Abstract:
A liquid-gas contactor which comprises a first liquid-gas contacting region of which the lower part is provided with a gas inlet and the upper part is provided with a liquid feed inlet and a second region through which flows a gaseous fluid comprised of particles of at least a portion of the liquid fed into said first contacting region suspended in the greater part of the gas supplied through the gas inlet. The first and second regions are interconnected in the form of an inverted U-shape, the bottom ends of both regions opening into a liquid tank and at least the bottom end of the first liquid-gas contacting region being submerged in the liquid in the tank. A liquid-gas contact process employs this contactor.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a liquid-gas contactor suitable for treating a relatively great volume of gas. To be more precise, the present invention relates to a liquid-gas contactor which is useful in the lime and limestone wet scrubbing process. It also is useful as a waste gas desulfurization apparatus, a wet dust scrubber, a gas absorption apparatus, a water quencher, a humidifier and a deodorization apparatus. Various liquid absorbents such as sodium carbonate solution and caustic soda solution can be used therein. 
     (b) Description of the Prior Art 
     As the conventional liquid-gas contactors, there are generally known plate columns, packed towers, etc., but all of these are attended with various troubles so that it is impossible to adopt a high gas superficial velocity because it would require a means for reducing the entrainment of liquid and the pressure loss, and there would occur deposition of scales within the column when a slurry is employed like in the case of the limestone wet scrubbing process, and so forth. 
     Another liquid-gas contactor known heretofore is the bubble column reactor. However, this contactor has drawbacks such that it is necessary to devise a dispersing plate for the purpose of dispersing the gas in the liquid. 
     SUMMARY OF THE INVENTION 
     The present invention provides a liquid-gas contactor which is simple in structure and effective in operation. It is devised to eliminate the aforementioned troubles in the prior art by dispensing with the plates, packings or the like within the column. Rather, entrainment is positively induced to cause a plentiful liquid holdup within the apparatus and to generate a liquid-gas mixture current therein in order to utilize it for the liquid-gas contact. The invention also provides a liquid-gas contact process employing said liquid-gas contactor. Such an apparatus as proposed in the present invention and a process employing it are unprecedented. 
     The present invention relates to a liquid-gas contactor which comprises a first liquid-gas contacting region of which the lower part is provided with a gas inlet and the upper part is provided with a liquid feed inlet. The contactor also comprises a second region through which flows a gaseous fluid comprised of particles of at least a portion of the liquid fed into said first liquid-gas contacting region, the particles being suspended in the greater part of the gas supplied through the gas inlet. The first and second regions are inter-connected in an inverted U-shape, with the bottom parts of both regions extending into a liquid tank and the bottom end of the first liquid-gas contacting region being submerged in the liquid within the liquid tank. The invention also relates to a process employing this contactor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective side view of an embodiment of the contactor according to the present invention. 
     FIG. 2 is a perspective view of another embodiment of the contactor according to the present invention. 
     FIG. 3 is a perspective view, partly broken away, of still another embodiment of the contactor according to the present invention. 
     FIG. 3A is a perspective view, partly broken away, of another embodiment of the contactor according to the present invention. In these figures, the same parts are indicated by identical reference numerals. 
     FIG. 4 shows the results of the experiments conducted in Example 3 and illustrates the relation between the operation conditions and the absorption efficiency. 
     FIG. 5 shows the results of the experiments conducted in Example 4 and illustrates the relation between the gas superficial velocity and the pressure loss. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following will be explained the details of the liquid-gas contactor according to the present invention as well as the way to use this contactor with reference to gas absorption by means of a liquid (solvent). 
     FIG. 1 shows a preferred mode of practicing the present invention. 
     In FIG. 1, the reference numeral 1 denotes a gas absorption column, 2 denotes a gas inlet line, 3 denotes a treated gas outlet line, 4 denotes a solvent tank, and 5 denotes a gas recycle line. The gas absorption column 1 is of an inverted U-shape, and the lower part of one leg thereof is connected with the gas inlet line 2, the other leg thereof is connected with the treated gas outlet line 3, and the bottoms of both legs of the column are submerged in the solvent contained in the solvent tank 4. Above a gas inlet 8 in the gas absorption column 1 which communicates with the gas inlet line 2, there is provided an inlet 7 for the solvent which is supplied from the solvent tank 4 by means of a pump 10. The gas recycle line 5 is connected at one end thereof to the gas recycle inlet 6 of the gas absorption column which inlet is located slightly above the solvent inlet 7. The other end of the gas recycle line 5 is connected to the upper part of the solvent tank 4. 
     In a liquid-gas contactor of the structure described above, the gas to be treated is introduced into the absorption column 1 from the gas inlet line 2 by way of the gas inlet 8. There are instances where rectification of the gas might be effected in the vicinity of the inlet 8 depending on the velocity of the flowing gas current. Meanwhile, the solvent within the solvent tank 4 is fed to the absorption column 1 from the solvent inlet 7 by means of the pump 10. The solvent falling from the solvent inlet 7 contacts the gas ascending from the gas inlet 8, and forms a counter-current contacting region located between the solvent inlet 7 and the gas inlet 8. Therefore, by properly adjusting the elevation of the solvent inlet 7 in proportion to the velocity of gas flow, the length of said counter-current contacting region is determined. The solvent continues to descend while drawing therein the gas fed from the inlet 8 and forming a liquid-gas mixture flow (concurrent flow contacting region), and flows into the solvent tank 4 together with a portion of the gas. A first liquid-gas contacting region is comprised of the above described two regions. Said liquid-gas mixture flow is injected into the solvent tank 4, and the gas contained therein forms bubbles within the solvent tank, thereby contacting same with the solvent. A part of the gas rises to the upper end of the solvent tank 4, ascends within the gas recycle line 5, and is recycled to the gas absorption column 1. The gas which rises within the column from the gas inlet 8 contacts the liquid supplied from the inlet 7. Some of the liquid is entrained in the gas so that a mixture of gas and the entrained liquid rises in the column to the overhead thereof. The recycle gas supplied from inlet 6 is added thereto. Then the mixture of gas and entrained liquid flows through the overhead to the other leg of the column and thence downwardly therein. Thus, a gaseous fluid containing liquid particles flows in the second region which extends from the liquid inlet 7 to the gas outlet 9. The liquid-gas mixture flow is separated into the treated gas and solvent at the treated gas outlet 9 by virtue of the difference of inertia force, and the thus-separated solvent falls in the solvent tank 4 thereby to be recovered. Further, the solvent is fed to the solvent tank 4 from the solvent inlet line 11 and is discharged to the outside of the tank through the solvent outlet line 12. 
     The bottom portions of both legs of the gas absorption column 1 extend into the solvent tank 4. The bottom part of the first contacting region is submerged in the solvent within the tank to a depth that provides a liquid seal corresponding to the head of said region. The bottom part of the second region also is submerged in the solvent to the same extent. 
     The treated gas separated at the treated gas outlet 9 is released through the treated gas outlet line 3, but there are instances where it is further treated by means of a demister or like apparatus as occasion demands. Moreover, in the present apparatus, the whole region of the column can be designed to be of a concurrent flow type by disposing the solvent inlet 7 in the vicinity of the gas inlet 8. The overhead of the solvent tank 4 is connected to the gas absorption column 1 at a location on said column which permits the gas that accumulates in the overhead of the solvent tank to flow into the gas absorption column. Further, the second region of the gas absorption column 1 can be provided with packings, plates and other internals as far as the pressure loss permits. 
     Referring to the conditions for operation of the liquid-gas contactor according to the present invention, as for the gas superficial velocity hereinafter sometimes referred to by the symbol Ug, it is appropriate to be 3 m/sec. or higher, preferably in the range of from 4 to 20 m/sec., and more preferably in the range of from 5 to 15 m/sec. The appropriate liquid mass velocity L is in the range of from 40,000 to 500,000 Kg/m 2  ·hr. 
     FIG. 2 illustrates another preferable mode of practicing the present invention. 
     In this example, separation of the treated gas from the solvent after passing through the second region is performed within the solvent tank 4. The solvent falls in the solvent tank 4 thereby to be recovered, while the treated gas is introduced into the solvent tank 4 and thereafter flows to the treated gas outlet line 3 through the treated gas outlet 9 provided in said tank 4. In order to prevent by-passing of the gas introduced into the solvent tank 4 through the bottom part of the first liquid-gas contacting region to the treated gas outlet 9, a partition plate 13 is provided in the solvent tank 4. This partition plate need not separate the solvent within the tank into separate compartments: it suffices to have a depth similar to the bottom part of the second region in the apparatus of FIG. 1. The gas introduced into the solvent tank passes to the outlet 9 after passing through the second region. Further, the pump 10 for recycling the solvent can be connected to the compartment on one side of the partition plate 13, as shown in FIG. 2, or it can be connected to the compartment on the opposite side of the partition plate. 
     FIG. 3 illustrates still another preferable mode of practicing the present invention. This mode of practicing is characterized in that the gas absorption column 1 is constructed to have a cylindrical external shape. The partition plate 13 which is installed within the solvent tank 4 and is integrated with the lower part of the absorption column and extends upwardly to the vicinity of the overhead of the absorption column 1 thereby forming an inverted U-shape liquid-gas contacting region along both sides of the partition plate. It is also characterized in that the first liquid-gas contacting region is divided into plural sub-regions parallel to the flow of the fluid by means of separate partition plates 14. 
     FIG. 3A illustrates another contactor for practicing the present invention. FIG. 3A is identical to FIG. 3, except that the treated gas outlet 9 and the outlet line 3 extend sidewardly from the lower end of the column 1, similar to FIG. 1. 
     By virtue of the foregoing structure, contact between a great volume of gas and a liquid can be performed by the use of a compact apparatus. 
     An apparatus according to the present invention makes it possible to enhance the gas superficial velocity as set forth above, and because there are no internal parts as plates, packings, etc. within the contactor, it is simple in structure, the pressure loss can be minimized, and it is free from choking due to deposition of scales. Besides, because the entire inside regions of the absorption column as well as the solvent tank can be utilized as the liquid-gas contacting region, the liquid-gas contact efficiency is enhanced. Moreover, because the present apparatus is of a compact structure, the cost of manufacturing thereof is moderate. 
     EXAMPLE 1 
     By employing a liquid-gas contactor equipped with a dust scrubber having an inside diameter of 500 mm such as illustrated in FIG. 1, a waste gas containing dust having an average particle size of 20μ and a true density of 2.4 g/cm 2  was treated. The result was as shown in the following table. 
     
         ______________________________________                  Flow ratio                  of liquidConcentration       Velocity of                  to gas     Dustof dust at  gas fed in within     collectioninlet       column     column     efficiencyg/Nm.sup.3  m/sec.     l/Nm.sup.3 %______________________________________0.02        3.8        5          990.27        4.6        6          980.45        5.2        5          95______________________________________ 
    
     EXAMPLE 2 
     By the use of the same liquid-gas contactor as employed in Example 1, a boiler waste gas containing 1,000 ppm of SO 2  and flowing at a gas superficial velocity of 5.2 m/sec. was made to contact with an aqueous solvent containing 5 wt.% of Na 2  CO 3  which was supplied at the rate of 68,000 Kg/m 2  ·hr. As the result, the SO 2  removal percent was 98.5%. 
     EXAMPLE 3 
     An experiment for making an aqueous solvent of sodium hydroxide absorb an organic acid contained in air was conducted by the use of the apparatus illustrated in FIG. 2. The height of the inverted U-shape gas absorption column was 1.5 m, the inside diameter thereof was 50 mm, and the amount of absorbed acid was measured according to the ratio of the concentration at the inlet to the concentration at the outlet. The conditions for the experiment were as shown in Table-1 below. 
     As regards the process for analysis, the concentration of inlet gas was measured by the ultraviolet absorption method (U.V. wave length: 230 nm) upon making ethanol absorb the organic acid contained in the inlet gas. The concentration of the organic acid contained in the outlet gas was measured by employing the gas chromatography [glass column: 2 m, PEG20M (10%)+H 3  PO 4  (0.5%), chromosorb WAW-DMCS]. The result of the experiment is shown in FIG. 4. The absorption efficiency was always more than 98%: this absorption increased with an increase in the amount of recycling liquid, but as to the effect of the gas superficial velocity, no conspicuous difference was observed. 
     
                       Table 1______________________________________Conditions for Experiment______________________________________temperature of feed gas              23° to 28° C.temperature of recycling liquid              41° C.gas superficial velocity Ug              6, 8, 10 m/sec.liquid mass velocity L              100,000 to 300,000              Kg/m.sup.2 . hr.concentration of feed alkali              1.71 mole/l(≈ 6.8 wt. %)              (constant)amount of alkali solution fed              1.8 l/hr.initial density of alkali              0.86 mole/l(≈ 3.4 wt. %) -within tankinlet concentration of organic              (content of tank: 150 l)acid and excess alkali ratio(Remark 1)______________________________________     Inlet concentration     of organic acidUg [m/sec.]     [wt. ppm]       Excess alkali ratio______________________________________6         3070 to 3170    1.17 to 1.20(Remarks 2)8         1960 to 5500    0.98 to 2.7510        2310 to 2580    1.45 to 1.62______________________________________ (Remark 1) Excess alkali ratio = mole flow rate of alkali fed/mole flow rate of organic acid fed. (Remark 2) Only in the case of Ug = 8 m/sec, the inlet concentration of organic acid varied within a wide range. 
    
     EXAMPLE 4 
     Liquid-gas contact between air and water was conducted by the use of an apparatus illustrated in FIG. 2, and the whole pressure loss in the column was measured. The inside diameter of the gas absorption column was 300 mm, and the height thereof was 4.5 m. 
     The values of pressure loss obtained through measurements conducted by varying the gas superficial velocity Ug and the liquid mass velocity L were as shown in FIG. 5. It is evident from this graph that when the gas superficial velocity Ug exceeds 5 m/sec, the pressure loss ΔP increases sharply and the liquid holdup within the column increases pursuant thereto, whereby a satisfactory liquid-gas contact was obtained.