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Oct-2011

Revamping FCC emissions control

An upgrade of FCC unit emissions reduction at a Tulsa refinery relied on computational modelling to develop optimal flow control systems

Merle Fritz HollyFrontier R&M Kevin Linfield, Airflow Sciences Corporation
Dennis Salbilla, Haldor Topsøe
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Article Summary
A selective catalytic reduction (SCR) unit was installed on the HollyFrontier (formerly Sinclair) Tulsa FCC unit, with flue gas entering the unit for the first time in late November 2009. The existing electrostatic precipitator (ESP) was removed from service and replaced with a high dust content flue gas SCR design. This required a custom SCR catalyst that could withstand the erosive environment and still perform well over a five-year continuous run.

The FCC SCR unit has been in service for over a year without any maintenance outages. NOx reduction is excellent, with outlet NOx values below 20 ppmvdc at 0% O2, and this is achieved without ammonia injection. Apparently, the amount of NH3 formed in the regenerator is in ~1:1 proportion with NOx. Pressure drop across each of the two separate SCR catalyst layers is less than 1.5 IWC.

Background
In January 2008, Sinclair entered into a Consent Decree with the US Environmental Protection Agency. The refiner agreed to reduce NOx emissions from the Tulsa refinery’s FCC unit to 20 ppmvdc on a 365-day rolling average and 40 ppmvdc on a seven-day rolling average, both at 0% reference O2. Reductions in SOx emissions were also agreed at 25 ppmvdc on a 36-day rolling average and 50 ppmvdc on a seven-day rolling average, again based on 0% O2.

In order to achieve these emission targets, Sinclair installed a SCR unit for NOx reduction immediately upstream of a wet gas scrubber, which removes particulates and SOx. An existing ESP was removed from service and dismantled prior to the installation of the new SCR and wet gas scrubber. It was decided that the ESP was expendable based on the high dust SCR unit offered by Haldor Topsøe and the flue gas scrubber offered by MECS.

Design
The design of a FCC unit SCR comes with some unique challenges. These include:
• Two-phase flow, as FCC unit catalyst fines are entrained in the flue gas
• Continuous operation targeting a five-year run life
• Low pressure drop in a dusty operating environment.

Selective catalytic reduction is an end-of-pipe technology used for NOx destruction and characterised by high single-pass removal efficiency. Ammonia is injected into the flue gas at slightly above the molar equivalent ratio as its NOx concentration to react on the catalyst, producing nitrogen and water. Ammonia flow is automatically controlled by feedback control, measuring outlet NOx downstream of the SCR catalyst.
Haldor Topsøe’s design for FCC unit SCR applications calls for a vertical down flow unit. This takes advantage of gravity to address the catalyst fines entrained in the flue gas. Turning vanes are required to prevent uneven stratification of the solids and maintain a uniform velocity profile leading up to the inlet face of the SCR catalyst.

The HollyFrontier Tulsa FCC unit SCR has these characteristics (see Figure 1). Two catalyst layers, each containing 20 modules with 1m depth of DNX-958 catalyst, are employed. The size of one module is approximately 2m wide by 1m high by 1m deep in the flow direction. A set of static mixers along with the NH3 injection lances are located well upstream of the SCR catalyst, to provide adequate mixing time for the ammonia to blend completely with the flue gas prior to reaction on the catalyst surface.

The DNX-958 catalyst utilises a trimodal pore size distribution containing macro pores, meso pores and micro pores for activity retention in this dust-laden environment. FCC catalyst entrained in the flue gas is typically fines with an average particle size below 10 microns, as well as full-range catalyst, with an average particle size of 70 microns during an upset. The fines are able to fill the macro pores, similar to how marbles fill a vase. At some point, the macro pores accept the maximum amount of catalyst dust, yet NOx and NH3 in the flue gas can still diffuse into these pores through the remaining void space and complete the reduction reaction of the active sites of the catalyst surface.

After deciding on a catalyst and determining the required volume configured in two identical layers, computational fluid dynamics is used to further develop the design. Root mean square maldistribution for flue gas flow, NH3: NOx and temperature are quantified and corrected within acceptable tolerances, +15%, +10% RMS and +20°F, respectively. Turning vanes, static mixers and adequate mixing time enable the even distribution of flow and NH3 prior to entering the first layer of SCR catalyst.

Flow modelling In order to optimise the design of the SCR, Haldor Topsøe contracted Airflow Sciences Corporation to perform a flow model study consisting of both computational fluid dynamics (CFD) and physical flow modelling. These tools were used to develop optimal flow control devices that targeted the following design objectives across the specified load range:

• Uniform gas velocity distribution upstream of the ammonia injection grid (AIG)
• Uniform gas velocity distribution upstream of the first catalyst layer
• Uniform ammonia distribution upstream of the first catalyst layer
• Maintain less than 10% flow angularity from vertical at the first catalyst level
• Locate, document and minimise particulate accumulations on all surfaces
• Minimise system pressure loss.

The CFD model was the main tool used to develop flow control devices throughout the system. Major internal elements, such as the mixers and turning vanes, are represented precisely in the model. Model results detailed the 3D velocity flow field, pressure and ammonia distribution.

Since the entire geometry can exist virtually within the computer, there are no scaling limitations. All elements are modelled full scale. Actual flow conditions such as temperature, density and viscosity are implemented so that matching of important flow parameters such as Reynolds numbers is attained.
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