Hydrogen PSA process product purity control method and controller

It is well known in the industry that a primary means for controlling the pressure swing adsorption (PSA) process product gas purity is the adjustment of PSA feed time or adsorption time. If the product impurity is too high, the feed time is shortened and if the impurity level is below the target the feed time is increased. Conventionally, the plant operator monitors the product purity and manually adjusts the feed time. Several control methodologies such as classical feedback and feedforward systems were suggested to automate this task with limited success. A novel control methodology based on the measurement of impurity fronts within the adsorber bed was developed by the Praxair Adsorption R&D team. The response of the concentration measurements inside the adsorber vessel to the process upsets and changes in feed time is more rapid than in the product stream. Consequently, closed loop control performance can be made much more effective and the operating impurity set points for product gas can be more aggressive resulting in longer PSA feed times, higher bed utilization and thus higher hydrogen recovery. The control methodology will be discussed in greater detail along with the advantages it has to offer such as improved process performance, disturbance rejection capability and improved process robustness. The control methodology will be illustrated using the hydrogen PSA process as an example.     

Solvent gradient operation of simulated moving beds. I. Linear isotherms

The simulated moving bed (SMB) is a multi-column chromatographic separation process, which--with respect to the single-column preparative batch process--allows for a continuous separation with larger productivity and smaller solvent consumption at the same time. The benefits of this process have been shown for several different applications in fine chemistry, particularly for the separation of enantiomers. In general, SMBs are operated under isocratic conditions. However, separation performance can be further improved by applying some sort of gradient mode operation, in order to optimize the operating conditions of each individual section of the unit. This can be achieved by tuning the retention behavior of the solutes to be separated along the unit, namely by enforcing weak adsorption conditions in sections 1 and 2, and strong adsorption conditions in sections 3 and 4. This can be achieved by applying a temperature gradient (high temperature in section 1, and low temperature in section 4), a pressure gradient (e.g. in the supercritical SMB, when pressure is high in section 1, and low in section 4), or a solvent gradient, which is the aim of this work. In the solvent gradient mode the mobile phase consists of a mixture of two or more solvents. To different mobile phase compositions corresponds a different retention behavior of the solutes, i.e. different adsorption isotherms. In this work we study a closed loop SMB unit with solvent mixtures of two different compositions entering the unit at the feed and desorbent inlet ports, respectively. Thereby two different mobile phase compositions are established in sections 1 and 2, and sections 3 and 4, respectively. To optimize this process the equilibrium theory design criteria for non-linear SMBs are extended to describe this operation mode. It is shown how the region of separation is derived and how the optimal operating conditions can be found. Finally the solvent gradient mode is compared with the isocratic mode in terms of productivity and solvent consumption.     

Parametric Study of a Pressure Swing Adsorption Process

The performance of a pressure swing adsorption (PSA) process for production of high purity hydrogen from a binary methane-hydrogen mixture is simulated using a detailed, adiabatic PSA model. An activated carbon is used for selective adsorption of methane over hydrogen. The effects of various independent process variables (feed gas pressure and composition, purge gas pressure and quantity, configuration of process steps) on the key dependent process variables (hydrogen recovery at high purity, hydrogen production capacity) are evaluated. It is demonstrated that many different combinations of PSA process steps, their operating conditions, and the feed gas conditions can be chosen to produce an identical product gas with different hydrogen recovery and productivity.     

Kinetic Separation of Oxygen and Argon Using Molecular Sieve Carbon

A pressure-swing adsorption (PSA) simulation study was performed for the separation of a mixture of 95% O₂ and 5% Ar using a molecular sieve carbon (MSC) as the adsorbent. Two PSA cycles have been outlined to maximize the recovery of either argon or oxygen as a high purity product. The effect of cycle parameters such as cocurrent depressurization pressure, purge/feed ratio, pressure ratio and adsorption pressure on the separation of O₂/Ar has been studied. It was found that it is feasible to obtain an argon product of purity in excess of 80% with reasonably high recovery using one of the cycles. The other cycle is capable of producing high purity oxygen (>99%) at high recovery (>50%) with reasonably high product throughputs. The PSA process can be conducted at room temperature and hence has an advantage over conventional processes like cryogenic distillation and cryogenic adsorption.     

Implementation of an automated on-line high-performance liquid chromatography monitoring system for ‘cycle to cycle’ control of simulated moving beds

In continuous chromatography simulated moving bed (SMB) is a firmly established powerful technique for the separation of fine chemicals and enantiomers. The use of a controller could improve the operation conditions and increase the productivity of an SMB unit. However, the performance of any controller is greatly affected by the reliability and the quality of the feedback information from the plant. Therefore, to overcome the limitations of optical detectors, such as UV and polarimeter, an automated on-line HPLC monitoring system was developed and installed to monitor the product streams. The performance of the system is tested experimentally separating a mixture of guaifenesin enantiomers on Chiralcel OD columns with ethanol as mobile phase in our laboratory SMB unit under both linear and nonlinear chromatographic conditions. The results show that the new monitoring system provides precise and accurate data about the concentration of the components in the two product streams. Moreover, they prove that despite disturbances a combination of the controller and the new on-line monitoring system allows to fulfill the product specifications and to improve the performance of the process in terms of feed throughput and solvent consumption.     

Experimental implementation of automatic ‘cycle to cycle’ control to a nonlinear chiral simulated moving bed separation

In order to better exploit the economic potential of the simulated moving bed chromatography a ‘cycle to cycle’ controller which only requires the information about the linear adsorption behavior and the overall average porosity of the columns has been proposed. Recently, an automated on-line HPLC monitoring system which determines the concentrations in the two product streams averaged over one cycle, and returns them as feedback information to the controller was implemented. The new system allows for an accurate determination of the average concentration of the product streams even if the plant is operated at high concentrations. This paper presents the experimental implementation of the ‘cycle to cycle’ control concept to the separation of guaifenesin enantiomers under nonlinear chromatographic conditions, i.e. at high feed concentrations. Different case studies have been carried out to challenge the controller under realistic operation conditions, e.g. introducing pump disturbances and changing the feed concentration during the operation. The experimental results clearly demonstrate that the controller can indeed deliver the specified purities and improve the process performance.     

Sensitivity of PSA process performance to input variables

Mathematical models for pressure swing adsorption (PSA) processes essentially require the simultaneous solutions of mass, heat and momentum balance equations for each step of the process using appropriate boundary conditions for the steps. The key model input variables needed for estimating the separation performance of the process are the multicomponent adsorption equilibria, kinetics and heats of adsorption for the system of interest. A very detailed model of an adiabatic Skarstrom PSA cycle for production of high purity methane from a ethylene-methane bulk mixture is developed to study the sensitivity of the process performance to the input variables. The adsorption equilibria are described by the heterogeneous Toth model which accounts for variations of isosteric heats of adsorption of the components with adsorbate loading. A linear driving force model is used to describe the kinetics. The study shows that small errors in the heats of adsorption of the components can severely alter the overall performance of the process (methane recovery and productivity). The adsorptive mass transfer coefficients of the components also must be known fairly accurately in order to obtain precise separation performance.     

Performance of a Two-Bed Pressure Swing Adsorption Process with Incomplete Pressure Equalization

Incomplete pressure equalization (PE) is practiced in a commercial oxygen concentrator for medical use by adopting simultaneous PE and feed-pressurization for pressurizing an adsorption bed. In such a cycle configuration, extent of equalization during PE affects process performance. In order to assess the effect, performance of pressure swing adsorption (PSA) process with incomplete PE was determined by both simulations and experiments. In simulations, an equilibrium model was used with the assumptions of multicomponent Langmuir isotherms, isothermal operation, and no pressure drop through a bed. The required parameters for simulations were measured in separate experiments. PSA experiments were performed for a two-bed cycle with PE. Two kinds of pressurization, feed and product, were examined. Effects of purge amount and extent of equalization on process performance were assessed in view of productivity and light-component recovery. From the obtained results performance contours were constructed. 95 oxygen mole percent production from air with zeolite 13× was considered as a case study. In both pressurizations, an optimal specific purge and an extent of equalization for the productivity and recovery were observed, but with a different level of equalization. For a maximum productivity feed-pressurization favored incomplete PE, while a maximum recovery occurred at complete PE for both pressurizations. The simulations depicted well existence of optimum conditions, though they showed quantitative disagreement with experiments.     

扩张床吸附技术

扩张床是流化床的一种特例。它具有流化床的特点,能处理含悬浮颗粒的液体。又具有固定床的优点,流动成活塞流;返混程度低,分离效率高。作为蛋白质的初步分离方法,它能取代固液分离、浓缩和初步纯化等三步操作。具有提高收率、降低投资费用、缩短操作时间等优点,成为生物工程下游过程的研究热点。本文综述了近年来扩张床吸附技术的发展。包括：1、原理：床层的分层稳定性、吸附剂和吸附柱;2、操作：吸附、洗涤、洗脱和再生4个步骤;3、流动动力学特性：床层扩展特征和停留时间分布;4．在蛋白质纯化中的应用。     

Adsorptive separation in the enhancement of butene dehydrogenation

Fixed-bed columns containing solid catalysts and adsorbents were employed for simultaneous reaction and separation. The models developed for butene dehydrogenation reaction were validated with experimental data. The model was then employed for variable bed configurations with and without the effect of pressure and vacuum swing reaction (PSR and VSR). The models for the mass and momentum transfer in the catalyst bed and adsorber were solved using orthogonal collocation within the method of lines. The reactor/separator performances were tested for beds with varying numbers of layers of catalysts and adsorbents arranged sequentially. The reaction columns behaved as reactor/separators in series. As the number of layers increased, a homogeneous distribution of the catalyst and adsorbent was approached in the limit. These configurations with variable catalyst/adsorbent distributions were investigated in terms of product purity, selectivity, conversion, recovery and yield. Improved reactor performance was observed with pressure and vacuum swing separation systems and in particular with close to well-mixed reactor/separator configurations.     

Step gradients in 3-zone simulated moving bed chromatography. Application to the purification of antibodies and bone morphogenetic protein-2

The simulated moving bed (SMB) technology is a proven tool for efficient separation of binary mixtures. However, relying on isocratic conditions limits the applicability of the classical SMB approach when considering the field of bioseparations. Here, the use of gradients opens up new possibilities. A gradient in a SMB process can be established by using different solvent strengths in the incoming feed and desorbent streams, resulting in two internal plateaus of elution strength. Thus, compared to the conventional process, the overall amount of solvent needed can be reduced, productivity can be increased and more concentrated product streams can be obtained. In this contribution, two case studies will be presented. At first, the separation of bovine IgG from lysozyme will be analyzed as a model system. Antibodies are a common target substance in bio-chromatography, as therapeutic monoclonal antibodies are among the most promising biopharmaceuticals. Using adsorption data obtained from single-column experiments, an appropriate SMB process was designed and implemented. The second target component is the active dimeric form of the bone morphogenetic protein-2 (BMP-2). This protein was isolated from a renaturation solution, which also contained its inactive monomeric form as well as other undefined proteins from the bacterial production strain. A 3-zone open-loop gradient-SMB approach was used successfully for both separations.     

Production of oxy-rich air by RPSA for combustion use

The capital and energy costs of production of oxygen enriched air by a rapid pressure swing adsorption (RPSA) process can be reduced by decoupling the air drying and the air separation duties of the process. Integration of the oxygen-RPSA process with an enhanced combustion application system allows thermal swing adsorption drying of air feed to the RPSA process. The air separation process then can be run using an ad(de)sorption pressure envelope of 2:1 atmospheres, which significantly reduces the cost and energy of operation of the air compressor.     

Fixed bed adsorption of CO2/H2 mixtures on activated carbon: experiments and modeling

We present breakthrough experiments in a fixed bed adsorber packed with commercial activated carbon involving feed mixtures of carbon dioxide and hydrogen of different compositions. The experiments are carried out at four different temperatures (25?°C, 45?°C, 65?°C and 100?°C) and seven different pressures (1?bar, 5?bar, 10?bar, 15?bar, 20?bar, 25?bar and 35?bar). The interpretation of the experimental data is done by describing the adsorption process with a detailed one-dimensional model consisting of mass and heat balances and several constitutive equations, such as an adsorption isotherm and an equation of state. The dynamic model parameters, i.e. mass and heat transfer, are fitted to one single experiment (reference experiment) and the model is then further validated by predicting the remaining experiments. Furthermore, the choice of the isotherm model is discussed. The assessment of the model accuracy is carried out by comparing simulation results and experimental data, and by discussing key features and critical aspects of the model. This study is valuable per se and a necessary step toward the design, development and optimization of a pressure swing adsorption process for the separation of CO₂ and H₂ for example in the context of a pre-combustion CO₂ capture process, such as the integrated gasification combined cycle technology.     

Optimal design of batch and simulated moving bed chromatographic separation processes

Preparative chromatography, especially simulated moving bed (SMB) chromatography, is a key technology for the separation of fine chemicals on a production scale. Most of the design methods for batch and SMB processes proposed in the open literature deal with the optimisation of the operating conditions for a given chromatographic unit only. Therefore, a comparison of the process economy may lead to incorrect results. In this contribution, an effective strategy for the optimal choice of all process parameters (operation and design parameters) is proposed. The main idea of this strategy is to apply a detailed and experimentally validated process model and to reduce the number of influencing parameters by introducing and optimising dimensionless process parameters. It is shown that there is an infinite choice of design and operating parameters to achieve maximum productivity or minimum separation costs and not at the maximum pressure drop only. The detailed design of the chromatographic unit and the selection of the operating conditions can be adjusted by considering the availability of columns and packing materials. As the model system, the separation of a racemic mixture (EMD53986) on Chiralpak AD was investigated. After complete optimisation of a batch and a SMB unit, a real comparison of the process economy can be achieved. Finally, the influences of two different objective functions, productivity and specific separation cost, are analysed.     

Optimization of throughput and desorbent consumption in simulated moving-bed chromatography for paclitaxel purification.

In simulated moving-bed (SMB) applications, throughput and desorbent consumption are two key factors that control process cost. For a given adsorbent volume and product purity requirements, throughput and desorbent consumption depend on desorbent composition, column configuration, column length to diameter ratio, and adsorbent particle size. In this study, these design parameters are systematically examined for paclitaxel purification. The results show that if adsorbent particle size, column dimensions and column configuration are fixed, the higher the product purity required, the lower the throughput. If product purity and yield are fixed, the larger the solute migration speed ratio, the higher the throughput, and the lower the desorbent consumption. If total bed volume and product purities are fixed, the longer the separation zones, the higher the throughput, but the higher the desorbent flow-rate. An intermediate configuration gives the minimum desorbent consumption. If there are no limits on pressure drop or zone flow-rate, the larger the column length to diameter ratio, the smaller the adsorbent particle size, the higher the throughput, and the lower the desorbent consumption. If the maximum zone flow-rate is controlled by the pressure drop limit and not by the standing waves requirement, the longer the columns, the lower the zone flow-rates and the lower the throughput. For 150 microns adsorbent particles and a maximum zone flow-rate of 300 ml/min, a design with optimal throughput and desorbent consumption is found for paclitaxel purification.     

Method and kit for adsorbent performance evaluation

Commercial gas separation plants running adsorption processes, including pressure swing adsorption, vacuum pressure swing adsorption and temperature swing adsorption are intended to operate continuously and meet design performance levels over the complete service life of the facility. Performance degradation of the adsorbent materials in extended commercial usage is a common problem. Issues such as adsorbent aging and poisoning by unwanted or unexpected contaminants, represent some of the causes of declining adsorbent performance. Lower adsorption capacity can result in declining productivity and/or product purity for the gas separation plant. Adsorbent troubleshooting is usually accomplished by taking samples from the plant and sending them to off-site laboratories for analyses such as moisture content determination by Karl Fischer titration, or BET surface area or other adsorption capacity measurements. The turnaround time for these laboratory analyses is on the order of days. To short-cut this lengthy process of analysis, we have developed a simple test method and kit for rapid diagnosis of adsorbent performance issues which can be used directly at a plant site. The test method involves the determination of the gas capacity of an adsorbent by equilibrating the adsorbent with an appropriate test gas, deactivating the adsorbent and measuring the amount of test gas released. Once a sample has been acquired, the test can be executed and results obtained in less than 15 min. We show that the test method is accurate to within 5 % of the adsorption capacity determined from isotherm measurements, at equivalent temperature and pressure, and can be used to test common commercial adsorbent types, including low silica zeolites and activated carbons.     

Piston-driven ultra rapid pressure swing adsorption

The piston-driven ultra rapid pressure swing adsorption (URPSA) equipment was developed and oxygen enrichment from air was examined as an example. The adsorbent bed is directly connected to the cylinder where a piston moves at high frequency. Thus pressurization and depressurization in the bed are driven by mechanical piston motion, which can achieve far more rapid cycles compared with the conventional pressure swing operation using valves. The cycle time is usually on the order of seconds or sub seconds. Oxygen enrichment from air up to about 60% or higher of oxygen concentration was achieved by small-scale equipment using zeolite 5A with a oxygen production capacity of 100 Nm³-product gas/m³-zeolite/hr, which is about ten times larger than those of commercialized PSAs for this purpose.A simplified numerical model describing the mass transfer taking place in URPSA was developed. The model could simulate fairly well the air separation characteristics in terms of oxygen concentration, oxygen production capacity and oxygen yield. The proposed model helps in the understanding of the basic nature of URPSA and possible applications. This novel PSA is promising as a compact yet high-capacity PSA to be utilized in a wide variety of applications.     

Optimizing control of simulated moving bed separations of mixtures subject to the generalized Langmuir isotherm

Simulated moving bed (SMB) is a cost-efficient separation technique that offers high productivity and low solvent consumption. SMB has gained importance in the pharmaceutical and fine chemical industry to perform complex separation tasks. However, an open and challenging problem is the optimal, robust operation of the SMB process. We have developed a control scheme that integrates the optimization and control of the SMB unit. A significant feature of the controller is that only minimal information of the system has to be provided, i.e. the linear adsorption behavior of the mixture to be separated and the average void fraction of the columns. Therefore, a full characterization of the adsorption behavior of the mixture and the columns is no longer required. In this ‘cycle to cycle’ control scheme, the measurements, optimization and control actions are performed once in every cycle. This paper presents simulation results of the control scheme applied to the separation of binary mixtures characterized by generalized Langmuir isotherms. The results are presented and analyzed in the frame of the triangle theory that has been recently extended to encompass these types of isotherms. Besides, online optimum performance of the SMB unit is compared with off-line optimization carried out using genetic algorithm. The results show that the controller fulfills the product and process specifications while operating the SMB unit optimally, regardless of the different types of Langmuir isotherms that the systems exhibit.     

CO2 removal by PSA: an industrial view on opportunities and challenges

CO₂ removal from gaseous streams is one of the most important separation tasks in this decade. Adsorption processes can contribute in a wide range to this topic, thus an enormous effort is performed respectively in research and industry. In two scenarios the competitiveness of pressure swing adsorption (PSA) and vacuum pressure swing adsorption technology is assessed: Carbon capture from hydrogen production by steam methane reforming for enhanced oil recovery and CO₂ removal from direct reduction processes for iron making. Additionally, industrial requirements, project as well as operation driven, have to be considered. Robustness and stable operation is as important as optimized captial expenditure and operational expenditure. Considering economical and operational aspects PSA processes are the most attractive alternatives in the presented scenarios.     

Effect of Operation Symmetry on Pressure Swing Adsorption Process

In a multi-bed pressure swing adsorption (PSA) process, cycle steps with gas flow transferring from one bed to another such as equalization, purge, etc. are generally practiced to enhance the product recovery. However, if the flows for the connected beds in these steps are not balanced, the PSA process may not operate in a symmetrical manner. In the modeling of the PSA process, most of the simulations consider only one bed and assume that the rest of the beds would behave in a same way. In order to assess the impact of bed symmetry on the PSA performance, a new PSA model capable of studying bed symmetry in a two-bed system is developed. Experimental results from this paper show that uneven equalization flow can result in a lower product purity and a peculiar purity curve at different equalization levels. This phenomenon can be successfully predicted by this model. Simulation results also show that in large-scale PSA units, asymmetrical operation can cause drastically different temperature profiles in different adsorbers and hence a much lower performance. This paper demonstrates the importance of maintaining operation symmetry in PSA processes.