The problem with solids is that they don’t want to move. This often results in solids settling out of slurries. The only thing preventing such settling is movement of the fluid, so we call slurries either settling or non-settling. The latter is defined by the saltation velocity or minimum suspension velocity. A 2006 CP article, “Avoid Trouble with Slurries,” describes some primary correlations. However, these correlations don’t address the issue of the pressure-drop/flow rate characteristics when working with solids that settle in pipelines or vessels. The flow of settling solids is a complicated affair.
Unfortunately, analytically based studies don’t offer practical guidance. Researchers have pushed back the boundaries of the unknown only a relatively short distance. Their correlations rely on between 10 and 20 pseudo-physical properties. However, the coefficients and exponents in these correlations are based on quasi-uniform particles over a narrow range of the relevant variables.
So, we must rely upon direct scale-up methods that have proven successful for non-settling solids. Increasing the velocity beyond the settling point (saltation or choking) is an option; however, this raises pressure drop and pipe erosion/particle breakage. Converting a settling solids suspension into a non-settling suspension is a better alternative to ensure successful scale-up.
You can consider several approaches to make solids that settle stop behaving so badly:
• Additives. Introducing high-molecular-weight polymers, soaps or deflocculating agents (generally electrolytes) into a slurry can reduce pressure drop. The use of additives is common in the mineral industry (drilling mud, phosphate rock, limestone cement and coal) to cut the pressure drop while keeping the solids concentration unchanged. Small amounts of polymers (10–100 ppm) have proven to decrease pressure drop in the transport of large coal particles. Soaps provide an option that doesn’t mechanically degrade as rapidly as high-molecular-weight polymers. Electrolytes reduce the zeta potential, which results in a lower head loss.
• Vibration. In certain situations, the oscillation of either the flow of slurry or the applied pressure can reduce head loss. Low frequency (5–10 Hz) tends to shift the peak velocity in a pipe from the center to nearer the wall, which aids in suspending the particles. This technique is most effective for non-Newtonian suspensions and decreases the terminal settling velocity. It has proven useful in inclined pipes where solids fold back on themselves. Vibration or oscillation of the pipe works but is much less effective than oscillation of the slurry.
• Air injection. For all Newtonian slurries in laminar or turbulent flow and for non-Newtonian slurries in turbulent flow, injecting gas into horizontal pipeline slurries will increase the pressure drop and shear on the slurry. However, this technique particularly suits shear-thinning fluids (i. e., pseudo-plastic non-Newtonian ones) because their viscosity decreases with increasing flow. The use of air injection on shear-thinning fluids can reduce the pressure drop or raise the capacity of a pipeline. Surprisingly, this technique hasn’t gained wide application.
• Fibers in suspension. One very specialized option is using high-aspect-ratio (10:1) fibers to reduce drag. The current experimental information on this subject is contradictory but studies in the pulp and paper industry show that low concentration slurries flow as a plug with a lubricating water annulus adjacent to the pipe wall. In some cases, the pressure drop is lower than that with liquid alone.
• Modified pipe geometry. Many studies have indicated that a circular cross-section pipe isn’t the most favorable for minimizing head losses in slurry flow. Among the alternatives evaluated are the use of segmented pipe of various geometries and the addition of helical ribs to the pipe. Some such configurations have reduced pressure drop by as much as a 20%. However, industrial acceptance has been poor, partially because these options increase both the weight of the pipe and installation costs. Also, there is fear that products may become contaminated or degraded.
If you have a troublesome settling slurry, there are many ways either to make it act like a non-settling slurry or at least increase the capacity of the pipeline. You can apply scale-up and design methods with confidence that the slurry pipeline will be trouble-free.
TOM BLACKWOOD is a Chemical Processing Contributing Editor. You can email him at TBlackwood@putman.net