Oct-2009

FCC flue gas scrubber alternatives: part II

The merits of combining additives and barrier filters as a cost-effective alternative to meeting SOx, NOx and particulate matter emissions targets

John Sawyer, Hanif Lakhani, Kurt Schuttenberg and Lindsay McRae, Pall Corporation
Ray Fletcher and Martin Evans, Intercat (Johnson Matthey)

Article Summary

Many refiners have been compelled to utilise platinum-based combustion promoters to control afterburn in the FCC regenerator. While highly effective at promoting the combustion of CO to CO2, these additives may also result in a substantial increase in NOx emissions. Intercat has developed an additive technology (COP-NP) that controls afterburn while effectively limiting the negative NOx impact. Typical COP-NP performance is shown in Table 1.

Figure 1 shows an example of NOx reduction achieved with Intercat’s non-platinum promoter, COP-NP, while maintaining a constant degree of afterburn. The industry’s experience with NOx reduction additives continues to evolve and mature. Intercat’s research and development efforts, combined with commercial testing, confirm that there is no single additive solution that will work in all FCCUs. The reduction of NOx from the FCCU is a complex process with many interdependencies. It is worth noting that several US refiners have recently stopped using NOx reduction additives, as they are finding that non-platinum combustion promoters are able to achieve similar levels of NOx reduction. These refiners find that once NOx is reduced by one additive, the second additive is unlikely to provide a significant additional benefit.

Combined hardware andadditive solution
Filters do not typically have an effect on the SOx/NOx content of effluent gas. Additives work well to reduce SOx and NOx, but can increase particle loads to the particulate matter collection device. But combining SOx/NOx control additives with particulate filters creates a powerful and flexible combination because the blowback filter will handle the increased particle loads, while never exceeding particulate matter  emissions limits, 
as optimal additive dosage rates 
are used.

For refiners who are not running very high sulphur feeds, or who do not need to reach ultra-low SO2 levels (<25 ppm), this combination of filtration and additives can be the most cost-effective solution. It not only offers the broadest alternative for emissions reduction at the most attractive capital cost, but also significantly reduces operating costs, depending on the level of SO2 reduction required.

The advantage of this approach is that the filter will meet all current and currently envisioned particulate matter limits, while additive usage can be adjusted to meet current limits. If today’s SOx limits are higher than they will be in five years, there is no need to pre-invest for tomorrow’s limit with additives. Additive technology is a continually changing field. New additives are always being developed, and refiners can take advantage of improved additive technology to reduce costs and improve performance as it becomes available.

The filter removes non-condensable particulates and provides a means of containing catalyst and additive fines, thereby permitting maximum dose rates to reduce the SOx/NOx content of the flue gas. The net result is essentially zero non-condensable particulate matter in the effluent gas, a reduction in condensable particulate emissions due to reduced sulphur emissions, and the near-elimination of ammonium sulphates since ammonia addition is not required.

Blowback filter operation
Pall’s jet-pulse blowback filter systems are designed to remove particulate matter from gas streams. To accomplish this, sintered metal 
or ceramic filter elements with sufficiently small pores, and sized at an appropriate flux (flow per unit area), are used to capture and retain FCC catalyst at the filter’s surface. As a result, a permeable cake of solids forms. The cake is dislodged at a predetermined time or differential pressure (∆P) by initiating a reverse gas pulse. In a single-vessel configuration, the filter elements within are arranged in sections, each with its own blowback gas mani-folding and pulse gas valves. Based on time or ∆P activation, the blow-back gas valves open sequentially, in rapid succession, blowing back adjacent sections, one at time, until all sections have been cleaned. The blowback pulse lasts only a fraction of a second, during which time process gas is diverted to the sections not being blown back. This permits uninterrupted system operation (See Figure 2).

Blowback requires the use of clean, dry, compressed air, typically at 850 kPag (125 psig) pressure for FCC. Once dislodged, the solids fall into 
the hopper section of the filter vessel and are periodically purged to the spent catalyst hopper. After the blowback cycle, a fine layer of particles remains on the filter media and serves as the fine, protective coat known as the permanent cake that aids filtration over subsequent filtration and blowback cycles. The filter then returns to full forward flow and, over an initial few cycles, a conditioned recovery or equilibrium ∆P becomes established (see 
Figure 3).

Figure 4 illustrates the structure and behaviour of the permeable cake that is formed on the filter surface. Over several initial filtration and blowback cycles, the fine permanent cake becomes established and, once equilibrated (due to weak electrostatic forces), provides an additional protective filtration layer.

Filter elements and experience
The FCC flue gas filter systems (gas solids separation, or GSS) are supplied with filter vessel(s), filter elements, automated controls (local PLC, data acquisition for DCS), instrumentation, valving, blowback manifolding and a pulse gas accumulator (see Figure 5).

Pall filter elements are at the heart of the blowback filter system. Measuring 60mm (2.38 inch) outside diameter and up to 2500mm (98 inch) in length, Pall has several element compositions (intermetallic, stainless steel and ceramic) with excellent high-temperature corrosion resistance. The filter elements have been used extensively at high temperature in both oxidising and reducing 
conditions at temperatures up to 787°C (1450°F). These element compositions have proven to be durable and very corrosion resistant.

Third-stage filter case study
In 2003, an Australian refinery found that flue gas particulate emissions from its RFCC varied from 150—400 mg/Nm3. Although this was in compliance with particulate emissions limits at the time, it was believed the refinery would not be compliant when tighter limits were adopted in 2005.The particulate emissions licence limits for the Australian refinery for 2005 and beyond are shown in Table 2.