March 2022

Environment and Safety

Cleaning mud oil incineration offgases

The sustainable management of petroleum exploration and refining requires appropriate treatment and disposal of the oily waste generated.

Herssens, Y., Safariyeganeh, S., Elessent Clean Technologies

The sustainable management of petroleum exploration and refining requires appropriate treatment and disposal of the oily waste generated. One common method for the disposal of hazardous oily sludge or mud oil is incineration, typically using a rotary kiln. However, mud incineration produces gas containing a high level of acid and particulate matter that needs to be removed before the gas can be released to the atmosphere. This is usually accomplished using a scrubber and a baghouse. However, one complicating factor is that the temperature of the gas exiting the kiln is 980°C–1,200°C (1,800°F–2,200°F). The scrubber must be able to cope with such high inlet temperatures.

A proprietary reverse jet scrubber technologya is a unique acid absorption and particulate removal system that can simultaneously quench and subcool the gas, and that can handle these very hot temperatures when fitted with a patented weir bowl technology (FIG. 1). The weir bowl can be attached to the top of the inlet barrel of the scrubber, enabling the scrubber to manage inlet gas temperatures up to 1,200°C (2,200°F). The bowl distributes a continuous stream of liquid that overflows a weir. This stream creates a thin protective liquid coating over the inner surface of the inlet barrel, thus preventing it from coming into direct contact with the hot gas and extending the life of the scrubber in this hot, corrosive environment.

FIG. 1. The weir bowl for the proprietary reverse jet scrubbera.

The proprietary reverse jet scrubbers have been used effectively in several incineration applications across industries due to their ability to not only scrub acid gas, but also to quench extremely hot incinerator effluent streams and to simultaneously achieve substantial particulate removal in a single vessel. Developed in the 1970s, more than 400 of these versatile reverse jet scrubbers are in use around the world to treat emissions generated in a wide range of industrial processes with many of these systems in operation for more than 20 yr.

The proprietary reverse jet scrubber consists of an inlet barrel in which a reagent solution is injected countercurrent to the gas flow through very large open-bore spray nozzles (FIG. 2). A standing wave of highly turbulent flow—called the “froth zone”—is created at the point where the liquid is reversed by the gas. The froth zone produces a very high rate of liquid surface renewal, efficiently quenches the gas to the adiabatic saturation temperature, absorbs acids such as sulfur dioxide (SO2) and efficiently removes particulate matter. After contacting the gas with the liquid reagent, the gas-liquid mixture enters a disengagement vessel where the liquid drops to the sump of the vessel and the gas exits the vessel through a demisting device.

FIG. 2. View of the proprietary reverse jet scrubber systema.

There are no moving parts or narrow passages that could restrict gas flow, which prevents plugging issues. The required maintenance and operator attendance is minimal and these tasks can be handled during any scheduled turnaround of the refinery.

Mud incineration offgas treatment case study

The following reviews a case study regarding SO2 and particulate removal.

SO2 removal. A refiner had installed a semi-dry lime scrubber and a baghouse for an existing mud incinerator to reduce SO2 and particulate emissions (FIG. 3A). While the semi-dry lime scrubber was able to attain an SO2 removal efficiency of approximately 90%, the plant needed to further reduce SO2 to achieve required levels below 20 ppm. To solve this issue, the refinery installed the proprietary reverse jet scrubber downstream of the bag filter (FIG. 3B). The objective was to have a highly efficient system that would also be simple to operate, as only two operators were available at a time and the scrubber was sited a long way from the plant.

FIG. 3. Typical scrubbing/baghouse flow diagram downstream of a mud incinerator (A) vs. the integration of the proprietary reverse jet scrubbera downstream of the mud incinerator to increase SO2 and particulate removal (B).

The flowrate of the incoming gas varied from 10,000 Nm3/hr–12,000 Nm3/hr. Although the proprietary reverse jet scrubber can be designed to handle much higher incinerator temperatures, in this instance, a pre-existing quench tower upstream of the scrubber meant that the inlet gas temperature to this 1.8-m-diameter scrubber vessel was approximately 150°C (302°F). The acid content on entry of the gas was roughly 2,000 ppm SO2, which had to be reduced to below 20 ppm.

At the startup of the jet scrubber, two test runs were conducted. The first run had 1,700 ppm SO2 coming into the scrubber with the incinerator offgas and measured less than 7 ppm SO2 on exit. The second run recorded 1,500 ppm SO2 on entry, with no SO2 detected on exit (TABLE 1).

Particulate removal. The proprietary reverse jet scrubber is also capable of efficient particulate removal from mud incinerator offgases. Particulate removal depends on two things: the gas-side pressure drop and the particulate size. FIG. 4 shows the relationship between the two and the resulting removal efficiency. The removal efficiency of the particulate depends on the particle size distribution.

FIG. 4. Removal efficiency of the proprietary reverse jet scrubber in relation to particulate size distribution and pressure drop.

In the refinery example, the gas leaving the bag filter still contained a certain amount of particulate, with sizes assumed to be between 0.5 µm–1 µm. The customer requested the flue gas treatment to be designed to remove at least 50% of that particulate matter, which the scrubber was able to guarantee (TABLE 2).

Increased scrubber efficiencies. A further benefit of the proprietary reverse jet scrubber is that, for higher removal efficiencies, one can simply add multiple stages of reverse jet nozzles in series, thus creating multiple separate froth zones and significantly increasing removal efficiencies. Designing the scrubber with two reverse jet stages within the same inlet barrel effectively doubles the acid gas removal efficiencies.

Such a design will obviously increase the overall scrubber pressure drop, but, since dust removal efficiency depends on the particle size distribution of the particulates coming in and also on the pressure drop across the system, adding another stage of reverse jet nozzles in the same inlet barrel will, at the same time, achieve a significantly improved removal efficiency for the particulate at a very limited incremental capital expenditure. It is worth noting that, with this technology, the pressure drop does not fluctuate or creep up, but remains constant. This is a very inexpensive way to increase scrubber efficiency.

What if the feed stream changes?

If refineries suddenly have a new contaminant in their mud incineration offgas, or if there is a spike in the acid content of the inlet gas, this system is extremely flexible and will adjust based on the pH. If there is more acid gas, it will be absorbed into the scrubbing liquid. The pH of the scrubbing liquid will drop. The system will add more of the reagent being used in that particular system to raise the pH to maintain the efficiency of the scrubber. The scrubber is usually operated at a pH of 6–7, although it can be operated at a higher pH if the acid content of the inlet gas is higher than expected. Since the system oxidizes the sump of the scrubber, the system can function at a higher pH without causing scaling.

If there is a baghouse upstream of the scrubber that should fail or break, the proprietary reverse jet scrubber can handle the additional particulate that would then consequently enter the scrubber system. The system can also handle considerable fluctuations in the inlet gas feed rate. Gas rates up to 120% above the design rate can be accommodated by adjusting the caustic rate and gas side pressure drop. As a rule of thumb, should gas rates drop, the turndown rate for the proprietary reverse jet scrubber is approximately 60%.

Takeaway

The correct treatment and disposal of refinery waste products is a complex but critical process for an industry beset not only with having to meet strict environmental regulations but also stakeholder expectations. As a result, organizations that opt for incineration as a disposal method utilize multiple and often elaborate gas cleaning processes to meet legislator requirements and improve the sustainability of their operations. While there are several options for treating mud-incineration offgases, a custom-designed versatile scrubber can simplify the process.

Due to its ability to handle very hot inlet gas temperatures, and thanks to its patented weir bowl technology, this proprietary reverse jet scrubber is an option that is capable of replacing all upstream gas treating equipment (such as the lime scrubber and bag filter) and still deliver low SO2 and particulate levels at the stack outlet (FIG. 5). HP

FIG. 5. Simplified scrubbing flow scheme downstream of the mud incinerator, with the proprietary reverse jet scrubber systema replacing both the lime scrubber and bag filter.

 

NOTE

a MECS® DynaWave® reverse jet scrubber

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