THIS MONTH’S PUZZLER
We have a troublesome dilute-phase pneumatic conveying system for the cylindrical soap pellets we make (see Figure 1). It uses an airflow of 6,000 lb/h to move 30,000 lb/h of pellets to whichever one of four cyclones we choose. The duct from the bin to the inlet of the blower runs the equivalent of 840 ft horizontally and 290 ft vertically. The system was designed for a total pressure drop to the blower inlet of 78.2 IWC; blower volumetric efficiency is 60%. Every few weeks, our feed screw conveyor clogs and no pellets go into the system. The problem is worst during thunderstorms and least troublesome in the winter, although when the temperature drops our blowers can’t keep up. In addition, we often see salting (dropping out) of pellets when the level in the bin is lowest. Other symptoms are blinding in the dust collector and bridging in the cyclone rotary valves. Our operators think we should increase the airflow, which will require a larger motor. The plant electrical engineer says no because there isn’t a larger bucket in the motor control center. What do you think we should do to improve the system reliability?
MOISTURE IS YOUR ENEMY
Soap pellets containing KOH and NaOH are hygroscopic and could cause conveying problems, especially with relatively wet air. The figure shows there is no means to remove moisture from air. As a short-term fix, look into providing insulation and heat tracing on the duct. In the long term, consider installing a refrigerated dryer and water separator. This should cool air down to roughly 35°–36°F. If the system has no means to remove moisture, it may work fine on a hot day. However, even day and night variations in temperatures could cause moisture to condense in conveying lines and cause fines to stick to the walls of the conveying ducts. Again, the key seems to be dry air. If the problems are frequent and severe, you might consider using instrument air with a dew point of, say, -40°F. Of course, this change could be pricy. The figure shows that pellets from the screw conveyor fall in the hopper. This is an open system and it will entrain outside air, which not surprisingly is wet on a rainy day. Consider installing a barrier to eliminate contact with the outside wet air. Also, provide vents in this chute and use dry air to remove moisture through the vents in the chute.
There are other problems. You mention that the air supply may be inadequate. Typically, for granular hygroscopic material, air velocities are in the range of 13–15 meter/sec through the duct. Check the blower curve to determine airflow.
G. C. Shah, Sr. HSE Advisor
Wood Group Mustang, Houston
VALIDATE THE DESIGN CONDITIONS
The symptoms described seem to indicate problems with light particles. All a designer can do is pick the heaviest, largest particle and size the equipment accordingly. If the process of making, storing or moving the material creates more fines, then there will be problems. Fines have more surface area. They more easily accumulate moisture and act as glue to stick pellets together; these particles salt out. Available equations don’t estimate the saltation velocity, µ, reliably. It can be estimated, poorly, with the Rizk equation, given in “Pneumatic Conveying of Solids,” by Klinzing, Rizk et al., 3rd edition, 2010, which many consider the most general of the equations. Based on the Rizk equation, I calculated a μ of 20.1, which is well below the 29.8 given in the book as the cutoff for dilute-phase conveying.
However, before you do anything, validate the design conditions for the pneumatic conveyor system. Determine the fractions of heavy materials that could salt out at the pickup point below the bin discharge screw conveyor. Also, find the fraction of fines. You may want to look at ways to reduce the variability upstream of the soap bin.
Because you don’t want to replace the system completely, consider the following:
• To keep moisture from getting into the soap bin, dry the air in the product stream feeding into bin. Dry the air coming into the building. Keep the relative humidity below 30%, perhaps even lower, to prevent the soap pellets from bonding together.
• As a short-term solution, add a nitrogen purge. A purge at the dust collector may provide some additional run time.
Now, let’s consider some more-expensive options:
• Replace the straight screw conveyor with a tapered design that doesn’t allow an avalanche of pellets or parts of pellets to flood into the screw. A tapered screw conveyor with an increasing pitch and shaft diameter will provide an even flow of material from the bottom of the bin.
• Consider sifting the pellets before they get into the soap bin. Recycle the dust back to the soap dryer producing the pellets. You could use a pneumatic conveyor for this.
Replace the screw conveyor feeding the pellets from the dryer. Install a vacuum conveyor. A vacuum conveyor prevents dust from collecting; using a blower instead of the screw conveyor will reduce pellet breakage. A sifter still may be a good idea, either after the dryer or at the feed to the soap bin.
If problems persist, install a dust collector in the pipe network. Then, you definitely will need a new blower but will avoid unnecessary accumulation of pellet dust in the system.
Dirk Willard, process engineer
A&B Process, Stratford, Wis.
We have a simple system for purging heat exchangers at our refinery shop before cleaning them: an upstream high-flow pressure regulator and a pressure gauge, as well as a downstream purge valve and flow meter. We don’t bother to vent the exhaust to a flare stack because it’s such a small amount. We usually operate the purge at about 1 IWC. Last week, a small fire broke out as one of our welders was cutting away an old lug from a purging exchanger. We reckon a spark set off the fire. It was a hot day, about 100°F. Now, the safety group investigating the incident is calling it a near-miss. Did we do anything wrong? Is there a better way to store these exchangers until we can clean them properly?
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