Molecular Sieve Silica (MSS) Membranes for Gas Separation and Reaction Processes

Weakly branched silica films formed by the two-step sol-gel process allow for the formation of high selectivity membranes for gas separation. 29Si NMR and gas permeation showed that reduced crosslinking leads to He/CH4 selectivity improvement from 300 to 1000. Applied in membrane reactor for cyclohexane conversion to benzene, conversions were achieved at 14 fold higher than a conventional reactor at 250°C. Hydrothermal stability studies showed that carbon templating of silica is required for hydrothermally stable membranes. From our work it was shown that with correct application of chemistry, practical membrane systems can be built to suit gas separation (e. g. hydrogen fuel) and reactor systems

A NTU-Based Model to Estimate Suction Superheating In Scroll Compressors

Suction superheating plays a major role in determining the efficiency degradation of hermetic scroll compressors. Current models to predict superheating are usually experimentally calibrated and therefore can only be applied to existing compressors. This paper presents a thermal model to estimate suction superheating in scroll compressors, based on the NTU method for heat exchangers design. The model considers an isothermal surface exchanging heat with the gas in the suction path and in the discharge plenum. Compared to other models, the new approach described herein has the advantage of not requiring any experimental input data. The thermal model is coupled to a thermodynamic model and applied to evaluate the performance of a scroll compressor. The model was capable to predict the suction gas temperature in good agreement with experimental data, making it particularly useful for compressor design

Enhanced Ethanol Dehydration with Hydrostable Inorganic Pervaporation Membranes

Membranes which allow water diffusion in favour of other substances can offer increased efficiency in processes to dehydrate ethanol. Silica membranes can perform this selective diffusion, but have mostly been reported for their gas high gas separating ability. This work investigates the effectiveness of carbonised template molecular sieve (CTMSS) membrane to dehydrate ethanol/water mixtures. The silica derived top layer of the membrane was measured at 20nm thickness by XPS sputtering technique. However, the silica enters the porous structure of -alumina layer in excess of 90nm. After 200 minutes of operation, H2O/EtOH selectivity increased to 5.6 from around 1 due to gradual pore filling by adsorbed water and ethanol which contributed to inhibiting ethanol transport. The smaller water molecules were thus favoured in transporting to the permeate side. Total mass flux using a 10% ethanol feed remained constant at around 1.5 kg.m-2.hr-1. Selectivity of up to 9.5 was achieved when azeotropic feed solutions of 95% were used, displaying the potential for this technology for a wide range of ethanol dehydration applications. Pressurising the feed up to 400 kPa doubled the permeate flux, but enhanced the transport of ethanol over water

A Neural Network to Predict the Temperature Distribution in Hermetic Refrigeration Compressors

The understanding of heat transfer interactions in refrigeration compressors is of fundamental importance to characterize their overall performance. Certain temperatures, such as those of the motor, oil, shell, and at suction and discharge chambers, have strong influence on the compressor electrical consumption and reliability. Experimental and numerical approaches have been successfully employed to characterize the thermal profile of compressors under different operating conditions. This paper presents a multi-layered feed-forward neural network developed to predict the main temperatures of a hermetic reciprocating compressor. Such a model can be used for different compressor layouts without major modifications, being a fast method for estimating temperatures without the solution of the compression cycle. Predictions of the neural network were compared with experimental data and numerical results from comprehensive thermodynamic simulations, and good agreement was observed in a wide range of evaporating and condensing temperatures. The neural network was found to predict the temperature distribution with sufficient accuracy for compressor analysis and development

An Experimental-Numerical Procedure to Characterize Compressor Performance under Cycling Operating Conditions of Refrigerators

The design of reciprocating compressors is commonly based on steady-state operating conditions. However, most household refrigerators operate under transient and periodic regimes, characterized by alternate periods in which the compressor is either operating (on) or not operating (off). The result is a decoupled design approach since stabilized conditions not necessarily represent the actual operating conditions of refrigerators. This paper presents a strategy in which a virtual refrigeration system is developed and coupled to an experimental facility in order to test compressors under on-off conditions typical of household refrigerators. The virtual refrigeration system simulates the dynamic behavior of a household refrigerator, except for the compressor, and provides the instantaneous operating condition to the compressor in the test bench. The developed procedure was used to emulate a refrigerator operating at ambient temperature of 32 °C and freezer cut-off temperature of -16 °C. The results were in good agreement with the experimental data, with the system energy consumption and the compressor run time predicted with a maximum deviation of 1.3%. The experimental facility is particularly useful to evaluate the effect that changes in the compressor design may have on the refrigerator energy consumption

High selectivity microporous silica membranes for lactic acid dehydration

Lactic acid (LA) has significant market potential for many industries including food, cosmetics, pharmaceuticals, medical and biodegradable materials. Production of LA usually begins with the fermentation of glucose but subsequent stages for the enrichment of lactic acid are complex and energy intensive and could be minimised using water selective membrane technology. In this work, we trialled a highly selective hydrostable carbonised template molecular sieve silica (CTMSS) membrane for the dehydration of a 15 vol% aqueous lactic acid solution with 0.1 vol% glucose. CTMSS membrane films were developed by dip-coating ceramic substrates with silica sols made using the acid catalysed sol-gel process. Permeation was performed by feeding LA/glucose solution to the membrane cell at 18°C in a standard pervaporation setup. The membrane showed selective transport of water from the aqueous feed to the permeate while glucose was not detected. CTMSS membrane permeate flux stabilised at 0.2 kg.m-2.hr-1 in 3.9 hours, and reduced LA to lower than 0.2 vol%. Flux through the CTMSS micropores was activated, displaying increased initial flux to 1.58 kg.m-2.hr-1 at 60°C. To enrich a 1 l.min-1 stream to 85% LA in a single stage, a minimum membrane area of 324 m2 would be required at 18°C. Increased operating temperature to 80°C significantly reduced this area to 24 m2 but LA levels in the permeate stream increased to 0.5 vol%. The highly selective CTMSS membrane technology is an ideal candidate for LA purification. CTMSS membrane systems operate stably in aqueous systems leading to potential cost reductions in LA processing for future markets

Thermal Cycling Stability of Silica Membranes for Gas Separation

Hydrogen is being seen as an alternative energy carrier to conventional hydrocarbons to reduce greenhouse gas emissions. High efficiency separation technologies to remove hydrogen from the greenhouse gas, carbon dioxide, are therefore in growing demand. Traditional thermodynamic separation systems utilise distillation, absorption and adsorption, but are limited in efficiency at compact scales. Molecular sieve silica (MSS) membranes can perform this separation as they have high permselectivity of hydrogen to carbon dioxide, but their stability under thermal cycling is not well reported. In this work we exposed a standard MSS membrane and a carbonised template MSS (CTMSS) membrane to thermal cycling from 100 to 450°C. The standard MSS and carbonised template CTMSS membranes both showed permselectivity of helium to nitrogen dropping from around 10 to 6 in the first set of cycles, remaining stable until the last test. The permselectivity drop was due to small micropore collapse, which occurred via structure movement during cycling. Simulating single stage membrane separation with a 50:50 molar feed of H2:CO2, H2 exiting the permeate stream would start at 79% and stabilise at 67%. Higher selectivity membranes showed less of a purity drop, indicating the margin at which to design a stable membrane separation unit for CO2 capture