Economic alternatives for controlling particulate and SOx pollutants in refining
The focus on the reduction of particulate and SO2 emissions in refineries has never been greater. Environmental pressures are leading towards lower and lower emission levels while changing process conditions can lead to higher emission levels requiring control.
Edwin H Weaver and Nicholas Confuorto
Belco Technologies Corporation
Viewed : 1955
As an example the most recent requirements in the United States of America have been 25 ppm for SO2 and 1Kg of particulate per Ton of Coke Burn. In Europe the SO2 regulations for new FCCUs are 20-150 mg/Nm3 and particulate regulations are 10-30 mg/Nm3. The need to meet these stringent emissions is coupled with the utmost need for equipment reliability to ensure continuous operation of the air pollution control system and to not effect the operation of the refinery.
In order to examine the alternatives available for the control of these emissions, the application of a wet scrubbing system to the fluid catalytic cracking unit (FCCU) is examined in detail. This particular process has been chosen since it is typically the largest source of emissions in a refinery and also has the utmost demands on performance and reliability. Wet scrubbing systems designed to operate on this process must operate reliably and continuous for at least five years. After the basic system designed is examined, the various alternatives that affect the overall system economics are examined in detail to illustrate the process and economic differences in various approaches.
In most refineries, the flue gas from the regenerator in the FCCU represents the greatest single air emission source. It contains significant quantities of catalyst fines and sulphur oxides.
Particulate (catalyst) emissions from this source vary depending on the number of stages of internal and external cyclones. Although cyclones are effective in collecting the greater constituent of catalyst recirculated in the FCCU regenerator, the attrition of catalyst causes a significant amount of finer catalyst to escape the cyclone system with relative ease. Typically emissions will range from 200 to 650 milligrams per normal cubic meter of gas (mg/Nm3).
Sulphur emissions in the form of SOx (SO2 and SO3) from the regenerator vary significantly depending on the feed sulphur content and the FCCU design. In the FCCU reactor, 70% to 95% of the incoming feed sulphur is transferred to the acid gas and product side in the form of H2S. The remaining 5% to 30% of the incoming feed sulphur is attached to the coke and is oxidised into SOx which is emitted with the regenerator flue gas.
The sulphur distribution is dependent on the sulphur species contained in the feed, and in particular the amount of thiophenic sulphur. SO2 can range from 500 to 9,000 milligrams per normal cubic meter of gas (mg/Nm3), whereas SO3 typically varies from 5% to 15% of the SO2 content.
Description of the EDV Wet Scrubbing System
The worldwide leading technology to control emissions from this process is the EDV wet scrubbing system. This wet scrubbing system controls both particulate and SO2. Primary particulate removal is accomplished in the absorber vessel where caustic soda (NaOH), or other reagents, are utilised to absorb SO2 and discharge it in the form of a soluble salt. Fine particulate control and significant reduction of SO3 in the form of sulfuric acid mist is accomplished in devices known as Filtering Modules. These, and the rest of the system, will be described in detail.
The EDV system consists of a Spray Tower (Figure 1), Filtering Modules (Figure 3), and Droplet Separators (Figure 4). The flue gas from the FCCU enters the spray tower where it is immediately quenched to saturation temperature. 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 at the temperature at which it exits the FCCU regenerator. The spray tower itself is an open tower with multiple levels of spray nozzles designed specifically for this application (Figure 2). 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 handles numerous process upsets where over 150 tons of catalyst has been sent to the wet scrubber in a very short period of time. An illustration of this spray tower and the spray nozzles is provided below (Figure 1).
These nozzles (Figure 2), used for both the quench and the spray tower, are G nozzles. They are a unique design and a key element of the system. They are non-plugging, constructed of abrasion and corrosion resistant material, and capable of handling high concentrated slurries. These nozzles remove coarse particulate by impacting on the water droplets. They also spray the reagent solution to reduce SO2 emissions. They produce relatively large water droplets, which prevent the formation of mist and therefore the need for a conventional mist eliminator that will be prone to plugging. This is unique in wet scrubbing system designs as any other design that uses conventional nozzles will produces mist size water droplets thus requiring mist eliminators.
Upon leaving the spray tower, the saturated gases are directed to the EDV Filtering Modules (Figure 3) 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. An F 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 and acid mist with an 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. This device is illustrated below in Figure 3.
To ensure droplet free gas at the stack, the flue gas then goes to a droplet separator (Figure 4). This is an open design that contains fixed spin vanes that induce a cyclonic flow to 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 a very low pressure drop with no internal components which could plug and force the stoppage of the FCCU. This device is illustrated in Figure 4.
Assuming that a sodium based system is used, purge from the wet scrubbing system contains catalyst fines as suspended solids, and sodium sulphite (NaSO3) and sodium sulphate (NaSO4) as dissolved solids. The purge treatment system removes the suspended solids and converts the sodium sulphite to sodium sulphate to reduce the Chemical Oxygen Demand (COD) so that the effluent can be safely discharged from the refinery.
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