Graphical approach for complex PSA cycle scheduling

A simple graphical approach for complex pressure swing adsorption (PSA) cycle scheduling has been developed. This new methodology involves a priori specifying the cycle steps, their sequence, and the number of beds, and then following a systematic procedure that requires filling in a 2-D grid based on a few simple rules, some heuristics and some experience. The outcome or solution is a grid comprised of columns that represent the total cycle time, rows that represent the total number of beds, and cells that represent the duration of each cycle step, i.e., the complete cycle schedule. This new approach has been tested successfully against several cycle schedules taken from the literature, including a two-bed four-step Skarstrom cycle, a four-bed nine-step process with two equalization steps, a nine-bed eleven-step process with three pressure equalization steps, and a six-bed thirteen-step process with four pressure equalization steps and four idle steps. This approach also revealed the existence of numerous cycle schedules for each bed and cycle step combination examined. Although it cannot identify the total number of permutations or which one is better, it does provide a very straightforward way to determine some of the possible cycle schedules of virtually any PSA process that can be conceived.     

Simplified graphical approach for complex PSA cycle scheduling

A simple, graphical, unit block approach for rapid complex pressure swing adsorption (PSA) cycle scheduling has been developed. This new methodology involves a priori specifying the cycle steps, their sequence, and the number of beds, and then following a systematic procedure that requires filling in a 2-D grid. The outcome or solution is a unit block which can easily be extended to form the complete cycle schedule. This new approach has been tested successfully against several multi-bed and multi-step cycle schedules taken from the literature. It should thus be very useful for quickly scrutinizing different PSA cycle schedules for further PSA process development.     

Heuristics for synthesis and design of pressure-swing adsorption processes

Simulation based synthesis and design of adsorptive enrichment of CO from tail gas having 51?% CO are presented. The adsorption breakthrough curve simulation using this feed gas composition, helped to provide a starting guess of the adsorption step duration in a pressure-swing adsorption cycle for meeting the purity and recovery targets. Use of smaller bed dimensions facilitated the simulation of many cycles. These simulations helped to decide the operating pressure range, operating temperature, constituent steps of the cycle, their sequence, direction of pressurization of the bed, number of beds in the cycle and the composition of the streams to be used for pressurization and/or rinse and/or purge steps. Only an optimally designed pressure-vacuum-swing adsorption cycle achieves the stiff separation targets of getting an extract having 80?% pure CO at 80?% recovery in a single-stage with an adsorbent that uses physical adsorption and offers CO/CH₄ sorption selectivity of just 2.44. Additional simulations are done wherein the bed sizes and velocities are increased to predict the performance of a large-scale unit. These require deciding only the durations of the steps that are finalized from the small-scale unit simulations. These durations were kept fixed and the bed dimensions were varied till the separation targets are obtained for the particular feed rate. The scale-up criteria was matching residence times in the bed. A strategy for treating a feed gas having only 30?% CO is also discussed and a novel concept of cascaded PSA is evaluated using simulations. Some heuristics are evolved from the studies.     

Backfill Cycle of a Layered Bed H2 PSA Process

The backfill cycle of two-bed PSA process using activated carbon beds, zeolite 5A beds, and layered beds was studied experimentally and theoretically to recover high purity H₂ from coke oven gas. In a layered bed PSA, a comparison was made between two PSA processes with/without a backfill step before the feed pressurization step. Since the backfill step made the adsorption bed rich in H₂ and this led to a rather steep concentration wave front at the feed pressurization step, incorporating a backfill step resulted in an increase in product purity with a decrease in recovery. Each step of the single-adsorbent and layered bed PSA processes with a backfill step was simulated with a dynamic model incorporating mass, energy, and momentum balances. The model agreed well with the experimental results in predicting the product H₂ purity and recovery, thus giving a basic understanding of the bed dynamics of a backfill cycle. While the concentration and temperature profiles of a layered bed in each step showed characteristic behavior of each adsorbent in each layer, the product purity of a layered bed was not between the limits of two single-adsorbent bed processes. The concentration profiles predicted by simulation showed that CO and N₂ played an important role in obtaining high H₂ purity.     

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.     

Use of 13Calpha chemical shifts in protein structure determination

A physics-based method aimed at determining protein structures by using NOE-derived distances together with observed and computed 13C chemical shifts is proposed. The approach makes use of 13Calpha chemical shifts, computed at the density functional level of theory, to obtain torsional constraints for all backbone and side-chain torsional angles without making a priori use of the occupancy of any region of the Ramachandran map by the amino acid residues. The torsional constraints are not fixed but are changed dynamically in each step of the procedure, following an iterative self-consistent approach intended to identify a set of conformations for which the computed 13Calpha chemical shifts match the experimental ones. A test is carried out on a 76-amino acid, all-alpha-helical protein; namely, the Bacillus subtilis acyl carrier protein. It is shown that, starting from randomly generated conformations, the final protein models are more accurate than an existing NMR-derived structure model of this protein, in terms of both the agreement between predicted and observed 13Calpha chemical shifts and some stereochemical quality indicators, and of similar accuracy as one of the protein models solved at a high level of resolution. The results provide evidence that this methodology can be used not only for structure determination but also for additional protein structure refinement of NMR-derived models deposited in the Protein Data Bank.     

Expedient synthesis of two structurally close tetrasaccharides corresponding to the O-antigens of Escherichia coli O127 and Salmonella enterica O13

Two structurally close tetrasaccharides corresponding to the O-antigens of Escherichia coli O127 and Salmonella enterica O13 have been synthesized using a ‘unichemo’ approach and minimum number of reaction steps. The yields of all glycosylation steps were excellent with a high stereochemical outcome. A common synthetic strategy has been adopted for the simultaneous synthesis of two tetrasaccharides.     

Multiple rate-determining steps for nonideal and fractal kinetics

We show that the kinetic model of a single rate-determining step in a reaction mechanism can be extended to systems with multiple overall reactions for which the elementary reactions obey nonideal or fractal kinetics. The following assumptions are necessary: (1) The system studied is either closed or open, but no constraints exist preventing the evolution toward equilibrium. (2) Elementary reactions occur in pairs of forward and backward steps. (3) The kinetics of the elementary steps are either nonideal or fractal and are compatible with equilibrium thermodynamics. (4) The number of reaction routes is identical with the number of rate-determining steps. If these hypotheses are valid, then the overall reaction rates can be explicitly evaluated: they have a form similar to the kinetic equations for the elementary reactions and the apparent reaction orders and fractal coefficients can be expressed analytically in terms of the kinetic parameters of the elementary reactions. We derive a set of relationships which connect the equilibrium constants of the reaction routes, the corresponding overall rate coefficients, and the stoichiometric numbers of the rate-determining steps. We also derive a set of generalized Boreskov relations among the apparent activation energies of the forward and backward overall processes, the corresponding reaction enthalpies, and the stoichiometric coefficients of the rate-determining steps. If the elementary reactions obey fractal kinetics, the same is true for the rate-determining steps. The fractal exponents of the forward and backward overall reactions are linear combinations of the fractal exponents of the fractal elementary reactions. Similar to the theory of single rate-determining steps, our approach can be used for selecting suitable reaction mechanisms from experimental data.     

Design of a H2 PSA for cogeneration of ultrapure hydrogen and power at an advanced integrated gasification combined cycle with pre-combustion capture

A novel hydrogen pressure swing adsorption system has been studied that is applied to an advanced integrated gasification combined cycle plant for cogenerating power and ultrapure hydrogen (99.99+ mol%) with CO₂ capture. In designing the H₂ PSA, it is essential to increase the recovery of ultrapure hydrogen product to its maximum since the power consumption for compressing the H₂ PSA tail gas up to the gas turbine operating pressure should be minimised to save the total auxiliary power consumption of the advanced IGCC plant. In this study, it is sought to increase the H₂ recovery by increasing the complexity of the PSA step configuration that enables a PSA cycle to have a lower feed flow to one column for adsorption and more pressure equalisation steps. As a result the H₂ recovery reaches a maximum around 93 % with a Polybed H₂ PSA system having twelve columns and the step configuration contains simultaneous adsorption at three columns and four-stage pressure equalisation.     

Heavy reflux PSA cycles for CO2 recovery from flue gas: Part I. Performance evaluation

This study evaluated nine stripping PSA cycle configurations, all with a heavy reflux (HR) step, some with a light reflux (LR) step, and some with a recovery (REC) or feed plus recycle (F+R) step, for concentrating CO₂ from stack and flue gas at high temperature (575 K) using a K-promoted HTlc. Under the process conditions studied, the addition of the LR step always resulted in a better process performance; and in all cases, the addition of a REC or F+R step surprisingly did not affect the process performance except at low feed throughputs, where either cycle step resulted in a similar diminished performance. The best cycle based on overall performance was a 5-bed 5-step stripping PSA cycle with LR and HR from countercurrent depressurization (CnD) (98.7% CO₂ purity, 98.7% CO₂ recovery and 5.8 L STP/hr/kg feed throughput). The next best cycle was a 5-bed 5-step stripping PSA cycle with LR and HR from LR purge (96.5% CO₂ purity, 71.1% CO₂ recovery and 57.6 L STP/hr/kg feed throughput). These improved performances were caused mainly by the use of a very small purge to feed ratio (γ=0.02) for the former cycle and a larger one (γ=0.50) for the latter cycle. The former cycle was good for producing CO₂ at high purities and recoveries but at lower feed throughputs, and the latter cycle was useful for obtaining CO₂ at high purities and feed throughputs but at lower recoveries. The best performance of a 4-bed 4-step stripping PSA cycle with HR from CnD was disappointing because of low CO₂ recoveries (99.2% CO₂ purity, 15.2% CO₂ recovery and 72.0 L STP/hr/kg feed throughput). This last result revealed that the recoveries of this cycle would always be much lower than the corresponding cycles with a LR step, no matter the process conditions, and that the LR step was very important to the performance of these HR cycles for this application and process conditions studied.     

Characterization of the Acid-Base Properties of Humic Substances by Chemometric Analysis of Synchronous Fluorescence and pH Potentiometric Data

Abstract

A procedure for the analysis of the acid-base characteristics of humic substances based on a self-modeling analysis of synchronous fluorescence spectra, collected at varying pH, and on a non-linear least squares adjustment of potentiometic pH data, is described. The data analysis methodology consists of two steps: first, the number of acid-base systems and the corresponding spectra and distribution diagrams are calculated by evolving factor analysis (EFA) with concentration constraints of the spectroscopic data; second, the potentiometric data is analyzed by a standard non-linear least square procedure using as fixed parameters the number of components and their pKas, determined in the first step of the analysis. As an example, for a sample of marine fulvic acids studied at pH between 2 and 11, four acid-base systems were found with average pKas: 3.1, 4.8, 8.0 and 10.0. The concentrations of the corresponding systems were: 2.55(5), 1.95(7), 0.14(4) and 1.8(3) meq/g.     

Tuning of Pressure Swing Adsorption Systems Based on Differential Pressure Profile

A method for tuning a Pressure Swing Adsorption (PSA) system aimed to achieve symmetrical operating conditions based on pressure differential in the adsorption vessels is developed in this study. Simulation of an oxygen Pressure-Vacuum Swing Adsorption (PVSA) process indicates that the pressure drop inside the adsorption vessel is closely related to the nitrogen concentration and gas velocity. The technique is applied to the tuning of an oxygen PVSA process. Adsorbent vessels of the PSA system are monitored and tuned by making corrective adjustments in each of the steps in a PSA cycle in response to imbalances in the differential pressure profiles in each of the adsorbent vessels. The method developed in this study provides a faster, easier, and more effective way to bring a PSA plant to its symmetrical, optimal state than those based on other parameters such as concentration, temperature, and pressure profile. This paper is dedicated to the memory of Professor Wolfgang Schirmer.     

Multinomial tau-leaping method for stochastic kinetic simulations

We introduce the multinomial tau-leaping (MtauL) method for general reaction networks with multichannel reactant dependencies. The MtauL method is an extension of the binomial tau-leaping method where efficiency is improved in several ways. First, tau-leaping steps are determined simply and efficiently using a priori information and Poisson distribution-based estimates of expectation values for reaction numbers over a tentative tau-leaping step. Second, networks are partitioned into closed groups of reactions and corresponding reactants in which no group reactant set is found in any other group. Third, product formation is factored into upper-bound estimation of the number of times a particular reaction occurs. Together, these features allow larger time steps where the numbers of reactions occurring simultaneously in a multichannel manner are estimated accurately using a multinomial distribution. Furthermore, we develop a simple procedure that places a specific upper bound on the total reaction number to ensure non-negativity of species populations over a single multiple-reaction step. Using two disparate test case problems involving cellular processes--epidermal growth factor receptor signaling and a lactose operon model--we show that the tau-leaping based methods such as the MtauL algorithm can significantly reduce the number of simulation steps thus increasing the numerical efficiency over the exact stochastic simulation algorithm by orders of magnitude.     

On the Optimisation of Periodic Adsorption Processes

A rigorous mathematical programming based approach to the optimisation of general periodic adsorption processes is presented. Detailed dynamic models taking account of the spatial variation of properties within the adsorption bed(s) are used. The resulting systems of partial differential and algebraic equations are reduced to sets of algebraic constraints by discretisation with respect to both spatial and temporal dimensions. Periodic boundary conditions are imposed to represent directly the cyclic steady-state of the system. Additional constraints are introduced to characterise the interactions between multiple beds in the process as well as any relevant design specifications and operating restrictions. The optimal operating and/or design decisions can be determined by solving an optimisation problem with constraints representing a single bed over a single cycle of operation, irrespective of the number of adsorption beds in the process.     

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.     

Carbon monoxide isotope enrichment and separation by pressure swing adsorption

Simulations of three different 3-bed 3-step pressure swing adsorption (PSA) cycles were carried out to study the enrichment and recovery of ¹⁴CO from an isotopic mixture of ¹⁴CO, ¹³CO and ¹²CO using NaX zeolite. Each PSA cycle included feed pressurization/feed (FP/P), heavy reflux (HR) and countercurrent depressurization (CnD) steps; they differed only in the way the CnD step was carried out: PSA Cycle I was carried out under total reflux (i.e., with no ¹⁴CO heavy product production); PSA Cycle II was carried out with discontinuous ¹⁴CO heavy product production; and PSA Cycle III was carried out with continuous ¹⁴CO heavy product production. The effects of the CnD step valve coefficient (c _v ), heavy reflux ratio (R _R ), and cycle time (t _cyc ) on the PSA process performance were determined in terms of the ¹⁴CO enrichment, ¹⁴CO recovery and CO feed throughput. The results showed that there was essentially no limit to the extent of the ¹⁴CO enrichment, despite the inherently low ¹⁴CO/¹²CO (1.05) and ¹⁴CO/¹³CO (1.12) separation factors for these isotopes on NaX zeolite. Under total reflux an optimum c _v was found for the CnD step and ¹⁴CO enrichments as high as 152 were obtained. Using the optimum c _v under finite reflux, a ¹⁴CO enrichment approaching 20 and a ¹⁴CO recovery approaching 100 % were easily achieved with discontinuous (PSA Cycle II) or continuous (PSA Cycle III) ¹⁴CO heavy product production. There was essentially no difference in the performance of PSA Cycles II and III, a counterintuitive result. The ¹⁴CO enrichment and the ¹⁴CO recovery both increased with decreasing CO feed throughputs and higher R _R , which were always very close to unity.     

Analysis of Equilibrium PSA Performance with an Analytical Solution

Five-step PSA cycles consisting of pressurization with product, adsorption, co-current depressurization, blowdown, and purge steps have been analyzed with equilibrium model assuming uncoupled linear isotherms and isothermal condition. Unlike the previous models, the proposed model is not restricted to the operating conditions that ensure a complete shock transition of concentration profile at the end of the high pressure adsorption step. The operating conditions could have two classifications: one is utilizing the column completely before blowdown, and the other is not. As the selectivity increases, it is more difficult to utilize the column completely before the blowdown step. There is an optimum co-current depressurization pressure which maximizes the recovery at the given extent of purge. The optimum co-current depressurization pressure decreases as the purge quantity decreases. On the less selective adsorbent, the recovery at the optimum co-current depressurization pressure increases with the decrease of purge quantity without much sacrifice of the throughput. But, on the highly selective adsorbent, there is an extent of purge and corresponding value of cocurrent depressurization pressure below which the recovery is not greatly improved while the throughput decreases rapidly, which limits the number of pressure equalization steps can be included.     

Deterministic global optimization of molecular structures using interval analysis

The search for the global minimum of a molecular potential energy surface is a challenging problem. The molecular structure corresponding to the global minimum is of particular importance because it usually dictates both the physical and chemical properties of the molecule. The existence of an extremely large number of local minima, the number of which may increase exponentially with the size of the molecule, makes this global minimization problem extremely difficult. A new strategy is described here for solving such global minimization problems deterministically. The methodology is based on interval analysis, and provides a mathematical and computational guarantee that the molecular structure with the global minimum potential energy will be found. The technique is demonstrated using two sets of example problems. The first set involves a relatively simple potential model, and problems with up to 40 atoms. The second set involves a more realistic potential energy function, representative of those in current use, and problems with up to 11 atoms.     

Concise synthesis of two pentasaccharides corresponding to the α-chain oligosaccharides of Neisseria gonorrhoeae and Neisseria meningitidis

Two pentasaccharides containing a common tetrasaccharide (lacto-N-neotetraose) core, and d-galactosamine and N-acetyl neuraminic acid in the non-reducing ends, respectively, corresponding to the lipooligosaccharides of Neisseria gonorrhoeae and Neisseria meningitidis were synthesized in a very concise manner from a common trisaccharide derivative using minimum number of steps. Thioglycosides and glycosyl trichloroacetimidate have been used as glycosyl donors for glycosylations and yields were excellent in every step.     

The art and science of forming packed analytical high-performance liquid chromatography columns

Columns of packed particles still are the most popular devices for high-performance liquid chromatography (HPLC) separations because of their great utility, excellent performance and wide variety. However, the forming of packed beds for efficient, stable columns traditionally has been an art where the basics of how to form optimum beds generally was not well understood. The recent development of monolith rods was introduced in part to overcome the difficulty of producing stable beds of packing particles. However, these materials are less versatile than packed particle columns. Technology developments in recent years have produced a better understanding among those skilled in the practice of how to form optimized packed beds, and this has led to widely available, high-quality commercial columns. This presentation discusses the developments that led to the present state of column packing technology. Important steps in the packing of efficient, stable beds are described. The key step of selecting the best solvent for the slurry packing method is emphasized. Factors affecting the mechanical stability of packed columns also are discussed. The early art of packing columns now has evolved into a more scientific approach that allows the packing of good columns with a minimum of effort and time.