1. Introduction
Radial flow reactors have been widely used in many vapor-phase catalytic processes. Since the rapid development in the field of air separation and gas–solid catalytic reaction, there has been an upward trend toward the widespread use of radial flow reactors or adsorbers [
1]. The primary benefit of radial flow reactors is that they have a lower bed pressure drop, lower energy consumption and smaller footprint compared with conventional axial flow reactors [
1]. The annular channel distributing header is the major internal part that provides the radial flow pattern inside the reactors, which consist of a perforated wall located between the reactor wall and the catalyst/adsorbent bed. The annular space between the annular channel and the central channel is packed with catalysts or adsorbents.
The flow direction of the radial flow reactor is generally as follows. The gas flows into the annular channel through the inlet, then passes through the packed bed radially from the annular flow channel, enters the central pipe, and is finally discharged from the outlet [
2].
Under the trend of super large-scale development of gas processing equipment, the demand for large-scale radial flow reactors is gradually increasing. The larger intake of air demand means the need to increase the height and adjust the diameter of the reactor. However, blindly increasing the height of the reactor increases the ratio of the height of the adsorbent bed to the equivalent diameter of the annular flow channel (height to diameter ratio), which could cause highly complex pressure changes along the height, which could result in flow non-uniformity through the bed [
3,
4]. Non-uniform flow could lead to a serious impact on the super large equipment, such as the gas short circuit, which causes earlier gas penetration. This may result in a significant drop in the utilization efficiency of the adsorbent, a more frequent replacement of the adsorbent, increased operating costs and reduced purification efficacy.
In view of the maldistribution of radial velocity in the reactor, researchers have applied computational fluid dynamics (CFD) methodologies to simulate and analyze gas flow characteristics inside the reactor. Prior work by Hlavacek and Kubicek [
5], Calo [
6], and Balakotaiah and Luss [
7] demonstrated that different flow directions in the radial flow reactor could affect the efficiency of the reaction conversion. Ponzi and Kaye [
8] conducted analytical studies on uniform radial flow distribution, addressing the effects on RFBR performance.
Chu et al. [
9] simulated and analyzed the variation of the inside radial flow reactor flow distribution with different inlet air velocity by using CFD software and it is concluded that the greater the inlet speed, the working time of the reactor will be reduced, the switching frequency will be faster, and the operation will be inconvenient (but there are different optimal inlet speeds for reactors of different sizes).
Miao and Pang [
10] draw the flow distribution traces line and come to the conclusion with the consideration of radial flow reactor should not only need to avoid the formation of a dead zone at the top of the adsorbent area but also needs to consider the uniformity when gas passes through the catalyst bed, the better uniformity it has the greater usage efficiency that the catalyst bed could have.
Li, Zhou and Lin [
11] conducted a simulation using the CP-z type molecular sieve adsorber model. The results show that although the adsorber has a more uniform pressure drop overall, the pressure drop at the bottom part is relatively small, and the middle part has the highest pressure drop. The velocity variety at the bottom of the adsorber is larger than that at the top, and its speed deviation value is 13–19%. In addition, the velocity change at the top and bottom of the molecular sieve layer is smaller than that in the middle, and the velocity deviation value is 17–26%.
To investigate the axial flow distribution, Lobanov and Skipin [
12] have operated an RFBR in three commercial reforming units and all the reactors are CP-z types. The test result shows that the uniform flow distribution in the reactor only occurs at the lower part of the catalyst bed; this phenomenon indicates that most of the upper catalyst bed is not fully utilized.
In the simulation results based on a CP-z type radial flow reactor, Chen et al. [
13] found that when the diameter of the reactor is constant, as the height of the reactor increases, the greater the deviation of the gas velocity distribution curve from the ideal curve, the more uneven the gas distribution along the axis of the catalyst fixed bed. Moreover, the research team also indicated that if the ratio of the radius of the annular channel to the radius of the center pipe is too small, this will cause a significant increase in the flow rate at the upper part of the catalyst bed and could lead to gas penetrating faster than the lower part.
In research on the fluid distribution design of the radial reactor, Zhang et al. [
14,
15] proposed four calculation models: the momentum exchange model, the friction control model, the momentum exchange term dominance model and the friction term dominate model. These four models are used to calculate and design the flow inside the reactor. Furthermore, suitable design dimensions for fluid distribution forms and flow channels under various flow models are obtained.
Based on four calculation models, Zhu, Zhang and Xu [
16] further studied the cross-sectional ratio relationship between the annular flow channel and the center pipe. In the 1990s, a mathematical model that can reflect the fluid flow characteristics of the radial flow reactor was established by Xu [
17] based on Zhang’s research [
14,
15] and the article shows that for the π-flow type radial reactor, the centripetal flow type can have more uniform gas distribution than the centrifugal flow under the same conditions.
By studying four different types of radial flow reactors, Kareeri [
18] found that the cross-sectional ratio relationship between the annular flow channel and the center pipe has a greater impact on the uniformity of the flow distribution. Moreover, Wang, Liu and Meng [
19] modeled the laboratory small radial reactor flow experimental device and conducted numerical simulation research through the 3D modeling method. By simulating the different influences, including the flow form of the gas flow, the cross-sectional area ratio of the outer flow to the center pipe, and the opening rate of the center pipe, the results indicate that the uniformity of centripetal flow is better than centrifugal flow. Furthermore, the smaller the opening rate of the center pipe, the better the air distribution effect in the radial reactor.
In recent studies, Li et al. [
20] used CFD software to implement numerical simulation to check the uniformity of the gas flow inside the CP-z type RFBF. Li first studied the relationship between the meridional pressure drop of the bed and the ratio of the cross-sectional area of the central tube to the cross-sectional area of the annular channel. By monitoring the variety of pressure drops inside the RFBR in the next step, three improved forms of gas flow distribution (including lower distributor, upper distributor, and distributor added to the upper and lower sides) are proposed. The lower distributor is a solid conical frustum, and the upper distributor is a cylindrical tube. The distributor further improves the gas flow distribution inside tFhe RFBR. Finally, Li concluded that under this RFBR model, the diameter of the cross-sectional area of the central pipe and annular channel was 0.27 m and 0.84 m, respectively; the adsorber achieved the best performance. Moreover, the pressure drop of the bed has the best uniformity when the lower distributor is a conical frustum with a length of 0.37 m and its top and bottom diameters are 0.2 m and 0.26 m, respectively; the upper distributor is a cylindrical tube with a length of 0.54 m and a diameter of 0.2 m.
On the basis of Li’s research, Chen et al. [
21] continued to study the airflow distribution of the CP-z type RFBR in the process of carbon dioxide adsorption and desorption by observing the velocity changes in the contour and came to the conclusion that adding conical frustum and cylindrical tubes can improve the uniformity of adsorption and desorption processes to 97.13% and 90.07%, respectively.
Celik [
22] claimed that it can reduce the maldistribution inside the radial reactor by equally dividing the distributor into three sections and in each section, the perforated annular distributor should have a different number of holes to achieve better uniform gas distribution. In the article, Celik believes that the opening rate should be divided into sections, as shown in
Figure 1 The first section is 1–10%, the second section is 10–25%, and the third section is 25–50%. The gas distribution in the radial reactor can be better when the opening rate is opened in the above-mentioned style.
Thus, based on the research of the above-mentioned scholars in related fields, the application of the computational fluid (CFD) method to analyze the gas flow in the radial flow reactor can more clearly demonstrate the form of gas flow distribution inside the reactor.
Previous research results indicate that traffic distribution has a greater impact on the operation of RFBR. In the above-mentioned literature on the research of RFBR fluidity, there are few studies on the influence of the opening rate of the wall distributor on airflow distribution, and there are also fewer studies on the opening rate in large-scale industrial equipment. This article will adopt three-dimensional CFD modeling on a large-scale industrial radial flow denitrification tower. The simulation model was established by changing the area of the annular flow channel, the opening strategy of the center distributor, and the opening strategy of the annular distributor, which were studied to optimize the structure of the radial flow reactor.
Figure 2 shows a typical flow distribution pattern in a radial flow reactor. Only when the gas mass flow is equally distributed along the axial height of the packed bed can a more uniform radial flow distribution be obtained. The radial gas flow distribution on the axial height of the catalyst bed in the radial flow reactor determines the operating efficiency of the reactor. When the gas mass flow is equally divided along the axial height of the catalyst bed, a relatively uniform radial flow distribution can be obtained. On the contrary, if the gas mass flow along the fixed catalyst bed is not relatively equal, this could cause some part of the catalyst bed to flow more gas and the other part to flow less gas, which will lead to a decrease in the utilization rate of part of the bed [
12].
As Kareeri, Zughbi and Al-Ali [
18] mentioned in their article, for a CP-z or a CP-π type radial flow reactor, when the non-uniform flow distribution occurs, the possibility of forming a cavity between the annular channel and the catalyst bed at the bottom or top of the catalyst bed increases, respectively, as shown in
Figure 2b. Thus, the CF-z or CF-π configuration of the reactor could also have the same problem shown in
Figure 2e,f; the only difference is that the cavity is located between the center pipe and the catalyst bed. The CP type and the CF type configuration shown in
Figure 2a,d indicate that having a radial pressure independent of the axial coordinate is an important criterion in the design of a radial flow reactor because this criterion allows the gas flow to be equally distributed as it passes through the catalyst bed. Therefore, the pressure distribution in the reactor determines the optimum utilization of the catalyst.
4. Conclusions
Fluid flow in a “Z” type centripetal flow radial flow fixed bed reactor (CP-z-RFBR) is simulated using computational fluid dynamics. The variation of several parameters including changing center pipe perforated plate porosity, annular channel perforated plate porosity and the width of the annular channel have been introduced to investigate the form of gas distribution inside the reactor. The porosity strategies are based on Celik’s research and the aim of this article is to further investigate the influence of Celik’s three-section porosity strategy in actual reactor parameters. Through the simulation results, this research reduced the range of opening ratio of each part of the perforated plate when using the three-section strategy. For the center pipe perforated plate, the porosity should be 10%, 16% and 29% from top to bottom. For annular channel perforated plates, the porosity should be 10–12%, 21–25% and 30–40% from top to bottom.
CFD calculation results indicate that the change in center pipe perforated plate porosity has a more obvious impact on the gas distribution in the reactor. Compared with the change in the center pipe perforated plate porosity, the change in the circular channel perforated plate porosity has a relatively lower effect on the gas distribution in the reactor. For the annular channel width, the simulation result shows that the larger the width of the reactor, the better its performance. This result also verifies the conclusion drawn by Kareeri [
18] that when the ratio of the cross-sectional area of the central tube to the cross-sectional area of the annular channel is greater than 1, the CP-Z-RFBR has a more uniform gas flow distribution. However, the cross-sectional area of the central tube and the cross-sectional area of the annular channel should not be too far apart. In this paper, the gas distribution inside the reactor becomes less uniform after reducing the width of the annular channel. The non-uniformity result also shows the trend that reducing annular channel width leads to lower uniformity. The reason for this trend could be that excessive reduction of the annulus channel cross-sectional area results in a high velocity of gas flow into the reactor, which may increase the difficulty of perforated plates regulating gas flow uniformity.
In the design stage of CP-Z-RFBR, when the height and diameter of the reactor cannot be changed, the optimal configuration cannot be achieved through the ratio of the cross-sectional area of the central pipe to the cross-sectional area of the annular channel. At that moment, the porosity of the center pipe perforated plate (the three-section porosity strategy described in the article) should first be changed to improve the uniformity of the flow and reduce maldistribution. Second, by changing the porosity of the circular channel perforated plate to further optimize the uniformity of the gas flow in the reactor, the optimal solution can be obtained as much as possible.
Due to the large volume of the reactor used in this paper, the height of 8 m and the width of 2.7 m made the field experiment more difficult. Most of the research on radial flow reactors, for experimental research, uses reactor sizes that basically stay on small laboratory reactors, and the height of the reactor basically does not exceed 2 m. Experiments in small reactors can indeed reflect the gas flow patterns inside to some extent, but few articles have studied gas flow inside using large-scale or even super-large reactors. Similar studies, such as Kareeri and Zughbi’s simulations, use a model with a height of 2 m and a width of 0.5 m and he does not cover the experimental research. Therefore, this paper uses CFD simulation as the main form to explore the structure of the internal flow distribution of the super-large scale radial flow reactor to obtain the best flow distribution and to reduce the mal-distribution problem. At the same time, provide more references for subsequent research.