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GENERAL RESEARCH Improved Drag Reduction by Control of Polymer Particle Size Ralph Little,+Suzanne Smidt,t Paul Huang,§James Romans,”Joseph Dedrick,§and Jan S. Matuszko*it Navy Technology Center for Safety and Survivability, Naval Research Laboratory, Washington, D.C. 20375-5000, Versar Inc., Springfield, Virginia 22151, Geocenters Inc., Newton Centre, Massachusetts 02159, and Hughes Associates Inc., Wheaton, Maryland 20902
Solutions and slurries of the drag-reducing polymer poly(ethy1ene oxide) were injected into a turbulent flow of water in a piping system to determine if a polymer slurry provides an enhanced drag reduction lifetime over a solution. Drag reduction over time and distance was determined by measuring pressure differentials along the pipe length of the system. The slurry systems that possessed different particle size cuts were then compared with the solution injection results. It was found that the particle size content of the injected polymer slurry could be optimized to yield a reduction in skin friction (drag reduction) which was 17-19% greater, overall, than that observed for the injected polymer solution in the piping system under the same flow conditions. It is felt that the rate of production of freshly dissolved polymer arising from the dispersed slurry particles compensates for the degradation rate of the sheared dissolved polymer, thus maintaining a higher level of drag reduction per unit length of pipe.
Introduction Certain water-soluble polymers, by nature of their molecular structure and physical properties, when added to water in minute quantitites, have the ability to reduce friction, or the level of resistance to flow. Such polymers are generally characterized by the following properties: (a) high molecular weight, (b) overall linear structure, and (c) high solubility in water. Polyethylene oxide (Polyox, PEO) is one such substance. While it initially possesses excellent drag-reducing properties, it is one of the most fragile of all the water-soluble drag-reducing polymers. It is therefore a “worst-case polymer” with respect to its susceptibility to shear-induced degradation. In fact, degradation of this polymer has been demonstrated to occur at friction velocities as low as 0.03, 0.04, 0.07, and 0.10 m/s respectively by Paterson and Abernathy (1970), Ting and Little (1973), Fisher and Rodriguez (1971), and White (1970). It is generally accepted that the high shear field associated with turbulence is primarily responsible for polymer degradation. It is further presumed that the turbulent shear stress induces scission of molecular entanglements or even of individual molecules associated with the very high local shear rates produced by the turbulent flow. Ting and Little (1973) found that this degradation could be characterized through the development of a “degradation index” when plots of normalized molecular weight vs a dissipated energy function were analyzed. The most probable point for the scission of the molecule has been shown to be a t midpoint in the polymer chain (Ode11 et al., 1984), effectively halving the molecular weight and greatly reducing the drag reduction (drag reduction is not directly proportional to molecular weight although its in+ Naval
Research Laboratory. Versar Inc. Geocenters Inc. 11 Hughes Associates Inc.
t
*
dex is when corrections are made for the onset molecular weight (Little, 1971; Ting and Little, 1973)). The practical application of drag reduction is determined by the properties of the polymer-fluid system which is to be added to the boundary layer of the hydrodynamic flow in either external or internal flow configurations. In the case of high molecular weight polymer solutions, the concentration of polymer that may be added to the boundary layer, for example, by injection techniques, is frequently viscosity limited since polymer solutions of high molecular weight become extremely viscous-even at concentrations of less than 1% by weight of polymer. Moreover, since dissolved polymer is subject to mechanical degradation, early injection into the flow will result in ever decreasing drag reduction per unit distance away from the injection point. To overcome this difficulty, much higher concentrations of dissolved polymer would be required for injection than would be otherwise needed had mechanical degradation of the polymer not taken place. This then results in a dilemma for practical application of drag-reducing polymers, i.e., the dual problems of the mechanical degradation of the polymer and the viscosity limitations on polymer concentrations for practical boundary-layer injection. The resolution of the viscosity-limited polymer solution injection situation with its attendant high molecular weight degradation problems may lie in the preparation of “solution-rate-controlled particulates”. That is to say, while the solution rate of low molecular weight substances is roughly proportional to the total area of the dispersed particulates, the solution rate of high molecular weight polymers is confounded by the rapid solvation of polymer segment functional groups on the surface of the particles. Solvation of these segment functional groups on the particle surface quickly leads t~ a well-anchored gellike coating (due to polymer entanglements) surrounding each particle that inhibits further dissolution of the particle (Voyutsky,
This article not subject to U.S. Copyright. Published 1991 by the American Chemical Society
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1978). In other words, the rate of solution of high molecular weight polymer particulates may effectively be terminated by the gel layer if the particle dimensions are significantly large, Le., of the order of several tenths of a millimeter or larger. On the other hand, solution will be extremely rapid if the particle dimensions are of the order of hundreths of a millimeter or less (as observed under the microscope at this laboratory for poly(ethy1ene oxide) of approximately 4 million in molecular weight). If the rate of particle dissolution is determined by its size or diameter, then it is conceivable that the rate of solution of a sample polymer may be controlled by specifying its particle size and particle size range or distribution. Small particles, almost immediately after injection into the boundary layer, should nearly instantly dissolve to produce the optimum drag reduction effect (corresponding to their effective concentration in the dissolved state). Larger particles should dissolve more slowly (as determined by their sizes and numbers) to replenish polymer lost by mechanical degradation induced by shear stress on the dissolved polymer molecules. In this manner, “roll-off” (defined as the decrease of polymer drag reduction with increase with in the flow parameter) might be eliminated or at least minimized. This technology has several applications that could be of use in both the military and civilian sectors. The most obvious application is to increase the speed of a vessel. PEO has good drag-reducing performance even at temperatures as low as 2 “C (Pike and Morgan, 1972), which could be an important consideration for cruising in Arctic ocean waters. The slurry can be formulated so that when injected into the hydrodynamic sublayer, drag reduction occurs along the length of the marine vessel and the last particles would dissolve just as the stern of the vessel is reached by the remaining slurry. When drag-reducing polymer is used, the power consumption is less than it is for a vessel moving through water, and energy costs can be temporarily reduced. In another case, introducing drag-reducing polymers into sewer piping systems would greatly decrease the friction through the line and increase the flow rate through the system. This might prevent backup by overloaded sewer systems during flood situations and perhaps negate the need for an expanded or replacement system. This report specifically deals with preliminary experimental designs that attempt to test the concept of “dissolving particulates matched to a specific hydrodynamic length” and to determine if drag reduction can be sustained along this hydrodynamic length. If the solution rates of polymer particulates are properly chosen, it should be possible to produce a viable concentration of freshly dissolving polymer sufficient to maintain the desired drag reduction level and to compensate for those polymer molecules that have been shear-degraded by their interaction with the flow. Experimental Section Experimental Considerations. A controlled rate of drag reduction in a pipe flow system can be obtained by injection of a slurry containing polymer particulates in an appropriate size distribution. The smaller particles dissolve faster than the larger ones, thereby allowing continuous release of drag-reducing polymer into the flow stream. Mechanical degradation, or “roll-off“, of the dissolved polymer resin also occurs as a function of shear stress and time, following dissolution of the particles, and the two effects counteract each other. Under steady-state conditions, these two factors may be equalized through particle size control in order to maintain a constant level
PROPOSED CONCEPT
UNCONTROLLED SIZE DISTRIBUTION
a1
TIME OR DISTANCE FROM SLURRY INJECTION
0
F 0
3
PREDICTION FOR CONTROLLED SIZE DISTRIBUTION
t3E
~
P
I
TIME OR DISTANCE FROM SLURRY INJECTION
Figure 1. Comparison of slurry types: uncontrolled wide distribution vs a selected controlled size distribution relevant to the hydrodynamic length.
of drag reduction. This concept is illustrated in Figure 1. Suspending the polymer particles in a slurry form provides the initial basis for the proposed time-release mechanism. The particles do not dissolve until injected into the water stream. Since undissolved polymers in particulate form will not interact with the flow, they will be protected from shear-induced degradation. Selection of an optimum particle size distribution may allow the drag reduction performance to be optimized and maintained for a specified time and distance. The slurry materials and preparation techniques, however, must be selected and executed carefully. The slurry medium must be of the proper density so that the particles remain suspended without settling yet sufficiently noninteractive with the polymer so that significant solvation of the particulates (which would result in a solution-resistant gelled surface coating) does not take place. The slurry must be mixed with a minimum amount of energy and kept protected from atmospheric contamination-especially by water-so that no degradation or particle solvation occurs before injection. Previous work by others involved large quantities of slurries consisting of uncontrolled particle sizes that were mixed with water and allowed to hydrolyze before being injected. This procedure caused the polymer to degrade prematurely. The current approach avoids this problem by injecting the slurry directly into the flow stream at the appropriate time. Another advantage of using slurry instead of solution is that higher concentrations of polymer can be readily suspended in the slurry than can be dissolved in the solution. Polymer Solution Preparation. The Polyox WSR-301 sample was obtained from Union Carbide Corporation. It was dissolved according to procedures instituted at this laboratory to prevent mechanical degradation of the polymer in the solution preparation stage (Little, 1971). Slurry Medium/Polymer. In this experiment, slurries consisted of a 25% anhydrous ethanol/75% anhydrous glycerine mixture (by volume), plus the amount of Polyox required for the run. The glycerine was of certified ACS grade (Fisher Chemical Co.) and the ethanol was of USP grade (Warner-Graham Co.). The polymer is essentially insoluble in this slurry medium. The density of the slurry
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DRAG REDUCTION APPARATUS C - - -_
I
-__ _ _ - _ _ _ - - -10
FT-
- _ _ _ _ - - - - - __----I
I
I
PRESSURE GAUGE WATER SUPPLY
SLURRY INJECTOR
... .................................................. Figure 2. Simplified diagram of drag reduction apparatus. Table 1. Polyox 301 Particle Size Distribution As Received size, pm weight, g % of sample
> 106 >90 >75 >63 >53 >45
>38
