Experiments in Coalescence by Flow through Fibrous Mats Lloyd A. Spielman*' and Simon 1. Goren Department of Chemical Engineering University of California, Berkeley, Calij. 947.20
Dilute oil-in-aqueous suspensions at high ionic strength have been coalesced by flow through glass fibrous mats having uniform fiber diameters. Incoming mean drop diameter (1-7 p), fiber diameter (3.5, 6.0, and 12.0 p), suspension superficial velocity (0.01 -1 .O cm/sec), mat thickness (0.1 and 0.3 cm), oil viscosity (44 and 462 cP), and preferential wettability were independently varied while measuring the degree of coalescence by Coulter counting and corresponding pressure drops. Measured single phase pressure drops are about 30% smaller than predicted b y theory. Pressure drops during coalescence were found to be independent of both incoming drop size and oil viscosity, as theory predicts. The relative permeability of the aqueous-wetted mats i s about 0.1 2, corresponding to a uniform coalesced oil saturation of 0.30 in the mats. Semiempirical correlations of a dimensionless filter coefficient against a dimensionless adhesion group are presented to describe the extent of coalescence.
Elsewhere (Spielman and Goren 1970b), we reviewed the literature dealing with coalescence induced by flow through porous media. We found that much careful experimentation was still needed to understand this complex process. N o s t investigators had not measured coalescence efficiency and pressure drop with systematic variation of crucial parameters such as drop diameter, medium fiber or grain diameter, flow velocity, wettability, and fluid properties. While the influence of some of these variables on coalescer performance was qualitatively understood, the available data were not sufficient for reliable design of practical coalescers. Especially lacking Lvere studies in which incoming drop size was varied and incoming and outgoing drop size distributions were measured. Even though drop size has been long recognized as a factor crucial to determining the likelihood of capture, most investigations were confined to measuring only overall removal or just qualitative evaluation of the degree of separation without regard to drop dimensions. Where drop size has been considered, the size range was narrow with counting and sizing techniques difficult and of limited reliability. I n the work to be described we coalesced well characterized oil in water suspensions by flow through glass fiber mats, the fiber diameter, mat thickness, preferential wettability, oil viscosity, flow rate, and incoming drop size being systematically and independently varied to study their influences on coalescence efficiency and pressure drop. Electrolyte type and concentration can also significantly influence coalescence through their control of double layer phenomena. I n our experiments we used electrolyte solutions sufficiently concentrated to suppress double layer repulsive forces. (Experimentation primarily aimed a t elucidating these effects a t lower electrolyte concentrations are now in progress.) Elsewhere (Spielman and Goren, 1970b), we presented a theoretical framework for coalescence induced by flow through porous media; in the preceding paper (Spielman and Goren, 1972) we derived expressions for the pressure drop and degree 1 Division of Engineering and Applied Physics, Harvard Vniversity, Cambridge, Mass. 02138
of phase separation. Our experiments will be interpreted by the predictions of the preceding paper. It is found t h a t for aqueous wetted fibers the measured filter coefficient, when made dimensionless, can be correlated with a single dimensionless group suggested by theory. Experimental Methods
Suspension Preparation. Relatively stable suspensions of submicron droplets were first formed by precipitation when solutions of silicone oil in acetone were rapidly mixed with much larger volumes of water. The mean droplet diameter was varied by adding varying small amounts of "03 t o the suspensions and then allowing coagulation of the (partially) destabilized suspensions to proceed for about 12 hr. Coagulation rates were sufficiently small that negligible further selfcoagulation of the suspensions occurred during their use in the coalescence experiments. While flowing to the filter mats, suspensions were further acidified with "03 t o eliminate electrolyte concentration differences between suspensions of differing nominal drop size. The final "03 concentration (0.6% by weight) was sufficiently large that the self-coagulation rate of suspensions in this concentration range was observed to be very rapid and independent of the acid concentration, indicating the effective suppression of electrical doublelayer repulsion (see, e.g., Kruyt, 1950). The composition and physical properties of the suspensions during coalescence are given in Table I. The oil phase volume fraction of 5 X was known from the amount of oil initially dissolved in the acetone, assuming virtually all the oil precipitated because of its small solubility. The oils were Dow Corning 510 silicone fluids, selected for several reasons. They are Xewtonian fluids available with different viscosities but similar in other properties. Their specific gravities are very nearly t h a t of the aqueous phase (see Table I), so that influences resulting from density difference were effectively eliminated in the coalescence study. The negligibly small density difference prevented settling and so aided in preserving the suspensions during their preparation and storage. The solubility characteristics of the oilInd. Eng. Chem. Fundom., Vol. 11, No. 1 , 1972
73
Table 1. Suspension Properties at 23°C
Composition of aqueous phase (vel %) Composition of oil phase
Water, 90% Acetone, 10% Silicone oil? in equilibrium with continuous phase 5 x 10-4
Volume fraction oil Weight % "03 introduced to prepare initial suspension of 1-p droplets (nominal) 3-p droplets (nominal) 7 - p droplets (nominal) Weight % " 0 1 in total system Aqueous phase density Phase density difference Aqueous phase vixosity Oil phase viscosities Interfacial tension
0.0 0.006 0.060 0.6 0.994 g/cm3 -0.01 g/cm3 1.07 cP 44 cp, 462 CP 40 i= 15a dyn/cm
Uncertainty is due to large correction for density difference between the two phases. (I
Table II. Properties of Glass Materials Used in Preparing Filter M e d i a Geometric Properties
Nominal Yarn fiber filament diameter, fi count
3 5 6 12
Mean diameter of filaments, P
Calcd root mean square diameter,
Calcd std dev of diameter,
P
1240. 408b 204b
3.5a 3.66 5.95 6.34 12.50 12.60 Physical Properties Composition : O w e n d o r n i n g "E" Specific gravity : 2.55b Softening temperature: 1240"Fb
P
1.3 2.3 17
Measd yarn weight/ length, 10-2 g/m
3.33 3.31 6.48
Glassb
a Hollingsworth (Owens-Corning). * Owens-Corning, Publication No. 1-GT-1375.
water-acetone system were such that the precipitation method initially produced very small droplets. The resulting drop size distributions of suspensions entering the filter were found to be approximately the self-preserving form predicted b y Friedlander and Wang (1966) for Brownian coagulation. The standard deviation in drop diameter was about 0.4 of the average diameter. Filter Mats. Filter mats were formed from low twist yarns of continuous filament glass fiber supplied by Owens Corning. Besides having importance in commercial filtration applications, media made from glass fibers are ideally suited to well defined filtration experiments because the fibers are smooth circular cylinders available with a wide selection of uniform diameters. Glass can be exposed to high temperatures for thermal treatment giving effective cleaning and mat formation. The properties of the fibrous materials are given in Table 11. The root mean square diameter was calculated from the measured yarn weight per unit length and the specified glass density. M a t s were formed by techniques similar to those of papermaking. The fibers were first thermally cleaned by baking them for several minutes a t about 750°C. A weighed quantity of fibers was then dispersed in a dilute aqueous solution of hydrochloric acid (about p H 3) using a Warning Blendor to 74 Ind. Eng. Chem. Fundam., Vol. 11, No. 1, 1972
form a slurry. The acid made dispersal easier. The criterion for proper dispersal was that the slurry be made free of fiber clumps but without chopping the fibers to excessively short lengths. The slurry was then poured into a Buchner funnel and drawn slowly by vacuum through a fine-mesh steel screen, leaving the fibers deposited on the screen as a 15-cm diameter mat. While still moist, the mats were removed intact from the screen and baked for about 45 min at a temperature of 825-850°C. This heat treatment provided the mats with sufficient wet-strength for the coalescence study presumably by annealing and slightly sintering the fibers. Observing the fibrous structure under the microscope revealed the fibers to be almost undistorted if the mats were not overbaked. If underbaked, the mats tended to redisperse when wet. From each of the 15-cm diameter mats two to four smaller mats of 5.00 cm diameter were cut using a circular cutter. The smaller mats were cut from those regions which appeared most uniform within the larger mat. Fiber volume fraction and mat thickness for each of the 5-cm diameter mats were calculated from the measured weight of dry mat and mat totally saturated with water and the known densities of water and glass. Fiber volume fractions are accurate to about 5% of their values. The accuracy of the calculated fiber volume fraction was considered to be governed by the reproducibility of the water-soaked mat weights as determined by repeated soaking and weighing. Direct measurement of mat thickness by caliper while in agreement with the above values was found to be less reproducible than the indirect procedure described above because the mats were only about 1-3 m m thick and exhibited small surface irregularities which averaged out in the indirect determination. Clean glass fibers were found to be preferentially wet by the aqueous phase. A silicone varnish treatment was applied to some of the mats to make them preferentially wet by the oil phase. The coating was Dow Corning 902 silicone varnish initiated with catalyst XY-61. Thermally cleaned mats were soaked in a solution of l o + volume fraction varnish and IO+ volume fraction catalyst in reagent grade toluene, air-dried to remove the toluene, and then baked a t 150°C for 10 min. The resulting mats had about 7 4 % varnish solids by weight and were preferentially oil-wetted. When treated mats were examined under the microscope, what appeared to be very slight local caking of the varnish on the fibers was observed; otherwise the fibers appeared to be unchanged by the treatment. The permanence of the treatment was tested by treating a Pyrex watch glass, having glass composition very similar to that of the fibers, with the varnish in order to give it a visible coating. The coated watch glass was then contacted separately with the (acetone-containing) oil and aqueous phases of the suspensions for a period of 1week each. No deterioration of the film was observed, Flow System.A schematic of the flow system is shown in Figure 1. I t s important features are the suspension and electrolyte reservoirs, the mixing system for combining " 0 3 with the suspension, the automatic flow rate control system, and the test section. With this apparatus the rate of flow of acidified suspension into the test section was controlled and maintained constant. The acid and suspension were mixed in fixed proportion independent of their combined flow rate. Mixing occurred just prior to passage into the test section. The suspension in the reservoir was subjected to minimal agitation. The system consisted of glass with short connecting sections of vinyl tubing. The test section consisted of a 2.40-cm i.d. Pyrex flange with faces machined flat. Dual pressure and suspension sampling
taps were positioned with their interior ends located a few millimeters from the upstream and downstream flange faces. T h e filter mats were supported with cotton cheese cloth drawn t a u t and fastened over each of the flange faces. Thermally cleaned glass paper rings fabricated from submicron glass fibers and also having a n inner diameter of 2.40 cm served as seals between the mat and flange. Counting and Sizing of Suspended Droplets. Droplet counting and size distribution analysis was performed with a Coulter counting system consisting of: a Coulter sampling stand with facility for 50-, 500-, and 2000-p1 sampling volumes, and fitted with 30- or 50-p diameter apertures; a NuclearChicago Nodel 55-1 particle counting system comprised of the power supply, aperture current supply, preamplifier, amplifier, discriminator, and pulse shaper; a Northern Scientific Model NS 601 pulse-height analyzer having 256 channels and having teletype and punched paper tape outputs. Calibration was made with Dow polystyrene and polyvinyltoluene latexes with 1,305-, 2.68-, and 3.49-p mean diameters. The minimum drop size reliably measured without elaborate precautions was about 1 p diameter. Because the Coulter apparatus was limited to drops having dimensions in the micron range, it gave no information about the distinctly larger (millimeter) coalesced globules released at the downstream face. The dimensions of the latter were, therefore, measured by observing samples of effluent under the microscope. Operating Procedures. When unvarnished mats were used they were thermally cleaned at 750°C just prior to mounting in the flange. Pressure drop as a function of flow rate was then measured using distilled, 0.45-p Millipore filtered water. When unfiltered water was used, the pressure drop was observed to increase significantly with time a t fixed flow rate, indicating the presence of particulate contamination of the distilled water supply. Prefiltering effectively eliminated this difficulty. Acidified suspension was then passed at a n arbitrary intermediate flow rate through the unvarnished mat until steady state was observed and the flow7 was stopped. The fiter mat was then allowed t o remain in the stationary suspension environment for about 12 hr. If this last procedure was not followed, the steady pressure drops observed during coalescence immediately following the clean water tests were found to be 10 to 20% lower than those obtained in subsequent experiments with the same mat under otherwise similar conditions. The mats apparently required several hours to adjust to the acidified suspensions and preliminary exposure of the mats greatly improved reproducibility of results. With varnished mats no preliminary single phase experiments using water were performed because the mats did not spontaneously saturate with water in the presence of air. Before mounting in the test section these mats were soaked thoroughly with the silicone oil used in the suspension. Fully acidified suspension was passed through the mats after mounting as before, and the mats were allowed to remain in the suspension environment for at least 12 h r prior to coalescence and pressure drop measurements. T h e total suspension throughput of each mat was limited to about 20 1. to minimize effects resulting from slow accumulation of solid particulate contamination. I n addition, all distilled water passed through the mats and used in preparing suspensions was filtered through 0.45-p Rfillipore filters. All acetone used was reagent grade. Flow rates were determined by collecting measured volumes of effluent from the test section over timed intervals. Pressure drops across the mats were measured using the manometer
VALVE POSITWS WRING OPERATION OPEN
CLOSED
I 2
6 10 I1
3 4 5
8
S 12
WRING SUSPENSION FILL
TO DRAIN
6
DURING 6 7
S 10
OPEN
Figure 1.
ACID
a
FI'&
4
5
CLOSED
Schematic diagram of flow system
which consisted of two vertical glass tubes connected, respectively, t o the upstream and downstream taps. T h e tops of the tubes were open to the atmosphere and t h e flowing liquid was the manometer fluid. A t moderate t o high flow rates, steady state was verified through the observed constancy of pressure drop and steady flow of outlet oil globules. Observations of the transient pressure drop in approaching steady state from an initially oil free state indicated the total throughput of suspension required t o attain steady state. At the lowest flow rates where steady state was difficult to discern, its establishment was based on throughputting a sufficient volume of suspension. Flow rates were varied in ascending order with occasional exceptions to confirm reproducibility. Upon establishment of steady-state conditions for each flow rate, samples for droplet count and size distribution analysis were taken from the downstream and upstream sample taps. The sampling rates were kept small compared with the throughput rates, so that flow conditions were insignificantly disturbed. Quantitatively diluted samples were analyzed using the Coulter apparatus. Droplet size was calibrated using monodisperse latexes with diameters close to those of t h e nominal drop diameters and electrolyte of composition the same as those of the analyzed samples. Measurement of the oil held up in each mat was done subsequent to mat dismantling after coalescence experiments were completed for a given mat. T h a t portion of each mat through which the suspension flowed was carefully cut out with a 2.40-cm diameter cutter, air dried a t room temperature, and further oven dried a t 110°C 0.5 hr to evaporate all the water but practically none of the nonvolatile oil. T h e sections were then weighed t o determine their total content of silicone oil. Ind. Eng. Chem. Fundam., Vol. 11, No. 1 , 1972
75
Table 111. Summary of Data Runs Symbol f o r Figures 1 1 and 1 2
Run no.a
33-35 3941 4244 4547 51-53 57-59 60-62
Nominal fiber diameter, dF, P
M a t thickness, L, cm
0 0
0
A V
v
Oil viscosity, CP
Range o f superficial velocities, q l , cm/secb
M a t s Unvarnished (Aqueous Wetted) 0,327 0.055 44 0.025-0.7 0.229 0.076 44 0.02-1 . 7 0.127 0.054 44 0.015-1.4 0.301 0.058 44 0.012-1.3 0.130 0,056 44 0.023-1.2 0.279 0.058 462 0,025-1.0 0.136 0.052 462 0.023-1.3
3.5 12.0 6.0 6.0 3.5 6.0 6.0
A
Volume fraction fibers, a
Range of filter coefficient, A, cm
10.0-30.0 0.6-25,O 1 .o-80.0 2.0-30.0 6.0-80.0 3,0-35,0 2.0-45.0
Max pressure drop, (Apl)tot, cm
of H20
Oil saturation after dismantling, Sz
87 31 31 57 53 57 34
0,289 0.317 0,303 0.261 0.304 0.285 0.320
M a t s Silicone Varnished (Oil Wetted) 36-38 0 6.0 0.313 0,059 44 0.01-1.5 0,8-25.0 56 0.330 48-50 A 12.0 0.235 0.076 44 0,026-1,8 0.2- 3 . 0 15 0,429 54-56 0 6.0 0,327 0.053 462 0.02-1.5 0.5-33.0 55 0,321 Each set of runs includes data for three nominal inlet drop diameters. b Flow rates corrected for flange geometry (Spielman, 1968).
12
I I U n t r r a t r d Mat Fiber Dlam * 6 p Nominal Drop Dlam * 7 p Oil Viscasity = 44 cp
-
E 10-
Q $
a
I
I a = 0.057 Mat T h i c k n r s r = 0.136 em S u p r r t i c l a l VI l o c i t y
0.072 cm/sae
A
A
L
1
AgAAfP
8-
A A
6 -
Effluent 011 Firs1 Observod
A
i
0’ a g 4 ; a 2
-JHomopeneour
A Ah’
0:
-
A , +
A O
I
I
Flow
I
I
I
I
Timr (minl
Figure 2. Change of pressure drop with time showing approach to steady state for an initially oil-free mat. Note considerable increase in pressure drop above that for singlephase flow
Table I11 outlines the ranges of flow rates, oils, and mat properties used in the coalescence study. Two oil viscosities (44 and 462 cP) were studied with more attention given to the 44-CPoil. Most of the experiments were performed using unvarnished mats, varnished mats were used mainly for comparison. A single mat was normally used for three data runs, each employing one of the three different suspension drop sizes (nominally 1 , 3 , and 7 p ) . A data run is defined as a complete spanning of flow rates with a given mat and a given batch of suspension. Experimental Results and Interpretation
General Observations. When a suspension of nonwetting oil was passed at constant flow rate through an initially oil free filter, the pressure drop was observed to increase, first gradually and then suddenly, before becoming constant a t steady state. A typical transient pressure drop curve is shown in Figure 2. The large increase in pressure drop during coalesence over that for water alone a t the same flow rate is characteristic. I n approaching steady state, effluent turbidity was observed to decrease. Globules of coalesced oil did not appear 76 Ind. Eng. Chem. Fundam., Vol. 1 1 , No. 1 , 1 9 7 2
in the effluent until shortly before steady state was attained (Figure 2). At sufficiently small flow rates coalesced oil was not a t first released as globules downstream but instead as visible globules exuding from and accumulating a t the inlet face. After a time, globiiles were released a t the outlet and accumulation a t the inlet ceased. The exiting globules were suprisingly uniform in size and appeared a t fixed points on the outlet face a t regular time intervals. This condition is interpreted as the steady state without oil backflow discussed in the preceding paper (Spielman and Goren, 1972). Upon increasing the flow rate, a portion of the accumulated oil a t the inlet flowed through the mat, as evidenced by the observation that the oil on the inlet face diminished in volume while a flood of oil globules appeared downstream prior to reaching a new steady state. ilt extremely small flow rates, oil was observed to accumulate a t the inlet without ever producing downstream globules, probably because very long times were required to reach steady state without backflow. This condition is interpreted as a pseudo-steady state with oil backflow, also discussed in the preceding paper. I n such cases, the total suspension throughput was more than sufficient to produce downstream oil globules if the flow rate had been larger, but sufficient oil apparently had not yet accumulated on the inlet face to give a pressure drop as large as ~,(SZ,) across the inlet. With preferentially oil-wetted mats, oil was observed to adhere to the downstream face, a globule of centimeter size occasionally separating from the adherent oil. At larger flow rates the nonwetting aqueous phase could be seen to form discontinuous globules within the adhering oil. These aqueous globules seemed to be produced in a manner very similar to that by which nonwetting oil was released into the effluent aqueous phase when unvarnished mats were used. The formation of water globules dispersed within the adhering oil is undesirable in practice and could be the mechanism of formation of “foam” encountered by Rose (1963). Oil Saturation. Measured oil saturations (fraction pore space occupied by oil) for the seven unvarnished mats are listed in Table 111. These saturations are constant to within about 10% of their average value of 0.30, independent of mat thickness, fiber diameter, and oil viscosity. The analysis of the preceding paper predicts a uniform saturation SZ Sz0 throughout the oil nonwetting mats, provided the suspended
IV. Summary of Pressure Drop Results
Table Single phase permeability, ko, I O 6 X cm2
Run no.
33-35 39-4 1 4244 45-47 51-53 57-59 60-61
Intercept, cm of HzO
Slope (Ap/q), cm of HzO/cm sec-l Two-phase One-phase
Relative permeability,
X a t s Unvarnished (Aqueous Wetted). 20.0 96.0 13.32 6.5 14.3 1.592 9.2 15.8 1.968 10.0 35.8 4.77 12.0 39.4 4.31 12.2 36.0 4.44. 8.5 18.9 2.12.
0.187 1.02 0.564 0.518 0.258 -
-
Dimensionless capillary pressure,
kl(SZc)/ko
fzlSzo)
0.139 0.111 0.125 0.133 0.112 0.123 0.112
0.66 0.62 0.53 0.55 0.39 0.68 0.50
M a t s Silicone Varnished (Oil Wetted)b 36-38 18.8 25.8 4.77. 0.19 0.71 8.3 3.9 1.592. 0.41 0.66 48-50 54-56 20.6 22.2 4.77. 0.22 0.76 a Average relative permeability, kl(S2,),lko = 0.12: average value of j2(S2c) = 0.59. Average relative permeability, k ~ ( S ~ , ) l l=z 0.27; average value of f2(S,,) = 0.71. c Computed from mats having properties very similar to those tested during coalescence.
60I
I
-
RUNS 4 2 . 4 3 . 4 4 OIL NON -WETTING d p 6 p L:O.I27cm
-
k
-
$501
RUNS 36.37,38 OIL WETTING
44 *: 0.059 6 2 L:O.3I3cm 4 4 cp
L
a
-m
SUPERFICIAL VELOCITY
e B
O
L
-~
SINGLE PHASE FLOW\
’
I
I
I
oil volume fraction is very small and the oil-to-water viscosity ratio not too large, i.e., fo
