Experimental Investigation of Liquid Axial and Radial Dispersion in

Article pubs.acs.org/IECR

Experimental Investigation of Liquid Axial and Radial Dispersion in Winpak Modular Catalytic Structured Packing Wen-Yu Xiang,† Jin-Ming Li,† Hui-Dian Ding,‡ and Chun-Jiang Liu*,† †

State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ‡ Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China ABSTRACT: The response technique was employed to investigate the axial and radial dispersion characteristics of the liquid phase in modular catalytic structured packing (MCSP) reaction elements. Residence time distribution (RTD) experiments were performed at ambient temperature and atmospheric pressure. A two-dimensional dispersion model was solved analytically to determine the axial and radial dispersion coefficients and the dynamic liquid holdup from the RTD curves. Three different reaction elements containing Winpak packing sheets, Mellapak packing sheets, or only catalytic particles were employed. The radial dispersion coefficient in these structures is approximately 1−2 orders of magnitude higher than the axial dispersion coefficient. The axial and radial dispersion coefficients in the Winpak elements are greater than those in the other structures. Thus, it can be concluded that the liquid dispersion characteristics in the Winpak elements are satisfactory. These measured dispersion coefficients enable the reliable and predictive design and construction of scale-up models. elements through which most of the liquid flows downward. The separation element is composed of packing sheets, whereas the reaction element is basically a catalyst bag. This bag, which contains packing sheets, is a wire gauze envelope that immobilizes particles in the MCSP and ensures liquid access.20 Catalytic distillation is widely utilized in the production of MTBE,21 TAME,22 and methyl acetate.23 These heterogeneous catalytic reactions occur in the liquid phase. Viva et al. studied MCSP systems using a nonintrusive high energy X-ray tomograph.12,13,19 The tomographic images obtained at various axial positions showed that the assumption that the liquid phase only flows downward through the reaction elements is reasonable. This finding highlights the importance of understanding dispersion characteristics in reaction elements, for designing column internals, and CFD modeling. It is critical to know axial and radial dispersion coefficients because they affect interphase mass transfer, concentration and energy distributions, conversions, selections, and yields. The radial and axial dispersion of the liquid phase has been studied by experiments and CFD simulations. In numerous studies, the axial dispersion coefficients in MCSPs were calculated using a dispersion model, usually one-dimensional.8,17,18 The liquid holdup and axial dispersion coefficients can be derived from the experimental RTD curve. The effects of the systems, MCSP type, gas flow, and liquid flow have been investigated.8,11,17 In these studies, only one test point centered at the bottom of the MCSP was used. Consequently, the radial dispersion coefficients are difficult to derive from conventional salt tracer test data.

1. INTRODUCTION Catalytic distillation, which combines heterogeneous catalysis and continuous distillation, is one of the most widely used intensification processes.1−4 It has received increasing attention due to relatively low investment and operating costs, low energy consumption, steady multiphase operation, and environmental friendless.5−7 Modern structured catalytic column internals have played a significant role in expending the fields of application of reactive distillation and extending its use to new applications.1,8,9 The selection of appropriate column internals for specific catalytic distillation processes is an essential design element.10 The application of modular catalytic structured packing (MCSP), for example, Multipak,6,11 Katapak-SP12,13 is development tendency. Kołodziej conducted an extensive experimental study5 to investigate the mass transfer and separation efficiency hydrodynamic behavior in Katapak-S and Multipak. For MCSP, the residence time distribution (RTD) is one of the most important properties because it affects both the reaction and separation processes. Generally, RTD curves can be obtained by experimental or CFD methods.8,14 Pulse-response techniques, in which the time-response of an injected tracer pluse is measured by monitoring its concentration, have been extensively used to determine RTD.8,15,16 Dispersion coefficients can be obtained from RTD curves by solving a dispersion model.11,17 Positron emission particle tracking (PEPT) is a novel technique18 that can also be used to obtain dispersion coefficients. In PEPT, not only is the average spread of the tracer recorded but detailed information about the motion of particles as they pass through these opaque systems can also be obtained. To prevent tracer particles becoming trapped in the system, the flow rates cannot be very low. The dispersion coefficients obtained by PEPT are similar to those obtained from salt tracer tests. Currently, MCSPs consist of two parts:19 separation elements, which can provide open channels for the gas flow, and reaction © 2016 American Chemical Society

Received: Revised: Accepted: Published: 1768

July 22, 2015 December 22, 2015 January 15, 2016 January 15, 2016 DOI: 10.1021/acs.iecr.5b02685 Ind. Eng. Chem. Res. 2016, 55, 1768−1777

Article

Industrial & Engineering Chemistry Research

The main objective of this work is to study the axial and radial dispersion coefficients of the liquid phase in winpak-C reaction elements. The pulse response technique was employed to obtain the RTD curves. The axial and radial dispersion coefficients were determined by solving a two-dimensional dispersion model. The holdup of the reaction elements could be derived from the RTD curves. The effects of the geometry and liquid flow rate on the dispersion coefficients were investigated. For comparison, the experiments and simulations were also performed using traditional reaction element structures.

Van Baten et al. studied the radial and axial dispersion in a Katapak-S structure.15 In these experiments, RTD curves were recorded below the reaction elements at five different positions, and the radial and axial dispersion coefficients were obtained from a 2D dispersion model. CFD simulations were also performed to determine the radial dispersion coefficients, and the results were compared to the experimental data. In recent years, our group has developed a novel structured packing with diversion windows named Winpak. The diversion windows in Winpak structured lead to an increase in the effective mass transfer area, and the gas−liquid mass transfer efficiency.24 A new MCSP named Winpak-C was also introduced in the previous work.1 The separation element is composed of Winpak packing sheets, and the reaction element also contains Winpak packing sheets (Figure 1).

2. EXPERIMENTAL SECTION 2.1. Reaction Elements. The RTDs in three different reaction elements, where each had two variants, were extensively studied. The basic structure of the reaction elements was a catalyst bag, i.e., a wire gauze structure containing two packing sheets facing opposite directions. The bag was filled with small spherical catalyst particles. Different types of packing sheets form different reaction element structures. The reaction elements employed in this study contained Winpak sheets (Winpak 250X or 500X), Mellapak sheets (Mellapak 250X or 500X), or no packing sheets (the catalyst bag was the same dimension as the 250X or 500X bag and was loaded with catalyst particles). Instead of using a transparent shell, a stainless steel shelf was designed to immobilize the reaction elements, effectively avoiding the wall effect. This effect is also avoidable in the horizontal direction when the reaction element length is sufficiently long. Hence, the reaction element length was 400 mm. Moreover, the reaction element height was 100 mm, 150 mm, or 200 mm for all elements. Details of the reaction elements are listed in Table 1, and the packing sheet characteristics are listed in Table 2. The dispersion characteristics of the reaction elements were investigated using resin particles with a diameter range of 0.8--1.0 mm. 2.2. Experimental Setup. Figure 2 shows a schematic of the experimental setup. The liquid flow rate was controlled by a peristaltic pump, to give a spray density of 5, 10, 15, or 20 m3/(m2h). Some reports in the literature examined the effect of the gas-phase flow rate on the RTD curve. Most of these studies demonstrated that neither the gas speed nor the gas flow direction has a significant effect on the RTD curves.17,26,27 Therefore, the liquid flow rate was the only variable for each reaction element in this study. A multistage groove-tray distributor with more than 2935 irrigation points per square meter was designed to provide a homogeneous inlet velocity (Figure 2b). The liquid collector (Figure 2c) was designed to provide seven test points in the radial direction. 2.3. Experimental Procedures. The pulse response technique was employed to investigate the liquid dispersion characteristics. In the experiments, the liquid phase was distilled water, while a saturated sodium chloride solution was used as the pulse tracer. It is difficult to completely wet the reaction elements after they were loaded with resin particles. Therefore,

Figure 1. Winpak-C structure: (a) separation and reaction elements, (b) schematic depiction of Winpak.

As computational power has improved, CFD has played an increasingly important role in the study of MSCPs. CFD methods were employed in several studies to obtain RTD data for packed bed and porous media systems.15,18,25 Ding et al.1 built a CFD modeling of the separation element, to investigate the pressure drop contributing mechanism of Winpak-C. In another work, van Beten15 divided the reaction element into a set of intersecting, connecting, triangular tubes. However, CFD is not an appropriate method for RTD studies of MSCP fields because they involve complex geometries. Table 1. Detailed Information for All Investigated Reaction Elements abbreviation

packing type

number of packing sheets

W500 W250 M500 M250 C500 C250

Winpak500X Winpak250X Mellapak500X Mellapak250X

2 2 2 2

W mm

L mm

ε

12 23 12 23 12 23

400 400 400 400 400 400

0.548 0.576 0.548 0.576 0.477 0.477

Table 2. Packing Sheet Characteristics Winpak250X L (mm) H (mm) W (mm) a (m2/m3) α (deg)

length height thickness specific surface area inclination angle

Winpak500X

400 100/150/200 11.5 250 60 1769

400 100/150/200 6 500 60

Mellapak250X 400 100/150/200 11.5 250 60

Mellapak500X 400 100/150/200 6 500 60

DOI: 10.1021/acs.iecr.5b02685 Ind. Eng. Chem. Res. 2016, 55, 1768−1777

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Figure 2. Experimental setup: (a) photograph of the setup, (b) schematic diagram of the designed liquid distributor, and (c) schematic diagram of the designed liquid collector.

Distilled water was pumped by a peristaltic pump from a reservoir into the reaction element via the distributor. The liquid was collected by a Perspex collector, which was divided into seven parts equally. A DJC-1C platinum electrode probe was interfaced with a computer by a data line for data acquisition. The probe was located perpendicularly in the collector (see Figure 2) at seven radial positions, to measure the effluent salt concentration (the conductivity is directly proportional to the tracer concentration). A conductivity meter was linked to a data acquisition system to record the liquidphase effluent conductivity each second. Water containing sodium chloride was collected by the liquid collector and removed from the system. At each liquid velocity, the conductivity was measured at seven different lateral positions. The tracer injection is crucial to the experimental accuracy. For each liquid velocity, a pulse signal was introduced to the system only after steady state was reached. Then, the data acquisition system processes were started simultaneously, and conductivity of the liquid-phase was monitored. The tracer was injected at the center of the reaction element upper surface (Figure 3) by a syringe. Moreover, the injection position was just below the distributor (Figure 2b), which should prevent the distributor from the RTD curves. The injection time was less than 0.5 s to ensure that it was negligible compared to the mean residence time. The injected volume was 1 mL for the 500X sheets and 1.9 mL for the 250X sheet. Compared to the liquid flow rate, the injected volume was negligible. The experiment was repeated with different radial positions, reaction element types, heights and liquid velocities. All the experiments were repeated at least three times to ensure that they were reproducible.

3. METHODS 3.1. Liquid Mean Residence Time. The measured tracer response curve c(t) was recorded during the experiments. The exit age distribution E(t) was obtained by normalizing c(t) (eq 1), and was used to determine the RTD curves.28 The mean residence time tm was calculated using eq 2.

Figure 3. RTD curves measured at different positions in each reaction element (W250, L = 5 m3/(m2 h)).

E (t ) =

to thoroughly wet them, they were immersed in distillated water for more than 1 h before the experiments. 1770

c(t ) ∞

∫0 c(t ) dt

(1) DOI: 10.1021/acs.iecr.5b02685 Ind. Eng. Chem. Res. 2016, 55, 1768−1777

Article

Industrial & Engineering Chemistry Research ∞

tm = =

∫0 tE(t ) dt ∞

∫0 E(t ) dt

=

∫0

∞

tE(t ) dt =

elements is anisotropic. The eq 7 defines the standardized velocity û.

VR hd ,RTD QL

û =

AR ·H ·hd , RTD AR ·uOL

(2)

f (u)̂ =

HF 4πt

⎛ y2 ⎞ ⎛ (x − u t )2 ⎞ 1 L ⎟ ⎟ exp⎜ − exp⎜ − 4Dax t ⎠ Dax Dr ⎝ ⎝ 4Dr t ⎠

1 −u2̂ /2 e 2π

(8)

The PDF has two important characteristic parameters: skewness and kurtosis. Skewness represents the symmetry, and the skewness value is the relative length of density function curve tail. The kurtosis value reflects the thickness of the density function curve tail. Thes parameters were calculated using eqs 9 and 10. S=

K=

⟨u3̂ ⟩ ⟨u 2̂ ⟩3/2

(9)

⟨u 4̂ ⟩ ⟨u 2̂ ⟩2

(10)

For the standard Gaussian distribution function (eq 8), S = 0 and K = 3. When S → 0 and K → 3, the velocity field statistics follow an approximately Gaussian distribution.

4. RESULTS AND DISCUSSION 4.1. Dispersion Characteristics. The RTD curves were recorded by a computer each second under different operating conditions. Figure 3 shows the response curves obtained at different positions in each reaction element. As show in Figure 4,

(3)

The liquid interstitial velocity inside the reaction element is related to the liquid spray density by uL = V*/(3600ε). The axial and radial dispersion coefficients (Dax and Dr, respectively) in eq 3 have the same units (e.g., m2/s) as diffusivities, which represent the effective dispersion coefficients during the experiments. Generally, the effective dispersion coefficient comprises the turbulent and diffusional contributions. The RTD curves were used to estimate the two model parameters. Employing the separation of variables method, the 2D dispersion model was solved analytically by Fourier transform and Fourier inversion. The axial and radial dispersion coefficients were determined by fitting the analytical solution to the exit age distribution E(t) (eq 4). E (t ) =

(7)

Then, the standardized Gaussian distribution function can be simplified to

3.2. Liquid Holdup. Equation 2 shows the relationship between the dynamic liquid holdup and mean residence time tm. The dynamic liquid holdup derived from the RTD curves, is denoted by hd,RTD in eq 2 to distinguish between the derived value and experimental data. The variable hd,RTD could be compared to the dynamic liquid holdup hd measured during hydraulic experiments. However, unexpected errors due to the static liquid holdup ht might be observed because ht also contributes to mass transfer in the reaction element. In eq 2, uOL is the superficial velocity. 3.3. Two-Dimensional Dispersion Model. Instead of solving the dispersion model numerically as done in previous studies,7,14,18,26 the dispersion model was solved analytically in this study. The physical arrangement used in this work was best described by a rectangular coordinate system, where x and y denote the axial and radial direction, respectively. A two-dimensional dispersion model that assumed that the flow field was axially symmetric was employed. eq 3 describes material balance for the tracer in the reaction element based on the 2D dispersion model. ∂c ∂ 2c ∂c ∂ 2c = Dax 2 − uL + Dr 2 ∂t ∂x ∂x ∂y

u(t ) − μ σ

(4)

3.4. Probability Distribution Function. In a turbulent, viscous fluid, the velocity field statistics at a fixed point are approximately Gaussian.29 The Gaussian distribution function is one of the most important PDFs (probability distribution functions) in turbulence (eq 5). f (u) =

2 2 1 e−(u(t ) − μ) /2σ σ 2π

(5)

Figure 4. RTD curves measured at the center of each reaction element.

where μ = L/εtm is the mean interstitial liquid velocity, μ(t) = L/εt is the instantaneous interstitial liquid velocity, and σ is the standard deviation, which was calculated using eq 6. σ2 =

∑ (u(t ) − μ)2 E(t )

all the curves have similar shapes with long tails, which indicate the existence of dead zones in the reaction elements. It can be deduced from Figure 4 that the radial dispersion in the Winpak reaction elements is higher than those in the other reaction element. Medium diffusivity is observed in the reaction elements containing Mellapak sheets. The reaction

(6)

A high variance indicates that the velocity distribution is nonuniform and that liquid-phase flow though the reaction 1771


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Figure 5. Characteristic PDF parameters: (a) variance, (b) skewness, and (c) kurtosis.

elements without any sheets inside behave similarly to packedbed structures; the liquid is poorly dispersed in these reaction elements. 4.2. Residence Time Distribution Curves. Figure 4 presents typical RTD curves measured by the pulse response technique. As show in Figure 4, the liquid velocity strongly affects the RTD curves. The RTD curves become taller and narrower as the spray density increases, which indicates that the velocity field curves are also steeper. This result is consistent with the PDF results. The kurtosis values increase with the liquid flux (Figure 5c), except when flooding occurs. According to previous studies,8,11,17 this phenomenon is common and expected. It should also be noted that bimodal RTD curves can be obtained at high liquid velocities. The presence of a liquid film outside the element, which is caused by flooding, is the origin of the first peak of the bimodal curve. The RTD curves of the reaction elements with 250X packing sheets are taller and narrower than those of the reaction elements with 500X sheets. This result can be explained by the fact that the reaction elements with 250X packing sheets have smaller void fractions than those with 500X sheets. As a spray density, the liquid velocity is higher when the void fraction is small. Therefore, the elements with 250X packing sheets have taller and narrower RTD curves due to turbulence. The RTD curves also vary with the system geometry. Although the variations are obvious, more analyses and calculations are needed to explain the results. The curves of the Winpak reaction elements are not as steep as those of the other reaction elements. Figure 6 illustrates the differences between the response curves measured at various radial locations. Responses are only observed at the test points far from the center of the elements (e.g., P2 and P6) when the liquid velocity is low. The curves recorded at symmetric test positions (e.g., P3 and P5) are reasonably symmetric about the center axis. These experimental data are consistent with the fluid particle paths generated by the Langevin equation.30 Along the radial direction, the RTD curves have different patterns. The response to the input pulse appears at the center (P4) before than other positions. 4.3. Mean Residence Time. Because the RTD affects both the reaction and separation performances, it is one of the most important packing properties. The tm values were derived for all the tested operating conditions using eq 2. As shown in Figure 7, tm decreases with increasing liquid flow rate for all the reaction elements. Moreover, the tm values of the elements with 250X packing sheets are smaller than those of the elements with 500X sheets. Figure 6 shows the significant increase in the variances of the velocity fields, which is

Figure 6. RTD curves for W500 measured at different positions.

Figure 7. Mean residence time of each reaction element.

consistent with the explanation in section 4.2. A smaller void fraction leads to a higher liquid velocity, which greatly enhances the turbulence. Therefore, the liquid flows through the reaction elements faster. The effect of the geometry is complex: (i) the use 1772


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Industrial & Engineering Chemistry Research of packing sheets complicates the reaction element constructions, leading to tortuous flow channels. Tortuous channels, in turn, lead to higher interstitial liquid velocity; hence, the tm values of the Winpak elements are higher than the other elements. (ii) Making holes or fabricating diversion windows on the packing sheets can change the flow pattern and enhance the renewal of the liquid film at the surface to increase the liquidphase residence time. Because these phenomena are too complex to study exhaustively, precise quantitative descriptions of these process cannot be obtained. It is clear, however, that the Winpak reaction elements have higher tm values, which are beneficial for the catalytic reaction, than the other elements. The values of tm along the radial direction of the reaction elements are compared in Figure 8. The tm values are smaller at

Figure 8. Mean residence times at different radial positions (W250).

the center test points than at the other test points. Moreover, the results for all the elements have acceptable symmetry considering the generally high errors in the second moment experiment.9 Therefore, the assumption of axis symmetric used in the previous analysis is reasonable. Figure 9 shows tm at all the test points in the Winpak-250X reaction element. 4.4. Liquid Holdup. In MCSPs both dynamic liquid holdup hd and static liquid holdup ht contribute to mass transfer. The dynamic liquid holdups calculated from the RTD curves (hd,RTD) were compared to the total holdups ht (ht = hd + ht) published in the literature. However, it should be noted that, only the holdups of the MCSP reaction elements were determined in this study. For example, in the Winpak-C-500X structure, the separation element consists of two Winpak-500X packing sheets, and the reaction element also contains two Winpak-500X packing sheets. The holdup of the Winpak-C500X reaction elements could be concluded using eq 11, which involves the dynamic liquid holdup of the packing sheets hd,S in the hydraulic experiments and hd,RTD derived from the RTD curves. hd,RTD = 0.5hd,S + 0.5(hd,E + hs,E) = 0.5(hd,S + ht,E) (11)

Figure 9. Mean residence times at different positions (L = 5 m3/(m2 h)): (a) W500, (b) M500, and (c) C500.

A comparison of the different liquid holdup types in the reaction elements is presented in Figure 10. The results of this comparison are reasonable consistent with the experimental data obtained at a low liquid spray density. When L = 20 m3/(m2 h),

the holdup is much higher for W500 than for Winpak-C-500X and is even higher than the void fraction (ε = 0.576). Flooding, 1773


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4.5. Axial and Radial Dispersion Coefficients. Catalytic reactions occur in the liquid phase, and ensuring that the reactants are uniformly mixed in the reaction element is important. Therefore, for catalytic distillation column design and calculations, a detailed knowledge of the axial and radial dispersion coefficients in the reaction element is essential. Figure 11 compares the dispersion coefficients obtained by solving eq 4 for different operating conditions. An optimization routine in Origin was employed to obtain the best-fit axial and radial dispersion coefficients. The axial dispersion and radial dispersion exhibit nearly opposite trends with increasing liquid velocity. The axial dispersion coefficients generally increase with increasing liquid spray density, whereas the Dr values decrease (see Figure 11). Convective mass transfer, which is markedly affected by the flow pattern, dominates the axial transfer process. Increasing the liquid spray density leads to strong turbulence, which enhances mass transfer and subsequently results in an increase in the axial liquid dispersion. Moreover, at a high-liquid velocity, liquid flow occurs outside the reaction elements due to flooding, further enhancing axial dispersion. In contrast, increasing uL results in a shorter residence time for the tracer particles, which negatively impacts the radial dispersion. The tracer particles flow along the reaction element too quickly to allow significant radial dispersion to occur (see Figures 3 and 6). Therefore, the Dr values decrease with increasing uL. Figure 12 shows the effect of the reaction element height on the dispersion coefficients and reveals the dispersion characteristics of the different elements. At the given liquid spray density, the axial dispersion coefficient increases as the element heights increases. A long axial distance allows for a long residence time, which enhances the axial dispersion and partly explains the higher axial liquid dispersion coefficients. Similarly, the radial dispersion coefficients increase with increasing reaction element height. As mentioned previously, the mass transport of the tracer is limited by the shorter residence time. Therefore, lower radial dispersion coefficients are obtained when the reaction element heights are smaller. Figure 11 shows that the packing sheets enhance the liquid dispersion characteristics in the reaction elements. The liquid channels in the packing structure are more tortuous, and the elements have a higher void fraction, aiding in the liquid dispersion. It should be noted that the Winpak packing sheets improve the liquid-phase mixing in the reaction elements as

Figure 10. Comparison of different types of liquid holdup: (a) liquid holdup in Winpak-C-500X and (b) comparison of the holdup in different reaction elements.

which enhances the liquid backmixing and results in the liquid film flow outside the element, occurs in the reaction element when the liquid spray density is 20 m3/(m2 h). However, the device with Winpak-C-500X does not flooding, even at 20 m3/(m2 h). Differences in the flows cause this unexpected result.

Figure 11. (a) Radial and (b) axial dispersion coefficients at different liquid spray densities. 1774


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Figure 12. (a) Axial and (b) radial dispersion coefficients for reaction elements with different heights.

Figure 13. Ratio of the radial and axial dispersion coefficients.

Figure 14. Radial dispersion coefficient ratio for different reaction elements.

demonstrated by the fact that both the axial and radial dispersion coefficients are higher in the Winpak elements than in the other elements. This result shows that the liquid dispersion performance of the Winpak structures is superior to that of the Mellapak structures. The diversion windows on Winpak, significantly accelerate the renewal of liquid films and direct liquid flow along the packing sheets to enhance the liquid mixing. The lower variances and skewness values (Figure 6) of the liquid velocity PDFs for the Winpak reaction elements show that these distributions are nearly Gaussian, indicating that the turbulence in the Winpak structure tends to be isotropic. This isotropic behavior results in a nearly normal distribution. Furthermore, the liquid flowing through the Winpak reaction elements has a longer residence time, which also contributes to the liquid mixing.

The Dr values in the investigated systems are approximately 1−2 orders of magnitude higher than the axial dispersion coefficients, and the difference between these types of dispersion coefficients as the liquid flow increases (Figure 13). The radial dispersion in the Winpak reaction elements is approximately a factor of 2 higher than those in the other reaction elements, as shown in Figure 14. The Dr values are in the range of 10−3−10−2 (m2 s−1), which is significantly higher than the range of values for packed bed reactor reported in the literature. The Dax values in this study are on the same order of magnitude as those reported by van Baten et al.,15 but they are significantly lower than those reported by Kołodziej et al.8 for Katapak-S and those reported by Hoffmann6 for Multipak. The differences in the results of these studies are due to differences 1775


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Industrial & Engineering Chemistry Research t u uL uOL û ⟨u⟩̂ W α ε σ σ2

in the experimental setup; higher axial dispersion coefficients were obtained when entire column or packed bed reactors were used, whereas the lower values in this study were obtained when only reaction elements were employed. Because of the high level of liquid dispersion in the Winpak reaction elements, good reactant mixing and a uniform heat distribution can be quickly achieved in these systems. Then, the heat and reactant distributions are more uniform, which helps prevent the formation of hot spots.31 These properties are desirable for chemical reactors.

5. CONCLUSIONS The axial and radial dispersion of the liquid phase in reaction elements with various MCSPs was investigated experimentally using the pulse response technique. A two-dimensional dispersion model was developed assuming that the flow field exhibited axial symmetry of the flow field. The RTD curves obtained from the experiments were used to estimate Dax and Dr by analytically solving the 2D dispersion model. The holdup in the reaction elements was also determined from the RTD curves and compared to other experimental results. The PDF was determined to study the fluid velocity. The Winpak reaction elements exhibit superior dispersion characteristics and have longer residence times. The dispersion coefficients for these reaction elements are approximately a factor of 2 higher than those for the other studied reaction elements. This work shows that the geometric configuration, liquid spray density, and void fraction of the elements affect the liquid axial and radial dispersion coefficients significantly. Additional work is needed to develop a CFD model of the system and to gain insight into the axial and radial dispersion mechanisms.

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[s] time [m s−1] instantaneous interstitial liquid velocity [m s−1] interstitial liquid velocity [m s−1] superficial liquid velocity [m s−1] standardized velocity [m s−1] expectation of û [m] thickness of the reaction element [°] inclination angle of the packing sheets [-] void fraction [m s−1] standard deviation [m2·s−2] variance

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-J.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge financial support by the National Basic Research Program of China (973 Program No.2012CB720500).

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SYMBOLS a [m2 m−3] specific surface area AR [m2] cross-sectional area of the reaction element c(t) [mol m−3] tracer concentration Dax [m2 s−1] axial dispersion coefficient Dr [m2 s−1] radial dispersion coefficient E(t) [s−1] exit age distribution F [m] length of the reaction element H [m] height of the reaction element hd,R [-] dynamic liquid holdup of the reaction element hs,R [-] static liquid holdup of the reaction element ht,R [-] total liquid holdup of the reaction element hd,S [-] dynamic liquid holdup of the separation element hd,RTD [-] dynamic liquid holdup derived from the RTD L [m3 m−2 h−1] liquid spray density K [-] kurtosis QL [m3 s−1] liquid flux S [-] skewness tm [s] mean residence time 1776


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Experimental Investigation of Liquid Axial and Radial Dispersion in

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