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REVIEWS Gas-Liquid Distributors for Trickle-Bed Reactors: A Review R. N. Maiti and K. D. P. Nigam* Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi-110016, India
A concise review of the gas-liquid distributors used in trickle-bed reactors (TBRs) is presented. The following topics are considered: distributors in a large-scale reactor, quench box/redistributor, inert particle layer, application of fluid flow modeling (CFD) in distributor studies, and distributors used in a laboratory-scale reactor. Mainly four types of distributors used in a large-scale reactor (e.g., perforated plate, multiport chimney, bubble cap, and gas-lift distributors) are described along with their advantages and disadvantages. Effects of various types of weep hole, such as inverted V notch and rectangular slot at the distributor tube wall and fluid distributing device at downcomer outlet, are discussed. Sizing methodology of multiport downcomer in chimney type distributors is presented. The performance of a gas-lift distributor is found to be more promising compared to other distributors. It provides intimate mixing of vapor and liquid, is less vulnerable to fouling, is insensitive to tray levelness, and distributes liquid uniformly at a large turndown ratio. This is also reflected in the increasing use of gas-lift distributors with increasingly stringent product specifications. This review presents all the information available in the literature to the best of the author’s knowledge and focuses the attention on enhancing the further understanding of internals toward uniform distribution of liquid in TBRs. It also focuses the future directions of work in designing of gas-liquid distributors to further facilitate the understanding of the design of TBRs to meet the challenges of the stringent sulfur specification in transportation fuel (10 ppmw in EURO V by 2009). 1. Introduction Trickle-bed reactors (TBRs) are one of the important classes of multiphase flow reactors. It consists of a fixed bed of solid catalyst particles contacted by a cocurrent downward gas-liquid flow carrying both reactants and products. In some cases, upward-flow TBRs are used but, because of flooding problems, downward flow is the most widely preferred. TBRs have been widely used in the petroleum industry for many years and are now gaining widespread use in several other fields. They are employed in petroleum, petrochemical, and chemical industries, in waste treatment, and in biochemical and electrochemical processing, as well as in other applications.1-4 In general, the reaction occurs between the dissolved gas and the liquid-phase reactant at the interior surface of the catalyst. In some cases, the liquid phase may be an inert medium for contacting the dissolved gaseous reactant with the catalyst. The observed and expected reaction rates, when the particles are fully covered with liquid, are directly related to partial wetting of the catalyst.5 For this reason, it is desirable to have external surfaces of the catalyst fully covered with liquid (as it is perceived that the pore gets filled with liquid by capillary force) for maximum utilization of catalyst. In some cases, gas is sparingly soluble (gas-limiting reactions) and incomplete particle wetting is desirable because it increase the effectiveness factor, owing to reduced gas-to-particle resistance. Obviously, there is a balance that must be maintained to avoid particle dry out, local temperature gradients, and vapor-phase reactions. Since the introduction of the first commercial hydrotreating units in the 1950s, catalyst manufacturers have developed and * Corresponding author. Tel.: 011-2659 1020. E-mail: nigamkdp@ gmail.com.
commercialized catalysts with the ever-increasing activities required to meet the stringent low sulfur, nitrogen, and aromatics specifications of environmentally friendly fuels. The key parameter for further improving unit performance with highly active catalyst is the efficient distribution of reactants at the microlevel, i.e., wetting of catalyst. There are several factors that affect the macro- and microlevel liquid distribution and flow textures. Macrolevel flow distribution is mainly affected by inlet liquid distribution, particle shape and size of the particle, fluid velocity, and packing method. At the microlevel, liquid distribution and flow textures are affected by start-up procedures, fluid velocity, wettability, flow modulations, and coordination number of particle as reviewed by Maiti et al.4 However, the extent of uniform distribution of liquid through the catalyst bed at the microlevel is grossly affected by proper design and functioning of reactor internals. The internal elements include (i) liquid entry devices, (ii) top distribution plate, (iii) quench box, and (iv) redistributor, i.e., redistributes the liquid and gaseous reactants evenly across each subsequent catalyst bed (Figure 1). The purpose of the liquid entry device and distribution tray is to establish an even liquid distribution radially across the catalyst bed. Poor liquid distribution introduces gross nonwetting in the bed, as shown in Figure 2parts a-c.6 The catalyst particles on the upper left side of packing (Figure 2a) are not wetted and are not utilized. This is in agreement with the observations of Christensen et al.,7 Szady and Sundaresan,8 and Marchot et al.9,10 The latter authors studied the distribution of liquid in a laboratory trickling filter and observed that about half of the bed cross section did not receive any liquid and the distribution was not uniform in the remaining cross section.
10.1021/ie070255m CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007
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Figure 1. Schematic drawing of a trickle-bed reactor used in hydrocracking.
Figure 2. (a-c) Liquid maldistribution in trickle-bed reactors.
A catalyst bed is hydraulically unstable in the sense that, if a restriction develops somewhere in the bed, then it will always become worse until the time of catalyst replacement. This may happen as a result of uneven distribution of gas and liquid in the catalyst bed where pockets containing mainly liquid and insufficient hydrogen can cause coking. Temperature maldistribution in exothermic processes generally indicates greater fluid flow in one part of the bed versus another. Rapid pressure drop buildup sometimes reveals coking in the bed caused by regions of stagnant flow or insufficient reactants. The restriction may be developed because of mechanical degradation of catalyst particle or corrosion materials, pipe scales/foreign material that entered with the feed. Fresh (not discolored) catalyst is sometimes found when fixed-bed units are opened for servicing after 2-3 years in operation, indicating flow bypassing. These findings indicate that at least some aspects of fluid flow in gasliquid distributors have not been well-understood. Yet in the petroleum refining and other industries, public demand and government regulations have dictated the removal of certain compounds from chemical products, necessitating more severe operation and greater need for optimal and reliable reactor performance. Effective distribution in reactors is critical to meeting these demands. Most of the designs of internals in TBRs packed with millimeter-sized particles are influenced by hardware used in a packed and trayed fractionation column. These designs are not necessarily well-suited for trickle-bed reactors because of large
center-to-center spacing between distributors and poor liquid discharge pattern. Moreover, with increasing demand of removal of certain specific compounds from petroleum refining products (e.g., ultralow sulfur diesel as specified in EURO III, EURO IV, and EURO V (10 ppm) norms by 2009), a greater need exists for optimum and reliable reactor performance. For example, in a DHDS (diesel hydro desulfurization) reactor, only 1% of the untreated feed (∼1.0 wt % sulfur) mixed with the product because of wall flow or flow channeling keeps the product sulfur specification (100 wt ppm) off by ∼100%, even after using a highly active catalyst. Effective uniform distribution of liquid in the macroscopic level is critical to meeting the above demands,5 and it demonstrates to have a good distribution especially when such a low sulfur specification in the product is targeted. Until recently, very little work has been undertaken to study and significantly improve the performance of existing distribution tray designs. Typically, catalyst manufacturers are wellequipped to test and develop new catalysts but have neither the testing facility nor the expertise to study flow distribution devices. Only a limited group of companies with the combined expertise from both catalysts manufacturing and licensing of technology possess these capabilities, vz. Haldor Topsoe, IFP, UOP, etc. Engineering companies do not have the facilities nor the interest to undertake reactor internals development studies, which fall outside the scope of their activities. In view of the rapid advances that are being realized in the area of improvement of reactor internals, it is deemed appropriate to supplement the information and description of varieties of liquid distributors used in TBRs. The present review aims to discuss the different types of internals used broadly in the industrial scale and in which improvements have been made over the years in terms of (1) distributors in large-scale reactors, (2) quench box/mixing device, (3) inert layer, (4) application of fluid flow modeling (CFD) in distributors studies, and (5) distributors used in the laboratory scale. Internals for cocurrent upflow reactors are also discussed in brief. It is hoped that the paper will stimulate additional research and development activities on design and selection of reactor internals (mainly used in the industrial scale) with a view to obtain uniform distribution of gas and liquid on the macroscopic level in the case of trickle-bed reactors. 2. Distributors in Large-Scale Reactors Most of the known designs of vapor-liquid distributors fall into one of the four categories. The first kind of distributor is a perforated plate or sieve tray (Figure 3a). This may or may not have notched weirs around the perforations. The tray may also have chimneys for vapor flow. This type of distributor is used for rough liquid distribution in conjunction with a more sophisticated final liquid distribution tray. The second common type of liquid distribution device is a chimney tray. This device uses a number of standpipes, typically on a regular square or triangular pitch pattern on a horizontal tray. The standpipes have holes in the sides of the pipes for the passage of liquid (Figure 3b). The tops of the standpipes are open to allow vapor flow down through the center of the chimneys. The third type of liquid distribution device is a bubble cap tray. This device uses a number of bubble caps laid out on a regular pitched pattern on a horizontal tray. A cap is centered concentrically on a standpipe (Figure 3c), and sides of the cap are slotted for vapor flow. Liquid flows under the cap and, together with the vapor, flows upward in the annular area and then down through the center of the standpipe. The fourth type of distributor is the
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Figure 3. Different types of distributors: (a) perforated tray, (b) multiport chimney, (c) bubble cap, and (d) vapor-lift tube.
vapor assist lift tube (Figure 3d). One leg (downflow tube) of the inverted “U” fits through a perforation in the support tray. The other leg (upflow tube) is shorter so that it is elevated above the tray. The ends of both legs are open. At the top of the inverted “U”, there is an internal opening between the legs. The device thereby provides a flow path across the tray, from the inlet through the end of the short leg, with vertical flow through the short leg, a direction change at the top of the inverted “U”, downflow through the long leg, and discharge through the open end of the long leg below the tray. A vertical slot is cut into the side of the short leg opposite the longer leg. The top of the slot is at or below the bottom of the internal opening between the legs. In many processes, e.g., hydroprocessing reactors, there can be wide variations in the flow rates of vapor and liquid phases and physical properties over time and during turndown operations. Because of fabricating tolerances and the care of installation, there will be unavoidable variations in the distribution tray levelness. Liquids dropping onto the distribution tray from an inlet distributor or quench zone mixer may be unevenly distributed and could result in liquid height gradients across the tray due to splashing, waves, or hydraulic head. Therefore, to have the optimized liquid distribution, the following important elements must be considered during the design of the gasliquid distributor tray: (a) Drip point spacing. The dense spacing of drip points is a key parameter in optimum radial dispersion of liquid coming out of distributor. The liquid dripping on to the catalyst bed may be visualized as a point source below each tube in the tray, and it disperses radially as it passes through the bed. So part of the bed may be used to compensate the larger drip point spacing toward uniform distribution of the liquid. Therefore, for uniform distribution of liquid, closer spacing and a greater number of drip points should be provided. (b) Vaporization over the run cycle. Vaporization over the run cycle increases the vapor/liquid ratio, which can reduce the liquid level on the tray below a point where liquid can flow through some of the distributors. The tray design should be able to handle various vapor/liquid ratios. (c) Tray levelness. Tray levelness must be carefully considered so that liquid does not preferentially flow through only
Figure 4. Impact of tray levelness for (a) perforated-plate distributor, (b) chimney distributor, and (c) vapor-lift distributor.
some of the distribution points, as shown in Figure 4a. The design of the distributor should be able to overcome out-oflevelness of the tray. Fabrication tolerance, poor inclination, deflection under load, mishandling, etc. cause tray out-oflevelness. The impact of tray levelness is reduced by the choice of a proper distributor, as shown in parts b and c of Figure 4. (d) Vulnerability to plugging. Vulnerability to plugging by coke or corrosion products must be considered to ensure equal liquid flow from all distribution points. (e) Vapor-liquid mixing. Vapor-liquid mixing is also an important feature for ensuring that the reactants reaching the catalyst surface are at an equilibrium temperature to have a uniform reaction throughout the entire catalyst bed. So the distributor providing a higher degree of vapor/liquid mixing will be advantageous, especially for trays located downstream of quench zones. (f) Pressure across the distributor. The pressure across the distributor should be low. In the following sections, the key features of different distributor designs that have been published and patented over the years and how well these devices address the above design considerations are discussed. 2.1. Perforated Tray. This distributor tray is provided with a large number of liquid downflow apertures. Generally, a pool
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Figure 5. Distribution systems for use in multiple beds (U.S. Patent No. 4,836,98912): (a) perforated tray and (b) perforated tray for rough distribution.
of liquid will accumulate on the tray and cover these apertures so that the flow of vapor through them is not possible. Normally, a large size chimney is provided to pass vapor to the tray/bed below this. The top of each chimney is provided with a number of slots to act as a weir for liquid flow if the liquid on the tray builds up and a flooding situation occurs (Figure 5a). This tray is rather simple to construct and is capable of providing the greatest number of drip points over the cross section of the catalyst bed. It is used in isolation for a rough distribution of liquid or in combination with other distributors (i.e., chimney tray, bubble cap, and gas-lift tubes) for a finer distribution of liquid. In the case of multiple beds, there is a collection tray below the catalyst bed and a rough distributor tray below the collection tray, which is basically a perforated tray type. After the perforated tray, a second, final distributor tray is provided with downcomers for flow of liquid and vapor onto the lower catalyst bed. Smith et al.11 developed a perforated distribution tray with a small perforation for liquid flow along with a central opening with cylindrical weirs for gas flow. Perforations were 5-15 cm in diameter, and the total open area was sufficient for passage of liquid and accumulation of liquid up to a certain level with pressure drop not exceeding 5 cm. As an example, Figure 5b shows the perforated distributor system developed by Aly et al.12 for an initial, rough distribution of liquid to the second distributor tray along with the chimney for vapor flow. Grott et al.13 also used a perforated tray (with a cylindrical wall at the outer periphery of the tray) for distributing liquid effluent from the mixing chamber. The tray has a uniformly distributed perforation of size 16 mm. Vapor passes through the annular passageway. The performance of this type of distribution device will not properly satisfy all the required design considerations. Liquid on the unleveled tray will gravitate to the low points, and consequently, the sensitivity of tray levelness will be very high. The perforation can easily become plugged by coke, corrosion products, or other debris carried into the reactor by the feed.
Finally, the flexibility to liquid load is very poor. Typically, this type of distribution tray can be designed to give good performance at either the design conditions or at turndown conditions, but not at both situations. Consequently, the tray has a tendency to run dry as vaporization increases toward the end of the cycle. This type of design may, therefore, not be seriously considered to provide good uniform distribution. However, this type of distributor may be used as rough distribution in a multilevel distributor system. 2.2. Chimney Tray. These types of distributors consist of a horizontal tray fitted with vertical downpipes called risers (both sides open-ended) having holes (liquid openings) drilled in the sides. These lateral liquid opening(s) may be at one or more elevations with varying sizes and shapes. The total flow area of the liquid openings is selected to hold a certain liquid level on the tray, and the total cross-sectional area of the vapor chimneys is normally selected to obtain a low pressure drop across the tray to ensure that the driving force for liquid flow through the liquid openings is mainly the static head of the liquid column above the liquid opening and not the pressure drop caused by vapor flow through the chimneys. The bulk of the liquid flow would pass through the holes as a jet, which is sheared by the gas passing vertically downward. The shearing actions break up the liquid and thereby improve gas-liquid contact before reaching the catalyst bed. This type of liquid distributor is generally designed to control liquid level on the tray as well as proper mixing of two phases depending upon the types of holes. Over the years, chimney trays have been patented along with the constant updation by several investigators. Details of some of the typical chimney distributors with the development as reported in the patent are compiled in Table 1. As en example, Riopelle and Scarsdale14 disclosed in U.S. Patent No. 3,353,924 a gas-liquid distributor, consisting of pipes with long vertical slots/notches on the sides so that liquid flow through the distributor increases as liquid level on the tray increases (Figure 6). A simple fluid mechanical analysis of such a device shows that the flow through the slot is proportional to the height of the head of the liquid above the slot base raised to the power of more than one (∼1.5). This behavior is undesirable because the 1.5 power dependence on liquid height makes the distributor very sensitive to variations in levelness. In addition, this device uses separate, larger chimneys for gas flow, which restricts the number of liquid irrigation points on the tray. Effron et al.15 disclosed in U.S. Patent No. 3,524,731 a distributor that comprises a plate having short tubes and long tubes inserted through the plate (Figure 7). The upper ends of the longer tubes are provided with notches and gas caps at the uppermost extremity. At low flow rate (i.e., at minimum feed throughputs), the flow of liquid is entirely through the short tubes with the gas flowing through the notched tubes. Uniformity of distribution is readily achieved by sizing the short tubes so that the head of liquid existing above it is at least 38 mm. At higher flow rates, the liquid builds up to the notches provided in the longer tubes and some of the liquid then begins to flow through the longer tubes. This in effect serves to “spread out” the increased flow over a greater number of points and serves to maintain the desired uniformity of distribution. The flow through the short tubes still remains uniform, and the gas phase still continues to flow through the notched tubes. Thus, the notched tubes serve two important functions: first, they act as gas chimneys to provide good uniformity in the distribution of the gas phase, and second, they prevent the building up of the liquid level over the plate beyond a desired height. With this
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Table 1. Comparison of Different Chimney-Type Distributors author Riopelle and
Scarsdale14
type of distributor
size details of the distributors
chimney type
long vertical down pipe for gas; small liquid down pipe with long vertical notch wide open at top
Effron and Hochman15
riser type
long down pipes with notch at the upper end; short down pipe with overflow boxes at outlet; pitch ) 7.5-30 cm; shorter tube: dia (mm) ) 10-20, height ) 50; longer tube: dia (mm) ) 25-50, height ) 165; triangular cutouts 25 mm × 50 (height) × 6 mm
Grosboll et al.16
chimney type
Derr et al.17
weir type
chimney with circular hole at wall; dia of tube ) 70-218 mm; 10-30 no. of chimney/m2; 3-8 aperture around the perimeter wall, 5-15 cm above the plate; aperture size ) 0.6-6 cm2 gas liquid downcomer pipe with circular holes at wall and rectangular notches at the upper end
Campagnolo et al.18
riser type; a pair of distribution trays
distributors at 1st tray are riser types with weir slots for liquid flow; distributors at second tray are both riser types for both gas and liquid and liquid down pipes
Koros et al. 19
chimney type
vertical slots of different lengths around the circumference of the chimney; spraygenerating device at outlet of distributors
Muldowney et al.20
open-ended down pipes with holes at two different levels
Wrisberg21
riser type
Muller22
down pipes with liquid conduit
two types of down pipes, first type with holes at two levels and second type has holes at higher level corresponding to first type; lowermost holes are at least 6 mm above the tray open-ended tubes with aperture at various elevations (3-4 levels); tube pitch ) 50-120 mm; aperture at various levels with lowermost at 50 mm from tray downcomers with holes at wall at elevations, a reduced flow area section, a liquid conduit, and a device for improved liquid spread
feature, reactors of shorter overall lengths may be employed since large buildups in liquid levels in the case of large turndown ratios (i.e., maximum flow rate/minimum flow rate) do not occur. The slots in the long tubes are designed so that, at maximum flow rates, they take up to 50% of the total flow rate. Furthermore, by maintaining a head of 38 mm above the uppermost end of the shorter tubes, distribution becomes relatively insensitive to out-of-level variations, which may occur in the transverse direction of the reactor. Overflow boxes are provided at the end of the shorter tube to reduce the effective distance between drip points with slots to distribute liquid. The preferred configurations for the slots are triangular cutouts. The advantage of using such a slot, as compared to a slot of rectangular cross section or one of triangular cross section with the apex downwardly oriented, lies in the fact that it insures greater uniformity of distribution when the liquid level above the plate is not parallel to it because flow through the slot is proportional to the height of the head of liquid above the slot base raised to the power
