Abstract:
Disclosed is a method and apparatus for conducting a mass transfer between a gas and a liquid, or for conducting a chemical reaction between a gas and a liquid. The method comprises supplying a liquid into a dynamic mixer, supplying a stripping gas or a reaction gas into the dynamic mixer, and flowing the liquid and the stripping gas or the reaction gas through the dynamic mixer in a turbulent co-current flow. The dynamic mixer includes a columnar casing, a rotor within the casing, which rotor has blades along substantially an entire length thereof, and stator blades positioned between the rotor blades within the casing along substantially an entire length of the casing.

Description:
This application is a 371 of PCT/NO98/00288 filed Sep. 30, 1998. 
     BACKGROUND OF THE INVENTION 
     The present invention concerns a method for mass transfer between gas and liquid or a chemical reaction with or without a catalyst between gas and liquid, comprising one or more process stages at each of which stripping gas or reaction gas is added. The invention is particularly well suited for use in connection with the deoxygenation of sea water which is to be injected into petroliferous formations beneath the sea bed as it is very compact and has a low weight. 
     Norwegian patent no. 158283 describes a system in which a circulating stripping gas, for example N 2 , is purified catalytically before being introduced into a mass transfer unit which may be in the form of static mixers, a serpentine pipe or a tower. These prior art solutions have, until today, been considered to be both effective and compact. 
     SUMMARY OF THE INVENTION 
     However, the present invention represents a mass transfer solution which is much more effective, more compact and lighter and cheaper than the prior art solutions. 
     The method in accordance with the present invention is characterized by one or more process stages at each of which one or more reactors are used in the form of driven dynamic mixers and at each of which gas is added at one or more locations so that the gas and the liquid move in a turbulent co-current through each mixer. 
     Moreover, the equipment in accordance with the present invention is characterized by one or more process stages at each of which one or more dynamic mixers are used and gas is added at one or more locations in connection with the mixers so that the gas and the liquid pass in a turbulent co-current flow through the mixers. 
     The reason for the very effective mass transfer with the present invention is the design of the dynamic mixers which ensure the formation of very small gas bubbles, which produces a relatively large contact surface between the gas and the liquid and effective agitation, together with a virtual “piston current” through the mixers. 
     As the dynamic mixers in a multi-stage system in accordance with the present invention may be placed horizontally and above one another, the design of a multi-stage system is much simpler than that of the prior art solutions as: 
     intermediate pressure pumps are eliminated, 
     gas compressors can be replaced by simple blowers, 
     the energy consumption is reduced, 
     the volume and weight of the equipment are reduced considerably, 
     the height, width and length of the installation can be selected to a greater degree, which makes the installation more flexible in terms of design. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be described in further detail in the following using examples and with reference to the attached drawings, where: 
     FIG. 1 shows a flow diagram for a two-stage system with circulating gas for mass transfer between gas and liquid. 
     FIG. 2 shows a flow diagram for a two-stage system with fresh gas supply and integrated equipment for catalytic gas purification. 
     FIG. 3 shows a drawing of a dynamic mixer in accordance with the present invention. 
     FIG. 4 shows a two-stage flow diagram for a chemical reaction between gas and liquid, more precisely by means of hydrogenation or hardening of oil, fat or fatty acids. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As stated, FIG. 1 shows a flow diagram for a two-stage system in accordance with the present invention in which water which is to be deoxygenated is passed through a pipe  9  into a first-stage dynamic mixer  27  where stripping gas is introduced from a separator  8  through a pipe  14  and forms a turbulent co-current with the water, which flows on through a pipe  23  to a separator  7 . 
     From the separator  7 , the water flows through a level-controlled valve  25  to a second-stage dynamic mixer  28  and from there on through a pipe  24  to a second-stage separator  8 . 
     From here, the treated water flows through a level-controlled valve  26  and a pipe  11  to a water injection pump  12 . 
     The stripping gas is introduced to the system through a pipe  13  to the second-stage dynamic mixer  28  and passes from there to the second separator  8 . 
     From the separator  8 , the gas passes through a pipe  14  to the first-stage dynamic mixer  27  where it encounters the incoming water current and, together with this water, flows on to the separator  7  described above. 
     The circulation gas flows from the separator  7  through a pipe  15  to a catalytic gas purification system. 
     The gas purification system consists of a blower  16 , a heat exchanger  17 , a catalyst chamber with a heating unit  19 ,  20 , a methanol supply  18  and temperature control valves  21 ,  22 . 
     The purified stripping gas, for example N 2 , which is virtually free of O 2 , is recirculated through pipe  13  back to the second-stage dynamic mixer. 
     Additional gas to replace the N 2  lost with the injection water is added either as N 2  or as air through a pipeline  23 . 
     FIG. 2 shows an open system without recirculation and without a catalytic purification system integrated in the circuit. 
     Natural gas is used here both as the stripping gas and, if the used gas is to be purified catalytically, as the reaction gas. 
     The natural gas must be added as fresh gas which, after the stripping process, is removed from the system, either for combustion or consumption. 
     The treatment equipment for water is otherwise identical to that which is described above and shown in FIG.  1 . 
     It is calculated that a technical system of this type with a capacity for treating water, i.e. removing O 2  from water, equivalent to 650 m 3 /h will have a height, length and width of 6×5.5×2.5 m and a volume of approximately 83 m 3 . 
     This represents a considerable reduction in dimensions in relation to the size of the prior art systems, which are based on static mixers, serpentine pipes or stripping towers. 
     FIG. 3 shows an example of a dynamic mixer  27 ,  28  in accordance with the present invention. It consists of an external cylindrical casing  5  with internal guide vanes  2  and a rotor with rotor vanes  1  which are designed to be driven by a motor  4  (see FIGS.  1  and  2 ). Liquid flows into the mixer at the lower end and out through the other (upper) end (not shown). Gas is added through an inlet  13 ,  14  in the side of the casing near the water inlet. 
     By means of the rotation of the rotor with the rotor vanes  1  and the “cutting” of the liquid/gas current against the guide vanes, very good splitting and distribution of gas bubbles in the liquid are achieved. The advantages of such a dynamic mixer over a static mixer are manifold: 
     it has a wider working area, i.e. the quantity of liquid and gas and the ratio between them can be varied almost without limitation, 
     the intensity of the turbulence in the liquid flowing through can be adjusted freely by varying the RPM of the rotor, 
     the pressure drop through a dynamic mixer can be eliminated by means of an added agitation effect while it is relatively high through a static mixer. 
     Moreover, calculations show that the total investment costs will be lower for a solution with dynamic mixers as described in the present application than for conventional systems with, for example, static mixers. 
     Example: 
     A pilot system was tested with a view to removing O 2  from crude water. The test setup had the following data: 
     Number of stages: 
     Two dynamic mixers with a diameter of 190 mm, length of 2000 mm, and rotor speed of 300 RPM. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Capacity: 
                   
               
               
                   
                 Operating pressure 
                 Atmospheric 
               
               
                   
                 Circulating quantity of N 2   
                 55 Nm 3 /h 
               
               
                   
                 Reaction gas for catalytic N 2  purification 
                 H 2   
               
               
                   
                 O 2  content in crude water 
                 9 ppm 
               
               
                   
                   
               
             
          
         
       
     
     After the first stage, the O 2  content was reduced to 280 ppb. Moreover, after the second stage the O 2  content was reduced to 10 ppb, which is lower than the normal operating requirement of 20 ppb. 
     The present invention is not limited to mass transfer. It may also be used for a chemical reaction between gas and liquid with or without the use of a catalyst. 
     Thus FIG. 4 shows a flow diagram for a chemical reaction between gas and liquid, more precisely a system for hydrogenating or hardening oil, fat or fatty acids. 
     The liquid, oil or similar substance, which is to be treated is pumped, using pump  6 , through a quantity meter  29  and a pipe  30  to a first-stage dynamic mixer  27  with an operating motor  4  and a rotor  1  and from there, together with the gas, through a pipe  31  to a separator  32  and from there through a level-controlled valve  33  to a second-stage dynamic mixer  28  and from there, together with the gas, through a pipe  42  to a separator  49 . 
     From the separator  49 , the treated liquid flows through a level-controlled valve  34  and pipes with a stop valve  35  out of the system. The liquid may be recirculated back to the feed tank  37  through pipes and a stop valve  36 . The gas used in the process, H 2  or a mixture of H 2  and other gases, passes through a pipe  38 , a quantity meter  39  and valves  40  into the second-stage mixer  28  from where unreacted gas (together with the liquid) flows through the pipe  42  into the separator  49 . 
     From the separator  49 , the gas passes through a valve  41  to a first-stage mixer  27  and from there (together with the liquid) through pipe  31  into the separator  32 . 
     From the separator  32 , any unreacted gas flows through a valve  43  and out for reuse. 
     The system shown in FIG. 4 is of the continuous type with a feed tank  37 . This is provided with an agitator  44  which is driven by a motor  45 . Untreated liquid and a catalyst are added through pipes with stop valves  47 ,  48 . 
     The system may, instead of catalyst dispensed in liquid form, also be used with solid, fixed catalysts. In such a case, it will be expedient to arrange these catalysts in connection with the rotor and stator blades in the dynamic mixers. The latter mixers are, moreover, of the same type as stated above and shown in FIG.  3 . 
     Unlike all prior art reactors, the present system may be built as a multi-stage system with a counter-current between the gas and liquid from stage to stage and with a co-current in the individual stages. This provides great process-related advantages when the hydrogenation is to be taken so far that the oil is fully saturated with hydrogen in that there will be a large surplus of hydrogen in the last stage in relation to the unsaturated oil molecules. 
     With the present system with dynamic mixers, it is possible to achieve a reaction speed, calculated as the reduction in iodine value per m 3  per hour, which is 20 times as high as that of a conventional batch hydrogenation reactor, and the energy consumption is only ⅓ as high. 
     Another advantage is that the process can be controlled much better with regard to temperature, hydrogenation speed, etc. and the investment costs are lower. 
     Example 
     A test was performed with hydrogenation of oil in a  1 -stage pilot system in which the circulating quantity of oil was 300 litres per hour, the operating temperature in the mixers was between 180-195° C., the operating pressure was 5.5 bar and the dispersed quantity of catalyst was approximately 0.5%. 
     The performance figures, PR, which were measured were PR=340 for iodine values between 132 and 80 and PR=114 for iodine values between 60 and 45. The performance figures specify the reduction in iodine value per hour per m 3  reactor volume. 
     Compared with a conventional reactor, these are very high performance figures.