Plugging hampers solids' Removal

A reader seeks an answer to the best way to control solids' removal rate from an evaporator.

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Alcoa, Point Comfort, Texas

Flush the lines

Use a biodegradable degreaser as a solution flush for the lines in question. If the degreaser can be heated before it is sent through the line, you should do so. It should also be sent under pressure depending on the pressure rating for the system in question.
Tom Squires, aseptic production supervisor
Abbott Laboratories, Rocky Mount, N.C.

Change the pump type

Why not try a continuous cavity or screw-type pump? Maybe with looser tolerances and slower turn ratios you could extract the solids without damage to or excessive maintenance requirements on the pump.
Vann Raley, director of technical services
Mobile Process Technology, Memphis, Tenn.

Control solids content

I would suggest that a progress cavity pump/frequency drive be used to control this process. A mass-flow meter could be used to read the percent of solids in a side stream and connected to a single-loop process controller (SLPC), could do an excellent job of maintaining the percent of solids at set point, by varying the speed of the pump. A progressive cavity pump with a screw-type rotor handles solids easily without fracture of the crystals.
W.E. (Red) Lewis, production manager
Wright Corp., Riegelwoo, N.C.

Use a rotary valve or density device

I have seen rotary valves work when adequate pressure differential exists. If the solids are a similar density to that of the mother liquor and remain at a constant composition, the simple level sensor and controls already in place for the valve can be used to control the drive for a pump or a rotary valve. Otherwise, a density-sensing device in the evaporator can be used to manage a constant solids inventory in a well-mixed evaporator vessel. Finally, in a system in which the solids are continuously precipitating, a multi-point density measurement system can be used to develop a control strategy for the variable speed pump.
Tom Williams, senior technical manager
Honeywell, Colonial Heights, Va.

Minimize dead leg and recirculate

Minimize the piping between the plug valve and the evaporator. This may help reduce the "dead leg." For some installations, this may not be an easy change to implement. It may also help to draw off a slip-stream of the slurry from the bottom of the evaporator (closer to the plug valve) and continually recirculate the slurry. This would help minimize the potential for plugging.
G.C. Shah, environmental engineer/industrial hygienist
ATOFINA Petrochemicals Inc., LaPorte, Texas

Use a valve stop

Consider using a mechanical valve stop to limit valve travel. Not fully closing the valve might solve the frequent maintenance required in servicing this plug valve. If a valve stop was used to limit the upper opening of the valve a longer interval of time that the valve remains open would be realized. If the evaporator has level controls that might be used to control the plug valve, then opening and closing times could be further minimized, thus likely extending the life of the plug valve.
Chuck Stewart, ACT engineering team lead
BP Exploration, Alaska

Hydrostatic trap

One solution would be to form a U-shaped hydrostatic trap at the bottom of the evaporator so that the contents are continuously emptied without a valve or piping restrictions. If a valve was needed for emergency shut off, etc., it could be left in a normally open position.
Ev Scherrer, vice president of technical services
Rea Magnet Wire Co. Inc., Lafayette, Ind.

Use an air-operated diaphragm pump

Rather than throttle the flow with a valve, we have throttled the pump to control slurry flow. We use an air-operated diaphragm pump and control the air pressure (and/or air flow rate) to operate the pump as a means to control the slurry flow.
James Loar, engineering group leader
Ciba Specialty Chemicals, Newport, Del.

Resize the pump or valve

I would think all you need to do is resize the pump with a VFD and flow transmitter for feedback to control the flow rate. You can then do away with the plug valve or use it as a shutoff valve. Since the slurry will not "stick" essentially, the lower flow velocity will not pose a problem, otherwise the pipe could be sized to a smaller diameter, however, the required horsepower on the pump will increase, and so will the size of your VFD. Worst case, though, if money is an issue, you could resize the throttling valve and/or change the type. Keep in mind that the CV for the valve is crucial for flow regulation. Not to mention, it may be substantially larger for this particular stream characteristic. I would also suggest feedback control instead of on-off for the valve.
Ron Johnson, process engineer
Alpharma Pharmaceuticals, Baltimore

Use a rotary valve

My suggestion would be to use a rotary valve (common in powder applications) suitably modified for the process conditions. With a variable-speed drive, the flow of solids could be regulated. If the solids settled to the bottom of the chute leading to the rotary valve, excess water could be drained off the side.
Jim Darby, process design engineer
Bayer Polymers, Sarnia, Ontario
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