Apr-2002

Controlling flue gas emissions

This article reviews the latest developments in a wet scrubbing system that is used to reduce particulate SO2 and SO3 within one simple process

Nicholas Confuorto and Edwin H Weaver
Belco Technologies Corporation

Article Summary

Sulphur level reductions for gaseous emissions and gasoline/diesel are becoming more prevalent at the same time that refiners are looking to increase profitability by processing lower cost crudes. In most cases, these lower cost crudes contain more sulphur. These trends are bringing a great deal of attention to controlling emissions from FCCU, sulphur recovery unit (SRU) and other processes. For many years this focus on controlling emissions was limited to new FCCUs, while older units received little attention. This is changing worldwide. Regulations are being promulgated which will require existing units to also meet new environmental requirements for particulate and SO2.

The proprietary EDV Wet Scrubbing System, used to reduce particulate SO2 and SO3, has the ability to be applied as a particulate-only device (instead of an electrostatic precipitator, or ESP) and then later, as regulations become more stringent, to be converted to particulate and SOx, without another system having to be purchased. For high levels of SO2, the same technology can also be converted to a regenerative SOx control device, thus greatly minimising operating costs. This regenerative option, called Labsorb, has been in use at a refinery in Norway for incinerated SRU tailgas cleaning and is soon to be installed to control FCCU regenerator emissions at a refinery in Europe, as well as two refineries in the USA.

The system utilises the proprietary EDV technology, already installed (or in the process of being installed) as emissions control for 22 FCCUs worldwide with an aggregate capacity of nearly 1 million bpd. In addition, the proprietary third stage separator (TSS) technology will be discussed, which has the potential to achieve emissions below 50mg/Nm3, making it the only third stage system that can actually be considered in cases where the only requirement is particulate control.

Further discussion will address differences between caustic scrubbing, regenerative scrubbing and standard ESP applications for FCCU emissions. The system will be described in detail. Potential benefits beyond meeting regulatory requirements are also addressed.

Emissions and reliability
The European Commission’s pending release of the Best Available Techniques (BAT) report brings attention to reduction of particulate and SO2 emissions at refineries. Environmental pressures are leading towards lower emission levels, while changing process conditions can lead to higher emission levels requiring control. For example, the most recent requirements in the USA have been 25ppm for SO2 and 1lb of particulate per 1000lb of coke burn.

In Europe, the regulations for “new” FCC units are 20–150mg/Nm3) of SO2 and 10–30mg/Nm3 of particulates. The same regulations will apply to existing FCC units in Europe by 2007. 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 available alternatives for controlling particulate and SO2 emissions, the application of an FCCU wet scrubbing system is examined. 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.

Particulate (catalyst) emissions from this source vary depending on the number of stages of internal and external cyclones. Although internal cyclones are effective in collecting the greater constituent of catalyst re-circulated 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 650mg/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 per cent of the incoming feed sulphur is transferred to the acid gas and product side in the form of H2S. The remaining 5 to 30 per cent 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 9000mg/Nm3, whereas SO3 typically varies from 5 to 15 per cent of the total SO2 content.

Wet scrubbing system
The proprietary EDV wet scrubbing system controls both particulate and SO2 combined, and at a very low pressure drop, with an option to have no pressure drop at all for those applications where pressure is not available. The 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 sulphuric 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 2), and droplet separators (Figure 3) (on previous page).

The fluegas 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. 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.