Wet Gas Scrubber

Refinery Wet Gas Scrubber

My job requires that I travel around the country (United States) and as a result I get to see many different wastewater treatment systems. As I travel, 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 polymer application at the scrubbers and I’m always surprised by some of the poor choices made by chemical vendors and their polymer programs which are intended to help catalyst fines settle in a clarifier, commonly referred to as a purge treatment unit or PTU. So what I want to talk about here, but only in fairly general terms, is polymer selection for treating a wet gas scrubber. And please note this important point, something that a few, so-called, “water treatment experts,” actually have no idea of: a “Belco” scrubber and a wet gas scrubber are the same thing. Belco is the name of a particular manufacturer of wet gas scrubbers that can often be found in refineries. But not all wet gas scrubbers are Belco scrubbers. There are other wet gas scrubber manufacturers such as Hamon Research-Cottrell, Inc. and MECS Dynawave Wet Gas Scrubbers.

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. It’s the steam plume rising from the scrubber. You can see this plume from miles away, an example of which is shown in Figure 1.

Figure 1: Classic Steam Plume from a Wet Gas Scrubber

Steam plume from a wet gas scrubber

As with any application, 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, unlike evaluating the highly variable and site-specific polymer needs for activated sludge settling in clarifiers, you can be fairly confident that a particular cationic flocculant polymer will work very well in improving solids settling in a purge treatment unit allowing you to consistently achieve single digit to low-teen turbidity values in the overflow or effluent from a 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. As the cross-linking increases the water drainage rate will decrease as portrayed in Figure 2. 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.

Figure 2: Impact of Polymer Cross-linking on Water Drainage

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 comment 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. If that seems counterintuitive all you need to consider is the temperature of the wet gas scrubber flow.

The water leaving a wet gas scrubber will be in the range of 125 to 165F. At that temperature, we benefit quite significantly from the high reaction rate between the polymer and the catalyst fines. 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 Figure 3 which shows the cationic flocculant being injected into the discharge of a pump.

The schematic in Figure 3 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. The first reason is that you want to minimize the deposition of solids in the pipeline. The use of the wrong polymer, which I often see, combining with the catalyst fines, 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. 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.

Figure 3: Polymer Injection Point

Wet gas scrubber process flow diagram

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 product is significant. The drainage bin on the left (Figure 4) shows no drainage of water because the wrong polymer has turned the catalyst fines into a thick, viscous, gelatinous mass. After switching this refinery over to the correct chemical program the results were immediate and dramatic as shown by the complete drainage of water in the bin on the right (Figure 5). A close-up view of the dewatered catalyst fines is provided in Figures 6 & 7.

Figure 4: Wrong polymer -- poor drainage

Figure 5: Correct polymer -- rapid, complete drainage

Catalyst fines not draining
Drained catalyst fines

Figure 6: Close-up of dewatered catalyst fines

Figure 7: Catalyst fines closer still

Close-up view of dewatered catalyst fines
Dewatered catalyst

The more successful refineries actually find a way to “reuse” their spent catalyst fines. In Figure 8 you can see a dewatering bin, a modified haul-off container, with completely dewatered catalyst fines. This container will be transported to a cement kiln where the fines will be blended into a concrete mix resulting in the 100% recycle of this particulate waste. This reuse provides an economic benefit to the refiner as well as to the concrete producer as well as a benefit to the environment because the spent fines don’t end up in a landfill.

Figure 8: Catalyst fines with water completely drained, ready for final disposal - an additive to a concrete mix

Completely dewatered catalyst fines