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Oct-2013

Wet scrubbing modifications to 
reduce emissions

Modifications to existing wet scrubbing systems for FCC units can readily be made to achieve additional reductions in particulates, SO2 or NOx.

Edwin Weaver and Nicholas Confuorto
Belco Technologies Corporation
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Article Summary
Many FCC units have wet scrubbing systems installed on them to reduce air emissions. A large percentage of these were designed to address only particulate and SOx emissions. Some of them may have been designed to achieve older emission mandates that are higher than the ones presently being mandated by regulatory authorities. A modification of these systems may be required to reduce particulate and/or SOx emissions and possibly to also include a reduction in NOx emissions. This article addresses how these reductions can be achieved, considering the optimisation of economics and system outage time to fit within turnaround schedules.

First, a reduction in SOx emissions is discussed. Methods to lower SOx levels from a current wet scrubbing system are reviewed along with the amount of emissions reductions that can be achieved. Next, particulate emissions reductions are discussed. The modifications required to reduce particulate emissions in an existing wet scrubbing system are detailed. Finally, the addition of NOx control to an existing wet scrubbing system is discussed. Several approaches are presented, along with the merits and potential issues associated with the different approaches.

A basic wet scrubbing system
For many years, refiners have chosen to use the EDV wet scrubbing system to control both particulate and SO2 emissions. With this system, particulate and sulphur emissions are removed simultaneously and efficiently. This technology is well proven in providing flexibility to handle added capacity that may result from FCC unit expansions or to increase reduction efficiency as regulatory pressures increase and in providing uninterrupted operation/performance exceeding that of FCC units. 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 flexibility of wet scrubbing compared to other emission control options. 

The EDV wet scrubbing system is the state-of-the-art approach for controlling particulate and SO2 emissions from oil refinery FCC units, boilers and heaters. One arrangement of this system is shown in Figure 1.

The system treats hot flue gas-containing particulates (such as FCC catalyst fines) and SO2, and discharges cleaned gas to the atmosphere through an integral stack. At the scrubber inlet, FCC flue gas is quenched and saturated by 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, such as boiler tubes or a flue gas cooler. However, the system can also be designed to accept the full flue gas temperature directly from the high-temperature flue gas source without any heat reduction in cases where a CO boiler on an FCC unit application requires to be bypassed. The flue gas from the FCC unit can be diverted directly to the EDV wet scrubbing system without any concerns and without having to make any adjustments to the operation. This not only results in a more reliable and simpler operation, but also allows the plant to continuously reduce emissions even during bypass and upset conditions.

The EDV wet scrubbing system utilises proprietary nozzles to 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 are designed to handle highly concentrated slurries.

SO2 absorption and particulate removal begins at the quench section and continues as the flue gas rises up through the main spray tower, where the 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 Belco 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 and reversals without any concern.

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 alkalis such as soda ash and magnesium hydroxide have also been utilised with excellent results in terms of performance and reliability. In non-refinery applications and in extremely few refinery applications, lime has also been used. However, for FCC unit applications and other refinery applications where three-to-seven-year continuous operation is required, the use of lime as a reagent is strongly discouraged.

In the EDV wet scrubbing system, multiple levels of spray nozzles provide sufficient stages of gas/liquid contact to reduce both particulate and SO2. An illustration of the spray tower and the spray nozzles is shown in Figure 2.

Makeup water is added to the system to replace the water lost to evaporation in the quench zone and also the water purged from the system. Captured pollutants, including suspended catalyst fines and dissolved sulphites/sulphates (NaHSO3, Na2SO3 and Na2SO4 in cases when sodium-based reagents are used) resulting from the reduction in particulate and SOx, are purged from the spray tower recycle loop to maintain the proper balance. The treatment of this purge stream is addressed later.

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 the water that is present in the flue gas as moisture to condense onto the fine particulate and the acid mist (mostly H2SO4 from condensation of SO3 in the saturated flue gas), increasing both their size and mass by converting them to relatively large droplets. These droplets are then removed from the flue gas by proprietary F nozzles located at the exit of the filtering modules. Additional condensation occurs on the walls of the filtering modules, keeping them continuously clean. Some agglomeration also takes place in the filtering modules, further enhancing particulate removal.

As mentioned above, a F nozzle is located at the exit of the filtering module. This nozzle sprays counter-current to the gas flow and provides the mechanism for the collection of the fine particulate and mist, which has been enlarged by condensation and agglomerated. This device has the advantage of being able to remove fine particulate and acid mist with an extremely low-pressure drop and no internal components that can wear or cause unscheduled shutdowns. It is also relatively insensitive to fluctuations in gas flow. It is illustrated in Figure 3.
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