Method for separating immiscible fluids of different density
BACKGROUND OF THE INVENTION 
     The efficient separation of fluids of different density is central to the efforts of industry to improve our environment by reducing the pollutants entering the atmosphere and to maintain the safety of working environments. Typical of such materials are such things as iron and steel furnace fumes and dust, ground limestone, carbon black H 2  SO 4  mists, pulverized coal, pigments, oil smoke, fly ash, foundry dust, alkali fumes, metallurgical fumes and dust, magnesium oxide smoke, zinc oxide fumes and the like. 
     Some of these can be separated by means of dry mechanical collectors such as a settling chamber, cyclone, fabric collector (bag filter) and electrostatic precipitator. Others must be separated by means of a wet scrubber such as a spray tower, packed tower, flooded mesh scrubber, reverse jet scrubber, wet cyclone, venturi or orifice scrubber. 
     Frequently, wet and dry collectors are combined, e.g., by (1) scrubbing gases containing a dispersion of either fluid or solid particulate matter with a liquid and then (2) separating the resulting dispersed liquid phase in an inertial separator, such as a cyclone or vane demister. 
     One important consideration in the design and use of separator systems is the physical space which is available for the system. For example, any machine mounted system for the removal of coal dust from the air at the face of a coal mine must be extremely compact in order to be able to fit on a conventional mining machine. Thus, counter-current, non-atomizing scrubbers, because of their large space requirements, cannot be used in this application. On the other hand, compact co-current atomizing scrubbers with separators which would meet the limited space requirements tend to incur re-entrainment. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention is therefore directed to a system for the separation of immiscible fluids of different density by which a quite high degree of efficiency can be obtained from small centrifugal separators. 
     In particular, the invention is directed to a method and apparatus for separating a mixture of immiscrible fluids of different density in which a higher density fluid phase is dispersed within a lower density fluid phase comprising passing the mixture of fluids through a centrifugal separator having an N D  value of between 50 and 150 under specified conditions of non-atomizing flow. 
     More particularly, the invention is directed to a method for separating a flowing mixture of immiscible fluids of different density in which a higher density fluid phase is dispersed within a lower density fluid phase comprising passing the mixture through a centrifugal separator having an N D  value between 50 and 150 under conditions of non-atomizing flow such that the Froude Coefficient of the fluid mixture at the separator inlet lies within the enclosed area defined by points A, A&#39;, B&#39; and B in FIG. 1 hereto. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the invention, it has been found that efficient separations of immiscible fluids can be obtained in compact separation systems by passing the fluid mixture under conditions of non-atomizing annular flow into a centrifugal separator having an N D  value of between 50 and 150. 
     The invention will most often be used for separating mixtures in which the higher density fluid is a liquid and the lower density fluid is a gas. The invention can also be used for the removal of particulate solids contained in either the dispersed or continuous fluid phase. In certain instances, the particulates (whether solid or liquid) may be contained in the dry influent gas, become wetted by the scrubbing liquid and then be removed by separation of the liquid and gaseous phases in the separator. In other instances, particulates may be formed as a result of passage of a dry influent gas through a scrubber in which the particulates are formed by absorption, condensation due to heat transfer or chemical reaction with the scrubbing liquid and then dispersed within the liquid phase. The particulates are then removed by separation of the liquid and gaseous phases in the centrifugal separator. Thus, one criterion for the fluid systems which can be treated by the invention is that they be comprised of immiscible fluids of different density and that the flow to the separator be within the range of non-atomizing annular flow as defined hereinbelow. It is, however, preferred to employ fluid mixtures in which the ratio of the densities of the higher to the lower density fluid is at least about 2. 
     As used herein, the term &#34;non-atomizing annular flow&#34; refers to that region of flow enclosed within the area defined by points A, A&#39;, B&#39; and B of FIG. 1 hereto, which is a flow map which plots the dimensionless gas velocity through a duct versus the dimensionless liquid to gas ratio. Various other parameters may be contour plotted as a third dimension, such as flow regime, pressure drop, suspended liquid entrainment, entrained particle size distribution and so forth. It was, however, unexpectedly found that atomization within liquid-from-gas separators usually controlled their performance rather than flooding limitations. Thus, by design manipulations to minimize atomization both within the separator and within its feed duct, the potential capacity of various types of inertial separators can be substantially increased toward the flooding limit potential while still maintaining entrainment below 1,000 ppm by weight. The key parameters in the flow map are the gas and liquid Froude Coefficients (e.g., dimensionless velocities) and the property index (dimensionless combination of physical properties). 
     The various terms of this correlation are defined as follows: ##EQU1## ρ G  = density of lower density fluid (mass/length 3 ) ρ L  = density of higher density fluid (mass/length 3 ) 
     G = mass flow rate of lower density fluid (mass/length 2  ·time) 
     L = mass flow rate of higher density fluid (mass/length 2  ·time) 
     E = void fraction of the conduit (length 3  /length 3 ) 
     S = surface per unit volume (length 2  /length3) 
     g = local acceleration due to body force (length/time 2 ) 
     The property index-modified ratio of F L  to F G  is defined by the expression ##EQU2## , wherein 
     F L  and F G  are defined as is given hereinabove. ##EQU3## μ L  = viscosity of the higher density fluid (mass/length·time), and 
     δ = surface tension of the higher density fluid (mass/time 2 ). Unless expressly stated to the contrary, all Froude Coefficients herein are based upon an arbitrary entrainment threshold of 1,000 ppm by weight. 
     A further criterion of the fluid mixtures which may be separated in accordance with the invention is that they enter the separator in a state of non-atomized annular flow. 
     Those fluid mixtures which are not already in a state of non-atomized annular flow can be classified in three ways according to the nature of the dispersion: 
     1. streams to which higher density fluid must be added in order to place the higher density phase in non-atomized form; 
     2. streams from which higher density fluid must be removed in order to place the higher density phase in non-atomized form; and 
     3. streams which require mere coalescence of the higher density fluid in order to place the higher density phase in non-atomized form. 
     In the case of the first of the above three categories, it is frequently possible to add the requisite amount of liquid directly to the fluid conduit. However, the same function can also be carried out by passing the fluid through a non-atomizing scrubber. This type of operation is likely to arise when using the separator of the invention for removal of entrainment from a countercurrent contactor such as a spray tower. 
     In the case of the category requiring reduction of the liquid content of the fluid mixture, the reduction in liquid/gas ratio can be readily handled by means of a knockout chamber or trap of appropriate design to remove less than all the liquid. This situation is likely to arise when separating fluid mixtures from cocurrent heat exchangers such as partial evaporators or partial condensers. 
     For those streams requiring mere coalescence, simple mesh type coalescers may be used as well as non-atomizing scrubbers which would, of course, function merely as coalescers, not scrubbers. The situation in which the denser phase is dispersed in a non-equilibrium atomized form will arise mainly as a result of close coupling of the separator with an atomizing contacter such as an orifice scrubber. 
     Non-atomizing scrubbers, which normally operate within the range of annular flow, do not require prior atomization of the scrubbing liquid. Instead, such scrubbers utilize the turbulence of the flow to bring about intimate contact between the scrubbing medium and the lower density fluid (usually a gas). One preferred form of non-atomizing scrubber is the so-called &#34;flooded mesh scrubber&#34; disclosed in U.S. Pat. No. 3,370,401 to Lucas and Porter. In the flooded mesh scrubber, efficient contacting between the lower density fluid and the scrubbing medium is achieved by passing the gas cocurrently with the scrubbing medium through a fibrous bed at above the flooding velocity, i.e., within the regions of annular flow. 
     Another preferred form of non-atomizing scrubber is the so-called &#34;reverse jet scrubber&#34; disclosed in U.S. Pat. No. 3,803,805 to Low. In the reverse jet scrubber, efficient contacting between the lower density fluid and the scrubbing medium is achieved by passing the gas countercurrently to a non-atomized spray of liquid scrubbing medium and then cocurrently into a separator. Neither scrubber utilizes prior atomization of the scrubbing medium and both scrubbers utilize annular flow during normal operation. 
     An important advantage of the invention is that it is effective without respect to orientation. That is, it does not depend upon the direction of the local gravity vector with respect to the equipment axis. Thus, it can be used in a moving environment in which the device is pitching, rolling, yawing or bouncing. In fact, it can even be used in an environment of zero net acceleration due to gravity since differences in density requirement for separation (body force) can result from inertial as well as gravitational forces. 
     Separators which can be used in the method of the invention are centrifugal (cyclone type) separators characterized as having Diemer Numbers (N D ) of between 50 and 150. 
     The term &#34;N D  &#34; is a dimensionless number which is dependent upon the weight ratio of the higher density fluid to the lower density fluid and the geometry of the separator and is independent of both the throughput and inlet gas velocity. Thus, for a given fluid density ration, the value of N D  is constant whether the separator size is large and the pressure drop is low or the separator size is low and the pressure drop is high. 
     N D  is defined by the expression ##EQU4## , wherein 
     V = internal volume of the separator (length 3 ) 
     Q g  32  throughout volume (length 3  /time) 
     ΔP = pressure drop over the separator (force/length 2 ) 
     g C  = force mass conversion constant (mass·length/force ·time 2 ) 
     ρ G  = density of lower density fluid phase (mass/length 3 ) 
     As used herein, the term &#34;non-atomizing&#34; refers to a condition in which the expected particle relaxation time is at least about ##EQU5## 
     Particle relaxation time, which is equivalent to particle inertia relative to gas drag, is a measure of the rate at which a particle of higher density fluid responds to a change in the flow direction of the lower density fluid. By way of comparison, for ambient air and water, a particle relaxation time of 0.07 second corresponds to a particle size of about 150 micrometers. 
     Relaxation time is defined by the relationship ##EQU6## ,wherein 
     ρ L  = density of higher density fluid (mass/length 3 ) 
     μ G  32  viscosity of lower density fluid (mass/length·time) and 
     d = volume surface mean diameter of the higher density fluid (length) wherein ##EQU7## 
     A still further criterion of the invention is that the fluid flow therein, which enters in a condition of non-atomized annular flow, not become atomized during its passage through the separator. Such atomization is not, however, a problem so long as the separator does not contain regions in which the relaxation time of the dispersed higher density phase is maintained at a level below the atomization threshold. 
    
    
     The invention is exemplified by the Examples, each of which refers to the Drawing, which consists of three figures, as follows: 
     FIG. 1 is a graphical correlation (flow map) in which various flow regimes are plotted as a function of the Froude Coefficient of the lower density phase and property index-modified ratio of the Froude Coefficients of the higher density fluid phase to the lower density fluid phase. (In the examples below, this abscissa value is for convenience referred to as X.). 
     FIGS. 2 and 3 are cross sectional elevations of typical separator systems in which the method of the invention is used. In particular, FIG. 2 illustrates a centrifugal separator for use in separating very small amounts of liquid from the outlet of a venturi scrubber. FIG. 3 illustrates the use of a centrifugal separator in conjunction with a non-atomizing scrubber. 
    
    
     EXAMPLE I 
     A commercially available cylindrical non-reversing cyclone having an N D  value of 149 was used in a fluid system in which no more than 10 ppm by weight of liquid entrainment could be tolerated in the outlet gas. It was found quite unexpectedly that the performance of the separator was not significantly affected by gas throughput at the separator inlet. It was, however, drastically affected by conditions at the throat of a venturi scrubber located upstream of the cyclone. Upon testing the operation of the cyclone with throat sizes of from 2.2 to 4.4 cm, it was shown that operation with a relaxation time greater than about 0.07 second results in no measurable entrainment, i.e. below 10 ppm by weight liquid in gas. The maximum Froude Coefficient in the vessel (0.24) is not limiting. Thus, separator capacity could be increased. Apparent maximum practical Froude Coefficient for the vessel with no measurable entrainment would be about 0.44. This value indicates that operation of the cyclone according to the invention would permit throughputs more than three times as high as the nominal design for which the separator was sold. The corresponding Froude Coefficient at the cyclone inlet would be about 7 (X =  0.01 to 0.06). 
     EXAMPLE II 
     Centrifugal separators having N D  values of about 70 were constructed to take advantage of non-atomizing inlet conditions and the physical proportions were adjusted to obtain minimum entrainment. For instance, different outlet sizes (Dimension A in FIG. 3) were compared using constant inlet and body diameters of 5.1 and 15.2 cm respectively. The data for these tests, which are given in Table I below, when applied to the flow map of FIG. 1 indicate that these separators are operating at virtually maximum potential throughput using gravity drainage. Minimum superficial gas residence time is about 0.07 second, which is on the order one-tenth the design residence time for the commercial cyclone separator described in Example 1. 
     
                                           TABLE I                                 
__________________________________________________________________________
EFFECT OF OUTLET VARIATIONS ON ENTRAINMENT                                
Ratio               S/E                                                   
Outlet/Inlet                                                              
       Gas Flow                                                           
             Liquid Flow                                                  
                    (inlet)                                               
                        M   Inlet                                         
                                 Body                                     
                                    Entrainment                           
Diameter                                                                  
       cm.sup.3 /sec                                                      
             cm.sup.3 /sec                                                
                    cm.sup.-1                                             
                        (inlet)                                           
                            F.sub.G                                       
                              X  F.sub.G                                  
                                    ppm wt.                               
__________________________________________________________________________
2.0     88,000                                                            
             250    4.45                                                  
                        0.0025                                            
                            10                                            
                              0.085                                       
                                 0.91                                     
                                    10                                    
2.0    102,000                                                            
             300    &#34;   &#34;   12                                            
                              0.088                                       
                                 1.2                                      
                                    1,000                                 
2.5    102,000                                                            
             300    &#34;   &#34;   12                                            
                              0.088                                       
                                 1.2                                      
                                    10                                    
2.5    119,000                                                            
             350    &#34;   &#34;   14                                            
                              0.089                                       
                                 1.4                                      
                                    1,000                                 
__________________________________________________________________________
 
    
     EXAMPLE III 
     A plant producing metal halides by chlorination of ores recovers product by direct contact condensation using recirculated chilled metal halide. Tail gases must not contain significant amounts of liquid halide entrainment to avoid problems with the fume disposal system. To increase the capacity of existing non-reversing cyclones of the type shown in FIG. 2 (N D  = 97, F G  inlet = 7.5, X = 0.02), a reverse jet cooler is placed in the inlet to the separators by which X is increased to 0.4 and the Froude Coefficient can be increased to 13. This provides an increase in the capacity of the cylones of about 70%. 
     It will be recognized by those skilled in the art that the location of the boundaries of the regions in FIG. 1, other than the boundary of the atomizing region, are fixed within the usual scatter of two-phase flow data. On the other hand, the location of the boundary of the atomized region on the flow map is a function of the numerical value of M, the dimensionless property index, as is indicated above by the defining equations for relaxation time. 
     It will also be recognized that the term S/E as used herein is defined in a new and more general way than is customary. ##EQU8## wherein 
     (S/E) p  = surface/volume ratio of any packing present (length 2  /length 3 ) and D s  is the superficial duct diameter (length).