Virtual Fluid Dynamics

Flow-simulation software could save millions of dollars at chemical and process unit operations

By Franz Zdravistch and Ahmad Haidari

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The Rentech GTL conversion process uses an iron-based catalyst in a slurry reactor. The reactor vessel contains the catalyst suspended in liquid hydrocarbon, and the synthesis gas is brought in at the bottom of the unit and bubbled up through the reactor body. As the bubbles contact the catalyst, the Fischer-Tropsch reaction takes place. Studies suggest petrochemical companies could realize millions of dollars of additional revenues each year in every refinery if the technique were to be commercialized fully and used to convert current refinery residues.

One of the drawbacks in pioneering this technology has been the prohibitively expensive capital equipment costs for plant-scale prototype reactor testing. CFD modeling is a promising alternative for scale-up studies of slurry reactor vessels.

Plant personnel must understand the 3D hydrodynamics in the vessel to achieve a high reaction rate for commercially viable GTL conversion. This high rate inevitably is linked to a high gas-bubble distribution and large gas/liquid interfacial area in the reactor.

Gas holdup is another important factor affecting reaction rate and must be maximized. Through transient 3D CFD simulations using software, Rentech has been able to determine this parameter for various vessel geometries, bubble release points and bubble diameters. Rentech found a strong correlation between these CFD simulations and results of experimental work, thereby gaining greater confidence in this technique. (See Fig. 2.) With potential savings in the millions of dollars for a single refinery, the technology offers great promise.

 

Figure 2. CFD Models Bubble Column

Hydrodynamics in Large Slurry Reactors

The modeling performed at Rentech correlated strongly with experimental data, which helped with scale-up of small reactors to commercial-size equipment.

 

CFD solves spray pattern problem

A packed-bed tubular reactor within BP's chemical stream consists of a tube bundle of several thousand tubes loaded with catalyst particles. The tubes are arranged in an annulus with an outer diameter of 4.8 meters (m) around a central solid core with a diameter of 1.3 m. The headspace at the top of the tube bundle is 1 m high.

Reactant gases enter through an elliptical port located at one side of the headspace. The feed gases are hydrocarbons and air, and most of the hydrocarbons are well mixed with the air upstream of the reactor.

To avoid an explosive mixture in the pipework, a final addition of hydrocarbon ," injected as a liquid spray ," is provided just upstream of the reactor inlet. The intention had been for this spray to evaporate and mix with the feed before it enters the tube bundle.

However, during operation, temperature variations monitored in different tubes indicated different reaction rates. This phenomenon was believed to result from a maldistribution of the hydrocarbons, attributed to poor dispersion of the injected spray and asymmetry in the inlet velocity profile caused by upstream pipework bends.

To quantify this effect, the flow in the upstream pipework and the reactor headspace was modeled using CFD. First, the inlet pipework was modeled with the hydrocarbon spray to establish the reactor inlet velocity and hydrocarbon concentration profiles. Then, using these inlet profiles, the flow in the reactor headspace was modeled, and the distribution of hydrocarbons entering the tube bundle was established. This distribution then could be compared with the temperature distribution in the tubes to determine any correlation between the two.

The final two bends of the inlet pipework (800-millimeter [mm] inner diameter) and the contraction/expansion into the ellipsoidal inlet were modeled using a 3D model with 29,000 grid cells. The flow entering the ducting had a uniform velocity of 27 m/second (m/sec.) and a mass fraction of 0.03 hydrocarbons in air at 155 Degrees C and 1.3 bars. The turbulence model was the k-epsilon RNG (renormalized group theory) to accommodate possible separation at the bends.

The hydrocarbon spray, 2 m from the end of the ducting, was modeled using a dispersed-phase evaporation model, with momentum coupling to the continuous phase. The sprays were delivered via gas-atomized nozzles; previous experimental measurements had shown the mean drop-size to be 80 m.

The modeled sprays, therefore, were input as 45-degree full cones with drop sizes of 53, 80 and 120 m. Evaporation of the drops in the dispersed-phase model is a function of local vapor pressure, temperature and heat and mass transfer between the drops and the continuous phase. From the analysis of velocity magnitude along the inlet ducting, it was evident that the velocity was skewed too high at the top and to one side of the inlet. Some swirl component also is present, but it is small compared to the mean and axial velocities.

The CFD analysis showed that droplet trajectories of 80 microns all evaporate before entering the reactor. The larger drops evaporate to around 50 microns at the reactor inlet; therefore, they can be assumed to evaporate completely soon afterward.

Most of the evaporation occurs near the axis of the pipe, with little time for radial dispersion, which was seen from the concentration profiles of hydrocarbon vapor entering the reactor. Also, the skewed velocity profile resulting from the bends in the ducting concentrates the hydrocarbon to just above the inlet centerline.

The 3D model of the reactor headspace flow used the inlet profiles from the pipework model. The tube bundle was modeled truncated as porous media with enforced vertical flow and a porosity sufficient to ensure a pressure drop similar to the reactor. Velocity vectors in the inlet plane (mid-height in the headspace) show that flow enters with a high inlet velocity and splits around the central section. It then returns around the circumference of the head.

Some asymmetry is found in the slightly higher velocities around one side. The mass fraction of hydrocarbons entering the tube sheet varies from 0.03 immediately in front of the inlet to 0.04 around one side of the central section, with an overall imbalance of hydrocarbons to one side of the reactor.

The locations of the high and low concentrations correlate reasonably with the high and low temperature measurements in the reactor, confirming qualitatively the assumption of uneven dispersion of the injected hydrocarbons within the reactor. This was not only an explanation of the phenomenon, but also a form of validation of the model, which then could be used to design inserts for the reactor headspace to improve mixing of the incoming flow and good distribution across the reactor tubes.

Conclusions

Properly used and validated CFD simulations have demonstrated resultant major financial bottom-line and top-line benefits. They let engineers move beyond simple rules-of-thumb and empirical correlations to in-depth unit operation simulations that provide comprehensive field-flow predictions.

Zdravistch is the chemical and process team customer support team leader and Haidari is industry director for the chemical industry at Fluent Inc., Lebanon, N.H., a developer of CFD software. Contact them at fz@fluent.com and  ah@fluent.com, respectively.

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