Separation of Hydrogen-Lean Mixtures for a High-Purity Hydrogen by Vacuum Swing Adsorption

Abstract

High-purity hydrogen is commercially produced by pressure swing adsorption from hydrogen-rich mixtures. In this work, a vacuum pressure swing adsorption cycle is used to produce high purity hydrogen from a hydrogen-lean binary mixture (20/80 H₂/CO) using zeolite 5A as the sorbent. The effects of different process variables on separation performance have been studied. The purity of hydrogen product increases at low throughput, high feed pressure, high end pressure of cocurrent depressurization, low end pressure of countercurrent evacuation, and short cycle time. Also, it was found that for a H₂-lean mixture, the separation is improved at higher ambient temperature. In addition, a new “vacuum purge” step was found to improve the separation and is therefore a promising step for commercial application.     

Application of an adsorption non-flow exergy function to an exergy analysis of a pressure swing adsorption cycle

Pressure swing adsorption (PSA) is a popular gas separation technology for the process industries and is commonly used for air separation, hydrogen purification, and isomer separation. In this study, we apply a second law analysis to this technology to identify sources of irreversibility in the process and, in particular, identify which steps in the PSA cycle are responsible for the major losses. Unlike previous exergy analyses, we derive and use expressions for the exergy of the adsorbed phase using adsorption thermodynamics. In this way, exergy loss (or entropy generation) within the adsorption cycle in each step is clearly identified. We illustrate the use of these exergy functions with the application of binary linear isotherm (BLI) theory to a four-step Skarstrom cycle. Major losses in the process are shown to be the exergy loss across the valve in the blowdown step, and feed compressor aftercooler losses. Feed repressurisation is shown to be more efficient than product repressurisation for the separation factor examined in this study since part of the feed gas is introduced at a low pressure. During the cycle, bed exergy loss during the feed step is significant, while there is no exergy loss in the adsorbent bed during the blowdown or purge steps. The exergy functions derived in this study can readily be applied to more complex PSA cycles and provide a basis for cycle design.     

Feed Purge Cycles in Pressure Swing Adsorption

Abstract

Three new pressure swing adsorption cycles for the production of oxygen-enriched air at very high recoveries are developed. The cycles involve a purge step using low-pressure feed gas. Local equilibrium theory is used to predict purities and recoveries for air separation with 5A zeolite. Novel methods for drying the inlet air are developed. For low product purities these processes are predicted to have very high recoveries and adsorbent productivities. A simple new method for determining the location of a curved shock wave is illustrated. Other possible applications of PSA feed purge cycles are briefly discussed.     

DESIGN OF SOLIDS SEPARATION PROCESSES CONSISTING OF IDENTICAL STAGES

The separation of the constituent minerals of an ore is never achieved in one stage. It is carried out in multiple stages incorporating recycling of streams to increase the recovery of the required mineral. This paper outlines a methodology for designing a separation process system consisting of nonsharp separation stages and involving recycles and mixtures instead of pure component as products.

For given technical and economic criteria the regions of optimal configurations on the grade versus flowrate diagram have been recognised. This diagram is used for the selection of the optimal circuit configuration for given feed flow, feed composition and separation characteristics. The minimum economically processable grade of the feed has been identified. This allows a decision to be made about an ore deposit as to whether it carries potential value for further processing or not. The maximum economically achievable grade has also been identified. This places a limit on the upgrading of the product for maximum profit.

The paper reports a number of case studies from the view point of application perspective, using the grade-flowrate diagram, and compares this work with the work of Chan and Prince. This comparison provides essentially the same results as the work of Chan and Prince. Sensitivity analysis in regard to the use of parameter values is another key aspect of this paper. The effects of various technical and economical parameters on the grade-flowrate diagram have been studied in this particular paper.

Results obtained from different case studies show that the present study offers an effective means for design engineers to make an initial selection of a process configuration for stagewise separation processes prior to the detailed flowsheet design.     

TRANSIENT FLOW OF A COMPRESSIBLE FLUID THROUGH BEDS OF SOLID PARTICLES OR POROUS MATERIALS

The problem of transient gas flow through porous structures is a typical one in many practical applications e.g. gas production from an underground reservoir, gas flow through the soil after an underground nuclear explosion or gas flow through a packed column. This later application is important for separation processes called Rapid Pressure Swing Adsorption (RPSA) and Pressure Swing Adsorption (PSA) where the course of the column pressurization strongly affects the mass transfer between the gas and either the adsorbent or the molecular sieve.

A mathematical model for the isothermal transient flow of an ideal gas through a solid bed or porous material has been compared with experimental results both in Darcy's region and in the high velocity region. Numerical simulations have been used to study the effect of the fixed bed characteristic parameters on the pressurization course of a fixed bed column closed at one end and connected to the constant pressure vessel on the other end. In four limiting cases the results have been interpreted using simple algebraic equations.

The theoretical analysis proved that for the particles being tested (d_p smaller than 0·7  mm) the effect of macroscopic inertia forces and nonisothermal effects are negligible under the following conditions: Input pressure smaller than 600  kPa and the ratio of inlet to initial pressure smaller than 20.     

CO2 recovery from flue gas by PSA process using activated carbon

An experimental study was performed for the recovery of CO₂ from flue gas of the electric power plant by pressure swing adsorption process. Activated carbon was used as an adsorbent. The equilibrium adsorption isotherms of pure component and breakthrough curves of their mixture (CO₂ : N₂ : O₂=17 : 79 : 4 vol%) were measured. Pressure equalization step and product purge step were added to basic 4-step PSA for the recovery of strong adsorbates. Through investigation of the effects of each step and total feed rate, highly concentrated CO₂ could be obtained by increasing the adsorption time, product purge time, and evacuation time simultaneously with full pressure-equalization. Based on the basic results, the 3-bed, 8-step PSA cycle with the pressure equalization and product purge step was organized. Maximum product purity of CO₂ was 99.8% and recovery was 34%.     

GENERALIZED FINITE DIFFERENCE SOLUTION FOR HEAT AND MASS TRANSFER FROM A FINITE CYLINDER DURING QUENCH

A generalized nondimensional solution is presented that describes heat or mass transfer from a finite cylinder during quench. The solution is applicable to three important cases:

Conduction with convection heat transfer at the surface during any single step hot or cold quench.

Conduction with radiation heat transfer at the surface during a single step cold quench with negligible background radiation.

Diffusion with surface desorption of a diatomic gas from a metal specimen during a single step quench in a high vacuum with negligible background pressure.

Application of the generalized solution, which utilizes the numerical method of finite differences with forward stepping, is illustrated by determining a cylinder's transient temperature distribution and surface transfer rate (both instantaneous and cumulative) for an example L/D ratio of 2.0. Selected results are graphed and tabulated for the three cases. The results for the conduction/convection case are verified using the familiar analytical product solution as well as the lumped solution. For the conduction/radiation and diffusion/desorption cases, no analytical solutions are available other than the lumped limit which is in agreement.     

Separation of Binary Gas Mixture into Two High-Purity Products by a New Pressure Swing Adsorption Cycle

Abstract

Binary mixtures can be separated into two high-purity products by a new pressure swing adsorption (PSA) cycle. The product purity depends on the purge/feed ratio of the respective gases in the PSA cycle. The process characteristics of the new PSA cycle, using activated carbon as the sorbent, can be adequately predicted by an equilibrium model.     

Optimum structured adsorbents for gas separation processes

Recent developments in separation technology by adsorption have included the development of new structured adsorbents which offer some attractive characteristics compared to a typical packed bed. These improved features include lower energy consumption, higher throughput and superior recovery and purity of product. However, the exact combination of structural, geometric parameters which yields optimum performance is unknown. This study formulates a methodology for comparison based on a variety of analytical and numerical models and uses it to examine the performance of different adsorbent configurations. In particular, monolithic, laminate and foam structures are evaluated and compared to a packed bed of pellets. The effects of physical adsorbent parameters which govern the performance of a PSA process are considered during model development. Comparisons are carried out based on mass transfer kinetics, adsorbent loading and pressure drop of a PSA system for CO₂/N₂ separation. The results indicated that structured adsorbents can provide superior throughput to packed beds provided their geometrical parameters exceed certain values. For example, laminate structures can offer superior performance to a packed bed of pellets only if the critical sheet thickness and spacing are less than about 0.2 mm. Each adsorbent structure should be designed to operate at its “optimal” velocity. When operating at velocities higher than the “optimal” value, the increase in pressure drop and length of the mass transfer zone more than offsets gains accrued through reduction in cycle time.     

PROCESSES RELATED TO DRYING: PART II USE OF THE SAME MODEL TO SOLVE TRANSFERS BOTH IN SATURATED AND UNSATURATED POROUS MEDIA

From the mathematical formulation presented in part I, a numerical code is developed to simulate heat and: mass transfers in porous media. The aim of this· tool is to understand and to improve each process related to drying. The association of a comprehensive set of equations with a efficient 2-D computer code allows us to predict the comportment of several porous media even if submitted to severe drying conditions. A few runs have been selected with special attention paid to the effect of internal gaseous pressure:

Convective drying of softwood at high temperature illustrate the typical two-dimensional transfers that occur in an anisotropic medium.

Microwave drying of light concrete pinpoints liquid expulsion of water which is driven by the pressure due to internal heating.

Finally, appropriate physical behaviours of a bed of glass spheres allows one to deal with simple processes for which full saturation occurs.     

Design of a two‐step pulsed pressure‐swing adsorption‐based oxygen concentrator

A two‐step pulsed pressure‐swing adsorption (PPSA) process has been modeled to assess the extent to which an oxygen concentrator might be miniaturized for medical applications. The process consists of a single bed of packed adsorbent particles that is alternately pressurized and depressurized at the feed end. An enriched oxygen product is withdrawn at ambient pressure from the product end when the bed is pressurized at the feed end. The product end remains closed during depressurization. The model development addresses the manner in which axial dispersion enters into the describing equations and the formulation of proper boundary conditions, both of which have not been handled rigorously in some prior modeling studies. The describing equations are solved using COMSOL® Multiphysics software. The effect on the performance of the adsorption time, desorption time, bed length, particle diameter, and imposed pressure drop across the bed have been investigated. An interesting novel result is that for a chosen particle size, bed length, and applied pressure drop, there is an optimum combination of adsorption and desorption times that maximizes the product purity. The results suggest that there are operating windows for both 5A and partially Ag‐exchanged Li‐substituted 13X zeolite adsorbents wherein the product oxygen purity is greater than 90%. At a given product flow rate within this operating window, the extent of miniaturization is limited by the (maximum) cycling frequency that is practically achievable. Sizing of an oxygen concentrator for personal medical applications is also discussed. A principal conclusion is that a compact oxygen concentrator capable of producing a highly oxygen‐enriched product is possible using commercially available adsorbents and implementable operating conditions. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Efficient pressure swing adsorption for improving H<Subscript>2</Subscript> recovery in precombustion CO<Subscript>2</Subscript> capture

An efficient design for pressure swing adsorption (PSA) operations is introduced for CO₂ capture in the pre-combustion process to improve H₂ recovery and CO₂ purity at a low energy consumption. The proposed PSA sequence increases the H₂ recovery by introducing a purge step which uses a recycle of CO₂-rich stream and a pressure equalizing step. The H₂ recovery from the syngas can be increased over 98% by providing a sufficient purge flow of 48.8% of the initial syngas feeding rate. The bed size (375m³/(kmol CO₂/s)) and the energy consumption for the compression of recycled CO₂-rich gas (6 kW/(mol CO₂/s)) are much smaller than those of other PSA processes that have a CO₂ compression system to increase the product purity and recovery.     

Membrane separation process modeling for CO2 partial removal in prepurification of air separation units

A novel membrane/adsorption hybrid system was proposed for air prepurification in large scale air separation units. Mathematical models were established for cocurrent and countercurrent flow patterns with crude nitrogen as purge stream to describe the membrane separation performance. The experimental and predicted results are in good agreement confirming the validity of the mathematical models. Effects of membrane properties and operation parameters on O₂ recovery, N₂ recovery, and membrane area requirement were investigated. For countercurrent flow pattern, O₂ recovery and N₂ recovery were larger than 98 and 99%, respectively, and membrane area requirement was less than 0.25?m²/m³?h^?1 with feed side pressure of 0.6?MPa and the purge gas/feed gas ratio of 0.2.     

快速变压吸附制氢工艺的模拟与分析

目前工业上主要通过变压吸附技术从蒸汽甲烷重整气中制取氢产品气。然而,能源需求量的快速增加使得传统变压吸附技术在产量方面的不足越发明显。为此,进行了快速变压吸附从蒸汽甲烷重整气中制取氢气的模拟研究。采用活性炭和5A分子筛作为吸附剂,并以测得的原料气中各组分在两种吸附剂上的吸附数据为基础,进行了六塔快速变压吸附工艺的数值模拟与分析。在分析了塔内温度、压力和固相的浓度分布后,探究了进料流量、双层吸附剂高度比以及冲洗进料比三个操作参数对于快速变压吸附工艺性能的影响,结果表明：原料气组成为H₂/CH₄/CO/CO₂=76%/3.5%/0.5%/20%,吸附压力为22 bar（1 bar=10⁵ Pa）,解吸吹扫压力为1.0 bar,处理量为0.8875 mol·s^-1,吸附剂床层高度比为0.5∶0.5,冲洗进料比为22.37%时,可获得H₂纯度99.90%,回收率69.88%,此时H₂产量为0.4713 mol·s^-1。相比之下,氢气纯度为99.90%时,尽管PSA工艺回收率为83.40%,但处理量只有0.39 mol·s^-1,因此H₂产量仅为0.2472 mol·s^-1。     

Energy Minimization in Cryogenic Packed Beds during Purification of Natural Gas with High CO2 Content

Minimization of energy consumption was explored for countercurrent switched cryogenic packed beds in which separation of CO₂ and other components of natural gas can be achieved based on differences in freezing or desublimation points. Highly pure CO₂ and methane were obtained after separation. An experimental setup for CO₂ removal from natural gas was constructed and a detailed experimental study was conducted by changing different operating parameters. Compared to other cocurrent or jacket‐cooled constant‐temperature configurations, countercurrent switched beds provided optimal separation and energy efficiencies. The effects of important process parameters like initial temperature profiles of the cryogenic bed, feed composition, and feed flow rate on energy requirement, bed saturation, bed pressure, and cycling times were investigated. The energy requirement for cryogenic packed beds was compared with the conventional cryogenic distillation process.     

Production of dehydrated fuel ethanol by pressure swing adsorption process in the pilot plant

Fermented ethanol is gaining wide popularity as a car fuel additive. The pressure swing adsorption (PSA) process is attractive for the dehydration of bioethanol on the industrial scale, since its energy consumption is low and it is capable of producing a very pure product. 3 ? zeolite possesses micro-pores which, due to their small size, adsorb water molecules but not ethanol molecules. In this work, up to 2 kL/day of dehydrated ethanol (99.5% by weight) was successfully produced with 3 ? zeolite by the pressure swing adsorption process. The cycles of the PSA process were operated under the following variables: feed flow rate (27–62 Nm³/hr of 93.2% by weight), purge/feed ratio (30–24%), adsorption temperature (130–140 °C) and adsorption pressure (1.2 atm).     

STEAM DRYLNG OF WOOD FUELS

The objective of this work is to investigate some of the important aspects in the design of a steam band dryer for wood fuels. For this purpose the drying of the material in a bench-scale fixed bed dryer has been studied.

Drying times and thermal efficiencies for experiments conducted under different conditions are compared. The investigated materials are soft-and hardwood chips and softwood bark.

The thermal efficiency, the part of the sensible heat which is used during one passage of the steam through the bed, increases with increasing mass load (mass of dry matter per unit area) and with decreasing steam mass flux. At a mass load of 30 kg/m the thermal efficiency is about 0.85 even at steam mass fluxes as high as 0.6 kg/m² s (1.2 m/s). The thermal efficiency proves to be almost independent of pressure and temperature of the steam.

Due to the very inhomogeneous materials the steam mass flow distribution was uneven. This causes a decreased thermal efficiency. When bark     

Comparative study on pressure swing adsorption system for industrial hydrogen and fuel cell hydrogen

In order to improve the design of PSA system for fuel cell hydrogen production, a non-isothermal model of eight-bed PSA hydrogen process with five-component (H₂/N₂/CH₄/CO/CO₂=74.59%/0.01%/4.2%/2.5%/18.7% (vol)) four-stage pressure equalization was developed in this article. The model adopts a composite adsorption bed of activated carbon and zeolite 5A. In this article, pressure variation, temperature field and separation performance are stimulated, and also effect of providing purge (PP) differential pressure and the ratio of activated carbon to zeolite 5A on separation performance in the process of producing industrial hydrogen (CO content in hydrogen is 10 μl·L^-1) and fuel cell hydrogen (CO content is 0.2 μl·L^-1) are compared. The results show that Run 3, when the CO content in hydrogen is 10 μl·L^-1, the hydrogen recovery is 89.8%, and the average flow rate of feed gas is 0.529 mol·s^-1; When the CO content in hydrogen is 0.2 μl·L^-1, the hydrogen recovery is 85.2%, and the average flow rate of feed gas is 0.43 mol·s^-1. With the increase of PP differential pressure, hydrogen recovery first increases and then decreases, reaching the maximum when PP differential pressure is 0.263 MPa; With the decrease of the ratio of activated carbon to zeolite 5A, the hydrogen recovery increases gradually. When the CO content in hydrogen is 0.2 μl·L^-1 the hydrogen recovery increases more obviously, from 83.96% to 86.37%, until the ratio of activated carbon to zeolite 5A decreases to 1. At the end of PP step, no large amount of CO₂ in gas or solid phase enters the zeolite 5A adsorption bed, while when the CO content in hydrogen is 10 μl·L^-1, and the ratio of carbon to zeolite 5A is less than 1.4, more CO₂ will enter the zeolite 5A bed.     

Strategies for CO2 capture from different CO2 emission sources by vacuum swing adsorption technology☆

Different VSA(Vacuum Swing Adsorption) cycles and process schemes have been evaluated to find suitable process configurations for effectively separating CO2 from flue gases from different industrial sectors. The cycles were studied using an adsorption simulator developed in our research group, which has been successfully used to predict experimental results over several years. Commercial zeolite APGIII and granular activated carbon were used as the adsorbents. Three-bed VSA cycles with- and without-product purge and 2-stage VSA systems have been investigated. It was found that for a feed gas containing 15% CO2(representing flue gas from power plants), high CO2 purities and recoveries could be obtained using a three-bed zeolite APGIII VSA unit for one stage capture, but with more stringent conditions such as deeper vacuum pressures of 1–3 k Pa. 2-stage VSA process operated in series allowed us to use simple process steps and operate at more realistic vacuum pressures. With a vacuum pressure of 10 k Pa, final CO2 purity of 95.3% with a recovery of 98.2% were obtained at specific power consumption of 0.55 MJ·(kg CO2)-1from feed gas containing15% CO2. These numbers compare very well with those obtained from a single stage process operating at1 k Pa vacuum pressure. The feed CO2 concentration was very influential in determining the desorption pressure necessary to achieve high separation efficiency. For feed gases containing N 30% CO2, a singlestage VSA capture process operating at moderate vacuum pressure and without a product purge, can achieve very high product purities and recoveries.     

变压吸附空分制氮过程的研究

建立了一套中试装置 ,对以商业炭分子筛为吸附剂的变压吸附 (PSA)空分制氮循环过程进行了系统研究 .用所建立的数学模型对相应实验进行模拟并将模型预测与实验数据进行比较 ,结果表明模型是可靠的 .