposing catalyst such as active carbon and metal oxide catalyst. The gas which does not contain ozone can be discharged out of the system. In the conventional process, the unreacted ozone contained in the treated gas (12) discharged from the ozone reaction chamber (8) could be removed with the ozone-decomposing catalyst in principle. However, the rate of the treated gas (12) discharged through the ozone bubbling vessel (14) in the conventional process is more than about 1000 times of that in the present invention and the content of ozone in the treated gas (12) in the conventional process is more than 10 times of that in the present invention. Accordingly, it is not easy to remove the unreacted ozone in the conventional process in the practical operation. 
     When the alkali bromide is included in the make-up solution and it is added to the washing solution (3), ozone rapidly reacts with the alkali bromide and a dissolved ozone is not substantially remained in the washing solution and the unreacted ozone is not included in the treated gas (12), discharged out of the system. 
     FIG. 3 shows a diagram of the system of the other embodiment of the present invention. The washing solution (3) recycled through the wash tower (2) to react with the bad smell gas (1) is fed to the top of the ozone bubbling vessel (14). Hypobromite ion BrO -  in the washing solution in the wash tower (2) reacts with the bad smell gas (1) to form bromide ion Br - . The washing solution containing Br -  is brought into the gas-liquid contact with the ozone-containing gas (11) in the ozone bubbling vessel (14) to be oxidized to convert Br -  into BrO -  and the washing solution is recycled as the spraying solution (5) to the top of the wash tower (2). In accordance with the system having the above-mentioned structure, bad smell substances or organic compounds dissolved in the washing solution (3) and sodium bromide and its reaction products are oxidized with ozone in the oxizone bubbling vessel (14) to be non-noxious compounds. 
     Certain experiments for deodorization effects using the apparatus for the deodorization of the present invention will be illustrated. 
     The conditions in the experiments are as follows. 
     Bad smell gas 
     Bad smell gas of air containing hydrogen sulfide, methyl mercaptan, methyl sulfide and methyl disulfide at ratios of 7.3 to 10.2 ppm respectively at 18° to 21° C. 
     Wash tower 
     Glass column having an inner diameter of 65 mm in which ceramic Raschig rings having 1/4 inch are packed in a height of 500 mm; 
     Bad smell gas fed at a rate of 50 liter/min. 
     Recycling solution fed at a rate of 150 ml./min. 
     Fresh water fed at a rate of 3 ml./min. 
     pH of the recycling solution of 8.0-8.9. 
     Ozone bubbling vessel 
     Glass cylindrical column having an inner diameter of 55 mm and a depth of the solution of 750 mm. 
     Make-up solution 
     Aqueous solution containing 8.1 g./liter of BrO -  prepared by feeding 750 liter of air containing about 6800 ppm of ozone into an aqueous solution containing 20 g./liter of sodium bromide. 
     Control of concentration of BrO -   
     The feeding speed of the make-up solution is controlled. 
     The concentrations and the removal percents of the bad smell substances at the outlet of the wash tower to the variation of the concentration of BrO -  in the recycling washing solution are shown in the following table. ##EQU1## 
     The concentrations of BrO -  were analyzed by the KI-Na 2  S 2  O 3  titration. The concentration of ozone was measured by the non-dispersion ultraviolet absorption type ozone actinometer (commercially available) and the bad smell substances were analyzed by the flame photometry type gas chromatography (commercially available). 
     
                       TABLE                                                       
______________________________________                                    
Concentration and removal percent                                         
to variation of concentration of BrO.sup.- :                              
Concen-                                                                   
tration of                                                                
BrO.sup.-  (mg                                                            
/liter)                                                                   
Kinds of                                                                  
bad smell                                                                 
substances                                                                
and concen-                                                               
trations                                                                  
(ppm)   0        30       70     130    300                               
______________________________________                                    
H.sub. 2 S                                                                
        8.7 ppm  3.7 ppm  0.30 ppm                                        
                                 Nil    Nil                               
9.2 ppm (5%)     (60%)    (97%)  (100%) (100%)                            
CH.sub.3 SH                                                               
        9.2 ppm  3.7 ppm  0.48 ppm                                        
                                 0.19 ppm                                 
                                        Nil                               
9.5 ppm (3%)     (61%)    (95%)  (98%)  (100%)                            
CH.sub.3 SCH.sub.3                                                        
        10.2 ppm 5.5 ppm  1.8 ppm                                         
                                 0.31 ppm                                 
                                        Nil                               
10.2 ppm                                                                  
        (0%)     (46%)    (82%)  (97%)  (100%)                            
CH.sub.3 SSCH.sub.3                                                       
        7.3 ppm  5.8 ppm  4.2 ppm                                         
                                 1.7 ppm                                  
                                        0.41 ppm                          
7.3 ppm (0%)     (21%)    (42%)  (77%)  (94%)                             
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     In accordance with the process of the present invention, the bad smell gas is treated with the hypobromite obtained by bubbling an ozone-containing gas into an aqueous solution of an alkali bromide. The effect of deodorant is remarkably improved and the completely treated gas can be discharged out of the system and the unreacted ozone is not discharged out of the system.