Influence of hollow structure of honeycomb catalysts on the pressure drop in packed bed reactors

doi:10.11949/0438-1157.20211431

Abstract

Abstract:

The pressure drop is an important indicator to measure the pros and cons of the fixed bed reactor, which directly affects the reaction performance and comprehensive energy consumption. The shape and size of the catalyst particles are the key factors affecting the pressure drop of the fixed bed reactor. In this work, the effect of hollow structure of honeycomb catalyst pellets (one type of pellets commonly used in the industry) on the pressure drop in packed bed reactors is investigated by using particle-resolved computational fluid dynamics (PRCFD). Firstly, the PRCFD model built in this work is validated by comparing with the voidages and pressure drops obtained from experiments. The deviation between the pressure drops calculated by PRCFD model and obtained from experiments is less than 5%, proving the rationality and accuracy of the PRCFD model. Then, the effect of pore number is investigated. The results show that pore number only very slightly affects the voidage but significantly affects the pressure drop under the same volumes of pore and catalyst. With the increase of pore number, the pressure drop goes up, as the loss of momentum is higher when fluid flows through a smaller pore. Eventually, the effect of pore structure of a Raschig ring catalyst pellet is studied. The voidage and pressure drop can be significantly regulated by adjusting outer cylinder radius, inner pore radius, and height. When the wall of the Raschig ring is thinner, the voidage is higher, resulting in the lower pressure drop. This work can provide a powerful model and some theoretical guidance for the optimal design of catalyst pellet shape.

Key words: packed bed, particle-resolved computational fluid dynamics, pressure drop, catalyst, Raschig ring, seven-hole cylinder, simulation

摘要：

压降是衡量固定床反应器优劣的重要指标，直接影响了反应性能和综合能耗，催化剂颗粒的外形和尺寸是影响固定床反应器压降的关键因素。采用颗粒分辨计算流体力学模型，研究了工业上常用的蜂窝状催化剂颗粒上中空结构对固定床反应器压降的影响规律。首先，通过对比实验测量的催化剂床层空隙率和压降，验证了建立的颗粒分辨计算流体力学模型的合理性和准确性，其中模型计算获得的压降与实验值相差5%以内。接着，研究了蜂窝状催化剂颗粒开孔个数的影响，发现在催化剂颗粒体积和开孔体积相同的情况下，开孔个数不会显著影响催化剂床层的空隙率，但开孔个数增加会导致压降增大，这主要是由于孔径变小后增大了流体在孔内的动量损失。最后，考察了单孔柱催化剂颗粒尺寸的影响，发现可通过调变外圆柱半径、内孔半径和高度，进而大幅度改变催化剂床层空隙率和压降，当单孔柱壁面越薄时，空隙率越大，致使压降越低。研究结果可以为催化剂颗粒外形的优化设计提供强大的模型工具和一定的理论指导。

关键词: 固定床, 颗粒分辨计算流体力学, 压降, 催化剂, 单孔柱, 七孔柱, 模拟

CLC Number:

TQ 021.1

Junqi WENG, Xinlei LIU, Jiahao YU, Yao SHI, Guanghua YE, Jin QU, Xuezhi DUAN, Jinbing LI, Xinggui ZHOU. Influence of hollow structure of honeycomb catalysts on the pressure drop in packed bed reactors[J]. CIESC Journal, 2022, 73(1): 266-274.

翁俊旗, 刘鑫磊, 余佳豪, 施尧, 叶光华, 屈进, 段学志, 李金兵, 周兴贵. 蜂窝状催化剂中空结构对固定床反应器压降的影响[J]. 化工学报, 2022, 73(1): 266-274.

Figures/Tables 15

Fig.1 Flow chart for building a particle-resolved computational fluid dynamics model

Table 1 Boundary conditions and model parameters for flow simulations

参数	数值
进口温度T_in	220℃
出口压力P_out	10 atm(1 atm=101325 Pa)
进口流速u_in	1 m/s
进口氧气摩尔分数X_O $2$	8%
进口乙烯摩尔分数X_C $2$ _H $4$	36%
进口氮气摩尔分数X_N $2$	44%
进口二氧化碳摩尔分数X_CO $2$	12%
反应管内径D	35 mm
黏度μ	2.26×10^-5 Pa?s
密度ρ	6.41 kg/m³

Table 1 Boundary conditions and model parameters for flow simulations

参数	数值
进口温度T_in	220℃
出口压力P_out	10 atm(1 atm=101325 Pa)
进口流速u_in	1 m/s
进口氧气摩尔分数X_O $2$	8%
进口乙烯摩尔分数X_C $2$ _H $4$	36%
进口氮气摩尔分数X_N $2$	44%
进口二氧化碳摩尔分数X_CO $2$	12%
反应管内径D	35 mm
黏度μ	2.26×10^-5 Pa?s
密度ρ	6.41 kg/m³

Fig.2 Relation between the pressure drop of a catalyst bed and the base size of a cell

Fig.3 Comparison between calculated result and experimental result

Fig.4 Dimensions and packing structures of the Raschig ring and seven-hole cylinder catalyst pellets

Fig.5 Angle distributions and radial voidage distributions of Raschig ring and seven-hole cylinder catalyst pellets in the packed bed

Fig.6 Contours of flow rate and pressure in the beds packed with Raschig ring and seven-hole cylinder catalyst pellets

Table 2 Voidages， pressure drops， and apparent reaction rates of catalyst in the beds packed with Raschig ring and seven-hole cylinder catalyst pellets

外形	平均空隙率 $ε ˉ$	压降(ΔP/L)/(Pa/m)	表观反应速率/(mol/(m³·s)
单孔柱	0.582	3660	5.56
七孔柱	0.579	3982	7.73

Table 2 Voidages， pressure drops， and apparent reaction rates of catalyst in the beds packed with Raschig ring and seven-hole cylinder catalyst pellets

外形	平均空隙率 $ε ˉ$	压降(ΔP/L)/(Pa/m)	表观反应速率/(mol/(m³·s)
单孔柱	0.582	3660	5.56
七孔柱	0.579	3982	7.73

Fig.7 Contours of temperature and C2H4 concentration in the beds packed with Raschig ring and seven-hole cylinder catalyst pellets

Table 3 Dimensions of Raschig rings， as well as the voidages and pressure drops of their corresponding beds （R unchanged， but r and H varied）

外径R/mm	内径r/mm	高度H/mm	平均空隙率 $ε ˉ$	压降(ΔP/L)/(Pa/m)
3.86	1.56	5.60	0.514	4501
3.86	1.81	6.00	0.545	4320
3.86	2.00	6.40	0.582	3660
3.86	2.15	6.80	0.599	3226
3.86	2.28	7.20	0.634	2802

Table 3 Dimensions of Raschig rings， as well as the voidages and pressure drops of their corresponding beds （R unchanged， but r and H varied）

外径R/mm	内径r/mm	高度H/mm	平均空隙率 $ε ˉ$	压降(ΔP/L)/(Pa/m)
3.86	1.56	5.60	0.514	4501
3.86	1.81	6.00	0.545	4320
3.86	2.00	6.40	0.582	3660
3.86	2.15	6.80	0.599	3226
3.86	2.28	7.20	0.634	2802

Table 4 Dimensions of Raschig rings， as well as the voidages and pressure drops of their corresponding beds （r unchanged， but R and H varied）

外径R/mm	内径r/mm	高度H/mm	平均空隙率 $ε ˉ$	压降(ΔP/L)/(Pa/m)
5.09	2.00	3.20	0.524	4492
4.31	2.00	4.80	0.569	4002
3.86	2.00	6.40	0.582	3660
3.57	2.00	8.00	0.622	3500
3.36	2.00	9.60	0.643	3188

Table 4 Dimensions of Raschig rings， as well as the voidages and pressure drops of their corresponding beds （r unchanged， but R and H varied）

外径R/mm	内径r/mm	高度H/mm	平均空隙率 $ε ˉ$	压降(ΔP/L)/(Pa/m)
5.09	2.00	3.20	0.524	4492
4.31	2.00	4.80	0.569	4002
3.86	2.00	6.40	0.582	3660
3.57	2.00	8.00	0.622	3500
3.36	2.00	9.60	0.643	3188

Table 5 Dimensions of Raschig rings， as well as the voidages and pressure drops of their corresponding beds （H unchanged， but R and r varied）

外径R/mm	内径r/mm	高度H/mm	平均空隙率 $ε ˉ$	压降(ΔP/L)/(Pa/m)
3.46	1.00	6.40	0.461	5927
3.63	1.50	6.40	0.516	4728
3.86	2.00	6.40	0.582	3660
4.15	2.50	6.40	0.642	3199
4.46	3.00	6.40	0.699	2188

Table 5 Dimensions of Raschig rings， as well as the voidages and pressure drops of their corresponding beds （H unchanged， but R and r varied）

外径R/mm	内径r/mm	高度H/mm	平均空隙率 $ε ˉ$	压降(ΔP/L)/(Pa/m)
3.46	1.00	6.40	0.461	5927
3.63	1.50	6.40	0.516	4728
3.86	2.00	6.40	0.582	3660
4.15	2.50	6.40	0.642	3199
4.46	3.00	6.40	0.699	2188

Fig.8 Pressure drops and voidages of the beds packed with Raschig ring catalyst pellets (R unchanged, but r and H varied)

Fig.9 Pressure drops and voidages of the beds packed with Raschig ring catalyst pellets (r unchanged, but R and H varied)

Fig.10 Pressure drops and voidages of the beds packed with Raschig ring catalyst pellets (H unchanged, but R and r varied)

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