Troubleshooting refinery equipment with multiphase CFD modelling

Much of the common process equipment in refineries today was designed and built according to traditional empirical design

methodologies that were developed decades ago. Without an intimate understanding of the complex flow patterns present within a given system, designers had to rely on conservative assumptions and trial and error to ensure that equipment met design requirements. Modern computational fluid dynamics (CFD) tools allow designers to pull back the veil on complex inter-nal flows, but their use has been limited by available computing power. As computing power continues to increase, CFD is becoming a practi-cal tool for industrial scale problems. Through a deeper understanding of standard process equip-ment, it is now possible to identify opportunities to improve both the function and the capacity of installed systems.

In many cases, small modifications can elimi-nate the need to design and fabricate new equipment, resulting in significant cost savings without compromising performance. This article will discuss several cases demonstrating the application of CFD to ‘traditional’ process equip-ment. Each case presented will discuss the motivation for the use of CFD, the assumptions required to yield a practical and robust CFD simulation, some details pertaining to the CFD modelling itself, and — most importantly — the practical outcome of the simulation exercise.

Central to the growing popularity of CFD for industrial scale problems is an ability to simplify a simulation. An extremely detailed simulation incorporating all of the relevant physical minu-tiae is of little value if the results cannot be interpreted and applied to solve a real world

Grant Niccum and Steve White Process Consulting Services

problem. Furthermore, the additional complexity and computational expense required to perform an extremely high fidelity simulation is often unjustifiable or unattainable for many industrial problems. In most cases, a simplified modelling approach specifically developed to examine the variable(s) of interest is the most efficient prac-tice. By carefully considering all of the independent and dependent variables relevant to the design question at hand, the pain and expense of a CFD project can be greatly reduced. Just because an engineer can solve for every possible variable throughout an entire domain doesn’t mean that he or she should. Unnecessary physics complicates the setup of a simulation, significantly increases the computational time required, and may decrease the stability of the simulation to the point where a converged solu-tion is impossible.

Case 1: liquid knockout drumAfter performing a detailed dynamic process simulation study of a particular unit, it was discovered that, given the right circumstances, vapour/liquid rates could be far above the design capacity of an existing liquid knockout drum. In addition to incurring significant expense, replacement of the new drum would have been difficult due to space restrictions. It was hypoth-esised that internals could be added to the drum to adequately increase the vapour-liquid separa-tion. For verification, CFD could be used to confirm the effectiveness of any design changes that would see the drum operate while signifi-cantly under-sized according to traditional sizing methods. Particle sizes of a certain critical diam-eter were considered the break point for effective

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Computational fluid dynamics is a useful and increasingly practical tool for improving the design of and increasing the understanding of common process equipment

operation of the separator. Therefore, the CFD analysis was used to develop and test modifica-tions to allow the drum to effectively trap particles with larger than acceptable diameters within the drum.

A ‘brute force’ CFD approach to this design problem would have been to model all of the relevant physical phenomena at the same time: multiphase vapour/liquid flow at the inlet, breakup/coalescence of the liquid droplets, formation of a liquid film on the walls of the drum, collection/movement of free liquid in the bottom of the drum, and so on. The modelling task was greatly simplified, however, by carefully considering the variables of interest. The func-tions of the liquid knockout drum are to separate and collect liquid particles, and to prevent re-en-trainment of the free liquid phase that has collected in the bottom of the drum. The varia-bles that needed to be evaluated to verify that the drum would perform as required are the fates of liquid particles entrained with the gas at the inlet to the drum and the shape/size of the stable liquid area in the bottom of the drum. Variables such as liquid wall film thickness are not significant to the overall function of the drum and were not modelled, as their omission did not significantly affect the variables of interest.

Modelling was further simplified by segregat-ing the variables of interest, as they are independent of one another. Each design option was evaluated using one model for particle tracking and a second for monitoring the free liquid phase. Although it may seem counterintui-tive that two models would be more efficient than one, this arrangement allowed the designer to perform several design iterations using the less computationally intense particle tracking model before running the more complex gas/liquid interface tracking model. Furthermore, the separation allowed the two models to be set up quite differently to give the best answers for the variables that each was tasked with solving.

The first of the two simulations was used to track particles entrained with the vapour at the drum inlet. The discrete phase model (DPM) was chosen in this situation for its ability to track particles through the domain and because the volume fraction of liquid entrained within the vapour flow was low. Small, light particles follow vapour flow streamlines more closely than

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larger particles because they have less momen-tum relative to the drag caused by the bulk vapour flow (lower Stokes number). With this principle in mind, a conservative particle size should be smaller than the maximum allowable droplet size. These conservatively sized particles were injected with the vapour at the separator inlet and tracked throughout the domain as the vapour travelled from the separator inlet to the outlet. A simplifying assumption was that the particles underwent partially elastic collisions (some energy lost) when they encountered walls within the vessel. Thus, wall collisions tended to slow the liquid droplets down until they sepa-rated from the vapour flow and settled in the bottom of the drum. In reality, some of these collisions would have splashed to create multiple smaller droplets, but this phenomenon was ignored because escape of smaller droplets was acceptable and therefore not consequential to the design. The design was modified and simu-lated iteratively until no particles escaped through the drum outlet.

The second simulation was designed to model the stability of the free liquid phase in the bottom of the drum. This simulation employed the volume of fluid (VOF) model to track a well defined vapour-liquid interface. Liquid volumet-ric flow rates were significantly lower than gas volumetric flow rates, and liquid entering and exiting the drum was not significant to the prob-lem of maintaining a stable liquid layer. The model was therefore built with no liquid flow in/out, and a mass of liquid was manually placed within the drum at the start of the simulation and allowed to ‘slosh’ around due to interaction with the vapour flow. If the flow agitated the liquid layer to the point where liquid mass escaped through the outlet, the design was modi-fied. Stabilising the free liquid layer with high vapour flow rates proved more difficult than trapping the initially entrained particles. The high velocity vapour flow tended to re-entrain significant quantities of liquid. However, a design was developed that could satisfy both requirements.

The use of these two complementary CFD simu-lations led to a robust design and confidence that the knockout drum can perform adequately under the given set of operating conditions. The use of CFD in this case allowed many design iterations to be evaluated within a matter of days to arrive

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at an optimal solution. For comparison, a sizing exercise using the traditional sizing meth-odologies was performed for a new drum to handle the same loads. According to traditional sizing methods, a new drum for the same service would be approximately three times the diameter of the existing drum and would have a tangent-tan-gent length roughly equal to the current drum. The CFD opti-mised internals support the continued use of the existing drum.

Case 2: fractionator overhead receiverIn the case of an overhead receiver for an FCC fractionator, CFD was used to predict the conse-quences of an existing design that appeared intuitively flawed. The overhead receiver was designed with the inlet at the bottom of the drum, presumably to reduce the complexity of the large diameter piping to the inlet. Although there was a short inlet riser inside the drum, the normal liquid level could easily exceed the height of this riser. Intuitively, it was hypothesised that an inlet jet agitating the fluid in the drum would interrupt the separation process and lead to high levels of water in the hydrocarbon outlet. CFD was used to examine the effects of this inlet design in detail.

Horizontal gravity separators are designed based on cross-sectional flow velocities and fluid residence time. Traditional methods assume plug flow through the vessel, meaning that each phase has a uniform cross-sectional velocity equal to the volumetric flow rate divided by the cross-sec-tional area through which that fluid flows. In a given fluid, a droplet of a certain size has a fixed terminal velocity. The separator must be sized with sufficient residence time for that droplet to traverse into the proper phase at the outlet from any starting position at the inlet. If the inlet design leads to heavy agitation within the drum, the most basic design assumption — plug flow through the cross-section of the drum — fails to hold. If the plug flow assumption is not met, some fraction of the liquid in the separator may experience residence times significantly below

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the design value, and good separation will not be achieved.

A CFD simulation was designed to test the validity of the plug flow assumption with the bottom inlet design. The volume of fluid (VOF) model was chosen for this multiphase problem. The CFD model contained a set liquid volume inside the drum, and the vapour load was applied at the inlet of the drum, escaping through the gas outlet. Since the design veloci-ties within the liquid phase of the drum are extremely low, inclusion of liquid mass entering and exiting the drum would not have affected the variables of interest (anomalous cross-sec-tional velocities), so this was omitted from the simulation. Any regions with significant move-ment can be assumed to be operating away from the plug flow design assumption. Significant recirculation or shortcutting within the liquid region of the drum should be viewed as an indicator of poor separation that will result in significant amounts of water in the hydrocarbon outlet.

The results of the CFD simulation explained the problems that this FCC unit main fractiona-tor had been experiencing. The CFD showed that the vapour jet leaving the riser entrained a significant amount of liquid. Figure 1 shows the intense agitation of the liquid within the drum. This created a large recirculation current within the liquid portion of the drum, completely violat-ing the plug flow assumption used in the separator sizing process. Based on this result, it

Figure 1 Contours of the volume fraction of liquid within the drum. The red region is 100% liquid, and the blue region is 100% vapour. With the liquid level above the short inlet riser, liquid is entrained into the jet of vapour entering the drum

1.00e+000

0.00e+000

9.47e–001

8.95e–001

8.42e–001

7.89e–001

7.37e–001

6.64e–001

6.32e–001

5.79e–001

5.26e–001

4.74e–001

4.21e–001

3.68e–001

3.16e–001

2.63e–001

2.11e–001

1.58e–001

1.05e–001

5.26e–002

Raw gasoline volume fraction

was confidently predicted that the liquid-liquid separation was poor, and there was likely a large amount of water in the hydrocarbon outlet, which feeds the fractionator reflux pumps. This water contains ionic compounds that will deposit in the tower as salts as the water evaporates (see Figure 2). The tower was, indeed, experiencing issues related to salt in the over-head system, and the circulation of large quantities of water in the reflux is a contributing factor. Lab samples have confirmed that the weight percentage of water in the reflux stream is very high.

The next application of CFD to this drum will be to correct the deficiencies in the inlet design. By incorpo-rating CFD into the revamp process, modifications can be tested and an iterative design developed to ensure that the recirculation zones are eliminated and that the flow in the drum more closely resembles the plug flow assumed in the initial vessel sizing calculations. Correction of this problem will eliminate the salting problems occurring in the tower.

Case 3: FCC fractionator feed inlet deviceA common feed inlet device for FCC fractiona-tors is a series of pipes intended to break up and distribute the flow within the column (see Figure 3). Experience has shown that, when installed properly, this design improves tower perfor-mance and prevents coking on the tower wall opposite the inlet. A CFD simulation was performed to understand the mechanisms by which this device works and to look for simple improvements to the design that could increase performance or reduce complex-ity and cost. The intuitive theory going into the study was that the flow was redirected upward by the pipes, thus creating a

more uniform distribution across the tower according to the placement of the pipes. By fully understanding the mecha-nisms that allow this inlet device to achieve good results, better design decisions can be made in future installations.

The vapour was modelled as incompressible due to the rela-tively low pressure drop in the domain. Detailed simulations were run with and without the inlet distributor, and the results were compared in an effort to find meaningful differences that would explain why the distribu-tor had such a noticeable effect when installed. The discrete phase model (DPM) was used to simulate the fate of liquid drop-

lets entrained with the feed. With no inlet distributor, the CFD model predicted that the inlet vapour would form a well defined jet before impinging on the opposite wall of the tower. Above the jet, the flow upward through the tower was far from uniformly distributed. Furthermore, the stability of the jet suggested that the impingement point on the wall opposite the inlet was unlikely to move over time. This created a stagnation region at the impingement point with essentially zero flow velocity and no wall shear stress.

While the impingement point saw little vapour flow velocity, the rate at which it was impacted by any liquid particles in the feed was high. If these particles were sufficiently large (high Stokes number), the particles tended to detach from the vapour flow and follow a trajectory dictated by their momentum. Thus, as the vapour streamlines turned abruptly at the stagnation point, the liquid continued along the path of the jet and impacted the wall in the stagnation region. The high liquid impact rate combined with near zero vapour velocities, and low wall shear stress created favourable conditions

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Bottom inlet agitates liquid

in drum

Water in reflux carries salts

Salt deposition plugs trays

Figure 2 Poor separation in the overhead receiver leaves water in the reflux, which carries ionic compounds that will deposit in the tower as salts when the water evaporates. These salts can cause corrosion and plugging of tower internals

Reactor effluent

Quench

Inlet device

Figure 3 Field experience has shown that installing a feed inlet device similar to the one shown improves performance and reduces coking on the rear wall of the tower

for coke formation in the stag-nation region.

Contrary to expectations, the addition of the inlet distributor to the model did not greatly improve tower cross-sectional flow distribution (see Figure 4). The simulation nonetheless provided evidence to explain the decreased coke formation on the wall opposite the inlet. The inlet device caused the inlet jet to widen more in the horizontal direction, but had little effect on the vertical size of the jet – significant vapour quantities were not redirected upwards into the tower with each impact. This was due to each pipe only blocking a small part of the flow. Any vapour that was deflected near a pipe was easily ‘corrected’ back into the mean jet flow by the momentum of the surrounding jet that had not directly impacted one of the distributor pipes.

If the pipes did not effectively redistribute the feed within the tower, how did they decrease coke formation on the opposite wall of the tower (the stagnation region) as seen in the field? The CFD shows that the pipes acted to scatter the jet on the wall of the tower rather than to distribute the feed. As the jet flowed around the pipes, it became less defined, mean-ing that the stagnation region was not as apparent as it was without the pipes. The stag-nation region was broken up over the back wall of the tower rather than remaining stably focused on a single point. The disruption of the stagnation region greatly reduced coke forma-tion for two reasons: 1) the liquid impacting the back of the tower did so over a larger area, concentrating less material on one spot; and 2) the area immediately surrounding the impinge-ment point had the highest wall shear stresses of anywhere on the tower wall. As the impinge-ment region was less defined, the shear stresses were more evenly distributed, contributing to the shearing of any coke.

Droplet breakup is another factor that may contribute to a reduction in coke formation with the inlet device installed. As the vapour flowed around the pipes, larger droplets with high momentum could not change direction fast enough to avoid a collision with the pipes. High

impact velocities and high wall shear forces around the pipes caused the droplets to shatter and break up into many smaller droplets. These smaller droplets had lower Stokes numbers, meaning that their momentum was less signifi-cant in relation to the drag caused by the vapour, and the droplets were more likely to follow the path of the bulk vapour flow rather than impact-ing the rear wall of the drum. By impacting and shattering the droplets on the pipes, where wall shear is at its maximum, rather than in the stag-nation region at the point of jet impingement, coke formation is minimised. Armed with an understanding of the fundamental mechanisms behind this style of inlet device, future designs can be adjusted to maximise the positive func-tions while minimising negative consequences.

ConclusionCFD has been applied to these case studies to gain a better understanding of the phenomena that govern the performance of examples of common process equipment. The designer can take advantage of this better understanding of the underlying physical phenomena to improve performance. For example, in the case of the FCC unit fractionator inlet distributor, under-standing that droplet breakup and jet scattering are the two key principles that allow this device to prevent excessive coke formation will lead to future designs that specifically maximise these phenomena rather than designs that attempt to improve the device through misguided attempts to affect the distribution of the flow within the tower.

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0.00e+000

1.07e+002

1.00e+002

9.33e+001

8.67e+001

8.00e+001

7.33e+001

6.67e+001

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4.67e+001

4.00e+001

3.33e+001

2.67e+001

2.00e+001

1.33e+001

6.67e+000

Velocity, ft/s

Figure 4 Velocity profile on a vertical plane within the tower. The case with the inlet device (left) is similar to the base case (right). Significant quanti-ties of vapour are not directed upward by the inlet device

CFD is an extremely useful and increasingly practical tool for improving the design of and increasing the understanding of common process equipment. By carefully defining a problem and the variables of interest, simulations can be built that give good information where necessary without incorporating unnecessary complexity. Finally, it is worth mentioning here that CFD inputs, assumptions and results should be care-fully scrutinised. Commercial CFD software will work dutifully on any problem that it is given, and will often reach a ‘converged’ answer, but the software has no notion of ‘correctness’. Incorrect inputs or the failure to include impor-tant physical aspects of a simulation will lead to incorrect answers. Part of the engineer’s respon-sibility in running a CFD simulation is to consider all of the trade-offs between complexity and accuracy so that an adequate solution can be developed with the minimum level of computa-tional expense. The ultimate usefulness of a simulation lies in an engineer’s ability to use the

information to draw concrete conclusions and develop solutions that improve performance.

Grant Niccum is a Process Engineer with Process Consulting Services, Inc. in Houston, Texas. Process Consulting Services provides grassroots and revamp front-end process engineering to the refinery industry worldwide.Steve White is a Chemical Engineer with Process Consulting Services. He has more than 37 years of process design experience in refinery revamps and grassroots units including crude/vacuum, FCC, hydrotreater, alky, butamer, reformers and others. He previously worked for Jacobs Engineering, UOP and ARCO.

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