Wet Gas (Belco) Scrubber
- Rick Fuller
- Dec 18, 2016
- 7 min read
Updated: Aug 29
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 (or Belco) scrubber systems 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 and hydrogen peroxide to quickly oxidize reduced sulfur compounds, primarily sulfite, to sulfate.
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 (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 for refineries in the United States but there are other wet gas scrubber manufacturers such as Hamon Research-Cottrell, Inc. and MECS Dynawave Wet Gas Scrubbers (neither of which I have ever seen!). So I tend to prefer using the generic “wet gas" scrubber terminology rather than Belco scrubber though both descriptions are used interchangeably in this post.
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 often see this plume from miles away, an example of which is shown below.
Polymer Jar Testing
As with any chemical application used to bring solids closer together so they will settle better, 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 one refinery to another. 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 emulsion 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 graphic below. The optimal cationic flocculant polymer will result in well-defined pore spaces that enhance the release and drainage of water from the catalyst fines.
Polymer Injection Point in the PTU Influent Line
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 effluent turbidity, higher total suspended solids (TSS), higher sulfite concentration, and higher effluent chemical oxygen demand (COD) 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. This is usually not the case though. Here’s a 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 this might seem counterintuitive or contradictory, you need to consider the reaction benefit you get from the relatively high temperature of the wet gas scrubber flow.
The water leaving a wet gas scrubber (WGS) will typically be in the range of 125 to 165°F (51.7 to 73.9°C). At this temperature, we benefit quite significantly from the high reaction rate between the polymer and the catalyst fines. Any concern for destruction or breakdown 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 (or an injection quill installed in the piping) to improve mixing between the polymer and the catalyst fines. Doing so will result in a lower polymer dose and with greatly improved effluent quality from the PTU. This is portrayed in the graphic below, a common configuration, which shows the cationic flocculant being injected into the discharge side of a pump with several bends in the piping indicated before the "polymerized" scubber flow enters the PTU.
Note: The WGS/PTU schematic shown below is fairly typical. But I have to immediately contradict myself. I don't think there is a "standard" design or configuration. I will discuss this in more detail at the end of this blog post.
The schematic above is not intended to communicate that the polymer injection point must be at or close to 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 influent PTU pipeline. Injecting polymer too far back from the PTU can result in frequent maintenance/cleaning issues with the scrubber/PTU influent 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 of catalyst fines that will adhere to the pipe. Eventually, this can lead to plugging of the pipe and the need to bring in a contractor to clean the line out, causing an unnecessary expense and unit process downtime.
2) The second reason is that too much mixing over a long pipe length may break apart the bonds that form between the polymer and the catalyst fines. The graphic below shows a scubber flow rate of 40 gpm through a 2-inch steel line into a PTU that provides 4.2 hours of detention time, more than sufficient to maximize the settling of catalyst fines treated with a low dose of a cationic flocculant polymer. The green color in the PTU is actually what you get with an optimized polymer program.
Not only do I try to calculate detention time in the PTU, I also try to calculate fluid velocity to determine if solids are likely to stay in suspension in the PTU influent line. The untreated scrubber flow carrying catalyst fines would not be described as a slurry. But adding a high dose of coagulant plus an anionic flocculant can shift the scrubber liquid from behaving like water closer to behaving like a slurry. To keep solids in suspension in water, a fluid velocity of 2.0 to 2.5 ft/s is required wherein the flow is turbulent. As the solids concentration in the fluid increases, the fluid velocity also needs to increase. Using a neat little program called Pipe Flow Wizard (www.pipeflow.com), the fluid velocity is calculated to be 4.08 ft/s as shown below, well above the minimum velocity required.
A Google search provided the following information on pipeline velocities.
General Guidelines
Fine solids: 1-1.5 m/s (3.3-5 ft/s).
Sand: 1.5-2 m/s (5-6.5 ft/s).
Coarse solids: 2-3.25 m/s (6.5-10.7 ft/s).
Sludge: 3.25-4.25 m/s (10.7-14 ft/s).
Lime slurry: 8 ft/s (2.4 m/s) for 3" and larger pipes, 8-10 ft/s (2.4-3.1 m/s) for 2" pipes.
Oil sands slurry: Generally, a minimum of 3.0 m/s (10 ft/s) is recommended, with coarser slurries potentially needing 4.0 m/s (13 ft/s) or higher.
Emulsion Polymer Makedown/Dilution Ssystem
There is always debate about the time required for aging the emulsion polymer when it is “made down” or diluted. 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 (I prefer a 1% polymer solution) using a polymer make down panel, as shown below, 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.”
Below is a more detailed view of a simple, easy-to-fabricate, polymer makedown/dilution system.
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 that significantly reduces water release and drainage.
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 below.
One final point to make is that the right chemical program also allows the drainage bins to be filled much higher with solids, 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.
Disposal of Catalyst Fines
In my experience, three methods are used by refineries to collect, settle, and thicken or concentrate catalyst fines.
The most common method uses a standard "clarifier" or purge treatment unit with solids blowdown (automatic or manual operation) to bins or dumpsters that get hauled away for final disposal or reuse. Recently, I saw an interesting variation of this method, where a lamella clarifier was used in place of a conventional clarifier design. Also, the oxidation tank was placed in front of the clarifier rather than after, as shown below.
The second most common method uses a single, large, rectangular concrete basin that functions like a clarifier or PTU to slow the scrubber flow down to allow the catalyst fines to settle with the aid of an inefficient coagulant. This method takes up a lot of space, requiring a substantial basin volume to accumulate several feet of catalyst sludge solids that require periodic dredging with the scrubber water flowing over the top of the solids as it makes its way to the wastewater treatment plant and the biological system where sulfite is oxidized to sulfate by the aeration equipment, consuming a lot of oxygen along the way.
The third method I have seen is the use of vacuum-assisted dewatering basins or sludge drying beds.This method is quite uncommon.
Documents for Downloading
You can download a single page Wet Gas Scrubber case study, as a PDF, below.
You can download a 4-page Wet Gas Scrubber brochure from DuPont, as a PDF, below.
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