LIQUID ENTRAINMENT SEPARATORS G. M. KIRKPATRICK Blaw-Knox Company, Pittsburgh, Pa.
T
surface of water vigorously boiling in a teakettle or in an evaporator is characterized by discontinuous rather than uniform motion. Geysers boil up and subside. Surging waves swing from side to side and are broken up by geysers. Large bubbles of steam or vapor break through the surface and explode, throwing large, small, and infinitely minute drops of liquid into the vapor space. Large and small drops of liquid are projected from the surface, arching upward 2, 3, or more feet. Drops strike and combine, and are frequently broken into a mixture of small and large drops by the impact. Particles dance about in space invisibly supported, finally to fall or be carried away.
The purpose of this article is to delineate the major factors affecting the design and operation of liquid entrainment separators. Consideration will be divided between factors inherent in the specific separator and factors which influence the design but which are imposed by conditions surrounding the point of installation. Principally an attempt will be made to visualize what goes on within and about a few types of separators without benefit of mathematical analysis.
With increasing concentration of the liquor there is change of conditions. Foam may be seen collecting along the outer edges of the liquid surface. Eventually this may build up to a deep rolling viscous blanket or it may have the appearance of an unstable tenuous frothy mass. At the surface of this blanket bubbles burst, spraying droplets and minute .bubbles into the vapor space. A strong light projected through the observation window shows that few of these smaller particles fall back to the foam or liquid. They are swept away in the vapor passing to the outlet. These conditions have been observed in evaporators, and it seems logical to relate them to boilers and certain types of tubular evaporators. However, in so doing, the mental picture must be modified, especially considering the mass velocity of the mixture of vapor and liquid that must exist within the heating tubes. This, in turn, must be related to the resistance which this mass will encounter if released below the surface of a fluid mass composed of liquid and bubbles of vapor, or if released into a free vapor space above the liquid level.
Only a casual consideration will indicate that localized areas of high liquid and vapor velocity are inherent in this type of equipment. And higher velocity means more turbulence, smaller particle size of liquid, and more energy to be dispersed before gravity can effect partial separation of the liquid and vapor phases. Moving on from the liquid surface or the heating tube outlets, it is apparent that there is a change in field effects as approach is made to the separator or the vapor outlet connection. Localized zones of high velocity tend to be smoothed out. Average vapor velocity increases. Liquid drops, which a t lower elevations would fall, are now conveyed by the energy of the vapor stream. Drop or foam concentration per unit of volume may still be highly variable. The velocity of the conveyed particles becomes so high that visual observation of discrete particles is difficult or impossible; if the larger of these drops strikes a stationary surface it may break into smaller drops which are more easily conveyed. Although it may not be possible to see what is going on within the overhead vapor line (steam header or pipe line), we can visualize mentally what must be taking place. Economically, streamline or laminar flow is out of the picture. Turbulent. flow may be visualized as the vapor stream rolling over and over itself. Small drops of liquid are projected along paths of relatively small deviation; the heavier or agglomerated drops fall to the bottom of a horizontal pipe. Smaller drops of liquid are bounced about, colliding with other drops or with the pipe walls, bouncing away, breaking up into smaller drops, or being coalesced into larger drops. At points of high turbulence such as a valve, the energy of the vapor stream plus the pressure loss across the fitting may whip drops of liquid into fog particles.
Factors in Mechanical Liquid Separation If the background of these observations is related to the problem of mechanical liquid separation, then some or all of the following factors need consideration: I. Vapor phase A . True vapor phase 1. Specific gravity 2. Operating density 3. Proximity to critical temperature and pressure 4. Absolute viscosity 5 . Chemical nature especially related to its entrained liquid B. Velocity of vapor phase 1. Mass velocity 2. Kinetic energy 3. Turbulent flow 1207
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6 . Critical velocity a. Ratio of pipe wall circumference to cross-sectional
area
b. Roughness of conduit or walls c. Unit volume contact or ratio of surface contacting
vapor stream to unit volume flowing
C. Vapor pressure relations of liquid-phase and vapor-phase components
D. Thermodynamic changes of state by heat radiation to or
from vapor stream 11. Liquid phase A . Aerosols
B. Foe C. Mi% D. Droplets E. Large drops F. Liquid stream G. Slugs or gobs of fluid H. Combination of A to G , inclusive I. Foam J. Individual large and small bubbles K. Agglomeration of discrete particles and bubbles L. Atomization of drops and breaking of bubbles M . Surface tension of liquid N . Weight per cent in vapor 0. Volume per cent in vapor P. High specific surface, low specific surface os. small mass, Lrge mass and kinetic energy Q. Mass velocity R . Proximity of liquid velocity to vapor velocity III.-The above items must also be considered in relation t o the confining pipe walls or walls of separating unit or any separating elements interposed in the vapor stream with re- . spect-to : A . Relative roughness or smoothness of walls B. “Wettability” of walls or interposed baffles C . Disposition of surfaces influencing formation of eddies or vortices. or zones of high velocity D. Liqiiid immurkment zones E . Dispersion of kinetic energy of both liquid and vapor phase F . Possible high-impingement zones causing atomization G. Process characteristics which may cause fine atomization in front of separation device H . Collection or direction of dried vapor stream I . Collection or direction of separated liquid J . Possible reentrainment zones K. Natural adherence ofliquidtoseparating elements and magnitude of this adherence in relation to drag of vapor stream past relatively stationary liquid L. Length of time liquid is in contact with vapor and bulk of this liquid related to volume of immurement zones
FIQVRE 1. CONBTRUCTION OF A COMBINATION CENTRIFUGAL AND CONTACT SURFACESEPARATOR
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M . Effect of and loading imposed on successive separating stages N . Effect of flow distribution means after separating elements 0. Thermodynamic changes of state due to pressure loss across separating elements and across distribution means
Stated briefly, the design of a separator is principally concerned with velocity of a vapor or gas, amount of entrained liquid, size of the liquid particles, distance these particles must traverse the vapor stream to be caught by a separating baffle or surface, immurement of assembled particles from the vapor stream, ratio of liquid density to vapor density, and ratio of total contact surface t o volume of vapor within separating elements in unit time. If these principles are utilized as the basis for the analysis of any mechanical separator, they will help toward ascertaining the practical value of the apparatus. Most of the better separators take advantage of two or more separating forces. These forces may be gravity, centrifugal force, contact surface, fluid surface tension, impact separation (dependent on kinetic energy and surface tension), and electrostatic precipitation. I n general, however, mechanical separators are broadly classified as centrifugal or contact surface types. This discussion will.be confined to but two of the many in use. One is a combination centrifugal and contact surface type. The other is almost purely a contact surface type.
Combined Centrifugal and Contact Surface Separators I n this type (Figure 1) a moderate degree of centrifuga1 force is imposed on the particles transported in the vapor stream; at the same time they are exposed to contact with a
CATCH POCKET INTERMEDIATE
CATCH POCKET TERMINAL
VANE SPACER
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approximately 0.3 gallon per million cubic feet as the total plant usage, not measured as mechanical entrainment alone. This indicates gas that is 99.995 per cent drybyweight. Thoughts relating to capacity of this type must be modified by consideration of density ratios, allowable pressure loss, desired degree of separation efficiency, etc. Figure 2 illustrates a combination tar extractor and naphthalene absorber conditioning manufactured gas. The tar-laden gas enters a single-stage contacting or washing section in the lower part of the tower, where i t is washed with collected tar. It then passes through a swirl-head type separator, as described above, for separation of entrained tar. Leaving the separator it passes into a second contacting stage where the gas is forcibly intermixed with straw oil which has drained down from the wood grid section occupying the major upper portion of the tower. Although no quantitative determinations have been made a t the outlet of the swirl-head separator, i t has not been possible to discern any tar in the fat straw oil coming from the second-stage contactor.
FIGURE 2
relatively large amount of contact surface. Catch pockets or directed drainage areas are provided within and a t the end of each flow channel. I n this design careful consideration must be given to the velocity and relative density of the mixture entering the inlet throat, to accurate manufacture of the flow channels, and particularly to the design of the distribution means a t the exit of each channel. This distribution means must compensate for the kinetic energy of the inlet stream and for the change in the velocity head (from bottom to top) in the annulus receiving the gas or vapor from each channel. As compared to the high-velocity centrifugal types, this design is characterized by moderate velocities, high liquid capacity, high efficiency over a wide range of vapor velocities, freedom from serious conflicting eddy currents or vortices, relatively narrow flow channels, low over-all pressure loss, relatively large size, moderate cost. The approximate capacity of standard sizes of this type of separator in cubic feet per minute of standard gas flowing a t the gage pressures stated is as follows: Size Unit,
In.
18 24
30 36 48 60
0 Lb. 650 1,500 2,400 3,400 7,300 11,000
10Lb. 800
2 000 3:lOO 4,500 9,600 15,000
1OOLb. 1,800 4,200
6,600 9,600
2oooo 31:200
250 Lb. 2,700 6,500 10,200 14,500 30,800 48,200
These capacities are related to natural gas containing oil, and the separator is required to effect the delivery of gas containing not over 1 gallon of mechanically entrained oil per million cubic feet of gas. This may be stated as the delivery of gas containing not over 7 pounds of oil per 49,000 pounds of gas passed, or gas which is 99.986 per cent dry by weight. Average daily operation of many units in the field has shown
Contact Surface Separators In appearance and construction the contact surface type separator (Figures 3 and 4) is of necessity a precision-built machine. Its high separation efficiency is dependent on accurate spacing of all baffle rows and of the individual gutters and upon uniform distribution of steam or gas through the baffle banks. The four baffle rows illustrated are assembled into cartons of five standard sizes varying in length from 12 to 36 inches. The thickness and height of each carton are constant. These cartons may be mounted on special steam or gas conduits or they may be obtained as complete standardized sections con-
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FIQURE 3. PATH OF STEAMRIBBONSTHROUQH CONTACT SURFACE SEPARATOR
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FIGURE 4.
C O S T A C T SCRP.4CE SEPARATOR
sisting of two cartons and the collecting and distributing conduit for connection to a nozzle outlet. Complete sections may also be obtained for mounting below a diaphragm with a long variable slot in the top providing the vapor or gas passageway through the diaphragm. Both the sections and cartons may be easily dismounted, taken apart, cleaned if necessary, and reassembled. The individual baffle rows are made of alloy steel, the housing, of carbon steel and castings. As shown in Figure 3, this apparatus divides the vapor or gas stream into many thin ribbons, each 0.1 inch thick and 7 inches high. These ribbons flow between the gutters, combine in the expansion space back of the baffle row, redivide, and pass through the next baffle row; the openings through each row are staggered in relation to the openings of the preceding row. The surface of a cubic foot of steam passing through one baffle row and making contact with the stationary gutters amounts to 34,560 square inches. With four rows through which to pass, this would be multiplied four times; as a result of combining the ribbons three times, the mathematical chance for a droplet or bubble to remain in the center of a vapor stream or remaining out of contact with a gutter is exceedingly remote. Where operating conditions indicate the presence of very fine fog, it is practical to provide units of this type with one or more rows of baffles spaced to cause formation of ribbons of vapor only 0.05 inch thick. It would appear that pressure losses across the baffle bank would be extreme. Such is not the case a t the recommended operating capacities. In practice, the pressure loss across the baffle bank averages around 4 inches of water gage, with steam a t 150 pounds gage pressure. However, to obtain uniform flow of steam throughout each lineal inch of baffle surface, it is necessary to introduce means to effect uniform distribution of steam flow, and this usually increases the total pressure loss to about 1 pound per square inch. However, this factor is subject to a certain amount,of control, and satisfactory operation may be obtained with total pressure losses ranging from a few inches of water to a pound or more as service conditions require. It is difficult to outline the capacity of this apparatus with reasonable accuracy. Service requirements imposed on this device are usually severe. For instance, on a natural gas dehydration tower where the gas was thoroughly washed with a
ASSEMBLED AND
-4TTACHED T O
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DIAPHRAGM
calcium chloride-glycerol dehydrating solution, it was required that the exit gas contain not over 0.05 gallon (0.603 pound) of solution per million cubic feet (49,000 pounds) of gas, when passing 135 niillion cubic feet of standard gas per 24 hours flowing a t 200 pounds gage pressure. The scrubbing tower is 78 inches in diameter, and the separator of the above type is composed of 360 inches of equivalent baffle section. In 7 2 days of operation the total loss of solution from the plant amounted to 0.015 gallon or 0.181 pound per million cubic feet of gas passed. Assuming that this loss was all creditable to mechanical entrainment in the gas and not loss by leakage, it mould indicate a gas which mas 99.9996 per cent dry by weight. The guarantee amounted to 99.998 per cent dry. It is not always economical or necessary to design to such stringent requirements. Since this equipment is essentially a volumetric machine and is made up in multiples of standardized cartons, it is possible to fulfill most requirements withreasonable economy where fine separation is required, However, it is necessary to have initially a reasonably accurate knowledge of many of the factors influencing design, construction, and the end desired. Although the many elements of the problem of separation make it appear complicated, that intangible factor “experience” materially simplifies the task of obtaining the necessary data and designing to requirements. The contact surface separator described operates a t relatively low velocity, its characteristic curve of separation efficiency is maximum and flat up to a critical velocity, and space requirements are usually in excess of other devices; but because of precision construction and the multiplicity of small streams and baffles, it is possible to obtain a high degree of separation efficiency. Fine separation is not easy t o obtain. The expenditure necessary is economical or not, as the desired result is related to the plant or process. I n general, the more thoroughly the total problem is understood, the easier it is to arrive a t that “best compromise.” Entrainment losses may appear small numerically, but evaluated in terms of weeks and months of operation they will frequently reach surprising totals justifying a good investment in fine-separation equipment. RECEIVED June 13, 1938.
