Liquid Flow Behavior of a Seepage Catalytic Packing Internal for

Jul 23, 2014 - The general agreement of the measured and calculated values for the height of the liquid layer is very good. The model calculations (so...
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Liquid Flow Behavior of a Seepage Catalytic Packing Internal for Catalytic Distillation Column Xin Gao,†,‡ Fangzhou Wang,† Rui Zhang,† Hong Li,*,†,‡ and Xingang Li†,‡,§ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China National Engineering Research Center of Distillation Technology, Tianjin 300072, China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ‡

ABSTRACT: A seepage catalyst packing internal (SCPI) as a good alternative for catalytic distillation internals was investigated in this work. The seepage flow behavior and liquid holdup characteristics of the SCPI have a significant influence on the performance and industrial design of a catalytic distillation column with SCPI, and the present study focused on this issue. Measurements of the seepage flow and liquid holdup of the SCPI were achieved using a 100-mm-diameter plexiglas tube and a 600-mm-diameter Perspex column, respectively. The experimental results were used to develop a series of correlations to predict the liquid holdup of the SCPI. Again, a comparison to the experimental data showed good agreement. It was concluded that the model represents the real process with satisfactory accuracy, although some deviations were observed. Furthermore, compared with two classical catalytic packings, the experimental results show that the SCPI has a much larger liquid holdup, which is advantageous for the reaction. Finally, the basic flow regimes in SCPI can be distinguished based on the liquid flow behaviors, which can be used for the design and scaleup of catalytic distillation column with SCPI.

1. INTRODUCTION Reactive distillation (RD) is a combination of chemical reaction and distillation separation in a single device.1 The advantages of existing and potential applications of RD are to surpass equilibrium limitations,2 achieve high selectivity toward a desired product, achieve energy integration,3 perform difficult separations, and so on.4 Catalytic distillation (CD) is an RD process in which the chemical reactions occur over a solid catalyst. Catalytic distillation technology can improve the performance of RD processes; however, the installation of small solid particle catalysts (1−3 mm in diameter) inside a column is one major challenge in CD technology.5 The design of column internals with catalysts requires special attention. The “ideal” catalytic internal configuration is one that offers maximum catalyst holdup in the column, good gas−liquid and liquid− solid contact and mass transfer, and low pressure drop.6 Unfortunately, these three requirements cannot all be met by the same catalytic packing, and most hardware choices represent compromises. The new-generation catalytic packing called the seepage catalytic packing internal (SCPI) is characterized by a hybrid structure made of alternating reaction and separation zones.7 Figure 1 shows a schematic of the structure of the SCPI that examined in the present work. The vapor comes into contact with the liquid only at the corrugated metal sheets, which avoids contact between the gas and liquid in the bed of catalyst particles. Consequently, the pressure drop in the SCPI is significantly decreased compared to that in conventional catalytic packing8 with two-phase flow in a catalyst particle bed. However, this feature also limits the use of SCPIs to those applications in which the reaction takes place in the liquid phase. At a given liquid flow rate, installation of avert-overflow baffles at a proper height above the catalyst bed ensures that liquid seepage flows through the catalyst bed successfully © 2014 American Chemical Society

Figure 1. Laboratory-scale element of SCPI with four catalyst containers: (1) corrugated metal sheets, (2) catalyst particles, (3) metal mesh, (4) avert-overflow baffle, (5) underside of a catalyst container.

without overflow. Empirical flow models and computational fluid dynamics (CFD) simulations of the pressure drops of SCPIs were carried out by us for the industrial design of catalytic distillation columns with SCPIs.9,10 The results showed that the two models were able to describe the pressure drop of SCPIs very well. However, estimation of the pressure drop is not sufficient for the industrial design of a CD column with SCPI. The liquid flow behaviors and holdup inside the catalytic internals influence the reactive performance of the internals; hence, knowledge of the liquid holdup is also very important for the scaleup of a CD column with SCPI. This study investigates the flow behaviors and holdup of SCPI, as a supplement to our previous studies.9,10 In this work, after the production and characterization of the SCPI, the Received: Revised: Accepted: Published: 12793

February 16, 2014 July 5, 2014 July 23, 2014 July 23, 2014 dx.doi.org/10.1021/ie500665q | Ind. Eng. Chem. Res. 2014, 53, 12793−12801

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particle diameter distribution of 0.4−1.25 mm was filled into the tube, and the bulk void fraction was about 0.32. A circulating water system was used in the experiments. The pressure drop of the catalyst bed was provided by inert gas. During operation, the pressure drop of the catalyst bed was varied between 0 and 15 kPa. The pressure drop was measured using a U-tube manometer filled with water that was inclined to provide a more sensitive measurement (sensitivity of 10 Pa). The effect of the liquid flow rate on the height of the liquid layer over the catalyst bed was investigated at different heights and different pressure drops of the catalyst bed. The level of the liquid layer is referred from the upper surface of the catalyst bed to the top edge of the liquid over the catalyst bed. Prior to performing a series of measurements, the plexiglas tube was operated at a high liquid load for 20 min to ensure thorough wetting of the catalyst bed. Then, the pressure drop of the catalyst bed was set to the desired value, and after stabilization, the liquid load was increased in regular time intervals from an initial low value. In all of these experiments, the temperature of the recycled water was 25 °C. The liquid superficial velocity employed was in the range from 0.5 to 2.5 mm/s. 2.2. Liquid Holdup of the SCPI. The experimental system used for the measurement of the liquid holdup is shown in Figure 3. An air/water system was used to represent gas/liquid

actual flow behavior of the liquid in a catalyst container with an avert-overflow baffle was investigated in a 100-mm-diameter plexiglas tube. Liquid holdups at various gas and liquid flow rates were measured in a 600-mm-diameter Perspex column. A series of semiempirical model to compute the seepage flow behavior and the liquid holdup of the SCPI were developed not only for a better understanding the characteristics of the SCPI but also for the development of the necessary information for design and scaleup. Finally, three basic flow regimes in the SCPI were distinguished, which is also the key element for the scaleup and operation of a catalytic distillation column with SCPI.

2. EXPERIMENTS 2.1. Seepage Flow in a Catalyst Container. The height of the avert-overflow baffle is a very important parameter in the design of an SCPI for industrial applications. The design value of the height of the avert-overflow baffle depends on the effect of the liquid flow rate on the height of the liquid layer over the catalyst bed. To obtain the design parameters, an experiment on liquid seepage flow through the catalyst bed was carried out to investigate the effect of the liquid flow rate on the height of the liquid layer over the catalyst bed at different heights and different pressure drops of the catalyst bed. The facility used in the seepage flow study was previously reported in detail by Li et al.11 A brief description of the rig is presented here for convenience. A schematic diagram of the experimental facility is shown in Figure 2.

Figure 3. Schematic drawing of the cold model experimental apparatus: (1) air blower, (2) pitot-tube flowmeter, (3) U-tube manometer, (4) liquid distributor, (5) SCPI, (6) catalyst particles, (7) column, (8) liquid rotameter, (9) gas distributor, (10) liquid level meter, (11) water tank, (12) centrifugal pump.

Figure 2. Schematic diagram of the seepage flow experimental facility: (1) gas cylinder, (2) connecting flange, (3) catalyst bed, (4) metal wire mesh, (5) plexiglas tube, (6) liquid distributor, (7) U-tube manometer, (8) liquid rotameter, (9) liquid seal device, (10) water tank; (11) pump.

countercurrent flow in the catalytic internals. An air flow from the blower entered the bottom of the column through the gas distributor, then flowed upward through the catalytic internals, and finally vented from the top of the column. Similarly, a centrifugal pump was used to transport the liquid from a storage tank to the liquid distributor at the top of the column. A diagram of the liquid distributor is shown in Figure 4. The drippoint density of the liquid distributor was about 500−600 drip

The water stored in the bottom of the tank was pumped to a liquid rotameter before it entered the liquid distributor, flowed into the tube, went down the catalyst bed, and returned to the water tank. The tube was a cylindrical plexiglas vessel 100 mm in diameter and 1 m in height. For the catalyst bed, the cationexchange resin NKC-9, which is similar to Amberlyst 15, with a 12794

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Figure 4. Top view of the distributor for the initial liquid distribution. The circle corresponds to the column circumference.

points per square meter. The flow rates of gas and liquid were determined with a pitot tube flowmeter and a liquid rotameter, respectively. A U-tube manometer filled with water measured the pressure drop across the bed. The main portion of the column was a 2-m-long, 600-mm-internal-diameter Perspex tube in which two integrated elements of SCPI were installed. The specifications concerning the liquid holdup volumetric measurements carried out in the column filled with SCPI were as follows: Initially, the bed was operated at high gas and liquid flow rates for about 40 min to ensure the complete wetness of the SCPI, and afterward, the gas and liquid loads were set at the desired values and maintained to reach stationary flow. At the time of the stabilization run, the air inlet, water inlet, and outlet of the column were stopped simultaneously and instantaneously. The liquid was subsequently allowed to drain and collect at the gas distributor. In particular, a standard draining time of 30 min was set for the measurement of the dynamic liquid holdup draining from the catalytic internals. The volume of the liquid drained was measured by checking the liquid level difference in the level meter in the water tank. The liquid sprayed density was varied between 14.06 and 31.25 m3/m2·h, and the gas F factor was increased up to the flooding of the SCPI bed.

Figure 5. Schematic of the structure for an integrated element of SCPI: (1) corrugated metal sheets, (2) catalyst particles, (3) liquid layer, (4) gas flow, (5) liquid flow, (6) metal mesh, (7) avert-overflow baffle.

3. MODEL DEVELOPMENT The liquid holdup models for the SCPI reflect the geometric structure of the internal, which consists of catalyst containers and corrugated metal sheets as shown in Figure 5. The liquid holdup models are based on the following assumptions: • Gas flows only through the corrugated metal sheets. • Liquid flows uniformly through the corrugated metal sheets and the catalyst containers. • Liquid holdup in the corrugated metal sheets increases the pressure drop of the SCPI. 3.1. Seepage Flow in a Catalyst Container. The avertoverflow baffle prevents liquid overflow from the top of the catalyst container into the separation zone, which ensures that all of the liquid passes through the catalyst bed. However, using an excessively high avert-overflow baffle, instead of increasing the height of the CD column, is not useful. In general, the height of the avert-overflow baffle is designed according to the operating conditions of the CD process. To evaluate the geometrical parameters of the SCPI, we developed a model to calculate the height of the avert-overflow baffle.

where k is a constant, μl is the liquid viscosity, L is the length of the tubule, and ul′ is the liquid flow rate in the vertical tubule calculated by the equation

In a catalyst container, the catalyst bed resistance will resist the draining of from a compact catalyst bed until a balance is reached with counteracting liquid gravitational force. The catalyst bed is assumed to consist of a multiplicity of parallel, unconnected tubules with a hydraulic diameter (dh) of voids defined as12 4εc cross section available for flow dh = 4 × = wetted perimeter a(1 − εc) (1)

where εc is the porosity of the catalyst bed and a is the mean specific surface area of a catalyst particle. For parallel unconnected tubules, according to the Hagen− Poiseuille equation, the pressure drop (ΔP) caused by the resistance of a vertical tubule to liquid passing through is given by ul′ = k

dh 2(ΔP) μl L

ul′ = ul /εc

(2)

(3)

where ul is the liquid superficial velocity in the catalyst container. Substituting eqs 1 and 3 into eq 2 yields ul =

⎤ ⎡ ΔPc ⎥ ⎢ ra 2(1 − εc)2 ⎢⎣ μ l (Lc + Le) ⎥⎦ εc 3

(4)

where Lc is the height of the catalyst bed, Le is the equivalent height of a wire that fixed catalyst particles, r is a constant based on experiments [r = 1/(16k)], and ΔPc is the pressure difference of the catalyst bed. For steady-state operation in a catalyst container, where the catalyst bed resistance is balanced by gravitational forces, no external pressure difference exists because of vapor flow around the catalyst container. However, the external pressure must be taken into account because the pressure gradient around the 12795

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catalyst container due to vapor flow is not negligible. Therefore, under steady-state conditions, ΔPc comprises three individual contributions: the gravity of the liquid layer over the catalyst bed, the gravity of liquid inside the porous catalyst particle bed, and the pressure drop of the catalyst container ΔPc = ρl ghl + ρl gLc − ΔPp

pressure drop of the catalyst containers was zero. The results of seepage flow experiments without pressure drop for different catalyst bed heights are plotted in Figure 6. The hydrodynamic

(5)

where ρl is the density of the liquid, g is the gravitational acceleration, hl is the height of liquid layer over catalyst bed, and ΔPp is the pressure drop of catalyst container (gravity and ΔPp have different directions). 3.2. Liquid Holdup. The liquid holdup for the SCPI can be divided into two different parts, namely, catalyst container (hcc) and structured packing (hsp), where catalyst container consists of catalyst bed (hcb) and avert-overflow baffle (haob) H=

hspVsp + hccVcc VSCPI

(6)

Various models for the prediction of the liquid holdups of structured packings have been developed by several workers.13−15 These are the basis for the models developed here to predict the liquid holdup of the SCPI based on our experimental data. In this work, the mechanistically based model of Rocha et al.13 is used to describe the liquid holdup inside the structured packing, which takes into account the flow and physical properties of the system, as well as geometric variables such as corrugation angle and surface enhancement ⎤1/3 3μ l ul ⎛ 4Ft ⎞2/3⎡ ⎢ ⎥ ⎜ ⎟ hsp = ⎝ S ⎠ ⎢⎣ ρ (sin θ)εg ⎥⎦ l eff

Figure 6. Comparison between the model-calculated and experimental values for seepage flow in a catalyst particle bed.

behaviors were similar for all catalyst bed heights and displayed an increase in the height of the liquid layer with increasing superficial liquid velocity. The slopes of the curves gradually increased with increasing catalyst bed height, indicating that the residence time in the catalyst bed increased with increasing catalyst bed height at the same liquid load. The studies also showed that the avert-overflow baffle of catalyst container can be increased to improve the liquid load in the catalyst container under the conditions of constant pressure drop and catalyst bed height. Furthermore, the reaction time in the catalytic distillation column can be precisely controlled to satisfy the requirements of the reaction and separation according to this result. A comparison of the seepage flow model and the experimental values for the experiments without pressure drop is presented in Figure 6. The height of the liquid layer was calculated by eq 9 with ΔPp = 0. In general, the predicted values are only slightly different from the experimental results, with maximum deviations for catalyst bed heights of 25 and 100 mm. The deviations between the model and the experimental points can be explained by the unevenness of the particle size, which was not taken into account in the correlation. In Figure 7, the results of seepage flow for a catalyst container with pressure drop are plotted for three liquid superficial velocities from 0.00053 to 0.00088 m/s and two catalyst bed heights. The overall shape of the curves corresponds well with the theoretical seepage process for a porous medium. The height of the liquid layer increases proportionally with the pressure drop of the catalyst container at a given superficial liquid velocity. The height of the liquid layer is also influenced by the superficial liquid velocity, increasing as the velocity increases. The effect of the height of the catalyst bed on the height of the liquid layer is also shown in Figure 7. It can be seen that the height of the liquid layer increased with the height of the catalyst bed at the same superficial liquid velocity and pressure drop of catalyst container. Because the height of the catalyst bed is one of the main factors controlling the amount of catalyst in the CD column with the SCPI, an increase in the amount of catalyst is expected with a smaller column. However, the above findings show that the ratio of the height of the liquid layer to the height

(7)

where Ft is a correction factor for the total holdup due to the effective wetted area, S is the side dimension of corrugation, θ is the angle of the corrugation channel with respect to the horizontal, ε is the void fraction of the packing, and geff is the effective gravitational constant. For a catalyst container, the liquid holdup is the sum of internal and external contributions of the catalyst bed. The former is the liquid held in the void fraction of the catalyst bed, and the external liquid holdup is equivalent to the liquid layer over the catalyst bed mentioned in section 3.1. Thus, the total liquid holdup in a catalyst container is given by hcc = haob + hcb =

hl + Lcεc Z

(8)

where Z is the height of the catalyst container. hl is calculated by substituting eq 5 into eq 4, yielding hl = −

ΔPp ρl g

+

μ l rula 2(1 − εc)2 (Lc + Le) εc 3ρl g

− Lc

(9)

The constant r on the right-hand side of eq 9 depends on the ratio between the flow rate and pressure drop. This value is determined by the void fraction of the catalyst bed, the particle shape, the stacking manner, and the particle size distribution and is described by an empirical relation based on experiments. We obtained Le ≈ 0.02772 m and r ≈ 2.8 × 10−18 according to the experimental results.

4. RESULTS AND DISCUSSION 4.1. Seepage Flow in a Catalyst Container. During the experiments on seepage flow without pressure drop, the 12796

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Figure 8. Comparison of the height of the liquid layer at different superficial liquid velocities in the presence and absence of a pressure drop.

Figure 7. Comparison between model-calculated and experimental values. (The solid lines correspond to the solid points, and the dashed lines corresponds to the open points.)

mmHg pressure drop vs. 50 mm for no pressure drop); these results are just an indication of the trend for the influence of pressure drop caused by gas on the height of the liquid layer over the catalyst bed. 4.2. Liquid Holdup. Figures 9−11 show the experimental data on the influence of countercurrent gas flow on the

of the catalyst bed decreases with increasing catalyst bed height at the same superficial liquid velocity. Hence, an increased height of the catalyst bed through a reduced number of catalyst containers at the same amount of catalyst in the SCPI is beneficial for a reduced height of CD column. Nevertheless, the limited number of catalyst containers restricts the frequency of alternation for reaction and separation occurring in the CD column with SCPI, which reduces the performance of the CD process. Therefore, the design of the height of the catalyst bed and the number of catalyst containers should represent a tradeoff between the height and the performance of the CD column. To examine the applicability of the seepage flow model for the experiments with pressure drop, the experimental and calculated values for the process of seepage flow with different catalyst bed heights and different superficial liquid velocities were compared for a large range of pressure drops. The height of the liquid layer was calculated by eq 9. The results of these comparisons are also shown in Figure 7. The general agreement of the measured and calculated values for the height of the liquid layer is very good. The model calculations (solid lines) slightly overestimated the height of the liquid layer for high catalyst beds, indicating that the actual resistance of the catalyst bed is less than the theoretical value at greater catalyst bed heights in the presence of pressure drop. Nevertheless, the experimental points correspond well to the predictions of the model for a 32-mm catalyst bed height (dashed line). The deviation of the experimental points from the model predictions does not exceed ±7% except for several experimental values. In general, most calculated values are able to represent the process of seepage flow in the catalyst bed. The model for calculating seepage flow behavior can therefore be considered valid. A comparison of the height of the liquid layer over the catalyst bed at different liquid superficial velocities in the presence and absence of pressure drop is shown in Figure 8. The liquid superficial velocity passing through the catalyst bed without pressure drop is higher than that for a 70 mmHg pressure drop at the same liquid layer height over the catalyst bed. This can be explained by the fact that the resistance caused by gas bypassing the catalyst bed reduced the liquid superficial velocity seeping through the catalyst bed. However, caution is necessary when interpreting the results because the heights of the catalyst beds were not exactly the same (52 mm for 70

Figure 9. Relationship between the dynamic liquid holdup of SCPI-I and the F factor at different liquid loads.

Figure 10. Relationship between the dynamic liquid holdup of SCPI-II and the F factor at different liquid loads.

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under the same conditions for SCPI-III. Furthermore, a greater resistance to liquid flow through the catalyst bed was obtained in the catalyst container for SCPI-III. Therefore, the dynamic liquid holdup increased with increasing Rcc/cms value of the SCPI, which corresponds to an increasing pressure drop of the SCPI for the same gas and liquid flows. The dynamic liquid holdups of the SCPIs were calculated according to eq 6 at the flooding point. With the presented analogy, the liquid holdup parameters were correctly estimated and closely followed the experimental data, as shown in Figure 12. Here, it can be seen that the deviation of the experimental

Figure 11. Relationship between the dynamic liquid holdup of SCPIIII and the F factor at different liquid loads.

dynamic liquid holdup for three SCPIs with different ratios between the width of the catalyst container and that of the corrugated metal sheets (Rcc/cms). In these plots, the dynamic liquid holdup is shown as a function of the F factor and liquid load, and the SCPI types are grouped by Rcc/cms value. The Rcc/cms varies from 1 to 2 for these samples, and the heights of the catalyst bed and avert-overflow baffle of catalyst container remain constant around 0.1 and 0.25 m, respectively. A higher liquid load was found to lead to instantaneous flooding under the gas F factor when the column operation was investigated, so the range of experiments was limited for that investigation.7 Therefore, the gas F factor for the dynamic liquid holdup was investigated on selected different values for three SCPI types. In general, the hydrodynamic behaviors were similar for all SCPI types; the slopes of the curves were nearly the same and displayed an increase in liquid holdup with increasing liquid load and gas F factor. Furthermore, the influence of liquid flow on the liquid holdup was found to be greater, whereas the effect of the gas F factor was found to be small. The amount of liquid over the catalyst bed increased with increasing liquid load, which is the main contribution for increasing the liquid holdup of an SCPI. For the effect of the gas F factor on the liquid holdup of the SCPI, the liquid holdup in the structured packing increased with increasing F factor,13 whereas the liquid holdup of the catalyst container hardly increased with increasing F factor. A reason for this effect is that the gas will not flow through the catalyst bed, where the liquid holdup increases only slightly with increasing pressure drop of the catalyst container caused by the gas flow through the structured packing around the catalyst container. Therefore, the liquid flow is the main factor for the liquid holdup of an SCPI. The influence of the width ratio of the catalyst container relative to the corrugated metal sheets (Rcc/cms) is also clearly visible from Figures 9−11. For SCPI-I (Rcc/cms = 1), the liquid holdup values were always lower than those for other types of SCPI under the same conditions. The highest liquid holdup values were found for SCPI-III (Rcc/cms = 2). This could be due to two different reasons: First, the catalyst container for SCPI-I was narrower than those for the other types, whereas that for SCPI-III was widest. This led to the presence of more liquid in the wide catalyst container and, thus, to higher liquid holdup values. Second, the pressure drop of the structured packing around the catalyst container for SCPI-I was slightly lower than those for the other types, and that for SCPI-III was the highest. This indicates a higher pressure drop of the catalyst container

Figure 12. Comparison between model-calculated and experimental values for SCPI-I.

values from the model predictions did not exceed ±4%, except for the cases of larger liquid loads and F factors. The correlation of the liquid holdup presented in this work was able to predict the increase in holdup with increasing gas F factor at low liquid load, but it slightly overestimated the holdup at higher liquid loads and gas F factors. The results might be due to the fact that the calculated height of the liquid layer could be higher than the actual height of the avert-overflow baffle when the liquid had already filled the avert-overflow baffle. At that time, the calculated value of the liquid layer height would still increase with increasing pressure drop of the catalyst container caused by increasing the gas F factor, whereas this phenomenon would not happen at lower gas F factor, as the avert-overflow baffle would not be filled with liquid. At lower liquid load (i.e., 20.31 m3/m2·h), the height of the liquid layer was always lower than the height of the avert-overflow baffle within all ranges of F factors. That is why the experimental points correspond well to the predictions of the model for flow regime B (load point; see below). The influence of the gas and liquid loads on the flow regimes is further discussed in section 5. 4.3. Comparison with Other Catalytic Packings. Considering the air/water system, the liquid holdups of three types of SCPIs were also compared with two types of classical catalytic packings used for catalytic distillation columns. These were Sulzer Katapak-SP 12,16,17 widely used in industry, with a specific area of 250 m2/m3, and catalyst bundles.18 The disadvantage of catalyst bundles is the maldistribution and insufficient wetting of the catalyst particles in the bundles, making them industrially suitable only for conditions of low conversion rate. Katapak-SP was developed as a good alternative for conventional catalyst bundles, the hybrid structure of the catalytic structured packing determines the 12798

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flow development inside the packed bed and, consequently, the overall performance.19 Therefore, these two classical structured catalytic packings were chosen for comparison in this work. Figure 13 shows a comparison of the dynamic liquid holdups of the three types of packings, namely, SCPI, Katapak-SP 12,

Figure 14. Sketches of a vertical cross section of a catalyst container in an SCPI for the three different flow regimes.

through the catalyst bed. The space within the avert-overflow baffle is, however, incompletely filled with liquid; indeed, voids without liquid can be observed in Figure 14a. This redundant space is useless for the catalytic distillation column, which just increases the height of the column. This also leads to a decrease in the amount of catalyst filling, defined as the mass of catalyst per unit volume of the column. With increasing liquid load, the liquid flow velocity and the height of the liquid layer over the catalyst bed increase, leading to an increased liquid holdup. 5.2. Flow Regime B: Load Point. In this regime (r* = 1), the liquid load has reached the value at which the space within the avert-overflow baffle is completely filled with liquid. This regime is depicted in Figure 14b, which is called the load point. Obviously, from the standpoints of reaction, energy consumption, and process coupling technology, the regime around the load point is the most favorable operating region for a CD column with SCPI supports. 5.3. Flow Regime C: Above the Load Point. At liquid loads above the load point (r* > 1), as shown in Figure 14c, the avert-overflow baffle is completely filled with liquid, and the excess liquid flows outside the catalyst container as a bypass, meaning that the reaction is not sufficient for the excess liquid. The flow rate through the catalyst container is the same as in flow regime B, whereas the excess liquid flows through the packing section without reaction. Of course, only liquid that overflows from the catalyst container directly next to the catalyst container is able to enter; however, the extent of this is expected to be small. This regime was defined as the point where overflow of liquid from the catalyst container to the packing occurs. This will lead not only to an earlier flooding status of the separation section, but also to a great deal of liquid not contacting the catalyst. This situation is unfavorable in the operating procedures of the catalytic distillation column. Therefore, in the design process of an SCPI, one must not allow this situation to happen. 5.4. Flow Regime D: Flooding Point. In addition, another flow regime of an SCPI is defined in the structured packing as the flooding point, which is independent of the flow regimes in the catalyst container. The flooding point can appear in flow regime A and also can occur in flow regime B or C of the catalyst container, which depends on the Rcc/cms value of the SCPI. The reason for this is that the vapor comes into contact with the liquid only at the corrugated metal sheets. Even in flow regime A of the catalyst container, if Rcc/cms is sufficiently large, the proportion of the gas channel is too small, which could lead

Figure 13. Comparison of the liquid holdup for catalyst bundle Katapak-SP 12 and three SCPIs at a liquid sprayed density of 20 m3/ m2·h.

and catalyst bundles, where the experimental data for KatapakSP 12 and catalyst bundles are from Ratheesh et al.16 and Xu et al.,18 respectively. At similar liquid loads (liquid sprayed density of about 20 m3/m2·h), Katapak-SP 12 showed the lowest liquid holdup (about 11−12%) but the greatest operating range. At a gas F factor of 1.1, the liquid holdup of catalyst bundles could be rapidly increased by increasing the gas F factor, leading to flooding in the packing bed. In contrast, for an equivalent catalyst loading, the liquid holdup of the SCPI was always higher than those of the other catalytic packings within the range of acceptable performance. The liquid in the avertoverflow baffle mainly increased the liquid holdup of the SCPI but did not lead to an increase in the reaction time, as the liquid made insufficient contact with catalyst particles in the space over the catalyst bed. However, with the avert-overflow baffle configuration, an improved liquid flow through the catalyst bed was achieved that could lead to highly efficient liquid−solid contacting. It is furthermore believed that the high liquid holdup in the catalyst bed means good reactive performance, which is desirable for catalytic distillation processes.

5. FLOW REGIMES IN SCPI Figure 14 shows a sketch of a vertical cross section of a single catalyst container of SCPI. When trickled with liquid, three different flow regimes were observed depending on the liquid load and gas F factor. Three basic flow regimes (flow regimes A, B, and C) in the catalyst container can be distinguished according to the ratio (r*) between the liquid load and the height of the avert-overflow baffle, given by r* =

hlcal haob

(10)

hcal l

In eq 10, is the calculated height of the liquid layer based on the actual liquid load, and haob is the height of the avertoverflow baffle. 5.1. Flow Regime A: Below the Load Point. For r* < 1, the liquid flows mainly inside the avert-overflow baffle seeping 12799

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to flooding in the separation section. This feature requires special attention in the scaleup design of a CD column with SCPI. 5.5. Design of an SCPI. The performance of an SCPI depends on many parameters, the most important of which are hydrodynamic parameters for reaction and separation efficiency. From the aspect of hydrodynamic design, as the operation causes fluctuations in the material flow, the flow rate in the catalytic distillation column can exceed the design value. The design values of the liquid load and gas F factor are specified by the load point of the catalyst container and below the flooding point of the structured packing. This situation is reached when the catalyst containers are just completely filled with flowing liquid and flooding has not yet occurred in the corrugated metal sheets. Vapor is able to flow only through the separation section, and when it is flowing countercurrently, the maximum operating capacity is determined by the hydraulic load point. Furthermore, the operating range of the flow rate is ascertained from 80% to 120% of the design value. To prevent operation in flow regime C, the results obtained from the aspect of hydrodynamics clarify the principle of the design of an SCPI: The calculated height of the liquid layer based on the design liquid load is equal to the design height of 80% for the avert-overflow baffle. The best performance of SCPI types is expected at liquid loads just above the catalyst container load point but with a corresponding gas F factor around the hydraulic load point of the structured packing; then, both functions of this hybrid gas−liquid and liquid−solid contactor are optimally utilized.

the present work were also carried out with an industrial-scale SCPI, creating a basis for a reliable scaleup of catalytic distillation processes with SCPI. Details of the scaleup procedure including hydrodynamics performance, reaction, and separation efficiency are presently being developed. Furthermore, the implementation of these findings into procedure for the scaleup of catalytic distillation processes with SCPI is intensively being pursued.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-022-27404701. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0936) and the National Natural Science Foundation of China (Nos. 21306128, 21336007).

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6. CONCLUSIONS SCPIs qualify as CD column internals in gas−liquid and liquid−solid countercurrent flow, although their operating range is limited compared with Katapak-SP. Information on the influence of fluid dynamics on the interaction between chemical reaction and distillation separation is important for the design and scaleup of catalytic distillation columns. In the present work, detailed experimental studies on the seepage flow of a catalyst container and liquid holdup of SCPI at the laboratory scale were carried out. A seepage flow model for the catalyst container is presented, with which a suitable height for the avert-overflow baffle from the difference of the vapor and liquid loads of a catalytic distillation column can be predicted. This is essential for the design and scaleup of a column using such internals. Furthermore, models for the liquid holdup of the SCPI were developed. All models were successfully checked against the experimental data. However, the limitation of the assumption of a uniform liquid flow rate across the SCPI should be noted. This assumption needs to be verified in further studies on hydrodynamic behavior by measuring the liquid distribution in the SCPI. Comparisons with two classical catalytic packings show that the liquid holdups of all SCPI types are generally higher than those of catalytic packings. Then, three basic flow regimes were found in the SCPIs. At liquid loads up to the load point, the liquid runs almost exclusively within the catalyst container, whereas at higher liquid loads, there is an overflow from the avert-overflow baffle to packing section. The influence of the most important of these parameters was investigated for laboratory SCPIs. The basic findings from this work also hold true for industrial-scale SCPIs. However, because of differences in geometry, the numerical results need to be adapted. Therefore, all of the experiments described in

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NOTATION a = mean specific surface area of a catalyst particle (m2/m3) dh = hydraulic diameter of the voids (m) Ft = correction factor for the total holdup due to the effective wetted area g = constant of gravitational acceleration (9.8 m/s2) geff = effective gravity (m/s2) H = total liquid holdup of the SCPI haob = liquid holdup of the avert-overflow baffle hcb = liquid holdup of the catalyst bed hcc = liquid holdup of a catalyst container hl = height of the liquid layer over the catalyst bed (m) hsp = liquid holdup of the structured packing k = constant L = length of a tubule (m) Lc = height of the catalyst bed (m) Le = equivalent height of a wire (m) r = constant based on experiments Rcc/cms = width ratio of a catalyst container relative to the corrugated metal sheets S = side dimension of corrugation ul = liquid superficial velocity in the catalyst container (m/s) u′l = liquid flow rate in a vertical tubule (m/s) Vcc = volume of a catalyst container (m3) VSCPI = volume of the SCPI (m3) Vsp = volume of the structured packing (m3) Z = height of the catalyst container (m) ΔP = pressure drop of a vertical tubule (Pa) ΔPc = pressure difference of the catalyst bed (Pa) ΔPp = pressure drop of the catalyst container (Pa) ε = void fraction of the packing εc = porosity of the catalyst bed μl = liquid viscosity (Pa·s) θ = angle of the corrugation channel with respect to the horizontal (deg) ρl = density of the liquid (kg/m3) REFERENCES

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