CN105203440A - Method of measuring variable-pressure absorption gas separation performance of carbon molecular sieve on basis of liquid absorption gas flooding principle - Google Patents

Abstract

The invention discloses a method for evaluating the gas separation performance of carbon molecular sieve pressure swing adsorption by using the principle of liquid absorption and gas displacement. The carbon molecular sieve saturated with adsorbed gas probe was immersed in the liquid probe, and the liquid suction and gas displacement experiment was carried out under the condition of constant volume, and the kinetic curve of liquid suction and gas displacement and the equilibrium gas displacement were obtained. The kinetic mechanism and gas selectivity coefficient K are determined through the simulation of the adsorption kinetic model, which is used to judge the relative size of the carbon molecular sieve micropore size, distribution uniformity and the relative size of the micropore volume, and then establish an evaluation carbon molecular sieve. A method for pressure swing adsorption gas separation performance. The method of the invention can accurately determine the carbon molecular sieve whose micropore size and pore volume meet the requirements of gas separation from the perspective of adsorption mechanism and pore structure, and can not only ensure high selectivity, but also ensure high adsorption capacity.

Description

A method for measuring the gas separation performance of carbon molecular sieve pressure swing adsorption based on the principle of liquid suction and gas displacement

technical field

The invention relates to a new method for measuring the gas separation performance of carbon molecular sieve pressure swing adsorption based on the principle of liquid absorption and gas displacement, and belongs to the technical field of gas separation.

Background technique

Carbon Molecular Sieve (Carbon Molecular Sieve, CMS) is one of the common non-polar carbonaceous materials, which belongs to bimodal pore size distribution materials, mainly including ultramicropore (<0.7nm) structure and macropore structure with narrow pore size distribution. The orifice of ultra-micro pores is molecular size, which can screen molecules of different sizes, and is suitable for kinetically selective separation of gas mixtures. It is commonly used in industry for pressure swing adsorption air separation to produce nitrogen, air separation to produce oxygen, recover CO ₂ , recover and Refining hydrogen, purifying CH ₄ in coalbed methane (mixed gas of CH ₄ , N ₂ , etc.), etc. Therefore, it is particularly important to evaluate the structure characteristics and adsorption performance of ultramicropores.

Due to the molecular size of the ultramicropores, the fluid has diffusion problems at low temperature and it is difficult to achieve adsorption equilibrium. Therefore, the low-temperature gas adsorption method cannot be used to characterize its pore structure performance; and the gas at room temperature is mostly in a supercritical state. The study of adsorption theory is still immature. Therefore, the current research on the characterization and adsorption performance of ultramicroporous structures is still a big problem.

One of the evaluation methods of carbon molecular sieve pressure swing adsorption gas separation performance is the selectivity coefficient. US Patent 4933314A calculates the O ₂ /N ₂ selectivity coefficient α by measuring the adsorption amount of O ₂ and N ₂ on the carbon molecular sieve for 1 min respectively. The patent believes that the larger the α and the larger the _O2 adsorption, the better the air separation performance. However, this method cannot accurately reflect the actual gas separation performance of carbon molecular sieves; at the same time, it cannot characterize the internal pore structure of carbon molecular sieves. The gas separation test results of pressure swing adsorption are widely used in the industry to evaluate the gas separation performance of carbon molecular sieves. For example, in the air separation nitrogen production experiment, the outlet flow rate of gas production is constant, and the air separation performance is evaluated by examining the concentration of _N2 in the gas production, the pressure filling time, the highest _O2 concentration in the desorbed gas, and the amount of desorbed gas; or constant gas production N ₂ concentration, the air separation performance is evaluated by examining the nitrogen recovery rate of the produced gas, the highest O ₂ concentration in the desorbed gas and the amount of desorbed gas. However, this method has not formed a unified evaluation standard in the industry. Different pressure swing adsorption devices and different experimental conditions are not comparable in evaluating the gas separation performance of the same adsorbent; at the same time, different adsorbents on the same device require different optimal experimental conditions. .

Contents of the invention

In view of the above technical deficiencies, the purpose of the present invention is to determine the relative size of the carbon molecular sieve ultra-micropore size, uniformity of distribution, and relative size of the micropore volume by means of the principle of liquid absorption and gas displacement, and then establish a method for measuring the pressure swing adsorption of carbon molecular sieves. A method for gas separation performance to guide the preparation of carbon molecular sieves.

The technical scheme adopted in the present invention is:

A method for measuring the gas separation performance of carbon molecular sieve pressure swing adsorption based on the principle of liquid suction and gas displacement, the steps are as follows:

In this method, the liquid absorption and gas displacement device disclosed in Chinese Patent No. 2008100125980 is used to carry out the liquid absorption and gas displacement measurement of carbon molecular sieves. Two corresponding pure gases in the two-component gas mixture (such as O ₂ /N ₂ , CO ₂ /CH ₄ , CH ₄ /N ₂ , etc.) that require dynamic selective separation are used as gas probes, expressed as gas A, B, where the molecular size of A is smaller than the molecular size of B; the polar liquid whose molecular dynamic diameter is smaller than the gas molecule and the gas molecule is not easy to dissolve is used as the liquid probe. For example, the liquid probe used to measure the carbon molecular sieve pressure swing adsorption air separation nitrogen production performance is deionized water, and the gas probes are O ₂ and N ₂ . Firstly, the carbon molecular sieve after crushing, sieving, drying and degassing pretreatment is placed in the sample pool of the liquid suction and gas driving device, and the gas probe is continuously passed into the gas probe under the conditions of normal pressure and constant temperature of 303.2K for saturated adsorption; after that The liquid injection probe is completely immersed in the carbon molecular sieve, and the liquid suction and gas displacement test is carried out under constant volume conditions, and the liquid suction and gas displacement kinetic curve and the equilibrium gas displacement (ml/g) are measured.

The process of liquid suction and gas displacement is a process in which liquid spontaneously diffuses in the sample of equilibrium adsorbed gas, and the adsorbed gas is displaced out. It belongs to the gas\liquid\solid three-phase adsorption process, which mainly includes the following three steps: (1 ) external diffusion process: the diffusion process of liquid molecules and gas molecules outside the pores; (2) internal diffusion process: the diffusion process of liquid molecules and gas molecules in the pores and pores; (3) adsorption/desorption process: gas molecules from the carbon The solid surface of the molecular sieve is desorbed, and the liquid molecules are adsorbed on the solid surface of the carbon molecular sieve. The overall suction and displacement rate is controlled by the slowest process, the rate controlling step. Among them, the effect of external diffusion has been eliminated by continuously magnetically stirring the liquid probe to increase its mass flow rate; the large pores mainly play the role of channels, and their diffusion resistance to gas molecules can be ignored; the size of liquid molecules is smaller than that of gas molecules, so the liquid Molecules suffer from micropore orifice and pore diffusion resistance that is less than that of gas molecules; while carbon molecular sieves are non-polar carbon materials, the adsorption process on the surface of gas molecules is much faster than that on the surface of liquid molecules. To sum up, the liquid-absorbing and gas-driving process of carbon molecular sieves belongs to the three-resistance control process, which is mainly controlled by the surface adsorption resistance of liquid molecules, the diffusion resistance of gas molecules in the micropore orifice or the diffusion resistance in the pores, and the corresponding pseudo-second-order kinetics respectively. Adsorption (PSO) model, linear driving force (LDF) model and Fickian diffusion model. Through the study of a series of carbon molecular sieve liquid absorption and gas displacement kinetics, it is found that the diffusion resistance in the micropores is very small compared with other resistances, and it only plays a certain role when the micropore opening is small; as the opening size decreases Smaller, the orifice diffusion resistance suffered by gas molecules gradually increases, and its rate-controlling effect gradually increases.

The equilibrium purging volume Ve is equal to the gas saturation adsorption volume. Since the gas is mostly in a supercritical state at room temperature, it can only effectively fill the ultramicropores, so the equilibrium purging volume V _e can be used to judge and compare the relative size of the ultramicropore pore volume.

Based on the above principles, the pseudo- ^second -order kinetic adsorption (PSO) model, the linear driving force (LDF) model and the Fick diffusion model are used to fit the liquid-absorbing gas-displacement kinetic curve. The chemical parameters determine the rate control steps of the carbon molecular sieve in the process of liquid absorption and gas driving, and calculate the gas selectivity coefficient K of the carbon molecular sieve, which is used to qualitatively and quantitatively analyze the micropore size and distribution uniformity of the carbon molecular sieve, and then measure the carbon molecular sieve variable pressure. Adsorption gas separation performance; K has the following forms:

Among them, R ² _{(gas A, PSO)} represents the linear fitting correlation coefficient of the PSO model for the gas A process of liquid suction flooding; R ² _{(gas B, LDF)} represents the linear fitting correlation coefficient of the LDF model for the gas B process of liquid suction flooding. The smaller the orifice size of the micropore, the greater the diffusion resistance to gas molecules, that is, the more the kinetic curve fits the LDF model and deviates from the PSO model. Therefore, the K value can truly reflect the relative size of the micropores of carbon molecular sieves, that is, as the size of the micropores decreases, the K value decreases.

The specific measurement method is:

(1) First judge the uniformity of the size distribution of the micropores of the carbon molecular sieve: when the whole process is mainly controlled by a single resistance, the size distribution of the pores is uniform; otherwise, it is not suitable for the gas separation process, and the relative size of the pores is not judged .

(2) Judgment of the relative size of the micropore size of the carbon molecular sieve:

1) When K≈0, that is, the carbon molecular sieve liquid absorption and displacement gas A process conforms to the PSO model, which is mainly controlled by the liquid molecular adsorption process, and the liquid absorption displacement gas B process conforms to the LDF model, which is mainly controlled by the orifice diffusion process, indicating that its micropores The diffusion resistance of the orifice to gas B is much greater than that of gas A, the A/B selectivity is high, and the micropore orifice size is moderate, which is suitable for the pressure swing adsorption gas A/B separation process.

2) When K>0, and the processes of carbon molecular sieve liquid absorption and displacement of gas A and B are mainly controlled by the surface adsorption process of liquid molecules, it means that the average pore size of its micropores is too large, and the A/B selectivity is poor, so it is not suitable for gas A/B. B separation process; the smaller the K value, the better the separation performance.

3) When K<0, and the carbon molecular sieve liquid-absorbing and driving gas A process is mainly controlled by the diffusion resistance of the micropore orifice, it means that the average pore size of the micropore is too small, and it is difficult for the A molecule to fully diffuse into the micropore within the effective adsorption time. Into the product gas directly into the hole, resulting in a decrease in the concentration of B produced, which is not suitable for the gas A/B separation process; the larger the K value, the better the separation performance.

(3) Judging the pore volume of micropores, that is, the larger the equilibrium purging volume, the larger the pore volume of micropores, and the more suitable for the pressure swing adsorption gas separation process.

The beneficial effects of the present invention are:

(1) The present invention can be used to qualitatively evaluate the ultramicropore structure performance of porous materials, and make up for the deficiency that the low temperature gas adsorption method cannot measure the ultramicropore structure parameters due to diffusion limitation.

(2) Qualitatively and quantitatively evaluate the gas mixture separation performance of carbon molecular sieve pressure swing adsorption from the perspective of adsorption kinetics and micropore structure.

(3) It can be used to select the best preparation conditions of carbon molecular sieves and guide the preparation of carbon molecular sieves for the separation of gas mixtures.

Description of drawings

Fig. 1 Kinetic curves of carbon molecular sieves CMS-1~CMS-6 absorb water and flood _N2 .

Fig. ₂ Kinetic curves of carbon molecular sieves CMS-1~CMS-6 absorb water and drive O2.

Detailed ways

Below in conjunction with accompanying drawing, table and embodiment the present invention is further described.

Example 1

In this example, six kinds of carbon molecular sieves for air separation (CMS-1~CMS-6) were used to carry out water absorption and flooding O ₂ /N ₂ experiments at 30°C with a constant volume liquid absorption and gas drive device, and the measured liquid absorption The gas purging kinetic curve and the equilibrium purging gas volume _Ve (ml/g) are shown in Figures 1 and 2. With the help of pseudo-second-order kinetic adsorption (PSO) model and linear driving force (LDF) model to carry out linear simulation, analyze and compare their respective linear correlation coefficient R2 and kinetic parameters (shown in Tables 1 and ² ), and determine the Process dynamics speed control step, calculate selectivity coefficient K value, as shown in Table 3.

By analyzing the speed control step and K value of the six carbon molecular sieves in water absorption flooding O ₂ /N ₂ process in Table 3, the nitrogen production performance of pressure swing adsorption air separation is summarized as follows:

(1) Judgment on the uniformity of micropore size distribution: Only the CMS-5 micropore size distribution is not uniform, which does not meet the requirements of the pressure swing adsorption air separation nitrogen production process.

(2) Analyze the relative size of the micropores of the carbon molecular sieve, and evaluate its pressure swing adsorption air separation nitrogen production performance. K value size law: CMS-1>CMS-2>CMS-3>0>CMS-4>CMS-5>CMS-6, corresponding to air separation effect (nitrogen concentration) law: CMS-1<CMS-2< CMS-3≈CMS-4>CMS-5>CMS-6. CMS-3 and CMS-4 have relatively moderate micropore size and are suitable for pressure swing adsorption air separation nitrogen production process.

(3) As shown in Table 4, the equilibrium gas displacement of CMS-3 is the maximum value of 4.4667ml/g, which confirms that the micropore volume of CMS-3 is the largest. Therefore, it is finally concluded that: CMS-3 is most suitable as an adsorbent for pressure swing adsorption air separation nitrogen production process because of its moderate orifice size, uniform distribution and largest micropore pore volume.

In order to verify the correctness of the invention, a double-tower pressure swing adsorption unit was used to conduct an air separation nitrogen production experiment on the above six carbon molecular sieves. The selected experimental conditions are: inlet flow rate 950ml/min, constant gas production outlet flow rate 1.054ml/min/g; adsorption temperature 30°C, adsorption pressure 0.5MPa, desorption pressure 0.1MPa; adsorption time 64s, venting time 2s , pressure equalization time 1s. The experimental results are shown in Table 5. Comparing the law of N ₂ concentration in gas production is: CMS-1<CMS-2<CMS-3≈CMS-4>CMS-5>CMS-6, indicating that the law of air separation performance is: CMS-1<CMS-2<CMS -3≈CMS-4>CMS-5>CMS-6, where CMS-3 is the best adsorbent. The above conclusions are completely consistent with the results obtained in this example, which verifies the correctness and practicability of the method for measuring the gas separation performance of carbon molecular sieve pressure swing adsorption disclosed in the present invention.

Example 2

In this example, 4 kinds of carbon molecular sieves (CMS-1~CMS-4) were used to carry out water absorption flooding N ₂ /CH ₄ experiments at 30°C with a constant volume liquid absorption and gas displacement device, and the kinetics of liquid absorption and gas displacement were measured. The curve and the equilibrium purging volume _Ve (ml/g) are shown in Figures 1 and 2. The pseudo-second-order kinetic adsorption (PSO) model and the linear driving force (LDF) model were used to conduct linear simulations to determine the kinetic speed-controlling step of the process of water absorption and gas displacement, and to calculate the value of the selectivity coefficient K, as shown in Table 6. K value size rule: CMS-1>CMS-2>0>CMS-3>CMS-4, corresponding to gas separation effect rule: CMS-1<CMS-2>CMS-3>CMS-4. The K value of CMS-2 is closest to 0, so it is most suitable for pressure swing adsorption separation N ₂ /CH ₄ process.

Table 1 PSO model and LDF model simulate water flooding of carbon molecular sieves CMS-1~CMS-6 at 303.2K

Kinetic parameters and linear correlation coefficients of _O2 process

Table 2. Kinetic parameters and linear correlation coefficients of carbon molecular sieves CMS-1~CMS-6 water flooding _N2 process simulated by PSO model and LDF model at 303.2K

Table 3 Summary of rate control steps and selectivity coefficient K of carbon molecular sieves CMS-1~CMS-6 water absorption flooding O ₂ /N ₂ process

Sample CMS-1 CMS-2 CMS-3 CMS-4 CMS-5 CMS-6 Water absorption flooding O ₂ process S S S S S+M m Water flooding N ₂ process S S m m S+M m K(%) 5.304 2.445 0.097 -0.016 -1.076 -2.792

Symbol S stands for water molecule surface adsorption process control; M stands for orifice diffusion resistance control

Table 4 Carbon molecular sieves CMS-1~CMS-6 water absorption flooding O ₂ process equilibrium gas displacement V _e

Sample CMS-1 CMS-2 CMS-3 CMS-4 CMS-5 CMS-6 _Ve (ml/g) 2.5140 2.5052 4.4667 3.1131 3.4951 2.9801

Table 5 Carbon molecular sieve CMS-1～CMS-6 double-tower pressure swing adsorption air separation test data

Table 6 Summary of rate control steps and selectivity coefficient K of carbon molecular sieves CMS-1~CMS-4 water flooding N ₂ /CH ₄ process

Sample CMS-1 CMS-2 CMS-3 CMS-4 Water flooding N ₂ process S S m m Water flooding CH ₄ process S m m m K(%) 2.867 0.005 -1.744 -4.634

Claims (2)

1. measure a method for carbon molecular sieve PSA Gas separating property based on imbibition purging principle, it is characterized in that, step is as follows:

This method adopts imbibition purging device disclosed in ZL200810012598.0 to carry out carbon molecular sieve imbibition purging and measures;

Two kinds of pure gas corresponding in the bicomponent gas mixt adopting kinetic selectivity to be separated respectively are as gas probe, and be expressed as gas A and gas B, wherein gas A molecular dimension is less than gas B molecular dimension; Employing molecular dynamics diameter is less than gas molecule and gas molecule is difficult to the polar liquid of dissolving as liquid probe; First by through broken, screening, the sample cell that dry, degassed pretreated carbon molecular sieve is placed in imbibition purging device, under normal pressure, constant temperature 303.2K condition, pass into gas probe continuously carry out saturated adsorption; Inject the complete submergence carbon molecular sieve of liquid probe afterwards, under constant volume, carry out the test of imbibition purging, measure imbibition purging kinetic curve and balance purging amount ml/g; Balance purging amount V _eequal gas saturated extent of adsorption, under normal temperature condition, gas is in supercriticality, effectively fills ultramicropore, balance purging amount V _efor comparing ultramicropore pore volume relative size;

According to pseudo-second order kinetic Adsorption Model, linear pushing power model and Fick diffusion model, matching is carried out, by respective fitting correlation coefficient R to imbibition purging kinetic curve ²rate determining step, the calculating carbon molecular sieve selectivity factor K of this carbon molecular sieve imbibition purging process is determined with kinetic parameter, for qualitative, analyze carbon molecular sieve micropore port size and distributing homogeneity quantitatively, and then measure carbon molecular sieve PSA Gas separating property; K has following form:

Wherein, R ² _{(gas A, PSO)}represent the linear fitting correlation coefficient of PSO model of imbibition purging body A process; R ² _{(gas B, LDF)}represent the linear fitting correlation coefficient of LDF model of imbibition purging body B process; Micropore port size is less, larger to the diffusional resistance of gas molecule, and namely kinetic curve more meets LDF model, departs from PSO model; K value is reflection carbon molecular sieve micropore port size relative size truly, and namely along with the reduction of micropore port size, K value reduces thereupon;

Concrete assay method is:

(1) carbon molecular sieve micropore port size distributing homogeneity is first judged: when whole process is mainly by single draught control, then port size distribution is homogeneous; Otherwise, be not suitable for gas separating technology, do not give the judgement of port size relative size;

(2) carbon molecular sieve micropore port size relative size judges:

1) as K ≈ 0, namely carbon molecular sieve imbibition purging body A process meets PSO model and the process control of main liquid body molecular adsorption, imbibition purging body B process meets LDF model and namely mainly control by aperture diffusion process, illustrate that its micropore aperture will be far longer than gas A to the diffusional resistance of gas B, A/B selectivity is high, micropore port size is moderate, is applicable to PSA Gas A/B separating technology;

2) work as K>0, and all main liquid body molecular surface adsorption process of carbon molecular sieve imbibition purging body A, B process controls, and illustrates that the average port size of its micropore is bigger than normal, A/B poor selectivity, is not suitable for gas A/B separating technology; K value is less, and separating property is better;

3) as K < 0, and carbon molecular sieve imbibition purging body A process mainly controls by micropore aperture diffusional resistance, illustrate that the average port size of its micropore is less than normal, in effective adsorption time, A molecule is difficult to fully diffuse in micropore directly enter in gas product and causes producing B concentration and reduce, and is not suitable for gas A/B separating technology; K value is larger, and separating property is better;

(3) Micropore volume judges, namely balance purging amount larger, Micropore volume is larger, is more applicable to adsorption gas separating technology.

2. the method measuring carbon molecular sieve PSA Gas separating property based on imbibition purging principle according to claim 1, it is characterized in that, the method is utilized to measure carbon molecular sieve air separation by PSA nitrogen performance, using deionized water as liquid probe, respectively with O ₂and N ₂as gas probe.