Wet Gas Scrubber
Refinery Wet Gas Scrubber
This may seem a little odd, perhaps, but there is one unit process in particular that I really enjoy evaluating and that is wet gas scrubbers at refineries. Of most interest to me is the choice of chemistry used to settle solids (catalyst fines) in the scrubber blowdown. My focus here, but only in fairly general terms, is polymer selection for treating a wet gas scrubber.
Typically, the scrubber blowdown is directed to a small clarifier, due to the relatively low flow rate of the blowdown. A wet gas scrubber clarifier is called a purge treatment unit or PTU. The scrubbers themselves are often called “Belco” scrubbers but that isn’t technically correct. Belco has become a popular manufacturer of wet gas scrubbers in the United States for refineries but there are other wet gas scrubber manufacturers such as Hamon Research-Cottrell, Inc. and MECS Dynawave Wet Gas Scrubbers. So I prefer to use the more generic “wet gas scrubber” terminology.
Whenever you are in an industrial environment the presence of a wet gas scrubber will be fairly obvious if you know what to look for. What makes the scrubber so obvious is the steam plume rising from the exhaust stack. You can see this plume from miles away, an example of which is shown below.
As with any chemical application used to settle solids, polymer selection is crucial and jar testing is always required. Having said that, the nature and behavior of catalyst fines from wet gas scrubbers does tend to be relatively uniform as you go from plant to plant. Therefore, you can be fairly confident that a particular cationic flocculant polymer will work very well in improving solids settling in a purge treatment unit. With the right polymer you will consistently achieve single digit to low-teen turbidity values in the overflow or effluent from the purge treatment unit (clarifier).
You can begin your jar testing using a range of cationic polymers. The selection of a cationic polymer is based on the fact that the catalyst fines carry a net negative surface charge. The success of the cationic polymer is due to its limited degree of cross-linking combined with a carefully selected charge density. With the right charge, the polymer will rapidly and strongly attract the negatively charged catalyst fines. You do not want too much cross-linking in the polymer when treating catalyst fines. As the cross-linking in the polymer increases, the water drainage rate will drastically decrease, preventing the release of water, as portrayed in the image below. The optimal cationic flocculant polymer selection will result in well-defined pore spaces that enhance the release and drainage of water from the catalyst fines.
The importance of the polymer injection point is second only to the selection of the polymer product itself. If the polymer is injected too close to the purge treatment unit (PTU), a higher polymer dose, with higher turbidity, will be the result. But feeding the polymer too far upstream of the PTU will also reduce the efficiency and effectiveness of the polymer. This may lead you to think that a lot of trial-and-error is involved before the best polymer injection point can be identified. That is not the case though. Here’s a little rule-of-thumb for you to consider: Injecting the polymer too far away from the PTU is going to be less effective than injecting it too close to the PTU. Though that might seem counterintuitive or contradictory, you need to consider the benefit you get from the high temperature of the wet gas scrubber flow.
The water leaving a wet gas scrubber will be in the range of 125 to 165°F. At that temperature, we benefit quite significantly from the high reaction rate between the polymer and the catalyst fines. Any concern for destruction of the polymer due to this high temperature is unfounded. All you need to do is find an injection point for the polymer sufficiently far from the PTU to take advantage of several bends in the piping to improve mixing between the polymer and catalyst. Doing so will result in a lower polymer dose and lower turbidity in the PTU effluent. This is portrayed in the graphic below which shows the cationic flocculant being injected into the discharge of a pump.
The schematic is not intended to communicate that the polymer injection point needs to be, or should be, at the scrubber itself. Every refinery is different. In the case where there is a relatively long distance between the wet gas scrubber and the purge treatment unit you will want to move the polymer injection point away from the scrubber to bring it closer to the PTU. There are two important reasons for this.
1) You want to minimize the deposition of solids in the pipeline. Injecting polymer too far back from the PTU can result in frequent maintenance/cleaning issues with the scrubber line. Also, the use of the wrong chemical program (metal salt + anionic polymer combination), which I often see, can result in the creation of a sticky, gelatinous mass that will adhere to the pipe. Eventually, this can actually lead to plugging of the pipe and the need to bring in a contractor to clean the line out, causing an unnecessary expense.
2) The second reason is that two much mixing will break apart the relatively delicate bonds that form between the polymer and the catalyst fines, not unlike the formation of “floc” in an activated sludge system.
There is always debate about the time required for aging the emulsion polymer when it is “made down.” Here again temperature becomes very advantageous. We don’t have to worry about aging time when treating wet gas scrubbers. The polymer can be diluted using a polymer make down panel and can then be immediately injected directly into the wet gas scrubber line to the PTU. The high water temperature greatly reduces the time needed for the polymer to fully “uncoil.”
The difference between using the correct polymer and the incorrect chemical program is significant. The drainage bin below shows no drainage of water because the wrong chemical program (metal salt + anionic polymer) has turned the catalyst fines into a thick, viscous, gelatinous mass with little to no pore space.
After switching this refinery over to the correct chemical program, a single product consisting of a cationic polymer, the results were immediate and dramatic as shown by the complete drainage of water in the bin as shown below.
One final point to make is that the right chemical program also allows the drainage bins to be filled much higher, reducing the number of bins required for catalyst disposal. The reason is that with sufficient drainage the catalyst won't spill out of the bin when it is tilted and lifted onto to a truck.