Isabel A.A.C. Esteves

Natural convection heat transfer in horizontal eccentric elliptic annuli containing saturated porous media

The two-dimensional Darcy‐Boussinesq equations, governing natural convection heat transfer in a saturated porous medium, are solved in generalised orthogonal coordinates, using high-order compact finites diAerences on a very fine grid. The mesh is generated numerically using the orthogonal trajectory method. The code is thoroughly validated against results reported in the literature for concentric and eccentric cylinders, obtained using diAerent numerical techniques. The code is applied to horizontal eccentric elliptic annuli containing saturated porous media. The judicious stretching of one of the annular walls in the horizontal direction reduces the heat losses with respect to a concentric cylindrical annulus with the same amount of insulating material. The savings in heat transfer can be further improved if the elliptic annular shape is made eccentric. Previous studies show that, under certain conditions, eccentric cylinders may lead to a more eAective insulation than concentric ones. The results presented here provide an alternative approach to optimising the heat transfer rate by a proper choice of the annular shape. The energy savings are of the order of 10%. 7 2000 Elsevier Science Ltd. All rights reserved.

Simulation of a new hybrid membrane/pressure swing adsorption process for gas separation☆

A new hybrid gas separation process combining membrane permeation and pressure swing adsorption (PSA) is presented. An integrated model was formulated which successfully predicts all process characteristics. Our modeling work shows that the coupled process increases the efficiency of the pressurization and high-pressure adsorption steps, thereby improving the separation performance as compared to a standalone PSA. The new process has been applied successfully to the bulk separation of a mixture of 50/50 H 2 /CH 4 and preliminary results have been obtained for CO 2/ CH 4 and H 2 /CO 2 /CH 4 mixtures.

Dynamic modelling of an adsorption storage tank using a hybrid approach combining computational fluid dynamics and process simulation

Abstract A computational fluid dynamics (CFD) software package has been coupled with the dynamic process simulator of an adsorption storage tank for methane fuelled vehicles. The two solvers run as independent processes and handle non-overlapping portions of the computational domain. The codes exchange data on the boundary interface of the two domains to ensure continuity of the solution and of its gradient. A software interface was developed to dynamically suspend and activate each process as necessary, and be responsible for data exchange and process synchronization. This hybrid computational tool has been successfully employed to accurately simulate the discharge of a new tank design and evaluate its performance. The case study presented here shows that CFD and process simulation are highly complementary computational tools, and that there are clear benefits to be gained from a close integration of the two.

Hybrid Membrane/PSA Processes for CO2/N2 Separation:

New integrated schemes combining membrane permeation and pressure swing adsorption (PSA) have been developed for gas separation. By using the membrane as a pre-bulk separation unit and coupling it to the intrinsically dynamic periodic operation of the PSA, the separation performance of the hybrid scheme is enhanced with respect to that of the two stand-alone units. Instead of constant-composition regular feed, the PSA is fed with a mixture which is progressively enriched in the more adsorbed component during the pressurization and high-pressure adsorption steps of the cycle. This results in sharper concentration fronts. The hybrid scheme detailed here has been applied successfully to the bulk separation of an 30:70 mol% CO2/N2 mixture over activated carbon. Process performance is reported in terms of product recovery and purity at cyclic steady state. Numerical simulations were validated by experimental work on a composite membrane and a laboratory-scale PSA unit. For the examples studied, product purity and recovery for the hybrid process were increased by 23% and 58% for CO2, and by 14% and 5% for N2, compared to an equivalent stand-alone PSA.

Molecular Simulation of Adsorption Processes. 1. Isothermal Stirred-tank Adsorber

A new molecular simulation method is proposed to solve the governing equations  for a multicomponent, isothermal stirred-tank adsorber under equilibrium controlled conditions. The technique is formulated in the Gibbs ensemble, but is more appropriately viewed as a hybrid of a molecular simulation and continuum modeling. For the general case of an arbitrary multicomponent mixture, the total number of molecules of each species is allowed to fluctuate so that the ensemble average satisfies the macroscopic material balances to the adsorber. It is shown that if an analytical equation  of state for the fluid phase is known, the simulation procedure can be considerably simplified and acquires many characteristics of a Monte Carlo simulation conducted in the grand canonical ensemble. The technique is thoroughly validated and its usefulness is demonstrated through application to a gas separation problem encompassing the major steps of practical value to batch adsorption processes.

Maize cob waste pre-treatments to enhance biogas production through co-anaerobic digestion with OFMSW

In the present work, the enhancement of biogas and methane yields through anaerobic co-digestion of the pre-hydrolised Organic Fraction of Municipal Solid Wastes (hOFMSW) and Maize Cob Wastes (MCW) in a lab-scale thermophilic anaerobic reactor was tested. In order to increase its biodegradability, MCW were submitted to an initial pre-treatment screening phase as follows: (i) microwave (MW) irradiation catalysed by NaOH, (ii) MW catalysed by glycerol in water and alkaline water solutions, (iii) MW catalysed by H2O2 with pH of 9.8 and (iv) chemical pre-treatment at room temperature catalysed by H2O2 with 4 h reaction time. The pre-treatments cataysed by H2O2 were performed with 2% MCW (wMCW/v alkaline water) at ratios of 0.125, 0.25, 0.5 and 1.0 (wH2O2/wMCW). The pre-treatment that presented the most favourable balance between sugars, lignin, cellulose and hemicellulose solubilisations, as well as low production of phenolic compound and furfural (inhibitors), was the chemical pre-treatment catalysed by H2O2, at room temperature, with a ratio of 0.5 wH2O2/wMCW (Pre1). This Pre1 was then optimised testing reaction times of 1, 2 and 3 days at a different pH (11.5) and MCW percentage (10% w/v). The optimised pre-treatment that presented the best results, considering the same criteria defined above, was the one carried out during 3 days, at pH 9.8 and 10% MCW w/v (Pre2). The anaerobic reactor was initially fed with the hOFMSW obtained from the hydrolysis tank of an industrial AD plant. The hOFMSW was than co-digested with MCW submitted to the pre-treatment Pre1. In another assay, hOFMSW was co-digested with MCW submitted pre-treatment Pre 2. The co-digestion of hOFMSW + Pre1 increased the biogas yield by 38.9% and methane yield by 29.7%, when compared to the results obtained with hOFMSW alone. The co-digestion of hOFMSW + Pre2 increased biogas yield by 46.0% and CH4 yield by 36.3%. In both cases, the methane content obtained in the biogas streams was above 66% v/v. These results show that pre-treatment with H2O2, at room temperature, is a promising low cost way to valorize MCW through co-digestion with hOFMSW.

A Sensitive Method Approach for Chromatographic Analysis of Gas Streams in Separation Processes Based on Columns Packed with an Adsorbent Material

A sensitive method was developed and experimentally validated for the in-line analysis and quantification of gaseous feed and product streams of separation processes under research and development based on column chromatography. The analysis uses a specific mass spectrometry method coupled to engineering processes, such as Pressure Swing Adsorption (PSA) and Simulated Moving Bed (SMB), which are examples of popular continuous separation technologies that can be used in applications such as natural gas and biogas purifications or carbon dioxide sequestration. These processes employ column adsorption equilibria on adsorbent materials, thus requiring real-time gas stream composition quantification. For this assay, an internal standard is assumed and a single-point calibration is used in a simple mixture-specific algorithm. The accuracy of the method was found to be between 0.01% and 0.25% (-mol) for mixtures of CO2, CH4, and N2, tested as case-studies. This makes the method feasible for streams with quality control levels that can be used as a standard monitoring and analyzing procedure.

Chapter 9:Novel Hybrid Membrane/Pressure Swing Adsorption Processes for Gas Separation Applications

Two widely known standalone technologies, such as pressure swing adsorption (PSA) and membrane permeation, have been extensively developed and applied in industry for gas separation. Their process configurations have been widely studied to either minimize recompression work for reducing final operational costs, or to give a better reuse to some waste gases that are not usually recovered by using more conventional methods. Although there is some published work on the development of hybrid systems integrating these two technologies, the awareness for innovative hybrid permeation- and sorption-based processes was seen only recently as an opportunity to further reduce the amount of waste, recover its energy, and recycle as much as possible.In applications where membranes can provide a moderately pure product at low cost, the separation may actually be inexpensively upgraded by a subsequent process. Likewise, the separation performance of fully established non-permeating technologies can be improved with marginal costs, if one uses truly synergistically hybrid schemes involving membranes. This fact has motivated active research on the integration of membranes with other separation processes, such as PSA. Accordingly, the present chapter intends to show the better of these two technologies in a cooperative mode of operation, rather than in a competitive one. New advances in membrane-based hybrid processes for gas separation and waste reduction via adsorption are examined, and the future prospects for synergistic coupling of separation processes are also suggested. The areas of potential waste reduction focused are natural gas, landfill gas, synthesis gas, hydrogen from reformer off-gas and olefin/paraffin separation.