By D.Védrine and L.Donnat, Gonfreville Research Centre, Process & Refining Division, TOTAL France, Gonfreville, France

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Since the early 1990s, TOTAL has used CFD software to understand, in great detail, the fluid processes inside its many unit operations. By analyzing the flow fields, engineers have been able to identify the causes of problems and evaluate solutions. Over one hundred studies have been conducted during this time, and most of the major refining processes (fluid catalytic cracking (FCC), alkylation, desasphalting, hydrotreating, and reforming, for example) have benefited by way of improved reliability and expanded operating conditions. Two such simulations, described below, led to improved performance for an FCC regenerator and a hydrotreating unit at the Provence Refinery.

Primary and secondary cyclones’ erosion

During a recent ten-year period, the flow rates and catalyst loading inside the refinery's FCC regenerator were progressively increased, and as a result, erosion in the cyclones became more and more significant. To better understand the phenomenon, a FLUENT simulation was performed for one of the units, using approximately 1.6 million tetrahedral cells. The catalyst phase was modeled using a Lagrangian (DPM) calculation, which was coupled with the background gas phase through momentum transfer. Around 35,000 particles were tracked, representing 10 different diameter sizes from 10 to 130 microns based on a Rosin-Rammler distribution. The Reynolds stress model (RSM) was used for turbulence, because of its ability to capture the effects of curvature, swirl, rotation, and rapid change in strain rate. Stochastic tracking was used to include the effects of the gas phase turbulence on the particles, and a porous region, described by an adapted Ergun equation, was used to take into account the catalyst height in the cyclone diplegs. The simulations accurately predicted several observed characteristics of the cyclone, such as the separation efficiency, catalyst trajectories, and gas velocity profiles. They were also used to identify troublesome zones and compare them with observations made on site. This exercise led to a better understanding of the causes of erosion and pitting, and helped engineers to propose and evaluate a new cyclone design, which was implemented during the last major shutdown (early 2003).

In 1998, a transfer line composed of two successive elbows upstream of one of the hydrotreating units of the Provence Refinery was damaged by corrosion. The corrosion was believed to be the result of mass transfer of toxic ions to the pipe wall during water washing cycles, or of oxidants in droplets during the flow of vaporized feedstock. FLUENT was used to simulate the second of these conditions, namely the gas/liquid flow in the pipe.

Using the VOF approach and a mesh of 630,000 cells, an annular flow of liquid was shown to be disrupted by the elbows, causing drops to impact on the wall and a liquid film to be created on the wall near the exit of the second elbow. To accurately account for the friction between the liquid and gas phases, a source term was added to the momentum equations through a userdefined function (UDF), developed at TOTAL's Gonfreville Research Centre. The results showed the behavior of the flow field near the critical area, where droplets were known to impact on the wall. Variations in the static pressure on a cross section through the pipe suggested the locations where the mass transfer coefficient of the corrosive species would be high. The knowledge gained will help prevent corrosion from occurring on other similar units in the future. Indeed, the approach used (and validated) will be applied with confidence to evaluate risky geometries on industrial units and to propose modifications, such as increasing the distance between the two elbows, for example.

Damaged pipe in the elbow section upstream of the hydrotreating unit

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