THIS MONTH'S PUZZLER
We make an anionic surfactant by reacting oleum (SO3 in H2SO4) with alkyl benzene (C12H25C6H5) in a continuous stirred-tank reactor (CSTR); see Figure 1. The product from our spray dryer is 85% pure, with the remainder being sodium sulfate and water. Ever since we started up this process a year ago, we've had problems: 1) a residue in the product layer in our separator; 2) a heavy black residue in the spent acid at the bottom of the separator — the company that regenerates our H2SO4 has threatened not to take our waste if we don't clean it up; 3) our titration analyzers downstream of the reactor and digester sometimes have fouled tubing — a sample pump failed a few weeks ago because a suction strainer plugged; and 4) startup and shutdown are rough — we can't turn down the capacity of the sulfonator cooler effectively and the smaller jacket control valve can't handle the mid-range cooling requirements. We see a lot of what appears to be burnt product during these periods. Recently, our Coriolis meter, which measures alkyl benzene, corroded out. What can we do to address these problems?
ADJUST THE OLEUM RATE
The problem likely arises from the oleum feed rate being too high relative to the alkyl benzene feed rate causing charring of the alkyl benzene. Very tight molar ratio control and temperature control are necessary to minimize the charring — the blackish material seen in the spent sulfuric acid. Slowing down the oleum feed rate and increasing the residence time in the CSTRs is a potential solution. Additionally, make sure the oleum and alkyl benzene feed control systems are accurately measuring mass flow rates.
Wyatt Winkenwerder, senior research engineer
Akzo Nobel, Brewster, N.Y.
REVAMP THE PROCESS
Alkyl benzene sulfonation is an exothermic reaction. If the exotherm and stoichiometry are not controlled, the reaction will produce undesirable products that will create the described problems. Process stoichiometry, operating parameters and the chemistry must be precisely controlled. Sufficient residence time must be provided to ensure the desired conversion. Also, I would use a pump for oleum. Relative flow control is important (see Figure 2); many options are available. The processor should select the one that would prevent back flow suggested by corrosion of the Coriolis meter.
Because this is a continuous process, I strongly recommend using a gravity-based separator that will separate two layers and eliminate the use of two pumps: refer to McCabe and Smith's "Unit Operations of Chemical Engineering," McGraw-Hill, 2nd ed., pp. 40–41. The current organic/aqueous separation is fighting two opposing controllers that complicate processing. The addition of water may be necessary to change the densities and facilitate the separation. The existing system can be reconfigured to create a simpler process that will eliminate black residue that is formed due to over-sulfonation of alkyl benzenes.
Girish Malhotra, president
EPCOT International, Pepper Pike, Ohio
CHECK FOR CONTAMINATION
Maybe it's in the water or the materials of construction, i.e., piping, tubing or valves.
Frank Fox Jr., retired,
TAKE FOUR STEPS
I suggest the following actions:
1. Aerate the separator/settler sump to remove more of the product.
2. Increase the brine concentration to provide more cooling to the sulfonator, to avoid the over-heating that causes formation of black reside.
3. Replace the Coriolis meter with one using sulfuric-acid-resistant tubing.
4. Install an automatic flushing system for the titration analyzer loop. This will prevent suction strainer plugging.
Donald Phillips, manager
Phillips Engineering, Melbourne, Fla.
TACKLE THREE ISSUES
There are three distinct problems, in order of importance: 1) poor process control; 2) poor separation before neutralization, and 3) backflow from more resistant pipe to less resistant pipe (Figure 3).
Process control must operate effectively during all phases of plant operation. Too often, controls run only smoothly near the nominal capacity. The smaller jacket temperature control valve (TV) can't provide sufficient cooling during either startup or shutdown. The larger reactor chiller TV provides too much cooling during these periods. The obvious solution is a larger jacket TV and increased velocity through the reactor jacket, assuming the pressure drop is not a problem — I've used an inline booster pump to make up for added pressure-drop. Another problem with the reactor heat transfer is the agitation. Seldom is a single impeller enough for good mixing, let alone good heat transfer. More impellers are needed — this may require a new agitator. One way to reduce the load on the jacket TV would be to add a cooled-jacket static mixer; the reactor then becomes a second digester. This may not be enough.
If energy balance calculations show additional changes are needed, it may be necessary to add a smaller TV for split-range control of the reactor chiller temperature during the transition periods between steady operation and shutdown.
A threat from your regeneration service company should not be taken lightly. One word from them and you're out of business. I see filters in your future. You can approach the spent-acid-contamination problem in two ways: a side-stream filtration for a modest problem or full-stream filtration. A hot water wash prior to filtration could help improve separation by avoiding gelling in the acid.
Perform laboratory tests to identify the maximum pore size that will produce clear acid in combination with the optimum pre-treatment. Then, work backward to satisfy the service company, balancing economics and quality.
Cleaning up the product may be easy once the source of trouble is identified. The entire process should be shut down and flushed of contaminants. Pay attention to the heat exchangers, especially as they may have significant fouling that has gone undetected. After the temperature control problem is ameliorated, the contaminant in the solid should go away. If not, you may want to take samples from the separator, adding filtration prior to neutralization when a slurry is produced, and a wash-centrifuge step prior to the spray dryer.
There is no reasonable explanation for the corrosion failure of the Coriolis meter. Check valves leak, and if corroded badly, springs fail to close and acid flows back into the line supplying alkyl benzene. Install a backflow preventer to avoid liquid backflow of the H2SO4. It's doubtful that the acid vapor can be so easily contained. The only idea that comes to mind is a cold water wash followed by decanting; the acid vapor would go into the water but some trace alkyl benzene will be lost.
You will want to chase the corrosion backward to the piston pump. These pumps are expensive to replace and long on delivery. In the end, it may be much easier to upgrade the pipe specification if the corrosion problem continues.
Dirk Willard, consultant
My refinery is grappling with how to set up the coolers for reducing oil sample temperatures for safe handling. These coolers usually consist of a U-tube inside a water jacket. The tubing is rated for 2,000 psig at 600°F; the shell maximum allowable working pressure is 75 psig at 600°F. We're working with heavy gas oil from a coker at 400°F. An alternative that's been suggested is to use a tube inside a tube: ¼-in. tubing for the oil within 1-in.tubing for the cooling-tower water. The project engineer wants hydrostatic pressure relief valves (PSVs) installed for blocked-in water. The maintenance engineer is pushing for the tube-in-tube approach because he insists it won't need a PSV; he claims it is protected under ASME code because the cooler diameter is less than 6-inch. Who's right? How should we proceed?
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