Process Puzzler: Take Some Solid Steps

June 7, 2022
Improve evaporator level measurement and cut labor intensiveness

This Month’s Puzzler

We make an ethoxy compound using a water-based process. Our evaporator is supposed to operate continuously but instead the operators run it in manual — with a lot of babysitting. The operators complain the level switches and level transmitter don’t work as advertised. So, instead, they judge level by looking through three sight gauges. This requires distinguishing wet solids from mere splatter; new operators must learn the difference between actual level and a dirty glass. The design engineer somehow thought a 2-in. pipe made sense for pumping the product, a wet slurry.

I looked at the material balance. Continuous processing is possible, if the flow rate were 10 gal/min through a pipe where the solids settle out at 4 ft/sec — and with working level measurements! Obviously, a 2-in. pipe forces us to dump the product into the dryers in loads measured by estimating the level using the sight gauges.

Temperature control is nearly impossible. Measurements by a resistance temperature detector vary as much as 10°F from sample temperatures and readings with infrared guns.

What can we do to make this a continuous evaporation train? Should we do this? Is there anything we can do to make this process less labor-intensive as a batch process?

Review Your Installation

Consider the following:

1. Numerous technologies are available for level measurement and level switch applications — differential pressure (dP) cell, guided wave radar (GWR), ultrasonic and others — with each technology having pros and cons. Favor technology that could be implemented without a long shutdown and without having to do many mechanical modifications to the evaporator (e.g., adding extra nozzles). You also should also consider signal transmission (HART, Fieldbus, etc.) to make sure it is compatible with your current distributed control system.

2. If you are using a dP cell system, slurry tends to clog impulse lines. Changing to a capillary system (with diaphragm) will help minimize clogging of the impulse legs because it replaces impulse legs with a capillary system. Slurry still could coat the diaphragm, though. Good agitation near the diaphragm will reduce fouling of the diaphragm.

3. Other non-contact technologies such as GWR or ultrasonic could offer significant relief from the slurry (coating) problem but would require nozzles in the top of the crystallizer.

4. Slurry coating of the probe and interference from vapors would be key considerations for level switches as well. Modern technologies offer diagnostics and compensation for coating.

5. You indicated that resistance temperature detector (RTD) readings are off by as much as 10°F; this could cause problems with temperature control. Tuning (proportional, integral, derivative settings) could be another issue to consider. Slurry may be coating the RTD thermowell. Provide better agitation near the thermowell or relocate the thermowell to a spot that has better agitation and is representative of the process. Avoid dead-legs.
GC Shah, consultant

Think Unconventionally

Without level control, you can forget about automatic control, let alone continuous control!

You also should forget about some technologies for level that others have tried unsuccessfully: nuclear, radar and contact.

Nuclear, being a non-contact method, will at least report the level — but it will be wrong. Solids splattered on surfaces will show up as level because they absorb radiation.

Radar also is a wave-based method, which means it will suffer the same fate as nuclear.

The various contact level switches available won’t work because they can’t detect the difference between solids in solution and solids splattered on the switch.

Weigh cells might tell you the level if you can keep track of the residual product left between batches. You may want to deploy a second, check scale downstream and programming to track the residual. The trouble is that weigh cells are expensive.

Perhaps you need a more radical approach. Could you use cameras? You could place these either outside the evaporator, looking at the sight glasses, or even inside the evaporator? The operator could look at the feed to decide what the level is. Periodically, the camera lens would require cleaning but this could work. Cameras won’t make the system automatic but can reduce operator involvement to a minimum; their cost might be competitive with retrofitting weigh cells to the evaporator. Someday, software may even be available to operate the system in true automatic fashion.

As for the pipe size issue, I see two solutions: a different type of pump, or a smaller discharge pipe. Take this to the laboratory to determine if you can replace the current pump type by one with a larger motor to handle the smaller pipe or go with a progressive cavity or lobe pump; get the vendor of the new pump to provide pressure drop data for changes in the pipe diameter.

Remember, accurate sampling and storage are crucial if the sample is to represent the material being pumped.

Temperature control of a slurry always is difficult. It is best to pick a sample point and build the product quality control around it. Try several points to determine which is best for control. Ensuring accuracy in temperature measurement calls for an approach opposite to that for pressure readings: for temperature you need mixing; for pressure you require an established, steady but representative positioning — elbows for temperature instruments, straight pipe for pressure ones.
Dirk Willard, consultant
Wooster, Ohio

August’s Puzzler

A safety review has discovered serious flaws in our relief valve vent system and electrical area classification. The reliefs for our ammonia compressors vent on to the roof of a busy area in our plant. In addition, the compressor building lacks an exhaust fan, which makes it a Class 1, Div. 1, Group D zone.

I designed a 4-ft diameter scrubber with the relief vents going to a basin. I proposed 10% sulfuric acid. The superintendent doesn’t like operators working with acid. He also complains the scrubber will be ignored: the ammonia alarm goes off about six-to-ten times a year (when someone actually takes note of it). The safety director gripes about having to resize the relief valves for the pressure drop: it’s a 300-psig system! He’s also concerned the ammonia relief flows will blow right through the basin without being captured. The project group is troubled over the cost of the scrubber because of material selection.

Now, the superintendent has talked corporate into the idea of using the scrubber to handle the formaldehyde and methanol that escapes our storage tanks when we do inspections and repairs. We have a thermal oxidizer but it only captures about 90% of the vapor from the plant.

Are there any options other than sulfuric acid that would lower the project cost? How much of a concern is resizing the relief valves? Can you suggest other ideas to meet the scope of this project?

Send us your comments, suggestions or solutions for this question by July 8, 2022. We’ll include as many of them as possible in the August 2022 issue and all on Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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