Process Puzzler: Eliminate Evaporator Effect

Readers solve an evaporation pump problem.

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We’ve just discovered what appears to be a design flaw during the first hazard and operability study involving solvent-extraction evaporators. The evaporation system is a three-effect backward-feed design using long-tube vertical evaporators. The viscosities should be moderate, i.e., below 100 cP; there’s only a slight possibility of solid build-up unless the bulk temperature falls below 100°F. With backward feed, 150-psig steam enters the first unit as the feed while a dilute concentration of ferric chloride and hydrochloric acid enters the third unit.

The product, a concentrated aqueous solution of ferric chloride, exits the first effect. It’s passed with the evaporated steam from the first unit to the second and then to the third, where it’s condensed and decanted after the final effect. The problem is the level control and pump cavitation. Operating at the design capacity of 50 gpm, the pump can empty the reservoir, i.e., the receiver, below the pump in less than a minute. This problem is most severe during startup. At this stage of the design what can we do to make this process operate more smoothly? Unfortunately, the evaporators, which require expensive materials, already have been ordered.

The simple solution would be to install a 3-in. pad flange level controller on the reservoir and change out the pump starter to a VFD (variable frequency drive), then control the speed of the pump by level.
Ryan Cariveau, project engineer
Archer Daniels Midland, Cedar Rapids, Iowa

Recycle part of the pump output to the inlet of the effect feeding the pump. This would be controlled by the level in the bottoms. If the level gets too low, the recycle could be increased by a recycle control valve. The product would be removed based on a set-point level; should it get low, some material will be recycled until it gets back above the set point.

The feed rate to the high water effect would be controlled by the temperature or density or concentration of the material in the second or third effects. Control will be interesting, but I’m not motivated to figure it out right now.
Bennett Willis, professor
Brazosport College, Lake Jackson, Texas

I think there is a design error. Here are my thoughts:
1. Consider a minimum-flow bypass back to the evaporator or feed to the evaporator.
2. Program a cutoff for the flow rate from the evaporator if the level drops below a certain minimum value.
3. Review the basis of the pump capacity. Is the design capacity from the material balance or is it based on the maximum expected flow? If it is based on the maximum expected flow, it may be possible to trim the pump capacity.
G. C. Shah, HSE project manager
Mustang Engineers, Sunnyvale, Calif.

It sounds like the pump is oversized for your application if it can empty the reservoir in less than a minute. I recommend scaling down the pump size or slowing it down with a variable frequency drive. This will increase the time to empty the reservoir and the level control should be more accurate.

Regarding the pump cavitating, cavitation occurs when vapor pockets form in a liquid flow because of local reduction in pressure. Undissolved air can also cause cavitation where the local total pressure is quite close to the vapor pressure. Cavitation can occur in any machine handling liquid whenever the local static pressure falls below the vapor pressure of the liquid. (This is likely to be a problem with evaporators.)When this occurs, the liquid can locally flash to vapor, forming a vapor cavity and changing the flow pattern (to two-phase flow). The vapor cavity changes the effective shape of the flow passage, thus altering the local pressure field. Since the size and shape of the vapor cavity are influenced by the local pressure field, the flow may become unsteady. The unsteadiness may cause the entire flow to oscillate and the machine to vibrate.

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