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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We must remove solids (as a 40-to-60 wt.% slurry) from an evaporator. The solids are crystals that are not especially fragile; they don't pose adhesion or abrasion problems. But, the stream cannot easily be throttled without plugging. We currently change the open time of an on-off plug valve, which is on a 2 min. cycle. Unfortunately, maintenance of these valves is a problem due to the large number of cycles per day. Are there better ways to control the solids' removal rate?

,From January's Chemical Processing

 

Remove solids with cyclone

You might try considering a series of cyclones that would act as a wide spot in your piping run. Basically, the cyclones would take the slurry in the top, extract the solids out of the bottom, into a holding tank for removal and the remaining mixture (reduced by the percentage desired) would flow on through the process. I have seen this system used for solids removal in wastewater streams, and it is quite effective.
Bill Chakford, manager, construction engineering services
Phoenix Products LLP, Jacksonville, Fla.

Extrude slurry

A screw-type extruder (with a variable-speed motor) located at the evaporator bottom might be able to withdraw the slurry at the desired rate.
Perry K. Ho
UOP, Des Plaines, Ill.

Change valve type

Use a glass-lined metal body diaphragm valve with a urethane rubber or Teflon diaphragm for resistance to solids erosion. Another option is a Red valve that uses an appropriate rubber liner material. This valve consists of a metal shell with a pre-formed rubber liner that is squeezed closed by air pressure. The valve is a straight-through design. Both styles of valves can easily be equipped with automatic valve operators.
Nick T. Macchiarolo, principal engineer
Macchiarolo Consulting, El Dorado, Ark.

Several solutions

1. Keep the existing plug valve setup, but install a bank of four valves so it is easy to take worn valves out of service and replace them.

2. Keep the existing plug valve, but take some of the pressure drop in several hundred feet of pipe (either full size or reduced size if plugging is not a problem). This will reduce the flow rate and allow a longer time between cycles. Rather than cycling the valve every two minutes, size the extra piping to give a flow rate just greater than required. Then cycle the valve as necessary to reduce the average flow rate. Be careful that you do not end up cycling the valve just as frequently as before.

3. Install a length of piping that is sized to give exactly the flow rate required. In series with this length of pipe, install a plug valve that can be cycled shut if the average flow is greater than desired. In parallel with the pipe and valve is the existing (or similar) plug valve that can be cycled open if the average flow is less than desired.

4. Use any other arrangement of a series of pressure drop piping lengths. Cycle one or several loops on and off to control the flow. Perhaps a 70% (of target flow) loop, and two each 30% loops would be best. A lot depends on how constant the target flow rate is.
William M. Hall, reliability engineer
BP, Warrenville, Ill.

Keep valve open
I would suggest you only close the plug valve when you have to stop withdrawal for maintenance. The plug valve would feed a rotor and stator type progressing cavity pump. Rather than close the plug valve, divert the discharge of the pump back to the evaporator. I would assume you are separating the crystals from the solution via a screener or a continuous centrifuge whose liquor is being returned to the evaporator. I used this method when processing potassium nitrate, which was a byproduct from the manufacture of rocket fuel for the space shuttle in 1979.
Joe Sadlier, production supervisor
Reade Manufacturing Co., Lakehurst, N.J.

Improve solids suspension
Have you tried a blowback to better suspend the solid phase in the liquid phase (i.e. a regulated flow of air or nitrogen blowback immediately upstream of your valve)?
Bernard Morel, senior process engineer
Petro-Canada, Montreal

Change valve or pump type
Swap out the plug valve to a diaphragm or pinch valve. They tolerate solids better than a plug valve. Even a knife-gate might do better in this service. Assuming the evaporator is not running under a vacuum, run a self-priming pump to lift the solids-laden stream out of the evaporator on a timer or other controller. Have a vacuum break so the solution doesn't siphon after the pump quits. Appropriate construction materials for the pump are a must (rubber linings or abrasion-resistant alloys).
Brian R. Thomas, process engineer
Simplot Phosphates LLC, Rock Springs, Wyo.

Cycle valve at high frequency

The best way to avoid plugging of the valve is to cycle it at a very high frequency, with several (maybe 100) openings and closings per second or minute. This will keep the solids in suspension and vibrate the piping to avoid plugging and sedimentation.
Felix de la Vega, senior consultant
Kellogg Brown and Root Inc., Houston

Control flow with VFD pump

The system currently experiences problems due to the use of a valve to control the flow of solids from the evaporator. To solve this problem, you must eliminate the need for a valve. Assuming that the flow is coming from a pump, one way to solve this problem is to connect a variable frequency drive (VFD) to the pump motor and vary the pump motor speed to control the desired flow rate or level in the evaporator. This can be made into an automated feedback control loop where the level or flow controller sends a signal to the VFD. The VFD then makes a speed change that affects the controlled variable.
Dennis Eisele, chemicals technical superintendent
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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