QUESTION FROM APRIL'S CHEMICAL PROCESSING

Our flue gas scrubbing system is showing signs of wear; see Figure 1. The overall pressure drop has increased about 20%, the acid concentration produced in the absorber is dropping, the magnetic flow meter measuring the acid shows a higher flow rate than expected, it is becoming difficult to control the acid concentration in the scrubber and the speed of the exhaust fan is tracking upward. Upstream of the venturi is a fluid bed coke reactor. What can be done to eliminate the problem(s) What do you think is the root cause? Figure 1

 

Fouled absorber?

I would suspect that there is plugging or internal failure in the absorber column.  The decrease in the acid concentration suggests that there is a loss in the contact area between the scrubber water and the flue gas. The loss of contact area is increasing the load on the VFD exhaust fan. It is also possible that some material from the fluidized reactor has migrated to the absorber tower. I would suggest completing a pressure survey of the entire system first to isolate the source of the increased DP. If it is the absorber column, you could try the following: increase the scrubber water flow rate (make-up) to the absorber increasing the mass transfer in the column. This is a good short-term solution. If this does not work, it may be necessary to enter the tower and inspect the packing and internals and repair, or replace, as necessary.

Brad Piggott, Production Engineer
Air Liquide America, Freeport, Texas

Test the equipment to define problem

Assuming that the increased pressure drop and reduced acid concentration are related, the venturi may not be working properly, the packed bed unit may be plugged or absorber inlet gas flow may have increased.  If additional information is not available, I suggest that the operator repair the acid pump and replace the venturi nozzles.  I expect that the root cause of the problem is corrosion in the acid feed pump or venturi nozzle.  A second solution is to replace the acid production control valve.  Under this case, the root cause is a defective acid control valve that does not sufficiently restrict the product acid flow --- producing a low strength acid. There may be internal problems with the absorber. The root cause, in this case, is an increase in scrubbed gas (probably from a gas leak in the absorber or duct).  Increasing the scrubbed gas increases absorber pressure drop, dilutes the acid in the vapor phase and thereby reduces efficiency. Additional information could be obtained by determining the source of the pressure drop.
 
I would look at the venturi since this might not require a plant shutdown. The first step would be to check the acid flow to the venturi. The venturi spray nozzles are the key to inducing gas flow and good contact efficiency.  Assuming good level control in the acid tank and a steady acid production flow, the output pressure of the pump would be a good indication of the flow to the venturi (using the pump curve and baseline information on the venturi and pump). High output pressure indicates low flow. If the flow is low, then the piping or venturi spray nozzles may be plugged; the solid can be determined once the pipe is disassembled and inspected . Closing the acid control valve for a measurement can also confirm plugging. A reduced flow to the venturi lowers the pressure drop across the venturi (slightly) and reduces the venturi’s efficiency. Alternatively, a low output pressure indicates either the pump or the venturi needs repair.  If temporarily blocking in the pump (after the output pressure) raises the pressure to design shutoff head, then the pump is good and the venturi spray nozzles are eroded and in need of repair. Also, inspecting the acid tank bottoms may be useful --- look for brick from the venturi lining.  If the flow to the venturi is normal, the venturi throat may be plugged with solids carried over from the coke reactor.  Typically, this occurs where the liquid and gas contact: the throat.  Throat-plugging tends to separate the liquid flow from the gas flow --- this reduces gas-liquid contact. The efficiency of the absorber decreases and venturi pressure increases. A high pressure lance, near the throat, may be able to remove deposits from the venturi.
 
Pressure drop in the packed beds might occur in either the mist eliminator or the packed beds of column inlets.  A pressure survey should determine where the increased pressure drop is occurring.  Heavy washing of the mist eliminator or inlet piping might reduce the pressure drop.  If the increased pressure drop is in the packed beds, plugging and channeling might reduce absorption efficiency and increase pressure drop. Heavy washing of the packed beds might reduce the plugging.  Alternatively, dumping and washing or replacing the packing may be required.
 
David A. Milligan, P.E., C.C.E., Principal Engineer
Matches, Edmond, OK

 

Precipitate or carry-over?

With the limited information available, it is hard to be comprehensive. Let’s start with some questions. Is coke from the upstream fluid bed reactor allowing small coke particles to collect in the absorber?  On the next turn-around, carefully clean out the absorber, learning what is precipitates are in the interstices, if any, to cause the higher delta P. If the system is scrubbing SOx, then they must be getting precipitants. To reduce delta P, consider replacing the packing. Install a packing with a reduced pressure drop but be careful to consider performance and durability.