Simulation and analysis of CH4/N2 separation by vacuum pressure swing adsorption with structured composite adsorption media

doi:10.11949/0438-1157.20210650

Abstract

Abstract:

To reduce methane emissions and achieve effective resource utilization of low concentration coalbed methane, the process of using a structured composite adsorbent for vacuum pressure swing adsorption to enrich low-concentration coal-bed methane was explored. The equilibrium adsorption capacities of pure gases (CH₄ and N₂) on the structured composite adsorption medium were measured under different pressures at a series of fixed temperatures by using the static volumetric method. Thus, a rigorous and reasonable mathematical model, including a set of conservation equation of mass, energy and momentum balances, was developed to precisely describe the dynamic behavior of multiple components in adsorption bed. A typical three-bed VPSA process with continuous feeding was designed and simulated. A comprehensive analysis was presented, relating to the process characteristics and performance such as temperature and pressure distribution in axial adsorption bed at cycle steady state. Additionally, effects of feed flow rate, desorption pressure, feed concentration and adsorption pressure on purity, recovery, energy consumption and productivity were investigated. The results showed that an effective separation performance of 59.07% CH₄ purity, 93.64% CH₄ recovery and 4.56 mol·h^-1·kg^-1 productivity with an energy consumption of 18.70 kJ·mol^-1, was finally got with the optimal parameters, while the feed flow rate, desorption pressure, feed concentration and adsorption pressure were 100 L·min^-1, 0.1 bar, 30% and 3 bar, respectively. Overall, this study indicated there is an effective adsorption and separation performance on CH₄/N₂ of structured composite adsorption media, which can achieve high-efficiency enrichment of methane in low concentration coalbed methane.

Key words: structured composite adsorption media, vacuum pressure swing adsorption, coalbed methane enrichment, numerical simulation, methane

摘要：

为减少甲烷排放，实现低浓度煤层气有效资源化利用，探究了使用规整复合吸附剂真空变压吸附富集低浓度煤层气的工艺。采用静态容积法测定了甲烷、氮气在规整复合吸附剂上的吸附等温线，同时建立了包括质量、热量和动量守恒在内的严格吸附床数学模型，设计了三塔连续进料的真空变压吸附工艺并进行模拟。分析了工艺达到循环稳态后吸附床层轴向温度分布和压力变化，并且探究了进料量、解吸压力、原料气中甲烷浓度和吸附压力对纯度、回收率、工艺能耗和吸附剂产率等工艺性能的影响。模拟结果表明，在进料量为100 L·min^-1，解吸压力为0.1 bar（1 bar=0.1 MPa），原料气甲烷浓度为30%，吸附压力为3 bar时可以生产纯度为59.07%，回收率为93.64%的富CH₄产品气，同时单位能耗为18.70 kJ·mol^-1，吸附剂产率为4.56 mol·h^-1·kg^-1。表明规整吸附剂对CH₄/N₂具有良好的吸附分离效果，能够实现低浓度煤层气中甲烷高效富集。

关键词: 规整复合吸附剂, 真空变压吸附, 煤层气富集, 数值模拟, 甲烷

CLC Number:

TQ 028.1

Junpeng TIAN, Yuanhui SHEN, Donghui ZHANG, Zhongli TANG. Simulation and analysis of CH₄/N₂ separation by vacuum pressure swing adsorption with structured composite adsorption media[J]. CIESC Journal, 2021, 72(11): 5675-5685.

田军鹏, 沈圆辉, 张东辉, 唐忠利. 规整复合吸附剂真空变压吸附分离CH₄/N₂工艺模拟与分析[J]. 化工学报, 2021, 72(11): 5675-5685.

Figures/Tables 15

Fig.1 The schematic diagram of three bed VPSA process

Fig.2 Schematic diagram of cycle configuration and adsorption front of CH4 in bed

Table 1 Cycle sequence of three bed VPSA process

步骤	塔1	塔2	塔3	时间/s
1	AD	ER	ED	10
2	AD	PR	VU	120
3	ED	AD	ER	10
4	VU	AD	PR	120
5	ER	ED	AD	10
6	PR	VU	AD	120

Table 2 The detailed mathematical model equations used in adsorption bed

项目	数学模型
质量平衡	$- ε b D a x, i ? 2 c i ? z 2 + ? v g c i ? z + [ε b + 1 - ε b ε p] ? c i ? t + ρ p (1 - ε b) ? q i ? t = 0$	(1)
	$D a x, i = 0.73 D m, i + v g r p ε b 1 + 9.49 ε b D m, i 2 v g r p$	(2)
热量平衡	气相 $- k g ε b ? 2 T g ? z 2 + C v g v g ρ g ? T g ? z + ε p C v g ρ g ? T g ? t + P ? v g ? z + h f (T g - T s) + 4 h w D b (T g - T w) = 0$	(3)
	固相 $- k s ? 2 T s ? z 2 + C p s ρ b ? T s ? z + ρ b ∑ i = 1 n (C p a, i q i) ? T s ? t + ρ b ∑ i = 1 n (Δ H i ? q i ? t) - h f a p (T g - T s) = 0$	(4)
	塔壁 $- k w ? 2 T w ? z 2 + C p w ρ w ? T w ? z - h w 4 D b (D b + W t) 2 - D b (T g - T w) + h a m b 4 (D b + W t) 2 (D b + W t) 2 - D b 2 (T w - T a m b) = 0$	(5)
动量平衡	$? P ? z = ± 150 × 10 - 5 μ v g (1 - ε b) 2 ε b 3 (2 r p ψ) 2 + 1.75 × 10 - 5 M (1 - ε b) ρ g 2 r p ψ ε b 3 v g 2$	(6)
传质速率	$? q i ? t = k L D F, i (q i * - q i) = 15 D e, i r p 2 (q i * - q i)$	(7)
	$D e, i = ε p τ × D k, i D m, i D k, i + D m, i ?, ??? D k, i = 97.0 r p T M i$	(8)

Table 2 The detailed mathematical model equations used in adsorption bed

项目	数学模型
质量平衡	$- ε b D a x, i ? 2 c i ? z 2 + ? v g c i ? z + [ε b + 1 - ε b ε p] ? c i ? t + ρ p (1 - ε b) ? q i ? t = 0$	(1)
	$D a x, i = 0.73 D m, i + v g r p ε b 1 + 9.49 ε b D m, i 2 v g r p$	(2)
热量平衡	气相 $- k g ε b ? 2 T g ? z 2 + C v g v g ρ g ? T g ? z + ε p C v g ρ g ? T g ? t + P ? v g ? z + h f (T g - T s) + 4 h w D b (T g - T w) = 0$	(3)
	固相 $- k s ? 2 T s ? z 2 + C p s ρ b ? T s ? z + ρ b ∑ i = 1 n (C p a, i q i) ? T s ? t + ρ b ∑ i = 1 n (Δ H i ? q i ? t) - h f a p (T g - T s) = 0$	(4)
	塔壁 $- k w ? 2 T w ? z 2 + C p w ρ w ? T w ? z - h w 4 D b (D b + W t) 2 - D b (T g - T w) + h a m b 4 (D b + W t) 2 (D b + W t) 2 - D b 2 (T w - T a m b) = 0$	(5)
动量平衡	$? P ? z = ± 150 × 10 - 5 μ v g (1 - ε b) 2 ε b 3 (2 r p ψ) 2 + 1.75 × 10 - 5 M (1 - ε b) ρ g 2 r p ψ ε b 3 v g 2$	(6)
传质速率	$? q i ? t = k L D F, i (q i * - q i) = 15 D e, i r p 2 (q i * - q i)$	(7)
	$D e, i = ε p τ × D k, i D m, i D k, i + D m, i ?, ??? D k, i = 97.0 r p T M i$	(8)

Table 3 Process performance indicators

指标	方程
纯度	$∫ 0 t c y c l e F o u t y o u t, t a r g e t d t ∫ 0 t c y c l e F o u t d t$
回收率	$∫ 0 t c y c l e F o u t y o u t, t a r g e t d t ∫ 0 t c y c l e F i n y i n, t a r g e t d t$
能耗	$d W = ∫ 0 t c y c l e V i n P o u t γ η (γ + 1) P o u t / P i n 1 - (1 / γ) - 1 d t ∫ 0 t c y c l e V p r o d u c t y p r o d u c t d t$
产率	$3600 ∫ 0 t c y c l e F o u t y o u t, p r o d u c t d t t c y c l e m a d s o r b e n t s$

Table 3 Process performance indicators

指标	方程
纯度	$∫ 0 t c y c l e F o u t y o u t, t a r g e t d t ∫ 0 t c y c l e F o u t d t$
回收率	$∫ 0 t c y c l e F o u t y o u t, t a r g e t d t ∫ 0 t c y c l e F i n y i n, t a r g e t d t$
能耗	$d W = ∫ 0 t c y c l e V i n P o u t γ η (γ + 1) P o u t / P i n 1 - (1 / γ) - 1 d t ∫ 0 t c y c l e V p r o d u c t y p r o d u c t d t$
产率	$3600 ∫ 0 t c y c l e F o u t y o u t, p r o d u c t d t t c y c l e m a d s o r b e n t s$

Table 4 Parameters of adsorption bed

参数	数值
H_b/m	1.00
D_b /m	0.20
W_b/m	0.005
C_p_w/ (kJ·kg^-1·K^-1)	0.502
h_w/ (W·m^-2·K^-1)	60
ρ_w/ (kg·m^-3)	7800
h_amb/ (W·m^-2·K^-1)	60
h_f/(W·m^-2·K^-1)	217
k_g/(W·m^-1·K^-1)	0.342
k_s/(W·m^-1·K^-1)	0.3
T_f/K	298.15
P_f/bar	3.0

Table 5 Parameters of structured composite adsorption media adsorbent

参数	数值
ε_b	0.75
ε_p	0.64
S_BET/ (m²·g^-1)	880.06
V_t/(cm³·g^-1)	0.509
ρ_s/ (kg·m^-3)	175
W_a/m	3.5×10^-4
C_p_s /(kJ·kg^-1·K^-1)	0.857

Fig.3 The adsorption isotherms of CH4 and N2 on the structured composite adsorption media

Table 6 The parameters of Extended Langmuir 2 model by fitting

参数	CH₄	N₂
IP₁/(mol·kg^-1·kPa^-1)	1.32×10^-8	9.75×10^-7
IP₂/K	2476	2186
IP₃/kPa^-1	2.84×10^-7	2.59×10^-6
IP₄/K	2726	1740
ΔH/(kJ·mol^-1)	-19.05	-15.93

Table 7 The simulation results of three bed VPSA process

工况	进料量/ (L·min^-1)	吸附压力/ bar	解吸压力/ bar	原料气甲烷浓度/%	CH₄纯度/%	CH₄回收率/%	能耗/(kJ·mol^-1)	产率/ (mol·h^-1·kg^-1)
1	80	3	0.1	30	52.28	98.95	18.46	3.86
2	90	3	0.1	30	56.04	96.81	18.54	4.25
3	100	3	0.1	30	59.07	93.64	18.70	4.56
4	110	3	0.1	30	61.48	90.45	18.95	4.85
5	120	3	0.1	30	63.16	85.89	19.57	5.02
6	130	3	0.1	30	64.71	82.37	20.05	5.22
7	140	3	0.1	30	66.08	79.02	20.59	5.39
8	100	3	0.05	30	59.54	96.36	19.32	4.70
9	100	3	0.075	30	59.23	94.56	18.92	4.61
10	100	3	0.125	30	58.72	90.73	18.34	4.42
11	100	3	0.15	30	58.28	88.07	18.32	4.29
12	100	3	0.175	30	57.91	85.94	18.30	4.19
13	100	3	0.20	30	57.56	84.69	18.25	4.13
14	100	3	0.1	10	26.16	95.28	52.78	1.55
15	100	3	0.1	15	37.11	94.32	35.75	2.30
16	100	3	0.1	20	45.32	93.92	27.02	3.05
17	100	3	0.1	25	52.81	93.75	21.69	3.81
18	100	3	0.1	35	65.05	93.55	16.33	5.32
19	100	3	0.1	40	70.41	93.48	14.65	6.08
20	100	1	0.1	30	40.23	90.34	9.66	4.40
21	100	5	0.1	30	68.76	92.81	38.37	4.52

Fig.4 Temperature distribution within adsorption bed and pressure profile of VPSA process

Fig.5 The effect of feed flowrate on VPSA process performance

Fig.6 The effect of desorption pressure on VPSA process performance

Fig.7 The effect of feed concentration on VPSA process performance

Fig.8 The effect of adsorption pressure on VPSA process performance

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Simulation and analysis of CH₄/N₂ separation by vacuum pressure swing adsorption with structured composite adsorption media

规整复合吸附剂真空变压吸附分离CH₄/N₂工艺模拟与分析

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