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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Computational fluid dynamics (CFD) is helping to generate million of dollars of savings in chemical process applications. It gives process engineers a more complete understanding of the internal operation of individual unit operations. As a result, some chemical and process companies are equipping their engineers with CFD software and investing in training to improve efficiencies in fluid flow, heat and mass transfer processes.

CFD is a tool for analyzing fluid flow and transport phenomena. Most chemical and process engineers are familiar with Bernoulli's Equation and heat, mass and momentum conservation laws. CFD uses computers to solve the fundamental nonlinear differential equations that describe fluid flow (the Navier-Stokes and allied equations) for predefined geometries with a set of initial boundary conditions, process flow physics and chemistry. The result of CFD is a wealth of predictions for flow velocity, temperature and species concentrations for almost any piece of chemical and process equipment.

CFD is a very potent, nonintrusive, virtual modeling technique with powerful visualization capabilities.

Recent advances in CFD have made it possible to analyze flow problems of ever-increasing complexity, including those involving multiphase flows, mixing-related phenomena, intricate equipment geometries and detailed chemically reacting flows within process-relevant time scales. Frontiers in fluid flow, heat and mass transfer applications are pushed back regularly, and problems that were deemed untouchable just a few years ago now are being solved.

The technical benefits to engineers and the financial benefits to managers are driving the phenomenal rise in CFD use in chemical process applications, and this trend shows no sign of slowing down in the foreseeable future.

The question being asked today no longer is: "What is CFD and how can it be used?" Instead, the relevant question today is: "Why is CFD not being applied to various unit operation flow problems?"

Figure 1. Pipe Flow Modeling Reveals Design Flaw

CFD analysis revealed that airflow was skewed by an unexpected interaction between two bends and the valve wafer in the normal open position. As a result, flow was concentrated on one side when it entered the blower, reducing efficiency.

FCC case cracked with CFD

As a case in point, Mobil Corp., during the past five years, has achieved tens of millions of dollars in capacity and product quality gains by using CFD to improve performance of refinery process units such as reactors, separators and extractors. In one case, Mobil engineers studied flow in the suction line of the main air blower that feeds the regenerator of a fluid catalytic cracking (FCC) unit. The FCC cracking reactions that produce gasoline and heavier liquid fuels also form coke on the fluidized catalyst, which is burned off in the regenerator using air.

The blower in question was designed to draw air at approximately 90 feet per second through a 6-foot (ft)-diameter line and supply it to the regenerator internal air manifold. The calculated blower efficiency was below 90 percent, significantly less than that of similar units and of critical concern because the FCC was blower-limited. Thinking that some nonideal flow feature might exist in the suction line, Mobil analysts used CFD to simulate the entire line, from the ambient air inlet to the blower intake port.

The 2.1 million-cell CFD model included all significant suction line details such as a chambered weather hood, several mitered bends and a butterfly valve with a tapered wafer. A high cell count was required because of the large line diameter and length, coupled with relatively small cells (between 1/4 and 1/2 inch [in.] on a side) needed near the walls to resolve sharp gradients in this area.

The analysis revealed that airflow through the line was strongly skewed by an unexpected interaction between two bends and the valve wafer in its normal open position. (See Fig. 1.) As a result, the flow was concentrated on one side of the line at the point at which it entered the blower ," a feature known to reduce machine efficiency. In effect, only half of the line cross-section and blower inlet was being used.

After gaining an understanding of the problem from the CFD results, Mobil analysts immediately thought of adjusting the equilibrium open position of the valve wafer. The clear advantage of this approach was that it sidestepped much more expensive modifications to the blower inlet and/or less-reliable tactics such as internal turning vanes.

After analyzing air flow through the suction line with the wafer in various positions, the analysts found a configuration that eliminated almost all of the nonideal flow pattern and allowed air flow rates several percentage points higher through the line at the same fixed pressure drop. As a bonus, plant personnel could implement the change simply by re-indexing the wafer relative to the positioning axle. On a blower-limited FCC, such a rate increase has the potential for capacity benefits exceeding $6 million per year.

Slurry reactor enhancements

Denver-based Rentech Inc. achieved a world first by developing three-dimensional (3D) CFD models to simulate bubble-column hydrodynamics in large slurry reactors. The company then would be able to scale up validated models of small-scale reactors easily to commercial-size equipment.

Rentech's approach, based on CFD simulations, is an important step toward designing a full-scale reactor for gas-to-liquid (GTL) conversions using Fischer-Tropsch chemistry. This method has the potential for widespread application in the conversion of refinery residues, an ever-increasing challenge worldwide. Rentech owns, licenses and markets a proprietary and patented process that converts syngas, a mixture of hydrogen and carbon monoxide produced from any carbon containing materials, into valuable liquid hydrocarbons, including diesel fuel, naphthas and waxes.

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