CFD Simulation and Optimization of Winpak-based Modular Catalytic

Article pubs.acs.org/IECR

CFD Simulation and Optimization of Winpak-based Modular Catalytic Structured Packing Huidian Ding, Jinming Li, Wenyu Xiang, and Chunjiang Liu* School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China ABSTRACT: A recently designed packing sheet Winpak was assembled with catalysts in a modular catalytic structured packing (MCSP) column. A combined mesoscale−microscale methodology was used to analyze and determine the pressure drop mechanism in the MCSP. The proposed methodology differentiates the pressure drop into six different principles, which was determined and compared using computational fluid dynamics (CFD). The three major causes are gas flow confluence and diffluence through the packing windows, the sudden change in the effective gas channel area and the wall effect. Inspired by constructal theory, the height of packing sheets loaded with catalysts was reduced, which resulted in a less abrupt gas channel contraction and expansion at the packing layer junction and more favorable for the undisturbed gas flow. The optimized overall pressure drop was reconstructed in Fluent and was validated by both dry and irrigated packing experiments. Furthermore, irrigated packing experiments were also conducted. Result comparisons reveal that the proposed optimization method is reliable and accurate. This methodology is shown to be significant in the optimization of Winpak-based MCSP.

1. INTRODUCTION Process intensification is a critical method for achieving energy conservation and emissions reduction. Resource consumption and pollution can be reduced and high efficiency, sustainability and flexibility can be achieved. The combination of different unit operations is a logical method to approach process intensification. For instance, a distillation column can be integrated with a reactor, which is known as a catalytic distillation column. Catalytic distillation packing plays a significant role in a catalytic distillation column. Its structure, amount and installation method affect the pressure drop, liquid holdup and mass transfer. The most frequently used packing type is modular catalytic structured packing (MCSP), such as bale packing,1,2 Katapak3,4 and Multipak.5,6 In general, catalytic distillation processes that produce esters,7 acetal8 and gasoline additive9 use one or more prereactors before the catalytic distillation column, which ensures that the conversion rate of reactants in the prereactor(s) is at least 80%. Therefore, because of the good selectivity of the catalyst (e.g., MTBE,10 TAME,11 ethyl acetate12), the catalytic distillation column is limited by the flux capacity rather than by the reaction. The primary structure of an MCSP is built from conventional corrugated sheets or sheets with flutes, embossments, perforations, grooves, lances, etc. In this case, the flux capacity of an MCSP is dependent on the geometry and arrangement of the corrugated sheet: the lower the packing layer pressure drop, the larger the column flux capacity. Researchers have analyzed the effects of packing on the pressure drop through the column. From the perspective of packing performance under specific operating conditions, Olujić13 proposed the Delft overall performance model, which predicted the hydraulic and separation performance of corrugated sheets. The pressure drop of the packing layer consisted of three independent contributions: gas−liquid interactions, gas−gas interactions, and the direction change loss of adjacent packing elements and the near-wall region. © XXXX American Chemical Society

Other studies have focused on the packing structure and its optimization. The geometries of common commercial packing (e.g., Mellapak, Montz-pak) are repetitive and periodic. They are easily decomposed into representative elementary units (REUs), which makes analysis of the mechanism of the pressure drop simpler. Petre et al.14 developed a combined mesoscale− microscale predictive approach and identified packing dissipation mechanisms using REUs. These mechanisms included pressure head losses at the bed entrance and exit and at the column wall, element direction changes and criss-crossing junctions. Each REU was simulated using three-dimensional (3D) computational fluid dynamics (CFD) and the results were validated by experiments using five packing types. To enhance the column processing capacity, sharp bends between neighboring packing elements were replaced with moderate ones.15,16 Although the modification did not achieve the expected improvement, the increased flux was considerable for industrial applications (i.e., at least 10%). In addition, various inclination angles and channel heights,17 gas fluid types and appropriate turbulence models18 were also investigated. All optimization procedures can be ascribed to constructal law, which is a critical theory that predicts the shapes and structures of processes and designs. Developed by Bejan, constructal theory is a promising tool for the design of fluid engineering systems, such as reactors,19 heat exchangers20 and fuel cells.21 Nevertheless, an optimized structure still must be designed to achieve maximum efficiency. We propose to determine first some suitable transport phenomenon for catalytic packing. Then, by analyzing the proposed mechanism with CFD modeling and experiments, we can change and optimize the packing column and its internal Received: October 10, 2014 Revised: January 10, 2015 Accepted: February 6, 2015

A

DOI: 10.1021/ie503998v Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Geometry and dimensions of Winpak.

Table 1. Details of the Packing l

m

n

l′

m′

n′

2B

2h′

δ

α

β

ε

25 mm

23 mm

13 mm

8 mm

5 mm

4 mm

10 mm

12.5 mm

0.2 mm

60°

90°

0.837

dimensions and measurements of Winpak are presented in Table 1. The presented modular catalytic structured packing layer (Figure 2) is constructed using several tightly interlaced Winpak sheets with and without catalysts. Each layer is rotated to a certain degree to improve gas−liquid flow conditions. 2.2. Problem Specification. In a real catalytic distillation scenario, resistance in a catalyst can be high enough to prevent gas through. Thus, gas can only flow in the packing and the catalyst part can be treated as a wall in the CFD simulation.24 The dry pressure drop is the benchmark of irrigated pressure drop. The F factor (defined as F = ρ0.5Vspf) related pressure drop curves see similar tendencies in numerous works.3,25,26 In this case, it is reasonable to predict that the same optimization technique can be adopted in the irrigated scenarios as well as the dry ones. Experimental results will verify this later. Despite the rapid development of high-performance processors in recent years, the computational power required to model an entire MCSP is not readily available. Although the thickness of each sheet is only 0.2 mm, the packing layer is hundreds of millimeters in length, width and height. As a result, the total number of grid elements approaches a quarter of a billion for a single sheet in an already tedious and error-prone modeling procedure. A Winpak-based MCSP has similar and repetitive structures. To significantly reduce the modeling complexity and the overall number of calculations, a few individual unit representatives of

structure to promote positive factors and eliminate negative factors. This approach is not new in industrial applications. The latest generation of structured packing material to feature optimized fluid flow is Mellapak and its enhanced version Mellapak Plus.22 In this packing, the corrugation angle gradually changes to 90° at the packing element junction such that vapors flow more smoothly between two packing elements. Thus, a high mass transfer is maintained with a reduction in the pressure drop. In this work, we examine the pressure drop of an MCSP, which is designed with a new structured packing registered as Winpak. Here, we investigate the complex MCSP structure and how its geometry affects the flux capacity. Additionally, we identify fluid flow issues and address these limitations using CFD modeling and experiments.

2. METHODS 2.1. Packing Used. The geometry and dimensions of Winpak are shown in Figure 1. There are several flow-guiding cut-out windows on the crests and troughs of the packing sheet. When liquid flows through diversion windows, the liquid on the crests are guided to the troughs and vice versa. Liquid is evenly distributed in a thin-film layer to effectively facilitate gas−liquid mass transfer. The liquid holdup and mass transfer of Winpak is 10%−20% greater than that of the common corrugated sheet.23 Consequently, the utilization of Winpak in a catalytic distillation would benefit the both reaction and the separation. The B


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Figure 2. Principles of Winpak-based modular catalytic structured packing.

suddenly expands when the gas flows out of the packing layer (Principles 1 and 6). As evident in Figures 1 and 2, two packing sheets are arranged in a reversed orientation and at a 60° inclined angle. With this geometry, gas flows in two directions and converges and diverges through the diversion windows (Principle 2). At the packing layer junction, gas flow is more complex. Most industrial applications rotate the neighboring packing elements to ameliorate the flow pattern. As stated before, gas only flows through packings (distillation element27) instead of the catalyst (reaction element). Figure 3 illustrates the proportion of distillation element at the junction. Different packing diameters (DN 100 and 400 mm) and different specific surface areas (500 and 250 m2/m3) are presented. In most rotated cases, the area of distillation element is approximately 25% of the column cross section. For a rotation of 90° (Figure 4), when gas flows into the first packing, the distillation element is effectively halved because of the reaction element (red). When gas flows to the junction, half of the remaining channels are blocked by the reaction element of the second packing (blue), which indicates that if the

Winpak layer can be used to predict the entire MCSP behavior. Flow principles can be used to identify areas within the MCSP that significantly affect the pressure drop; focusing on these few key points readily permits analysis of the resistance mechanisms responsible for the pressure drop (see Figure 2). Principles: 1. Head loss at the entrance of the layer; 2. Head loss due to gas flow confluence and diffluence through windows; 3. Head loss of gas flows into the packing layer junction; 4. Head loss of gas flows out of the packing layer junction; 5. Head loss due to wall effects; 6. Head loss at the exit of the layer. Approximately half of the packings are filled with tightly packed catalyst particles. The resistance encountered in a packed catalyst bed is remarkably greater than that of the packings layers alone (i.e., without catalysts). Initially, gas occupies all sections of the column after being injected from the distributor. When gas flows into a packing layer, the gas flowstream effectively moves through a sudden contraction. Similarly, the gas flowstream C


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Gas flow near the wall is hard to be rigorously simulated. Each grid element is dependent on adjacent elements; determining appropriate boundary conditions for near-wall gas flow modeling is complicated. Reasonable hypotheses have been proposed14 to overcome this issue. Here, for conventional structured packing, the head loss coefficient at the wall was assumed to be equivalent to the head loss at a packing junction (Principle 5). 2.3. Pressure Drop Calculation Strategy. The superficial gas velocity Vspf is the nominal gas velocity of an empty column. The gas velocity in the packing, i.e., the gas interstitial velocity Vint is

Vint =

Vspf (1)

εsin α

The packing pressure drop ΔP is determined by the gas interstitial velocity Vint, gas density ρ and packing layer propertylike corrugation angle α, hydrodynamic diameter of distillation element dh, packing total height H, packing element height h, unit height l and so on:

Figure 3. Distillation element of different diameter (100 and 400 mm) and specific surface area (500 and 250 m2/m3).

ΔP = ζ

column superficial F factor is 1 (i.e., a moderate value), the local F factor at the junction can be as high as 4 (which indicates column flooding in most scenarios). When gas flows from the junction to the second packing, the gas channel expands to the entire distillation element (Principles 3 and 4). The resultant sudden change in the gas velocity at the packing layer junction is a significant factor in increasing the overall pressure drop.

2 ρVint Ω(α , dh , H , h , l , ...) 2

(2)

According to Bernoulli’s equation: V2 P + gz + = constant 2 ρ

(3)

The mechanical energy balance and momentum balance equation near the layer entrance can be listed as

Figure 4. Distillation element and reaction element of 90° rotation. D


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Figure 5. Boundary conditions of Principle 2.

Ven 2 + ρWe 2 Vlayer 2 + Player + ρ + ρ ∑ hf 2

ΔP2 = ζ2

ρgzen + Pen + ρ = ρgz layer

∑ hf =ζ1

Ven 2 2

ζ3,4 =

PenAen − PlayerAlayer + Pavg(Alayer − Aen ) (6)

(7)

ζ5 =

2

⎛ Alayer ⎞ ρVint 2 (εsin α)2 ⎜1 − ⎟ 4 Aen ⎠ ⎝

2 ⎛ Alayer ⎞ ρVint 2 2 ΔP6 = (εsin α) ⎜1 − ⎟ 2 Aex ⎠ ⎝

(8)

2ΔPA dh ρVint 2 l

ρVint 2 ⎛ H ⎞ ⎜ − 1⎟ ⎠ 2 ⎝h ⎞ ⎛H = (ΔPB + ΔPC)⎜ − 1⎟ ⎠ ⎝h

(14)

Ψwall ζ3,4 2

ΔP5 = ζ5

(9)

Ψwall =

(15)

ρVint 2 H ΔPB + ΔPC H Ψwall = Ψwall 2 h 2 h

where Ψwall is defined as

(16)

28

⎛ h ⎞ 2h h2 2 + arcsin⎜ dC 2 − ⎟ 2 2 π πdC tan α tan α ⎝ dc tan α ⎠

(17)

The total loss coefficient can be summarized as

(10)

⎛ ΔP + ΔP ⎞ ⎛ Alayer ⎞ 2ΔPA dh ζtot = 0.5⎜1 − + (2 + Ψwall)⎜ B 2 C ⎟ ⎟+ 2 Aen ⎠ ρVint l ⎝ ⎝ ρVint ⎠

The pressure drop loss coefficient caused by gas flow confluence and diffluence through diversion windows ζ2 is (Figure 5)

ζ2 =

(13)

Petre considered the wall-effect loss coefficient to be equivalent to that of a direction change. The pressure drop was similarly calculated, but included a correction factor Ψwall. In this work, as previously discussed, the gas channels at the junction are effectively reduced by a factor of 2. Therefore, the loss coefficient and corresponding pressure drop at the wall are expressed as

Thus, the pressure drops at the layer entrance and exit are ΔP1 =

ρVint 2

14

Similarly, the loss coefficient at the exit of the layer ζ6 can be calculated as ⎛ Alayer ⎞ ζ6 = ⎜1 − ⎟ Aex ⎠ ⎝

2(ΔPB + ΔPC)

ΔP3 + ΔP4 = ζ3,4

The gas is assumed to be an incompressible fluid. Additionally, no energy is introduced into the system, i.e., We = 0. Assuming negligible effects of friction and gravity, the loss coefficient at the entrance of the layer, ζ1, can be calculated as

⎛ Alayer ⎞ ζ1 = 0.5⎜1 − ⎟ Aen ⎠ ⎝

(12)

The sudden change in the gas channel area (Figure 6) results in a sudden velocity change and head loss. Therefore

(4)

(5)

= ρVlayer 2Alayer − ρVen 2Aen

ρVint 2 H ΔPAH = 2 dh sin α l

2 ⎛ Alayer ⎞ + ⎜1 − ⎟ Aex ⎠ ⎝

(11)

(18)

The pressure drop per meter of the packing layer is E


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Figure 6. Boundary conditions of Principles 3 and 4. 2 ⎡⎛ ⎛ Alayer ⎞ Alayer ⎞ ⎤ ⎛ ΔP ⎞ ρVint 2 ⎜ ⎟ (εsin α)2 ⎢⎜1 − = ⎟ + 2⎜1 − ⎟⎥ ⎝ H ⎠layer ⎢⎣⎝ 4H Aen ⎠ Aex ⎠ ⎥⎦ ⎝

+

⎤ (ΔPB + ΔPC) ⎡⎛ Ψwall ⎞ ΔPA ⎟N − 1⎥ + ⎢⎜1 + ⎣⎝ ⎦ 2 ⎠ l H

accuracy, we compared various viscous model in Figure 7. In the complex geometry of Winpak, the laminar model is less accurate than the turbulent models even at the low Reynolds number region, while the RNG k-ε model provides the highest accuracy (

CFD Simulation and Optimization of Winpak-based Modular Catalytic

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