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
Bill Chakford, manager, construction engineering services
Phoenix Products LLP, Jacksonville, Fla.
Perry K. Ho
UOP, Des Plaines, Ill.
Change valve type
Nick T. Macchiarolo, principal engineer
Macchiarolo Consulting, El Dorado, Ark.
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
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
Felix de la Vega, senior consultant
Kellogg Brown and Root Inc., Houston
Control flow with VFD pump
Dennis Eisele, chemicals technical superintendent