This Month’s Puzzler
We were contracted to design and install a water scrubber for acetic acid to allow our client to expand its reactor building (see figure). 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 meltdown of the packing and pleasing our customer?
Ditch The V-Notch!
There may not be a solution that plant or construction management wants to hear, but some points from the Puzzler description stand out.
First, the words “v-notch distributor” and “high-efficiency packing” don’t really belong in the same statement. V-notch distributors are known for their poor distribution quality. They only should be used in three types of systems where: (a) distribution quality is not important, (b) the systems foul easily and nothing else will do, or (c) liquid rates are very high and no other distributor can be used. Have you taken this into account in the contacting calculations?
Second, a tower properly filled with random packing should have no breakage or settling sufficient to cause noticeable voids that would require “spare fill.” Was the packing damaged during installation?
Third, the random packing should have only a small range in sizes. Nested pieces can lead to packing blockage. There are some exceptions to this but they are unlikely to apply here.
Fourth, is it possible that the mist eliminator has plugged, leading to high pressure drop? If there is even a small amount of packing breakage, then packing pieces might get into the mist eliminator and plug it.
Fifth, because solids are going overhead, can you get some of the solids on a filter? An analysis should show what they are. Even putting the particles under a microscope might tell you if they are pieces of packing.
Sixth, all random packing requires a hold-down to keep it in place. Without a hold-down, the beds will move around slightly, possibly damaging fragile packing. You didn’t mention a hold-down.
Seventh, the larger-than-expected packing size in the bottom doesn’t help — but there are enough questions already that suggest that this is probably not the decisive problem.
Finally, high and increasing blower power indicates plugging or fouling of some of the equipment. This may be the packing, packing support, packing hold-down, collectors or high liquid level. Are you sure the liquid is below the vapor feed line? There are many other potential problems as well but this list gives you a starting point of what to look at.
Andrew Sloley, principal consultant
Assess Various Options
Evaluate the following:
• If possible, try reducing the temperature of deionized water to the absorber.
• Check the discharge pressure and flows in the pump-around. Low level on the chimney tray could cause pumping problems, which will affect absorber performance.
• If the absorber has a large diameter, (>3 ft), consider a vapor sparger or gas distributor for the incoming acetic acid vapors.
• Determine the particle distribution of the dust in the incoming acetic acid vapor stream.
• Assess installing a filter before the acid stream enters the absorber.
• Because there is a concern about “packing meltdown,” consider cooling the incoming vapor and a high temperature alarm on that stream.
• Acetic acid is a weak acid with relatively low dissociation. One molar solution shows pH of approximately 2.4. If the pH is much lower than that, you must determine the cause. There may be strong acidic species that you may not have accounted for. Consider corrosion and ensure metallurgy is compatible with the range of compositions of the incoming vapor.
• Liquid distributor trays should be level.
• The problem also mentions that “fill material appears to be broken.” This could have happened because of a sudden increase in pressure or flow of the incoming stream, maybe during a startup. In addition, the control scheme for the incoming stream (say, on flow control) could cause swings in flows, e.g., high gain or low proportional band of the flow controller.
GC Shah, senior advisor
Wood Group Mustang, Houston
Try Some Tricks
Your customer isn’t going to want to hear it, but you designed the tower based on the customer’s design data and faith that the construction crew would build as designed. The project scope should have covered back-checking the design against construction but often this is the first item cut to save money. A few cheap options can improve performance if you can live with some modifications. The dust problem, I’ll table until the end.
The first option is to raise the circulation rates for the pumps as high as possible. This will give you the equivalent of a few more feet of packing by reducing the liquid film resistance to mass transfer. It won’t buy you much — and mass transfer actually could fall instead if foaming occurs. Unfortunately, increasing the impeller size for most pumps is only slightly cheaper than replacing the whole pump. Besides, the current pump probably wouldn’t be operating optimally with the larger impeller.