July 2017

Process Engineering

Develop reactor internals for optimizing reactor design: High-performance vapor-liquid distributor

Vapor-liquid distributors are a crucial element of reactor internals for maximizing catalyst utilization of fixed-bed reactors.

Ahn, E. S., Park, S. I., Song, I. C., Kim, G. B., Chang, G. S., GS Engineering and Construction

Vapor-liquid distributors are a crucial element of reactor internals for maximizing catalyst utilization of fixed-bed reactors. Any maldistribution in the catalyst bed will lead to vapor-liquid channeling that degrades catalyst performance and, by extension, overall reactor performance. A stepwise development of a high-performance distributor is detailed here, supported by computational fluid dynamics (CFD) analysis and a pilot test.

In the early stages of development, CFD simulation was performed to verify a conceptual design of the high-performance distributor. The developed design was then tested in a pilot plant of semi-commercial scale. The test results showed that the distributor has superior performance compared with a conventional distributor, in that it drastically improves liquid wetting on the top of the catalyst bed.

The high-performance distributor has been successfully applied to a hydrogenation reactor in a commercial plant in South Korea.

Study description

A fixed-bed reactor is commonly applied to equipment in the petroleum refining and petrochemical industries, especially as a hydroprocessing reactor. Most fixed-bed reactors are designed with a downward flow, where both liquid and vapor enter at the top of the reactor and are evenly distributed before flowing through the catalyst bed.

Reactor performance has been improved mainly by catalyst development; however, as catalyst technologies become more common, interest has grown in reactor internals as an alternative area of development. Advancements in reactor internals also contribute to the optimization of overall reactor performance. Most of these developments have been accomplished by a handful of leading technology companies.

Significant research has been carried out to understand and improve the performance of reactor internals. Information on the operating characteristics of various reactors has become available.1,2 One study identified a vapor-liquid distribution mechanism through scanning a pilot-scale reactor.3 Another experiment provided a methodology for evaluating distribution performance.4 The researchers pointed out that it is necessary to perform experiments with two-phase (vapor and liquid) flow to simulate the dispersion characteristics of a real operation.

Three main factors affect the performance of a reactor:

  1. Effective catalyst utilization
  2. Optimal vapor/liquid distribution
  3. Low radial temperature differences.

Most of these factors are influenced by the loading of the catalyst and the performance of the internals. Catalyst loading characteristics are complicated areas for engineers to predict. Information is available on experimental and CFD analysis cases for catalyst loading characteristics.5 This study compares the results of pressure drop and liquid holdup through the loading characteristics of trilobe catalysts. However, it provides engineers with background only on hydraulic performance and does not suggest information about the distribution of fluids.

CFD analysis is also used to understand flow characteristics in the catalyst bed. A study was carried out between the vapor-liquid phases in a trickle flow regime of a packed-bed reactor to understand the fluid phenomena and heat exchange.6 The study also delivers a number of parameters and the hydraulic information needed for fixed-bed analysis, providing additional methods for simulation.

Conventional simple modeling is unable to evaluate key design issues, such as maldistribution, channeling, wetting of catalyst and local temperature variation, while the developed CFD model shows promising results in understanding fluid dynamics and their interactions with chemical reactions.7

Evaluations of distributor performance have been carried out more frequently than in the past, probably due to the improvement of simulation computer performance and the rise of CFD software’s analytical reliability. CFD has proven to be a powerful tool for designing vapor-liquid distributors, and will continue to facilitate advanced design in the future.

The study described in this article introduces the development of a new, high-performance vapor-liquid distributor (HPVLD). It details the performance review of a conventional vapor-liquid distributor by CFD analysis. A new design of HPVLD was prepared and verified by CFD analysis, and an actual distributor was manufactured and tested in a semi-commercial-scale pilot plant. After the pilot test concluded, the CFD simulation was utilized again to implement new design parameters to improve the distributor performance. During the development, CFD was confirmed to be very effective in reducing cost and time.

Reactor internals

The fixed-bed reactor has numerous internals. The inlet diffuser is the first device to meet the fluid entering the reactor. The performance of the inlet diffuser has a strong impact on the distribution of the vapor-liquid distributor installed below it.8 It is preferable to use an inlet diffuser with a simple geometry to improve even distribution. A vapor-liquid distributor, a mixing tray and a quench sparger are the other reactor internals located above and between catalyst beds. They are used to maximize the utilization of catalysts and to increase reactor performance.

General criteria for maximizing the utilization of the catalyst bed include:

  • The loading of the catalyst should be uniform
  • The vapor and liquid must be evenly dispersed in the catalyst bed
  • The wetting area on the catalyst layer must be maximized to increase the utilization rate.

Finally, a collector is installed at the outlet nozzle. The collector helps discharge the product and also acts as a filter to remove solids that may form during operation.

Characteristics of vapor-liquid distributors

The vapor-liquid distributor design is crucial in reactor design. A poorly designed distributor leads to maldistribution and a radial hot spot in the catalyst bed. The reactor performance generates a decrease in yield, as well as an increase in maintenance costs due to the reduction of catalyst lifecycle.

Conventional distributors are classified according to the type of tray used for distribution.8 In simple distributors with perforated plates, both vapor and liquid pass through the hole at the same time. It is sometimes possible to separate the flow of vapor and liquid by installing a weir.

In chimney-riser-type distributors with constant pitch and various openings (top opening, side holes and slots), the openings create contact between the vapor and liquid for two phases inside the riser. The vapor is introduced into the top opening of the chimney riser, while the liquid flows through the side holes by the liquid head force.

Bubble cap trays are commonly applied in a vapor-liquid distributor. The trays are able to collect the liquid falling from the top, while the vapor flow is dragging the liquid into the slots of the bubble cap until it flows down to the tray.

Most distributors do not deviate from the basic geometry mentioned above, although minor design improvements exist and are often applied. Among the various commercial distributors, a chimney-riser-type distributor was selected as the base model of the HPVLD because its equipment showed better distribution performance than the perforated plate distributor.

While the perforated plate distributor is greatly influenced by the inlet diffuser, the distribution performance of the chimney-riser-type distributor is unaffected by the inlet diffuser. It also has a design advantage in that the liquid flow regime in the riser outlet is within a predictable range, compared to the perforated plate distributor.

The CFD study was constructed, based on four cases, to investigate the impact of certain design parameters on distribution performance. The design parameters of interest are:

  • Introduction of vapor flow
  • Presence of the top plate
  • Angle between upper holes and lower holes (Fig. 1).
Fig. 1. Chimney riser types with hole arrangement showing (A) top open chimney riser,  (B) chimney riser with top plate, (C) chimney riser with top plate and rotated hole.
Fig. 1. Chimney riser types with hole arrangement showing (A) top open chimney riser, (B) chimney riser with top plate, (C) chimney riser with top plate and rotated hole.

The four case designs encompass:

  1. Case 1: Liquid distribution without vapor flow (Fig. 1A)
  2. Case 2: Liquid distribution with vapor flow (Fig. 1A)
  3. Case 3: Liquid distribution with vapor flow and top plate (Fig. 1B)
  4. Case 4: Liquid distribution with vapor flow and top plate and lower hole rotation from upper hole by 45° (Fig. 1C).

The CFD simulation was conducted to examine the performance of each chimney riser. For vapor-liquid two-phase flow, the Eulerian-Eulerian approach was applied. In this approach, the conservation equations for mass and momentum were solved for each vapor and liquid phase. The fluid in the simulation was an air-water system for vapor-liquid. The simulation did not include the dispersion effect in the catalyst bed. The conditions for air and water are shown in Table 1.

Fig. 2. CFD geometry and monitoring plane at a 250-mm depth for (A) CFD geometry,  (B) a conventional distributor and (C) an HPVLD.
Fig. 2. CFD geometry and monitoring plane at a 250-mm depth for (A) CFD geometry, (B) a conventional distributor and (C) an HPVLD.


The simulation used a hexagonal shape to define a unit volume (Fig. 2). The dispersion characteristic of the liquid was compared with the wetting ratio (WL), which is defined as the ratio of the wetting area (AL) to the monitoring plane area (AT). The monitoring plane area was 250 mm below the chimney riser bottom (Fig. 2). The wetting area is defined as the area wetted by the liquid in the monitoring area. The wetting ratio can be calculated as shown in Eq. 1:

            (1)

where:

WL =  Wetting ratio of liquid phase, %
AL   =  Wetting area (area wetted by the liquid in the monitoring plane)
AT  =  Monitoring plane area.

The results are shown in Table 2. Case 1 and Case 2 were constructed to compare dispersion characteristics according to the presence of vapor flow. The wetting ratios of Case 1 and Case 2 were 10.8% and 14.21%, respectively. The wetting ratio increased by approximately 31% with the presence of vapor flow, which implies that the vapor flow must have a considerable impact on the liquid distribution performance and must not be excluded from the experimental parameters due to its small specific gravity.


This result is also supported by a study9 that showed that even if the specific gravity of the vapor is negligible compared to the liquid, it may sufficiently affect the flow pattern of the liquid flowing through the chimney riser, according to the V/L ratio. It is clear that gravity is a main driving force for the liquid, and that vapor provides for various changes to liquid flow patterns to improve distribution. Therefore, it is important to introduce vapor flow for liquid distribution performance in any analytical or experimental evaluation.

Case 3 was carried out to observe the impact of a top plate on the liquid distribution. The top plate prevented the liquid from falling through the top opening and was expected to change in the direction of vapor flow. The result showed that the wetting area was 15.36%, an increase of about 8% compared to the result of Case 2. This increase was mainly due to the liquid flow through the holes being periodically influenced by the turbulent vapor flow coming through the open area of the riser top.

Case 4 was different from Case 3 in that the lower holes were rotated from the upper holes by 45°, as shown in Fig. 1C. The wetting ratio for Case 4 was 17.07%, which represents an approximate 11.1% increase from the ratio for Case 3. In Case 4, the liquid flow from the upper holes had different flow directions from the lower holes, so the randomness of the liquid flow pattern increased. As a result, the dispersion efficiency was further improved by changing the hole arrangement.

These results were also confirmed by the distribution characteristics shown in Fig. 3. The features of the general flow pattern of the liquid inside the riser are different in the upper holes and lower holes. In the lower holes, the liquid is emitted by the stronger hydraulic pressure, making the velocity of the liquid high enough to be randomly shattered inside the riser. The liquid inlet from the upper hole has a lower velocity, and the liquid flows from four holes are mixed at the center of the riser, where they begin to fall through the riser in a more stable manner.

Fig. 3. Numerical liquid flow features with different hole angle on (A) a normal hole location  and (B) a rotated hole location.
Fig. 3. Numerical liquid flow features with different hole angle on (A) a normal hole location and (B) a rotated hole location.


This difference of liquid flow through the lower and upper holes is shown in the difference in the liquid flow patterns between Fig. 3A and Fig. 3B. Fig. 3A shows where the upper holes and lower holes are arranged in the same direction, although the interaction between the liquid from the upper and lower holes is limited. However, in Fig. 3B, the liquid flow from the upper holes is rotated by 45° from the lower holes, creating more complicated interactions between liquids from the upper and lower holes. This feature broadens the wetting area and improves the liquid distribution performance.

The liquid distribution performance of the four cases is examined according to three design parameters: the introduction of vapor flow, the presence of a top plate and changing the hole arrangement. The addition of each design parameter increases the wetting ratio, although the improvement of the wetting ratio is limited. The pressure drop exerted at the chimney riser holes is insufficient for the liquid to be spread or sprayed; rather, the liquids trickle down from the riser bottom ends. Therefore, the basic chimney riser type (Cases 1–4) is insufficient to wet the entire top surface of the catalyst bed.

High-performance vapor-liquid distributor

Based on the results of the previous studies, the high-performance distributor was devised to overcome the shortcomings of conventional chimney riser types. To maximize the distribution, while the 45° angle between the upper and lower holes is fixed as confirmed by the CFD simulations, a unique dispersion device was additionally installed beneath the chimney riser (Fig. 4).

Fig. 4. Concept of a high-performance  vapor-liquid distributor (HPVLD).
Fig. 4. Concept of a high-performance vapor-liquid distributor (HPVLD).


Prior to the manufacturing of the test sample, the dispersion characteristic was evaluated by the CFD at the same boundary conditions. The wetting ratio of the HPVLD was 82.43%, a 660% increase from that of conventional chimney riser types (Fig. 5). As previously mentioned, the shortcomings of the conventional chimney-type distributor are attributed to the low pressure drop and liquid trickling tendencies. The CFD study shows that the HPVLD successfully eliminates this negative flow mechanism.

Fig. 5. Influence of distributor types on percentage of wetting ratio.
Fig. 5. Influence of distributor types on percentage of wetting ratio.


To easily understand the improvements of distribution performance, the results of the monitoring plane are normalized by distance, as shown in Fig. 6. The conventional distributor shows very high liquid volume fraction at the center of the monitoring plane (Cases 1–4). This high fraction would require the distribution design to have a greater number of chimney risers, or necessitate the installation of a greater depth of inert materials at the top of the catalyst bed to achieve the acceptable dispersion efficiency. Conversely, the HPVLD shows that the liquid is dispersed over the entire area (Case 5), so the reactor design is expected to be more cost effective.

Fig. 6. Influence of distributor types on liquid volume fraction to the monitoring plane.
Fig. 6. Influence of distributor types on liquid volume fraction to the monitoring plane.

Experiment of distribution performance

The performance of the HPVLD was evaluated in a semi-commercial-scale pilot plant with an air-water system. The test facility consisted of an acrylic vessel equipped with a vapor inlet nozzle, a liquid inlet nozzle, a diffuser, a high-performance distributor, inert balls, catalysts and a collector (Fig. 7). The chimney riser and new dispersion device were installed at regular triangular pitches. A collector was installed below the inert balls and catalysts to monitor the distribution performance of the HPVLD in combination with the catalyst bed.

Fig. 7. Schematic of test facility.
Fig. 7. Schematic of test facility.


Two experiments were carried out to compare the performance of the conventional chimney riser distributor and that of the HPVLD. The distribution performance was evaluated by the amount of liquid in the collector. The collector was divided into 13 sections, including seven circular sections located under each chimney riser, and six fan-shaped sections divided into equal areas. The circular section occupied a share of 5% by area, and the fan-shaped section held 10.8%. The area was divided in such a way that the liquid distribution performance by the amount of liquid collected in each section could be observed. If the liquids are not spread enough from the chimney riser, then most of the liquid will fall on the circular area from Sections 1–7. If the liquids are perfectly distributed, then the liquid on the circular section will be 5%, while that of the fan-shaped area will be 10.8%.

Tests were carried out for 50%, 100% and 110% of liquid flowrates. The result at 100% is shown in Fig. 8A for the conventional chimney riser distributor, and Fig. 8B for the HPVLD. For the conventional distributor, the percentage of the liquid collection rate was approximately 10% in Sections 1–7 and approximately 5% in the other sections.

Fig. 8. Distribution efficiency at 100% liquid flowrate for (A) a conventional chimney riser distributor, (B) an HPVLD.
Fig. 8. Distribution efficiency at 100% liquid flowrate for (A) a conventional chimney riser distributor, (B) an HPVLD.


As previously mentioned, these results imply that most of the liquids from the chimney riser are not spread; rather, they fall directly below the chimney riser. The collection rate in each section in Fig 8B shows a contrary result to that of Fig. 8A. The collection rates were approximately 5% in the circular section and approximately 10% in the fan-shaped sections. The result is close to the ideal distribution, which implies that the liquids were evenly distributed in the entire area by the HPVLD.

The same trend was observed in the experiment with increasing or decreasing liquid flowrate (Fig. 9)—a slight difference was detected in the overall tendency of distribution by changing the flowrate for the two distributors. In the conventional chimney riser distributor, as liquid flowrate increased, the percentage of the liquid collection rate in the circular section also increased. However, as the flowrate increased in the HPVLD, the dispersion performance grew closer to the ideal distribution, showing a slight decrease in collection rate at the circular sections.

Fig. 9. Change in distribution percentage, according to the liquid loads.
Fig. 9. Change in distribution percentage, according to the liquid loads.


The dispersion performance of the HPVLD was much better than that of the conventional chimney riser distributor. It also showed robustness in distribution quality with various flowrates, indicating that it can achieve stable response to changes in flowrate during operation.

Based on the two-phase CFD simulations and experimental results, the advantages of the HPVLD can be summarized as follows:

  • Excellent performance over a wide range of operating variables, such as liquid and vapor loads
  • A minimum required reactor height for spray angle for distributor elements
  • More space for catalyst inside the reactor
  • Increased catalyst run length by eliminating partial deterioration exerted by poor distribution.

Takeaway

Maximizing the catalyst utilization of the fixed bed is the most important factor in optimizing reactor design. This article has introduced the development of an HPVLD to maximize catalyst utilization. To reduce the cost and time, various case studies were performed using CFD. The liquid flow behavior inside the chimney risers was observed to ensure that rotated lower holes have better liquid distribution performance than conventional ones. This approach provided useful information for HPVLD development.

The CFD analysis and experiment results are useful for process engineers to understand the importance of reactor internals. Engineers should consider the distributor design as one of the most crucial design options for the reactor system, even after considering classical design parameters such as reaction conversion, liquid hourly space velocity and operating schemes. HP

Literature cited

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  2. Raynal, L., F. Augier, F. Bazer-Bachi, Y. Haroun and C. Pereira da Fonte, “CFD applied to process development in the oil and gas industry—A review,” Oil & Gas Science and Technology Review, IFP Energies nouvelles, Vol. 71, No. 42, 2016.
  3. Al-Dahhan, M. H., A. Kemoun, A. R. Cartolano, S. Roy, R. Dobson and J. Williams, “Measuring gas-liquid distribution in a pilot scale monolith reactor via an industrial tomography scanner (ITS),” Chemical Engineering Journal, Vol. 130, No. 147–152, 2007.
  4. Marcandelli, C., A. S. Lamine, J. R. Bernard and G. Wild, “Liquid distribution in a trickle-bed reactor,” Oil & Gas Science and Technology Review, IFP Energies nouvelles, Vol. 55, No. 4, 2000.
  5. Bazmi, M., S. H. Hashemabadi and M. Bayat, “CFD simulation and experimental study for two-phase flow through the trickle bed reactors, sock and dense loaded by trilobe catalysts,” International Communications in Heat and Mass Transfer, Vol. 38, 2011.
  6. Heidari, A. and S. H. Hashemabadi, “Numerical evaluation of the gas-liquid interfacial heat transfer in the trickle flow regime of packed beds at the micro- and meso-scale,” Chemical Engineering Science, Vol. 104, 2013.
  7. Gunjal, P. R. and V. V. Ranade, “Modeling of laboratory and commercial scale hydro-processing reactors using CFD,” Chemical Engineering Science, Vol. 62, 2007.
  8. Maiti, R. N. and K. D. P. Nigam, “Gas-liquid distributors for trickle-bed reactors: A review,” Industrial & Engineering Chemistry Research, Vol. 46, 2007.
  9. Llamas, J.-D., F. Lesage and G. Wild, “Influence of gas flowrate on liquid distribution in trickle-beds using perforated plates as liquid distributors,” Laboratoire des Sciences du Génie Chimique, Nancy-Université, Nancy, France.

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