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Conquer Centrifuge Challenges

March 14, 2017
Assess a number of issues that can compromise performance

We’re trying to complete our functional acceptance test for a centrifuge for recovering solids after pharmaceutical fermentation. In our process, fermentation mash goes through a bead mill, then to disc centrifuges, a steam sterilizer, and a spray dryer. We’ve run into a few glitches: 1) initially, the motor on the variable-frequency-drive control trips out within a few seconds; 2) batches seem to run for a shorter period than desired before we have to switch to another centrifuge; 3) we seem to see carry-over into our spray dryer; and 4) we have trouble meeting our riboflavin testing. I went back to the scale-up design report that was used by an engineering contractor to design and build our fermentation process. It shows that a tubular bowl centrifuge was used to develop data — but we went with a disc, split-bowl centrifuge for the full-scale operation. How much of an issue is this? Is there a workaround to allow us to get the desired performance from our centrifuge?

Check Multiple Factors

I’ve worked with a few types of centrifuges in fermentation, mining and crystallizers. Starting and stopping a centrifuge is a challenge. From a dead start, the inertia can be very large. Typically, you start the centrifuge from zero and try to get it up to 6,000–10,000 rpm in a matter of seconds.

You don’t often see variable frequency drives (VFDs) with centrifuges. I ran centrifuges on star motors. These motors use three windings connected by a switch. During starting, the wye winding draws the current; when the motor reaches sufficient speed, usually in a fraction of a second, the current is passed to the delta windings. Obviously, heating is a function of current. In a wye connection, heating is 1:1 with current. In a delta connection, heating is 1.73:1 with the winding current; this requires more insulation than wye. Delta is chosen over wye because delta can better balance an off-voltage source and requires fewer wires. So, the motor sees less heat with wye wiring when the current is very high during startup. A current overload device is part of the circuit; this could be your problem. I tend to shy away from a complication like a VFD when the old three-winding motors worked so well. The only problem we ever had were fuses; we used time-delay fuses to avoid overload during startup.

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With the VFD and an induction motor, you have the complication of programming. The motor must produce enough torque to pass the high slip area of the motor speed versus torque curve. The challenge is initially the torque must be produced at extremely low frequency. Voltage boosters have been used to give the motor more torque at low frequency — typically at 5% of full-load current — and the effect is to at least double the torque available. Once the discs are spinning, ever so slowly, the frequency and voltage must increase steadily, boosting the disc speed while keeping the motor in the low slip area of the speed-torque curve.

Tripping may seem like an issue of the past but it will return. I suggest you look at the programming and the wiring with the VFD vendor and the motor vendor; consider a faster ramp initially.

Let’s consider how you can limp through until you solve the electrical problems.

One of the drawbacks with scale-up is that laboratory equipment works much more smoothly and efficiently than production equipment. With a new centrifuge, I would run a diluted feed, at least initially; then, increase the load gradually. With startups, always assume that the first batch of feed is garbage; filter it, then flush the feed tank if you can use it before assessing whether you need to add another layer of separation before the centrifuge.

When we started up a molasses mash system at Anheuser-Busch, we quickly found out that we needed a hydrocyclone to get rid of the clays in the beet molasses. Never believe everything the R&D department hands you.

Before you start thinking of new equipment, consider looking at your bead mill. The bead mill is there to break up the yeast cell wall to allow you to extract product. No doubt the lab has a better mill than you have. A rule of thumb is that 80% of the cell is waste. You need to quickly assess optimum feed rate, the wear rate of beads, what the heavy cut looks like — especially as the beads wear, and whether you’re getting a good light cut.

As for the riboflavin testing, I hope you’re looking at the process downstream of the steam sterilizer; if not, then you have a pocket of bugs living somewhere in your piping but more likely your equipment. You should go through the clean-in-place (CIP) system and clean-out-of place (COP) protocols in detail. To clear the steam sterilizer, I suggest looking at the product exit temperature and dwell time in the hot pipe. Also, sample before the product gets to the spray dryer.
Dirk Willard, consultant
Wooster, Ohio

May’s Puzzler

We were contracted to design and install a water scrubber for acetic acid to allow our client to expand its reactor building (Figure 1). The acetic acid is a byproduct of an acrylic acid process. The scrubber was installed a few weeks ago. The expanded plant just started up two days ago. Unfortunately, it isn’t meeting the emission target. I was sent to investigate.

The scrubber consists of three equal 10-ft beds of high-efficiency random packing. Each bed has a two-ply mist eliminator; v-notch troughs distribute the liquid. Three separate pumps ensure even distribution in each bed.

An initial sampling of the reactor vents shows about 10% dust by weight. This was not anticipated. Also, the pH is much lower than expected from the data sheet provided for our calculations.

On inspection, I found the bottom packing is larger than that specified in the design calculations and the scrubber packing doesn’t match what we used in those calculations. In addition, the packing I sampled from the spare fill material isn’t a uniform material but appears to be broken and off-brand; spare fill is added after the beds settle. After only two days of operation, the blower speed is nearing 100% and the temperature of the scrubber basin is higher than design.

What are our options for improving efficiency, avoiding a melt down of the packing and pleasing our customer?

Vent Scrubber

Figure 1. Unit is not providing expected acetic-acid removal efficiency.

Send us your comments, suggestions or solutions for this question by April 14, 2017. We’ll include as many of them as possible in the May 2017 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.

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