Adsorption separation of CO from syngas with CuCl@AC adsorbent by a VPSA process

Abstract

In this work, activated carbon (AC) supported CuCl (CuCl@AC) prepared with CuCl₂ as precursor is investigated for CO separation from the synthesis gas mixture. Firstly, the CuCl@AC adsorbents are investigated for their CO reversible adsorption capacity at an operation temperature of 303 K. And a vacuum pressure swing adsorption (VPSA) process of CO separation from syngas utilizing the prepared CuCl@AC adsorbent is investigated at ambient temperature and 0.79 MPa through a dynamic optimization with Aspen Adsorption software. The integrated model is closer to a realistic PSA process, making the results of the simulation and optimization more convincing. The adsorption result reveals that the obtained CuCl@AC adsorbent with the copper loading of 7 mmol g⁻¹ AC achieves a high reversible CO adsorption capacity and adsorption selectivity. The simulation result shows that, under optimal conditions, the CO product with the purity of 98.1 vol% can be separated from the syngas with the CO concentration of 32.3 vol% utilizing the prepared CuCl@AC adsorbent, and the recovery of CO is 92.9%.

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1 Introduction

Carbon monoxide is an important chemical used widely as a raw material for the production of various chemicals including methanol, ethylene glycol, formic acid, acetic acid, polyurethane foams, polycarbonate plastics, etc.¹ The production of CO mainly utilizes the methods of the steam reforming of natural gas and coal gasification, producing a synthesis gas mixture containing CO, CO₂, CH₄, N₂ and H₂.² And then the high purity CO must be separated from the gas mixture for the production of various chemicals. In addition, huge amounts of CO are released from the tail gases of some industrial oxidation processes, such as coke oven gas, blast furnace gas, carbon black manufacturing tail gas, etc.^3–5 The CO from these tail gases should be separated and used from both the industrial and environmental point of views.

Recently, extensive efforts have been devoted to develop the technologies for CO separation from gas mixtures, including cryogenic distillation, liquid absorption and adsorption processes.⁶ Cryogenic distillation is an extremely cost- and energy-intensive process, and suitable only when the CO-containing gas stream has a relatively low concentration of N₂ due to the similar boiling points.^7,8 Liquid absorption processes of CO utilizing ammoniacal cuprous chloride and aromatic CuAlCl₄ solution (COSORB) have the disadvantage of the instability of Cu⁺ ions, which always react to give Cu⁰ and Cu²⁺.^9,10 Among these technologies, adsorptive separation like the pressure swing adsorption (PSA) appears to be the most promising and energy-saving alternative due to low energy costs and easy operations.^11,12

For the adsorptive separation, CO adsorbents are the key to directly determine the separation performance.¹³ In the development of adsorption separation technologies, various porous solids including activated carbon,^14,15 zeolites,^16,17 and metal–organic frameworks (MOFs)^18,19 display the high CO adsorption capacity. However, these adsorbents do not have significant adsorption selectivity for CO. Recently, CO separation^20–27 based on the Cu(I) π-complexation adsorbents could achieve high adsorption capacity and high adsorption selectivity for CO, resulting from the stronger π-complexation interactions of CO with Cu⁺ ions supported on the porous solid materials. On the other hand, these types of bonds are still sufficiently weak to be broken by using simple engineering operations. Presently, the Cu(I)-based adsorbents are mainly prepared by dispersing CuCl onto the porous supports of zeolites or activated carbon (AC) using different dispersion methods. For the zeolites supported CuCl (CuCl@zeolites) adsorbents, the interactions of CO with CuCl@zeolites are relatively stronger than CuCl@AC resulting in a higher desorption temperature of ∼453 K.²⁸ In the previous work of the first author involved in Tianjin University,²⁹ the activated carbon (AC) supported CuCl (CuCl@AC) adsorbent was successfully prepared with CuCl₂ as precursor by a solid-state dispersion method, and the activation of CuCl₂ supported on AC to CuCl was realized at 543 K in N₂ without any reducing gas. The prepared CuCl@AC adsorbent displayed high CO adsorption capacity and adsorption selectivity, and the adsorbed CO on CuCl@AC could be desorbed at lower temperature of 303 K under vacuum. This lower desorption temperature can greatly reduce the operation costs in CO separation processes compared with the higher desorption temperature of CuCl@zeolites. Thus, this CuCl@AC adsorbent has broad prospects for separation of CO from the synthesis gas mixture. So it is necessary to investigate the vacuum pressure swing adsorption (VPSA) process of CO separation from syngas utilizing CuCl@AC for designing a realistic PSA process.

In this work, a five-bed and 7-step VPSA process at ambient temperature of CO separation from syngas utilizing the prepared CuCl@AC adsorbent was investigated with Aspen Adsorption software. The operating conditions were optimized to obtain the high CO recovery and CO product purity. After optimization, the PSA process was analyzed, and the influence of the parameters on the process was explained.

2 Experimental

The CuCl@AC adsorbent with copper loadings of 7 mmol g⁻¹ AC was prepared with CuCl₂ as precursor by a solid-state dispersion method according to the previous work of the first author involved in Tianjin University,²⁹ and denoted as CuCl(7.0)@AC. For an ideal adsorbent for gas separation in industrial application, it is required to be reversible for multiple adsorption and desorption cycles, and the adsorption and desorption cycle operations should be carried out at the same temperature. In the previous work,²⁹ the effect of the operation temperature on the reversible adsorption capacity of CO on CuCl@AC was investigated, and the optimum operation temperature was determined as 303 K. So, in this work, the reversible adsorptions of CO, CO₂, CH₄, N₂ and H₂ on CuCl(7.0)@AC at operation temperature of 303 K were measured using a static volumetric method^30,31 for investigating the PSA process. After adsorption of CO on CuCl(7.0)@AC, the desorption was carried out in situ by venting off the gas, vacuuming for 30 min with the mechanical vacuum pump at temperature of 303 K, and the adsorption equilibrium isotherms of CO, CO₂, CH₄, N₂ and H₂ on the desorbed adsorbents were measured again at the same temperature of 303 K, respectively.

3 Simulation of CO separation from syngas using a five-bed model

3.1 Process description

In this work, a five-bed and 7-step VPSA process at ambient temperature of CO separation from syngas utilizing the prepared CuCl@AC adsorbent was investigated with Aspen Adsorption software. Fig. 1 shows the flow diagram of this process, and the schedule is shown in Table 1. The relevant bed and adsorbent parameters used in the simulation are listed in Table 2, and the equilibrium and kinetic parameters for CO, CO₂, CH₄, N₂ and H₂ on CuCl@AC are listed in Tables 2 and 3. The overall process of each bed involves seven steps including adsorption (AD), pressure equalization for depressurization (ED), replacement (RP), blowdown (BD), vacuum (VU), pressure equalization for pressurization (EP) and feed repressurization (FR), and the total cycle time for the process is 20.0 min.^32,33 Take bed 1 for example, at the beginning of a cycle, the feed gas of syngas (32.3 vol% CO, 1.0 vol% CO₂, 2.4 vol% CH₄, 18.3 vol% N₂, 46.0 vol% H₂, T = 303.15 K, p = 101.325 kPa) passed through the adsorption bed after compression. During the adsorption step (AD), most of the CO from the mixture was selectively adsorbed on the CuCl@AC adsorbent, and the waste gas left from the adsorption bed. And at the first 60 s during the adsorption step, all the waste gas was introduced into pipe of gas waste. For the next 180 s during the adsorption step, one stream of the waste gas was drawn into the adsorption bed 2 to pressurize bed 2 in preparation for the adsorption. After the adsorption step, the ED step was conducted, and the gas of the bed 1 was introduced into bed 3 which was performing the EP step. Following the ED step, the replacement step (RP) was conducted utilizing part of the CO product to increase the concentration of CO and exclude the exhaust gas. And then the blowdown (BD) and vacuum (VU) steps were conducted to regenerate the adsorbent and extract the high purity CO product. Finally, the EP and FR steps were conducted to increase the pressure of the bed for a new cycle.

Fig. 1 Schematic flow diagram of five-bed VPSA process.

Table 1 Schedule of five-bed VPSA process

	60 s	180 s	60 s	180 s	60 s	180 s	60 s	180 s	60 s	180 s
Bed 1	AD↑	AD↑	ED↑	RP↑	RP↑	BD↓	VU↓	VU↓	EP↓	FR↓
Bed 2	EP↓	FR↓	AD↑	AD↑	ED↑	RP↑	RP↑	BD↓	VU↓	VU↓
Bed 3	VU↓	VU↓	EP↓	FR↓	AD↑	AD↑	ED↑	RP↑	RP↑	BD↓
Bed 4	RP↑	BD↓	VU↓	VU↓	EP↓	FR↓	AD↑	AD↑	ED↑	RP↑
Bed 5	ED↑	RP↑	RP↑	BD↓	VU↓	VU↓	EP↓	FR↓	AD↑	AD↑

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Table 2 Bed and adsorbent parameters used in the simulation

Parameters	Value	Units	Description
Hb	1.00	m	Height of bed
Db	0.25	m	Internal diameter of bed
WT	0.01	m	Wall thickness used of bed
εb	0.24	m^3 void/m^3 bed	Interparticle voidage
εp	0.21	m^3 void/m^3 bed	Intraparticle voidage
ρs	473	kg m^−3	Bulk solid density of adsorbent
Rp	0.001	m	Adsorbent particle radius
ψ	0.86	n/a	Adsorbent shape factor
MTC (“CH4”)	0.068	s^−1	Constant mass transfer coefficients
MTC (“CO”)	0.17	s^−1	Constant mass transfer coefficients
MTC (“CO2”)	0.1	s^−1	Constant mass transfer coefficients
MTC (“H2”)	0.7	s^−1	Constant mass transfer coefficients
MTC (“N2”)	0.19	s^−1	Constant mass transfer coefficients
Cps	0.857	kJ (kg K)^−1	Adsorbent specific heat capacity
Cpw	0.502	kJ (kg K)^−1	Heat capacity of wall
ap	477	m^2 g^−1	Specific surface area of adsorbent
Hw	65	W (m^2 K)^−1	Heat transfer coefficient between wall and gas
kg	0.242	W (m K)^−1	Heat conductivity of gas phase
ks	0.3	W (m K)^−1	Heat conductivity of solid phase
kw	17	W (m K)^−1	Heat conductivity of wall
ρw	7800	kg m^−3	Density of wall
Tamb	303.15	K	Ambient temperature

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Table 3 Parameters of adsorption isotherm and physical parameters of adsorbates

Parameters	CH4	CO	CO2	H2	N2
IP1/(kmol kg^−1 bar^−1)	1.79 × 10^−4	0.013	5.26 × 10^−4	4.52 × 10^−5	7.33 × 10^−5
IP2/bar^−1	0.171	3.551	0.228	0.191	0.215
ΔH/(kJ mol^−1)	−21.70	−31.40	−24.87	−11.84	−15.65
Cpa/(kJ kmol^−1 K^−1)	32.24	24.81	37.27	28.86	29.03

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3.2 Mathematical model

The following assumptions have been made for the mathematical models of the adsorption bed:^34–36 (i) the gas phase obey the ideal gas law; (ii) there are no radial variations in the temperature, pressure, and concentrations of gases in the solid and gas phase; (iii) gas flow can be described by the plug flow model; (iv) the kinetics of the adsorption are approximated by the LDF (linear driving force) model; (v) the void fractions in the bed and adsorbent were uniform.

Based on the above assumptions, the component and total mass balance with an estimated axial dispersion coefficient are established as follows:³⁷

The LDF (linear driving force) model is defined by:³⁸

The energy balance of the gas and solid phases are established in eqn (4)–(6).

The pressure drop along the bed is calculated by the Ergun equation:

Finally, the Langmuir model is used to describe the adsorption equilibrium isotherms of CO, CO₂, CH₄, N₂ and H₂ on the CuCl@AC adsorbent.

The PSA system is governed by the nonlinear equations listed in this section. In this work, the PDAEs were discretized in the spatial domain by a second-order central finite difference method with 50 axial nodes.³⁹

4 Results and discussion

4.1 Adsorption isotherms of CO, CO₂, CH₄, N₂ and H₂

Fig. 2 gives the adsorption equilibrium isotherms of pure CO, CO₂, CH₄, N₂ and H₂ on the CuCl(7.0)@AC adsorbent at operation temperature of 303 K. Apparently, the adsorbent achieves a high reversible CO adsorption capacity, and can preferentially adsorb CO over CO₂, CH₄, N₂ and H₂, which is ascribed to the stronger interaction between the highly dispersed CuCl and CO via π-complexation and the weaker van der Waals and electrostatic interactions of CO₂, CH₄, N₂ and H₂ with the adsorbent. In addition, the adsorption equilibrium isotherms were fitted by the Langmuir model, respectively, and fitting parameters are listed in Table 3.

Fig. 2 Adsorption isotherms of CO, CO₂, CH₄, N₂ and H₂ on CuCl(7.0)@AC at operation temperature of 303 K.

4.2 Computational results

The simulation a five-bed VPSA process at ambient temperature of CO separation from syngas utilizing the prepared CuCl@AC adsorbent was conducted using Aspen Adsorption software with the adsorption isotherm parameters and the relevant adsorption bed parameters. The main objective is to analyze a medium-scale VPSA process and optimize the operating conditions. And the high CO purity of higher than 98 vol% and high CO recovery of higher than 90% are considered as the requirements of this VPSA process. The outlet flow rate of the waste gas and the replacement gas flow rate are the important parameters affecting the CO purity and CO recovery. Thus, in this work, these two parameters were optimized to achieve the required the CO purity and CO recovery. And the optimized result is listed in Table 4. Under the feed gas treatment amounts of 39.7 N m³ h⁻¹, when the outlet flow rate of the waste gas and the replacement gas flow rate are 25.7 and 7.7 N m³ h⁻¹, respectively, a high CO purity of 98.1 vol% with CO recovery of 92.9% is achieved, and outlet flow rate of the product gas is 12.1 N m³ h⁻¹.

Table 4 Concentrations of components in streams and the recovery of components in the product gas at a cycle steady state of the five-bed model

Components	CH4	CO	CO2	H2	N2
Product gas	0.2	98.1	0.3	0.9	0.5
Gas waste	3.3	2.8	1.2	66.5	26.2
Replacement waste	4.8	7.2	3.4	58.7	25.9
Recovery	2.2	92.9	9.0	0.6	0.8

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4.2.1 Pressure change of the five adsorption beds in a cycle. Fig. 3 shows the pressure history curves of the five adsorption beds in a cycle. Take bed 1 for example to analyze the pressure change in this VPSA process. At the first 240 s, the compressed feed gas passes through the adsorption bed, and the pressure of bed 1 sharply increased to the required adsorption pressure of 0.79 MPa. After the adsorption (AD) step, the gas of the bed 1 was introduced into bed 3 to equalize pressure for 60 s, and the pressure decreased to 0.40 MPa. At the next 240 s, the replacement step (RP) was conducted utilizing CO product gas, and the pressure was kept at 0.41 MPa. And then the blowdown (BD) step was conducted for 180 s to vent the inside pressure for the next vacuum step, and the pressure decreased to 0.11 MPa. Compared to the direct vacuum step, the conducted blowdown step can reduce the energy consumption. And then the vacuum (VU) step was conducted for 240 s to regenerate the adsorbent and extract the high purity CO product, and the pressure decreased to 0.014 MPa. A low vacuum level in the regeneration step is critical for good performance of the VPSA process. Finally, the pressure equalization for pressurization (EP) and feed repressurization (FR) steps were conducted to increase the pressure of the bed for a new cycle, and the pressure increased to 0.62 MPa.

Fig. 3 Pressure history curves of five adsorption beds in a cycle.

4.2.2 Change of axial CO concentration on adsorption beds in a cycle. Fig. 4 shows the Change of axial CO concentration on adsorption bed 1 in a cycle. Seven measurement points are pinpointed among the adsorption bed with labeled Z/H for 0.00, 0.20, 0.40, 0.50, 0.60, 0.80 and 1.00, respectively, and Z/H = 0.00 is defined as the entrance of adsorption bed. It can be seen from Fig. 4 that, after the adsorption step, the adsorption bed at Z/H = 0.00–0.60 reached the saturation for the CO adsorption. During the ED step, some impurity gases were discharged from the adsorption bed, and the CO concentration increased slightly. During the replacement step, the CO concentration in adsorption bed increased gradually, and after the replacement step, the CO concentration in adsorption bed was higher than 98% except for the bed outlet, i.e., at the end of the replacement step, the adsorption bed at Z/H = 1.00 just going to be saturated for the CO adsorption. Thus, the adsorbents were fully utilized at the optimal condition.

Fig. 4 Change of axial CO concentration on adsorption beds in a cycle (Z, axial distance of adsorption beds; H, height of adsorption bed).

4.2.3 Axial concentration distribution in gas phase at the end of each step. Fig. 5 shows the axial concentration profiles of CO, CO₂, CH₄, N₂ and H₂ after the adsorption (AD) step. In can be seen that, after the adsorption step, the adsorption bed at Z/H = 0.00–0.60 reached the saturation, and the concentration of CO, CO₂, CH₄, N₂ and H₂ did not change with the axial distance of the bed, which consistent with the concentration of each component in feed gas. And in the unsaturated adsorption bed at Z/H > 0.60, with the increase of axial distance of the bed, CO concentration decreased gradually, and the concentration of other components increased gradually. Finally, the concentrations of all components tend to be stable.

Fig. 5 Concentration profiles in gas phase after the adsorption step.

Fig. 6 shows the axial concentration profiles of CO, CO₂, CH₄, N₂ and H₂ after the step of pressure equalization for depressurization (ED). During the ED step, the pressure was released to the adsorption bed which needs to be pressurized, and some impurity gases were discharged from the adsorption bed. Thus, compared with the adsorption step, at the end of ED step, the CO concentration increased, and the concentration of other components decreased. In addition, from Fig. 6, after the ED step, the axial distance of saturated adsorption bed moved from Z/H = 0.6 to 0.7. Fig. 7 shows the axial concentration profiles of CO, CO₂, CH₄, N₂ and H₂ after replacement (RP) step. The replacement step, also called the product purge step, aims to exclude impurity gases from the adsorption bed and increase the concentration of CO in the adsorption bed utilizing part of the CO product. The extent of replacement can greatly affect the purity and recovery of CO. It can be seen from Fig. 7 that, except for the outlet of adsorption bed, the CO concentration in adsorption bed was higher than 98% at the end of replacement step, and the concentration of other components was close to 0. The result indicates that the replacement process is sufficient at the optimal condition.

Fig. 6 Concentration profiles in gas phase after the step of pressure equalization for depressurization.

Fig. 7 Concentration profiles in gas phase after the replacement step.

The axial concentration profiles in gas phase after the blowdown (BD) step and the vacuum (VU) step are depicted in Fig. 8 and Fig. 9, respectively. For VPSA processes, the evacuated pressure is one of the critical benefits to improve the recovery of the product. The blowdown and vacuum steps play the same role in VPSA processes. And unlike the vacuum step, the blowdown step does not consume any energy. After the blowdown step, the pressure of the adsorption bed decreased to 0.11 MPa. From the adsorption isotherms of CO, CO₂, CH₄, N₂ and H₂ on CuCl(7.0)@AC (see Fig. 1), it can be seen that, during the blowdown step, most of the light component gas adsorbed on the adsorption bed was desorbed from the adsorbents, while only a small amount of adsorbed CO was desorbed. Thus, compared with replacement step, after the blowdown step, the concentration of light components increased, and the CO concentration decreased. In addition, it can be seen from Fig. 8 that the impurity gases were mainly concentrated on the top of the adsorption bed. During the vacuum step, the pressure of the adsorption bed further decreased to 0.014 MPa, and the adsorbed CO was almost completely desorbed from the adsorption bed and introduced to the product gas system. And it can be seen from Fig. 9 that, except for the outlet of adsorption bed, the CO concentration in adsorption bed was higher than 99% at the end of vacuum step, and the concentration of other components was close to 0.

Fig. 8 Concentration profiles in gas phase after the blowdown step.

Fig. 9 Concentration profiles in gas phase after the vacuum step.

Fig. 10 and 11 display the axial concentration distributions in gas phase after the step of pressure equalization for pressurization (EP) and the step of the feed repressurization (FR), respectively. These two steps were conducted to increase the pressure of the bed for a new cycle utilizing the pressurized gas from the beds performing the step of pressure equalization for depressurization and the step of adsorption, respectively, in which the CO concentration was at a low level, and the concentration of other components was high. Thus, at the optimal condition, the CO concentration of the bed was maintained at a low level after these two steps.

Fig. 10 Concentration profiles in gas phase after the step of pressure equalization for pressurization.

Fig. 11 Concentration profiles in gas phase after the feed repressurization step.

5 Conclusions

In this work, the activated carbon (AC) supported CuCl (CuCl@AC) is investigated for CO separation from the synthesis gas mixture. This CuCl/AC adsorbent achieves a high reversible CO adsorption capacity at lower operation temperature of 303 K. A simulation and optimization of five-bed VPSA for the CO separation from syngas on CuCl@AC has been described and discussed. The operation step of adsorbent beds in this process is governed by valves and dynamic boundaries that are close to realistic conditions. The operating conditions have been optimized for maximizing the CO recovery and CO product purity by adjusting the outlet flow rate of the waste gas and the replacement gas flow rate. The optimization result indicate that the performance of the VPSA process can be improved under the operating restrictions, and a significantly high recovery (92.9%) and purity (98.1 vol%) of CO can be achieved from the syngas with a low CO concentration (32.3 vol%). This VPSA process is operated at ambient temperature, which can great reduce the operation costs in CO separation processes. In addition, the systematic analysis of this VPSA process can provide significant insight for designing a realistic PSA process.

Conflicts of interest

There are no conflicts to declare.

Nomenclature

ci	Gas phase concentration of component i (mol m^−3)
Cvg	Heat capacity of the gas at constant volume (kJ kmol^−1 K^−1)
Cps	Adsorbent specific heat capacity (kJ kg^−1 K^−1)
Cpa	Heat capacity of the gas at constant pressure (kJ kmol^−1 K^−1)
Dax	Axial dispersion coefficient (m^2 s^−1)
Dm	Molecular diffusivity coefficient (m^2 s^−1)
Db	Internal diameter of bed (m)
Hw	Heat transfer coefficient between wall and gas (W m^−2 K^−1)
Hb	Height of bed (m)
IP1	Adsorption equilibrium constant (bar^−1)
IP2	Adsorption equilibrium constant (kmol kg^−1 bar^−1)
MTCi	Constant mass transfer coefficients (s^−1)
M	Molecular weight (kg mol^−1)
kg	Heat conductivity of gas phase (W m^−1 K^−1)
ks	Heat conductivity of solid phase (W m^−1 K^−1)
kw	Heat conductivity of wall (W m^−1 K^−1)
pi	Pressure (bar)
R	Gas constant (J mol^−1 K^−1)
Rp	Adsorbent particle radius (m)
Tamb	Ambient temperature (K)
Tg	Temperature of gas phase (K)
Ts	Temperature of solid phase (K)
T0	Temperature of wall (K)
vg	Gas velocity (m s^−1)
WT	Wall thickness (m)
yi	Mole fraction of component i
εb	Bed void fraction
εp	Particle void fraction
ρs	Particle density (kg m^−3)
ψ	Shape factor
μ	Gas viscosity (kg m^−1 s^−1)
ΔH	Isosteric heat of adsorption of component (kJ mol^−1)

Acknowledgements

This work has been supported by Natural Science Foundation of Shandong Province (No. ZR2018BB071) and a Project of Shandong Province Higher Educational Science and Technology Program (No. J17KA107).

Notes and references

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