This Month’s Puzzler
I run a mill (see figure) that processes different types of oil seeds, e.g., soybeans, into oils that our clients then sell. After five years of operation, we want to expand capacity and improve quality. This also will provide an opportunity to deal with some lingering problems in the process; the grinding part of the plant and the cleaning system is designed well but the owners seemingly ran out of money when it came to the solvent extraction set-up. The word has gotten out that we are looking for improvements to consider — and everyone has an idea. I need help sorting through these suggestions.
One of the owners says we should consider an enzyme addition to the hexane we use; during construction, he had pushed for going with an exotic solvent instead of tried-and-true hexane; at his behest, the salesperson continues to badger me.
Our young maintenance engineer has a fairly lengthy list: 1) cartridge filters to take care of fouling in the solvent recycle system (I reckon socks would work better); 2) a weir instead of spray nozzles that clog (but a weir can plug); 3) stronger vacuum on the percolator screen and in the meal dryer (which seems sensible until you consider flammability with hexane); 4) fouling in the steam deaerator tank (he blames the chemical treatment — but that needs more study); 5) decanter fouling in the solvent recovery prior to steam stripping; and 6) tearing of the percolator screen every month — he found a screen maker that invented an easier unzipping of the screen, which is a really good idea!
Our quality control manager’s agenda is to improve the clarity in the oil. I’m still waiting for his shopping list.
What do you think is the best approach for us to improve the plant performance?
Don’t Dismiss Enzymes
From a project management viewpoint, one notable point is that you have a very enthusiastic and supportive team. Consider:
1. Several research papers claim that enzyme addition or pretreatment has better yield than that with solvents consisting primarily of n-hexane and isomers. You need data from industrial-scale operations. An abrupt change of solvent system could cause unforeseen and, perhaps, major operational problems. To minimize operational risk, conduct a survey to determine plant-scale experience with enzyme systems. You might contact percolator manufacturers to see if they would conduct pilot runs to quantify the benefits of enzyme addition or pretreatment.
2. As environmental concerns escalate, you should consider a switch to “green solvents” as a long-range strategy.
3. Review current operating data and bottlenecks to see if changes in operation — such as solvent temperature, (soybean) flake size, crushing of the soybeans prior to their entry to percolator, flow rate from hopper and solvent recycle — could improve oil extraction yield and hexane consumption.
4. The suggestion about deeper vacuum on the percolator requires you to examine associated issues:
• Deeper vacuum on the percolator will reduce potential solvent leaks to atmosphere. Because leaks could form a flammable vapor cloud, deeper vacuum helps minimize flammability risk.
• Deeper vacuum lowers the boiling point of hexane, which, in turn, will lower the oil extraction rate from soybeans (and other seeds). Of course, you should keep solvent temperature as high as practical (considering hexane/water azeotrope) to enhance oil extraction rate from seeds.
• It’s a balancing act between vacuum and flammability risk. Effective ventilation in the percolator building helps with lowering the flammability risk.
5. Filters in the solvent recycle system should help reduce fouling as well as clogging of spray nozzles.
6. Frequent tears in the percolator screen obviously are a bottleneck. Check past records to determine possible causes of tearing. In addition, investigate improvements in screen attachment and maintenance.
As is common in any project work, prioritize key action items and assign projected completion times for these items. Keep all stakeholders in the loop.
GC Shah, consultant
Focus On The Leaching
The very heart of your process is the leaching operation. Anything you do to improve it will have multipliers that will save you money. Let’s consider what leaching is: close contact of a solvent with a solid. Contact is a factor. Time is a factor. Driving force is a factor. Everything else is a distraction from what makes you money.
Improved contact comes from mixing providing increased surface area between the solid and solvent. This means making the grind as fine as possible but avoiding clumping. It means removing spent solvent from the contact area between fresh solvent and fresh solid.
Time is a factor because this extractive process is between a solid and a liquid, not a liquid and a gas.
Driving force is the concentration differential between the oil and the solvent; it’s promoted by temperature.
Maintenance is a factor but only where leaching is involved. Filtering recycled solvent is a quick fix for fouling spray nozzles — and, perhaps, the steam deaerator tank. Cartridge filters, also called “depth filters,” are a landfill nightmare; sock filters, also known as “surface filters,” are limited to 1 micron, though. The typical range of cartridge filters is 0.1–500 microns; for sock filters, it’s 1–1,000 microns. Also, you can backwash and reuse sock filters; cartridge filters aren’t reusable and are limited to light loads, i.e., 0.01% solids by weight in a liquid. Perhaps a combination — the sock upstream of the cartridge filter — will work. More information can be found here.
I’m not big fan of weirs, which also foul. Fix the fouling.
Increasing the vacuum on the percolator seems like a good idea but, with higher vacuum, you also get more tramp air. Unless you’ve designed your vacuum system for this problem, this is an expensive option with questionable payback.
Improve the mixing over the percolator surface to expose more of the solid to the solvent.
Using the new screen is an excellent idea. But don’t lose sight of the main goal of improving leaching. Choose a screen that’s easily removed or adjusted with the right sieve resistance not just a screen that’s easy to replace.
Perhaps the enzyme is the way to go — but first run some tests in the laboratory. While you’re there, try optimizing your grind to improve the efficiency of the heart of the problem: the leaching.
Dirk Willard, consultant
We just replaced a high-density polypropylene (HDPP) mesh pad and glass-filled polypropylene packing at the top of our distillation tower with a metal mesh pad and metal packing. We had installed the packing and mesh pad because we needed greater efficiency than we were getting with the simple tray design below in the tower. Unfortunately, we now can’t seem to make purity in the condenser.
Operations was concerned about spikes in temperature beyond 200°F during start-up and maintenance’s desire to steam the packing and pad to speed up cleaning. Our condenser set point is 162°F. The project engineer fought long and hard against the metal replacement because he says it’s less efficient than the HDPP and can crust over, making removal difficult.
Maintenance also worries about carryover of broken mesh into the condenser and product. Quality control has seen no sign of mesh in the three years we have used HDPP packing.
Even with that concern, maintenance said a bed limiter wasn’t needed for metal packing like it was for plastic. So, one wasn’t installed with the metal packing.
Currently, cleaning the distillation tower involves washing with a detergent and then muriatic acid, followed by hot water. Maintenance insists that a high-pressure gun easily can steam clean the metal packing and metal mesh.
Did we make the right choice? Is there any other way to get higher efficiency in the upper section of the distillation tower? What kind of trouble might we face using metal packing and a metal mesh pad?
Send us your comments, suggestions or solutions for this question by June 11, 2021. We’ll include as many of them as possible in the July 2021 issue and all on ChemicalProcessing.com. 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.