Evaluating wet scrubbers

The use of both regenerative and non-regenerative wet scrubbing systems to control the emission of particulate and SO2 is well proven and established

Edwin H Weaver, Belco Technologies Corporation

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Article Summary

An operating system in a refinery using regenerative and non-regenerative wet scrubbing systems is examined in detail, as is the technology for each system. The reason for the selection of regenerative or non-regenerative wet scrubbing at each installation is also discussed.

In order to illustrate the different scenarios a refiner might face in technology selection, several possible situations are examined. These include situations of high uncontrolled SO2 emissions, low uncontrolled SO2 emissions, a location where effluent discharge is not an issue, a location where effluent discharge is an issue and the impact of particulate control needs. For each situation, the use of regenerative and non-regenerative wet scrubbing is discussed with respect to the technology required, the capital and operating costs, and any other pertinent factors.

The use of both regenerative and non-regenerative wet scrubbing systems to control the emission of particulate and SO2 is well proven and established. Systems in operation have proven to operate continuously and reliably while maintaining emission levels comfortably below 50 mg/Nm3 for particulate and the US Environmental Protection Agency (EPA) target level of 25ppm for SO2. However, the selection of the most appropriate system requires a detailed evaluation of both the technical aspects and the capital and operating costs of each technology. In addition to the air emission requirements, the liquid effluent discharge from the wet scrubbing system is an issue that must be closely examined in some locations.

Caustic-based non-regenerative wet scrubbing
For many years, refiners worldwide have decided to use wet scrubbing for control of particulate and SO2 emissions from FCCUs and other air emission sources within the refinery. For most of these sources, both particulate and sulphur emissions in the flue gas are controlled simultaneously. This scrubbing technology is well proven in providing the flexibility to handle the added capacity that comes with FCCU expansions and in providing uninterrupted operation/performance exceeding that of FCCUs. Each refiner’s specific reasons for choosing wet scrubbing differ, but these have generally been related to environmental compliance as well as relative costs, reliability and the flexibility of wet scrubbing compared to other emission control options.

One arrangement of the proprietary EDV wet scrubbing system for controlling particulate and SO2 emissions from FCCUs and other process units is shown in Figure 1. The system treats hot flue gas containing particulate and SO2 from an FCCU or other process unit and discharges cleaned gas to the atmosphere through an integral stack. At the scrubber inlet, flue gas is quenched and saturated by means of multiple water sprays in the spray tower’s horizontal quench section. Normally, the flue gas enters the wet scrubber after passing through a heat recovery device. However, the system is designed so that it can accept flue gas directly from the FCCU regenerator or another process unit at the temperature at which it exits the FCCU regenerator or process unit.

The proprietary LAB-G spray nozzles produce high-density water curtains through which the gas must pass. Each nozzle sprays water droplets that move in a cross-flow pattern relative to the flue gas. These cover the entire gas stream and uniformly flush the vessel’s surfaces clean. The spray nozzles are non-clogging and able to handle highly concentrated slurries.

SO2 absorption and particulate removal takes place immediately after quenching as the flue gas rises up through the main spray tower, where gas is again contacted with high-density water curtains produced by additional spray nozzles. The spray tower itself is an open tower with multiple levels of LAB-G spray nozzles. Since it is an open tower, there is nothing to clog or plug in the event of a process upset. In fact, this design has handled numerous process upsets where over 150 tons of catalyst has been sent to the wet scrubber in a very short period of time.

The scrubbing liquid is controlled to a neutral pH with reagent addition to drive SO2 absorption. Caustic soda (NaOH) is typically used as the alkaline reagent. However, other alkali, such as soda ash and magnesium hydroxide, have also been utilised with excellent results in terms of performance and reliability. Multiple levels of spray nozzles provide sufficient stages of gas/liquid contact to remove both particulate and SO2.

Makeup water is added to the system, replacing water lost to evaporation in the quench zone as well as water purged from the system. Captured pollutants, including suspended particulate and dissolved sulphites/sulphates (NaHSO3, Na2SO3 and Na2SO4) from SO2 removal, are purged from the spray tower recycle loop.

In order to remove very fine particulate, flue gas leaving the spray tower is distributed to a bank of parallel filtering modules. Within each module, the flue gas first accelerates (compresses) and then decelerates (expands). This action causes water to condense from the flue gas. The water uniformly washes the module’s walls. More importantly, water condenses on the fine particulate and acid mist (H2SO4) present in the flue gas, increasing both their size and mass. Some agglomeration also takes place.

A proprietary LAB-F nozzle, located at the exit of the filtering module and spraying counter-current to gas flow, provides the mechanism for the collection of the fine particulate and mist, which has been enlarged and agglomerated. This device has the unique advantage of being able to remove fine particulate and acid mist with an extremely low pressure drop and no internal components, which can wear and cause unscheduled shutdowns. It is also relatively insensitive to fluctuations in gas flow.

Prior to being discharged to the atmosphere through a stack, the flue gas enters a bank of parallel droplet separators. Each treats a portion of gas flow and separates/collects free water droplets. The gas entering each separator passes through a fixed spin vane, where centrifugal acceleration causes free water droplets to impinge on the separator’s walls. Collected water droplets flush the walls uniformly clean and drain to the bottom. Collected water is recycled for flue gas cleaning in the filtering modules or spray tower.

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