Powder Handling: Dryer Revamp Demands Care

Feb. 13, 2022
The need for speed doesn’t justify ignoring safety concerns

This Month’s Puzzler

We are re-tasking a batch fluid-bed dryer we had used for a different pharmaceutical product that has become a generic. I have several concerns: 1) the old particle size was larger than that of the new one which has a slightly higher density; 2) the new powder could be a fire risk — we don’t have safety data but still are pushing ahead with the project; 3) we only have incomplete maintenance data on the blower; 4) the screen and baghouse are severely corroded; 5) the electric heater leaks tramp air; and 6) the exhaust fan has been upsized twice and also is corroded.

If this dryer were a fish, I’d throw it back. Instead, I am tasked with getting this dryer up and running in three months. Because of the pandemic bottleneck, I’m not sure there’s anything I can do to make it run better.

In the future, I want to get a handle on these problems. The corrosion has me concerned. Another problem I notice is vibration in the corroded ductwork and poorly supported conduit — it jerks about an inch every time the blower starts and stops. I want to check out the fluidization against a Clark chart because I have heard of clumping complaints about the previous product.

What can I do to ensure this product campaign goes well? What do you think causes the corrosion? Why do you think the exhaust fan size was increased? Should I be concerned about preparing this dryer for use before the safety data are in?

Consider Eight Steps

Unfortunately, you seem to have a relatively short three-month window to accomplish a wide scope of work, the most crucial being addressing safety concerns. Perhaps your sales group or management is unaware of the accident-prone state the dryer is in. First, get a realistic time estimate to accomplish various tasks you listed. (Please don’t forget the time required for a hazard and operability study, pre-startup safety review, and operator training.) I used the word “realistic” because additional uncertainty created by recent supply chain problems throws a monkey wrench at our otherwise carefully developed estimates. [See, “Sensibly Estimate Project Time and Cost,” for tips on coming up with proper cost estimates amid the pandemic.] Communicate time and cost estimates as well as safety concerns to sales/management groups. In a strange way, this could prove to be an opportunity to fix long-standing mechanical and safety issues of the dryer. You also may use this opportunity to “educate” your sales staff on key safety issues of the dryer (and the plant) and the consequence of unsafe operation. In the long run, improve communications — keep people posted on key issues and opportunities. In addition, consider performing the following tasks, some of which can proceed in parallel:

1. Contact lab-service companies that can determine the vapor’s flammability properties (flash point, lower explosive limit (LEL) and upper explosive limit (UEL). Because the material is in powder state, the conventional tests for these properties probably will need modification.

2. Once you feel it’s safe to perform a lab-bench-scale test, if possible, collect relevant data including, for example, operating temperatures and pressures, fluidization velocity, moisture level in the product, and particle size distributions of feed and product.

3. Use the bench-scale data to scale up equipment sizes.

4. If the data collected point to flammability and toxicity concerns, you must have gas monitors and appropriate interlock systems to bring the system to a safe state in the event of a mishap. If you are using air as the fluidizing medium, you must replace it with nitrogen or another inert gas for flammable products. Similarly, toxic gases will require appropriate systems to minimize worker exposure.

5. Discharging exhaust gases directly to atmosphere may not be a concern now but, if the proposed product is flammable or toxic, you must change the routing to ensure discharge at a safe location (e.g., flare, or a high point). Because the particle size is smaller, you will need to look at the mesh size of the bag filters. Improper filtration could lead to exhaust gas with unacceptable levels of particulates. In addition to nuisance, this could become an environmental compliance problem or prompt complaints from irate neighbors.

6. If you can’t reduce moisture to the required levels, you may need to consider an additional heat source with higher temperature (assuming the product is not heat sensitive) or vacuum operation. The Clark drying curve has two zones: heat-transfer-controlled followed by diffusion-limited, which entails much higher temperatures to remove moisture to ppm levels. Vacuum operation will help in reducing the temperatures.

7. The problem description gives little information about the current product being handled. In basic terms, corrosion could result from improper material of construction, operation outside the allowable range, or a combination of the two. It seems the problem stems from the “cold end” and could arise from moisture condensation and poor material choice. Because time is short, see if you can apply a protective coating or paint at the corroded surfaces. Later, you can investigate possible causes of the corrosion problems.

8. As far as vibration goes, you might beef up the support structure for duct work and conduits. The 1-in. “jerk,” (at the start and stop of the blower) may result from sudden alterations in flow and drastic changes in flow path (say, short-radius ells and expansion/contraction). A control mechanism, e.g., louvers or a damper, can allow gradual change of flow at the start and stop of the blower.
GC Shah, Consultant,

Ditch The Dryer

The person raising these issues about the fluid-bed dryer deserves applause. Each item described should cause alarm to any engineer, especially one involved with a pharmaceutical product.

Unless the company has total command of the process, starting up the fluid-bed dryer puts the company in a precarious position. Actually, the company is putting you in an unacceptable and unethical situation.

Safety data are critical, even more so because the product may pose a fire risk. Blower maintenance data, corrosion issues, and the incorrect exhaust fan size indicate that this is a situation of trying to fit a square plug into a round hole — and one promising an unending streak of problems.

The existing equipment is inadequate and requires fixing or replacing to produce the desired product. Otherwise, dryer performance is going to differ each time and you will require multiple tests to achieve the desired product size. If the company insists on using the existing dryer, it should expect delays and batch-to-batch variation. Use of the existing dryer will be a perfect case of “quality by analysis” and insurmountable waste of product.

My recommendation would be either to outsource the production or replace the dryer.

If funding limitations prevent getting the right equipment and you are pressured to use the existing dryer, you must lay out every scenario to management. If the company still insists, look for another employment opportunity.
Girish Malhotra, EPCOT International
Pepper Pike, OH

Put Safety First

Production sometimes doesn’t make safety its top concern like it should. I remember the joke from the tv series The Office: “Remember, rule five: safety first.” Unfortunately, you’re in the unenviable position of yelling “Stop!” Without safety data, you can’t design a process that won’t endanger workers and the reputation of the company.

You’re stuck with two options if the company’s answer is: “Do it or you’re fired.” First, design the system for the worst case you can imagine. This might get you fired anyway because the project cost will balloon, perhaps out of control. Management won’t be happy. I once faced a similar situation as a contractor on a thermal oxidizer (TOX) project for a pharmaceutical company. The pharmaceutical maker downplayed the fire risk and wanted to remove $3 million of safety equipment from the project; it turned out the firm had blown up a TOX in Switzerland with this approach to safety. Better to lose a contract than ruin your company fighting law suits in court when (not, if) the new plant goes up in a blaze of ignominy.

If such a maximum risk strategy is acceptable, it still may be possible to swap out some of this purchased equipment if the risk of an explosive dust doesn’t materialize. Management may find this approach very sage.

Second, you can go slow. The idea here is to take care of things like utilities and ancillary equipment while leaving the safety issues until you can assess the risks. Most clients are agreeable to this option. If not, walk away! In another wise career move, I declined an offer as a project engineer where permit filings were due within a couple of weeks of my arrival.

However, the slowdown approach can lead to some scope creep. Defining utilities may prove hard and projects left open for a time often get unexpected additions to their scope.

As for the condition of the dryer, pay special attention to the corrosion. Corrosion leads to holes; they result in tramp air leaking in, reducing vacuum. The trouble is that ducts generally are made of sheet material not plate; properly repairing such sheet usually is impossible. Clamps depend on a solid pipe wall around the hole, so they won’t work well here. You might try sealing the holes with fiberglass, polymer or ceramic material. It’s important to run several pressure and vacuum tests.

Inspect the baghouse immediately! Replace all the bags and repair all the caging supporting the bags. Inspect and test all the bangers or air pulsers.

If you can afford it, consider installing a blower with greater head (pressure), after ensuring the dryer and duct can sustain a higher vacuum. Confirm the vacuum requirements by modeling the hydraulics. Here is where you will want to test the material and use a Clark chart as a basis for arguing for a more-powerful blower. Note that it may be possible to beef up the dryer and duct with guidance from a structural engineer.

Now the bad news. Electric heaters almost always are hard to repair and they’re difficult to get. These heaters usually get put in when you need tighter temperature control than possible with steam or a fuel gas. Perhaps you can seal the leaks for now with a ceramic and limp through the campaign while you wait for a replacement. Good luck!
Dirk Willard, consultant
Wooster, Ohio

April’s Puzzler

Our research team has changed the flow characteristics of a product. By my measurement, the viscosities of the ingredients now are in the 100-cP range, while previously 30-cP was the maximum. The researchers still are monkeying around with the batch mix schedule, so I don’t have a final value on the flow properties.

Thankfully, we use gear pumps for ingredients. The product ran about 20-cP, which allowed us to rely on our old 80-gal/min magnetic-drive centrifugal pump to send the product, which is a weak slurry, to a filter press. The total dynamic head is 148-ft-WC (about 64 psi). This pump is easily cleaned-in-place. Space around the 3,000-gal reactor is very tight, preventing me from installing a new pump off the suction line of the current pump. I also am concerned about the agitation. I have to be ready to change this reactor to the new product within a few weeks. What are your thoughts on the product delivery pump and reactor? Can we successfully make the product?

Send us your comments, suggestions or solutions for this question by March 11, 2022. We’ll include as many of them as possible in the April 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.