Fractionation of fine particle slurries using periodic flows in a spinning helix

Abstract

An unusual method of separating or fractionating fine-particle slurries has been proposed and partially explored by Landin (1980), Papanu (1983Papanu ( , 1986, Menon (1983), Lennartz (1984), and Lennartz et al (1987). A helically coiled tube spinning steadily on its major axis is connected to two agitated reservoir pumps which cycle a slurry bidirectionally through the helix as shown in Figure 1. With appropriate pumping steps, slurries can be concentrated or fractionated. The operation can be made continuous by adding feed at F and withdrawing products at I and 11.Research is motivated by the prospect of achieving sharp separations with fine particles smaller than about 10 pm. The technique is inherently multistage; the number of theoretical stages is approximately equal to the helix tube volume divided by the fluid volume pumped per cycle. The number of theoretical stages increases with reduced pumping volume.In this work, the method is further explored for fractionation.Early investigators found that separation was hindered by material accumulation in the helix, caused by ineffective particle resuspension, an essential step of each cycle. A new equipment design provides more effective particle resuspension by reorienting sedimented particles in the centrifugal field. A mathematical model is developed to describe and simulate the process. Data from a batch fractionation experiment using a sand/kaoh-clay slurry is reported. Fractionation-Six-Step SequenceA slurry containing two types of particles, A and B, with different settling rates, is fractionated by convecting particle-free fluid, slurry containing B, and slurry containing both A and B, a t By the end of this step, secondary flows are generated, due to an imbalance of centrifugal forces in the helix cross-section. These secondary flows resuspend sedimented solids in step 5. If these secondary flows are inadequate for particle resuspension (the particles collect at a stagnation point), the helix cross-section may be reoriented in the centrifugal field to move sedimented solids back into the fluid stream during step 5. Figure 2 shows these secondary flows, axial flows, and centrifugal forces, for cases where the fluid flow opposes or complements the spinning direction. In step 6, resuspended A and B particles are convected leftward with fluid, toward the A-rich reservoir. The sum of volumes pumped left in steps 4 and 6 equals the volume pumped right in step 2; no net fluid movement occurs. At the end of the sequence, there has been a net movement of B particles toward the right reservoir and A particles toward the left reservoir. Repeating the sequence indefinitely, increases the degree of fractionation until steady state is achieved with balance axial dispersion and axial separation effects. ModelThe six-step fractionation cycle is modeled by material balances for the suspended and the sedimented particles. Since the helix fluid velocity profiles are not known precisely, the model detail is limited to a one (space) dimensional axial dispersionconvection ap...

A Statistical Evaluation of the Frictional Pressure Losses Correlations for Slurry Flow

“…Their flow is very complex to model due to the existence of two-phase flow consisting of base gel and solid proppant. In coiled tubing, it is even more complex due to complex flow regimes and possible separation of solids from the suspending medium under the effect of centrifugal forces in the curved flow geometry [1,2]. This complexity may lead erroneous results when calculating friction pressure losses for slurry flow to calculate the resulted bottom hole treating pressure and hence the net pressure.…”