Intensification of low-grade methane enrichment in nitrogen mixture by CH4/CO2 displacement

doi:10.11949/0438-1157.20200416

Abstract

Abstract:

To solve the problem of CO₂ uncompleted desorption in the process of CO₂ displacement enhancing the adsorption separation of CH₄/N₂, a small amount of product gas CH₄ was used as purge gas to improve the CO₂ desorption. CH₄/CO₂ mixture gas obtained from desorption step was recycled as the displacement gas to enhance the enrichment of low-grade methane in nitrogen mixture. In this work, the research conducted the experiments for CH₄/N₂ separation using CH₄/CO₂ displacement intensification adsorption and the laboratory-made coconut shell activated carbon as sorbent. The mathematical models were built in gPROMS and the accuracy of models was verified by comparison of simulations and CH₄/N₂/CO₂ breakthrough experiments. The performance of enrichment of low-grade methane with displacement intensification using different displacer was compared. The result showed that the process with CH₄/CO₂ displacement had higher purity product than CO₂ displacement. The CH₄/ CO₂ mixed gas replacement enhanced vacuum pressure swing adsorption cycle experiment was carried out, which can jointly enrich 14% CH₄/ N₂ and 53% CH₄/CO₂ to 98.8%, and at the same time obtain a recovery rate of 77.8%.

Key words: activated carbon, methane, carbon dioxide, nitrogen, vacuum pressure swing adsorption, process intensification

摘要：

针对CO₂置换吸附分离CH₄/N₂过程中CO₂再生困难的问题，采用少量产品气CH₄真空吹扫以提高CO₂的解吸效果，并以解吸得到的CH₄/CO₂混合气为置换步骤的置换气，通过置换来强化含氮低品质甲烷的浓缩过程。以自制椰壳活性炭为吸附剂，对CH₄/CO₂混合气置换强化吸附回收含氮低品质甲烷工艺过程进行了实验与模拟研究。在gPROMS软件中建立并求解固定床吸附分离模型方程，预测了CH₄、N₂ 和CO₂在自制椰壳活性炭上的竞争吸附穿透曲线，通过预测结果和实验的对比，验证了数学模型方程的准确性。对比了不同置换气强化吸附分离低品质甲烷的效果，结果表明CH₄/CO₂混合气置换强化相对于CO₂置换强化可获得更高纯度产品。进行了CH₄/CO₂混合气置换强化真空变压吸附循环实验，可以将14%的CH₄/N₂和53%的CH₄/CO₂联合富集到98.8%，同时获得77.8%的回收率。

关键词: 活性炭, 甲烷, 二氧化碳, 氮气, 真空变压吸附, 过程强化

CLC Number:

TQ 028.1

QU Donglei,YANG Ying,QIAN Zhiling,LI Ping,YU Jianguo. Intensification of low-grade methane enrichment in nitrogen mixture by CH₄/CO₂ displacement[J]. CIESC Journal, 2020, 71(12): 5599-5609.

曲冬蕾,杨颖,钱智玲,李平,于建国. CH₄/CO₂混合气置换强化含氮低品质甲烷的浓缩[J]. 化工学报, 2020, 71(12): 5599-5609.

Figures/Tables 12

Fig.1 Fixed-bed adsorption separation experimental device

Fig.2 Flow chart of fixed-bed VPSA- CH4/CO2 DIS cycle

Table 1 Operating parameter of vacuum pressure swing adsorption with CH4/CO2 displacement intensification

步骤	时间/s	进料流率/(L·min^-1)
步骤	时间/s	CH₄	N₂	CO₂
充压	150	0.2	0.8	—
进料	400	0.2	0.8	—
置换	1400	0.5	—	0.5
抽真空	1300	—	—	—
吹扫1	3000	0.1	—	—
吹扫2	300	—	0.3	—

Table 2 Mathematical model, boundary conditions, initial conditions for fixed bed experiment

mass balance of column
$ε c ? C i ? t - ε c ? ? t D a x C i ? y i ? z + ? u C i ? z + 1 - ε c a' k f, i 1 + B i i C i - C i p ˉ = 0$ , $a' = S a + 1 / R p$ , $B i i = R p k f, i S a + 3 ε p D p, i$
mass balance of pellet
$ε p ? C i p ˉ ? t + ρ p ? q i ˉ ? t - ε p Ω p D p, i R p 2 B i i 1 + B i i C i - C i p ˉ = 0$
mass balance of micropore
$? q i ˉ ? t - S i + 1 S i + 3 r μ 2 D μ, i q i * - q i ˉ = 0$
energy balance of gas phase
$ε c C t c v ? T g ? t - ? ? t λ ? T g ? z + u C t c p ? T g ? z - ε c R g T g ? C i ? t + 1 - ε c a' h f T g - T s + 2 h w R w T g - T w = 0$
energy balance of solid phase
$1 - ε c ε p ∑ i = 1 n C i p ˉ c v, i + ρ p ∑ i = 1 n q i ˉ c v, a d s, i + ρ p c p s ? T s ? t - 1 - ε c ε p R g T s ? ∑ i = 1 n C i p ˉ ? t - ρ b ∑ i = 1 n - Δ H i ? q i ˉ ? t - 1 - ε c a' h f T g - T s = 0$
energy balance of column wall
$ρ w c p w ? T w ? t - D w e D w + e h w T g - T w + U T w - T ∞ D w + e l n D w + 2 e D w = 0$
momentum balance in the column
$-$ $? p ? z = 150 μ g 1 - ε c 2 ε c 2 d p 2 u + 1.75 1 - ε c ρ g ε c 2 d p u u$
initial conditions
$T z = T s z = T w z = T a d s$ , $C z = C i p ˉ = p a d s y i n i t i a l, i R T a d s$ , $q i ˉ = 0$ or $q i ˉ i ≠ N 2 = 0$ , $q N 2 ˉ = q i n i t i a l, N 2 *$
boundary conditions
pressurization step
$u 0 C i, 0 z - = u 0 C i, 0 z +$ , $- ε c D a x i u i 0 ? y i, 0 ? z z + - y i, 0 z - + y i, 0 z + = 0$ $- λ ? T 0 ? z z + - u C t c p T 0 z - + u C t c p T 0 z + = 0$ , $u L c = 0$ , $? C i, L c ? z z - = 0$ , $? T L c ? z z - = 0$
feed and displacement step
$p L c = p h i g h$ , $u 0 C i, 0 z - = u 0 C i, 0 z +$ , $- ε c D a x i u i 0 ? y i, 0 ? z z + - y i, 0 z - + y i, 0 z + = 0$ $- λ ? T 0 ? z z + - u C t c p T 0 z - + u C t c p T 0 z + = 0$ , $? C i, L c ? z z - = 0$ , $? T L c ? z z - = 0$
blowdown step
$p 0 = p h i g h + p h i g h - p b l o w 1 - e x p - θ t b l o w$ , $? C i, 0 ? z z + = 0$ , $? T 0 ? z z + = 0$ , $u L c = 0$ , $? C i, L c ? z z - = 0$ , $? T L c ? z z - = 0$

Table 2 Mathematical model, boundary conditions, initial conditions for fixed bed experiment

mass balance of column
$ε c ? C i ? t - ε c ? ? t D a x C i ? y i ? z + ? u C i ? z + 1 - ε c a' k f, i 1 + B i i C i - C i p ˉ = 0$ , $a' = S a + 1 / R p$ , $B i i = R p k f, i S a + 3 ε p D p, i$
mass balance of pellet
$ε p ? C i p ˉ ? t + ρ p ? q i ˉ ? t - ε p Ω p D p, i R p 2 B i i 1 + B i i C i - C i p ˉ = 0$
mass balance of micropore
$? q i ˉ ? t - S i + 1 S i + 3 r μ 2 D μ, i q i * - q i ˉ = 0$
energy balance of gas phase
$ε c C t c v ? T g ? t - ? ? t λ ? T g ? z + u C t c p ? T g ? z - ε c R g T g ? C i ? t + 1 - ε c a' h f T g - T s + 2 h w R w T g - T w = 0$
energy balance of solid phase
$1 - ε c ε p ∑ i = 1 n C i p ˉ c v, i + ρ p ∑ i = 1 n q i ˉ c v, a d s, i + ρ p c p s ? T s ? t - 1 - ε c ε p R g T s ? ∑ i = 1 n C i p ˉ ? t - ρ b ∑ i = 1 n - Δ H i ? q i ˉ ? t - 1 - ε c a' h f T g - T s = 0$
energy balance of column wall
$ρ w c p w ? T w ? t - D w e D w + e h w T g - T w + U T w - T ∞ D w + e l n D w + 2 e D w = 0$
momentum balance in the column
$-$ $? p ? z = 150 μ g 1 - ε c 2 ε c 2 d p 2 u + 1.75 1 - ε c ρ g ε c 2 d p u u$
initial conditions
$T z = T s z = T w z = T a d s$ , $C z = C i p ˉ = p a d s y i n i t i a l, i R T a d s$ , $q i ˉ = 0$ or $q i ˉ i ≠ N 2 = 0$ , $q N 2 ˉ = q i n i t i a l, N 2 *$
boundary conditions
pressurization step
$u 0 C i, 0 z - = u 0 C i, 0 z +$ , $- ε c D a x i u i 0 ? y i, 0 ? z z + - y i, 0 z - + y i, 0 z + = 0$ $- λ ? T 0 ? z z + - u C t c p T 0 z - + u C t c p T 0 z + = 0$ , $u L c = 0$ , $? C i, L c ? z z - = 0$ , $? T L c ? z z - = 0$
feed and displacement step
$p L c = p h i g h$ , $u 0 C i, 0 z - = u 0 C i, 0 z +$ , $- ε c D a x i u i 0 ? y i, 0 ? z z + - y i, 0 z - + y i, 0 z + = 0$ $- λ ? T 0 ? z z + - u C t c p T 0 z - + u C t c p T 0 z + = 0$ , $? C i, L c ? z z - = 0$ , $? T L c ? z z - = 0$
blowdown step
$p 0 = p h i g h + p h i g h - p b l o w 1 - e x p - θ t b l o w$ , $? C i, 0 ? z z + = 0$ , $? T 0 ? z z + = 0$ , $u L c = 0$ , $? C i, L c ? z z - = 0$ , $? T L c ? z z - = 0$

purge step

$p (0) = p e x i t$ , $? C (i, 0) ? z z + = 0$ , $? T (0) ? z z + = 0$ , $u (L c) C (i, L c) z - = u (L c) C (i, L c) z +$ , $- ε c D a x (i) u i (L c) ? y (i, L c) ? z z - + y (i, L c) z - - y (i, L c) z + = 0$ ,

$- λ ? T (L c) ? z z - + u C t c p T (L c) z - - u C t c p T (L c) z + = 0$

adsorption equilibrium isotherms

IAST-Sips model

q i q m, i = K i p i 1 / n i 1 + ∑ i K i p i 1 / n i

p i = p y i = p i 0 x

q i = q T x i

∑ i = 1 n x i = 1, ∑ i = 1 n y i = 1

1 q T = ∑ i - 1 n x i q i 0

π A R g T = q m, i n i l n 1 + K i p i 0 1 / n i = c o n s t a n t

purge step

$- λ ? T (L c) ? z z - + u C t c p T (L c) z - - u C t c p T (L c) z + = 0$

adsorption equilibrium isotherms

IAST-Sips model

q i q m, i = K i p i 1 / n i 1 + ∑ i K i p i 1 / n i

p i = p y i = p i 0 x

q i = q T x i

∑ i = 1 n x i = 1, ∑ i = 1 n y i = 1

1 q T = ∑ i - 1 n x i q i 0

π A R g T = q m, i n i l n 1 + K i p i 0 1 / n i = c o n s t a n t

Table 3 Transport parameters and gas phase properties

Diffusion coefficient	CH₄	N₂	CO₂
D_ax /(m²·s^-1)	1.57×10^-5	1.57×10^-5	1.54×10^-5
D_p /(m²·s^-1)	5.74×10^-6	5.65×10^-6	4.05×10^-6
k_f /(m²·s^-1)	3.77×10^-2	3.77×10^-2	2.82×10^-2
D_μ0 /(m²·s^-1)	23.54	5.57	6.86
E_a /(J·mol^-1)	13062	7162	4280
Thermal conductivity		Gas properties
Parameter	Value	Parameter	Value
h_f /(W·m^-2·K^-1)	91.13	μ_g /(Pa·s)	1.61×10^-5
h_w /(W·m^-2·K^-1)	60	c_p/(J·mol^-1·K^-1)	30.44
λ /(W·m^-2·K^-1)	0.20	c_v/(J·mol^-1·K^-1)	22.13
U /(W·m^-2·K^-1)	180

Fig.3 Adsorption equilibrium isotherms of CH4, N2 and CO2 on GAC

Table 4 Fitting parameters of Sips model for adsorption equilibrium isotherms of pure CH4， N2 and CO2 on GAC

Gas	q_m,_i/(mol·kg^-1)	K_0,_i/kPa^-1	-ΔH_i/(kJ·mol^-1)	n_i
CH₄	6.466	1.10×10^-6	19.556	1.112
N₂	3.834	2.02×10^-6	16.381	1.011
CO₂	15.459	1.52×10^-7	23.698	1.344

Fig.4 Breakthrough curves of CH4/N2/CO2 in column packed with GAC

Fig.5 Concentration distribution curves and elution curves of adsorption separation CH4/N2 by displacement on fixed bed

Fig.6 Pressure, temperature, outlet concentration and flowrate histories in VPSA-CH4/CO2 DIS process

Fig.7 Material balance of VPSA-CH4/CO2 DIS process

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Intensification of low-grade methane enrichment in nitrogen mixture by CH₄/CO₂ displacement

CH₄/CO₂混合气置换强化含氮低品质甲烷的浓缩

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