Jan-1999

Flue gas scrubbing of FCCU regenerator flue gas performance, reliability, and flexibility

In 1997 Pennzoil Products Company, now Pennzoil-Quaker State Company, brought into operation a new FCCU at their Shreveport, Louisiana refinery.

Nicholas Confuorto and Edwin H Weaver
Belco Technologies Corporation (Now BELCO Clean Air Technologies)

Article Summary

The new FCCU was a Reduced Crude Conversion Unit (RCC) with a design capacity of 10,500 bpsd. It was designed to process a variety of feedstocks including resids, gas oils, slack waxes and off-spec waxy lubestocks. The RCC has two stages of internal cyclones in the regenerator. The regenerator is a two stage, partial burn unit, so a CO-boiler was installed after the regenerator for the combustion of CO. In order to comply with New Source Performance Standards (NSPS) for particulate and SO2 emissions, a wet scrubber was provided. The requirements of NSPS for particulate and SO2 emissions are provided in Figure below.

Pennzoil selected the Belco Technologies Corporation’s EDV wet scrubbing system to control particulate and SO2 emissions. In addition to the EDV wet scrubbing system, BELCO also included a purge treatment system designed to reduce the suspended solids and Chemical Oxygen Demand (COD) in the purge stream from the EDV Wet Scrubbing system. The system design, operation history, and performance are examined in detail in this article.

System design
As previously noted, the process design capacity of this installation is 10,500 bpsd. This translates to a flue gas flow rate to the EDV wet scrubber of 261,886 lbs/hr of flue gas. The design inlet catalyst (particulate) loading to the scrubbing system was 0.178 gr/dscf, or 5.57 lbs/1,000 lbs of coke burn-off. Design inlet SO2 emissions to the scrubber were 626 ppmvd. The scrubber inlet design conditions are summarised above.

The emissions performance of the EDV wet scrubbing system was determined keeping in mind that operational flexibility was a key issue. While particulate outlet emissions were designed at 0.95 lbs/1,000 lb of coke burn-off, the SO2 removal was designed to provide performance significantly better that NSPS requirements. The design outlet emission level for SO2 was 13.1 ppmvd or approximately 26% of the emission level allowed in NSPS.

In order to have an acceptable wastewater from the scrubbing system, the purge treatment system was designed to reduce suspended solids to less than 50 ppm and to reduce the COD level to less than 50 ppm. A summary of these design performance levels is provided above.

Description of the EDV Wet Scrubbing System
The wet scrubbing system provided for this installation is the EDV technology from Belco Technologies Corporation. This wet scrubbing system controls both particulate and SO2. Primary particulate removal is accomplished in the absorber vessel where caustic soda (NaOH) is utilised to absorb SO2 and discharge it in the form of a soluble sodium sulphate salt. Fine particulate control is accomplished in devices known as Filtering Modules. These, and the rest of the system, will be described in detail.

The EDV system at this installation consists of a spray tower, filtering modules and droplet separators. This general configuration is shown below. The flue gas from the CO boiler enters the spray tower where it is immediately quenched to saturation temperature. The spray tower itself is an open tower with multiple levels of spray nozzles. These nozzles remove coarse particulate by impacting on the water droplets. These nozzles also spray the caustic solution to reduce SO2 emissions. These nozzles, used for both the quench and the spray tower, are Belco nozzles. Their unique design is a key element of the system. They are non-plugging, constructed of abrasion and corrosion resistant material, and capable of handling high concentrated slurries. They produce relatively large water droplets, which prevent the formation of mist and the need for a conventional mist eliminator which can be prone to plugage. An illustration of this nozzle is provided opposite.

 Upon leaving the spray tower, the saturated gases are directed to the EDV Filtering Modules for removal of the fine particulate. This is achieved through saturation, condensation, and filtration. Since the gas is already saturated, condensation is the first step in the filtering modules. The gases are accelerated slightly to cause a change in their energy state and a state of super saturation is achieved through adiabatic expansion. Condensation occurs on the fine particulate and acid mist. This causes a dramatic increase in size of the fine particulate and acid mist, which significantly reduces the required energy and complexity of its removal. A nozzle located at the bottom of the filtering module and spraying upward provides the mechanism for the collection of the fine particulate and mist. This device has the unique advantage of being able to remove fine particulate with extremely low pressure drop and no internal components which can wear and be the cause of unscheduled shutdowns. It is also relatively insensitive to fluctuations in gas flow. An illustration of this device is provided above.

To ensure droplet free stack, the flue gas then goes to a droplet separator. This is an open design that contains fixed spin vanes that induce a cyclonic flow of the gas. As the gases spiral down the droplet separator, the centrifugal forces drive any free droplets to the wall, separating them from the gas stream. This device has very low pressure-drop with no internal components which could plug and force the stoppage of the FCCU. This device is also illustrated on the previous page.
 
Purge treatment system
Purge from the EDV Wet Scrubbing system contains catalyst fines as suspended solids, and sodium sulphite (NaSO3) and sodium sulfate (Na2SO4) as dissolved solids. The purge treatment system removes the suspended solids and oxidises the sodium sulphite to sodium sulphate to reduce the COD so that the effluent can be safely discharged into the refinery water system or off site.

In order to remove the suspended solids, the purge treatment system contains a clarifier to separate the suspended solids and a filterpress to concentrate the solids into a filter-cake which is cohesive and can be readily disposed. The oxidation system consists of a tower where air is forced into the effluent to oxidise the sodium sulphite to sodium sulphate.

As can be seen in the diagram provided above, the scrubber purge enters the clarifier from a deaeration tank. The solids settle out in the clarifier and are removed from the clarifier in the underflow. The underflow from the clarifier is sent to the filter press where the excess water is removed. The solids are sent to disposal while the water is returned to the clarifier. The overflow from the clarifier, which has had the suspended solids removed, is sent to the oxidation tower where air is forced into the system to contact the liquid and oxidise the sodium sulphite to sodium sulfate. The effluent is then discharged to the refinery water system.