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Design, operation, and performance of a recently installed wet scrubbing System for Motiva

The need to reduce SO2 emissions from the FCCU regenerator flue gas is essential for a refinery to minimise its emissions of the pollutant. In 1997 Star Enterprise Port Arthur refinery decided to minimise these emissions through the installation of a wet scrubbing system on their 3 FCCU.

Edwin H Weaver and Bill Boska, Belco Technologies Corporation (Now BELCO Clean Air Technologies)
John Grice Motiva, Enterprises LLC
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Article Summary
This paper examines the decision making process in the selection of wet scrubbing as the optimum technology for this installation. The design of the wet scrubbing system is also examined, including the specific design requirements and features for this installation. Treatment of the purge stream from the wet scrubbing system is also examined. Finally, the tie-in, startup, and performance of the system is discussed along with operating data from the system.

In 1997, Star Enterprise in Port Arthur, Texas decided to install a wet scrubbing system to reduce SO2 emissions from their 3 FCCU. The 3 FCCU has been in service for many years. The original technology for the FCCU was licensed from Kellog. Originally, the flue gas went to a CO boiler after leaving the regenerator and then directly to the stack. It was planned to install the wet scrubbing system after the CO boiler and then send the flue gas to a new stack. The project was planned so that the scrubber would be made operational during planned turnaround activities in the 2nd quarter of 1999. The system was designed to meet 300 ppmdv SO2. This represented an SO2 reduction of approximately 26.1% of the SO2 leaving the FCCU CO Boiler stack.

In early 1998 Star Enterprise selected the EDV wet scrubbing system from Belco Technologies to control SO2 emissions. The system design and operation history are examined in detail in this article.

System design
Based on the design capacity for the various operating conditions, the flue gas flow rate to the wet scrubbing system varied from 1,044,221 lb/hr of flue gas up to 1,122,195 lb/hr of flue gas at a gas temperature of 550°F. When the CO boiler was bypassed, the flue gas flow rate ranged from 812,569 lb/hr up to 890,554 lb/hr at a gas temperature of 1320°F. These conditions are summarised in Figure 1.

As previously noted, the emissions performance of the system was designed to ensure that the 300 ppm SO2 emissions could easily be achieved. In addition, continuous system operation between unit turnarounds was a key factor in the system design. With a planned time between turnarounds of 5 years there were a couple of key design features included in the system to ensure its continuous operability and performance. First, the nozzles in the lower part of the scrubber vessel, including the quench, were put on a separate pump loop. This allowed these nozzles, which are subjected to the highest wear factors to be operated independently of the other nozzles. This reduces the wear on these nozzles and ensures the 5 years of continuous operation. Also, one spare pumps on each of the pump loops was designed as a steam turbine driven pump. This provided for a very high reliability factor. The system should have no total pumps failures, even in the situation where there is a total loss of power.

Because of the capabilities of the existing waste water treatment facility the purge from the scrubbing system is pumped directly to the refinery waste water treatment facility where an oxidation/aeration pond was built to pre-treat the scrubber effluent prior to its introduction into the existing Activated Sludge Treating Unit. An unusual part of this system is that the refinery waste water treatment facility is located almost one mile from the scrubbing system so the purge is being pumped almost one mile before treatment.

Description of the Wet Scrubbing System
The wet scrubbing system provided for this installation is the EDV technology from Belco Technologies Corporation. This wet scrubbing system controls SO2. SO2 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.

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 spray the caustic solution to reduce SO2 emissions. These nozzles, used for both the quench and the spray tower, are 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 pluggage. An illustration of this nozzle and the general system configuration is provided in Figure 2.

Upon leaving the spray tower, the saturated gases are directed to the EDV Filtering Modules. 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 acid mist. This causes a dramatic increase in size of the 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 mist. It is relatively insensitive to fluctuations in gas flow. An illustration of this device is provided in Figure 3.

To ensure droplet free gas, the flue gas then goes to a droplet separator. This is an open design that contains fixed spin vanes which 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 in Figure 4.

System purge
Purge from the wet scrubbing system contains sodium sulphite (NaSO3) and sodium sulfate (NaSO4) as dissolved solids. This purge is sent to the refinery waste water facility where the sulphite are oxidised to lower the CODs to an acceptable level. The characteristics of the purge stream from the scrubber is provided in Figure 5.
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