Giuseppe Barbieri

Direct contact membrane distillation: modelling and concentration experiments

Abstract Direct contact membrane distillation (DCMD) process was chosen to produce a highly concentrated apple juice using hollow fiber modules. A high 64°Brix concentration was achieved. The trans-membrane driving force decreased with increasing extra-fiber temperature but increased with higher feed and distillate flow rates in the intra- and extra-fiber volumes, respectively. Flux inversion was sensitive to small differences in temperature between the intra- and extra-fiber volumes and could be prevented by increasing the intra-fiber feed temperature by 2–4°C. Flux rates were dependent upon the temperature polarisation coefficient (TPC) and the effect of the concentration polarisation coefficient (CPC) was negligible. Trans-membrane flux was also significantly increased by thermal-osmotic distillation (TOD) using a high concentration of CaCl 2 as the permeate solution. A new model describing the fluid dynamics and membrane behaviour within the DCMD system is presented. In particular, the influence of various properties of membrane morphology, such as the distribution of pores of different diameters and elastic and other mechanical properties upon flux were taken into account in this model.

Integrated membrane operations in desalination processes

Abstract Today there are many reverse osmosis (RO) plants in operation all over the world for desalination processes. The operating costs for both seawater and brackish water desalination are already competitive with those of thermal operations. These costs are mainly related to the costs of the pretreatment steps, which for seawater desalination might reach 60% of overall costs. By introducing integrated membrane operations, a possible reduction might be possible with an increase of water quality. For example, cross-flow microfiltration (MF), ultrafiltration (UF) and eventually nanofiltration (NF) units might be introduced in the pretreatment line for purifying and clarifying the feed streams and for reducing bivalent ions concentration. If the removal of dissolved gases (CO2, O2, etc.) is suggested, the possibility of also introducing membrane contactors before the RO treatment might be considered. Moreover, the recovery factor of the RO process could be enhanced by introducing membrane distillation (MD) units to treat the RO brine. In the present work, integrated membrane operations such as the ones described above are analyzed for a seawater desalination system. Preliminary experimental results will be discussed confirming the possibility of reaching a seawater recovery factor of 87%.

Effect of energy transport on a palladium-based membrane reactor for methane steam reforming process

Energy transport in a Pd-based membrane reactor (MR) was analysed for an annular and a tubular configuration with a one-dimensional mathematical model. This model takes into account also the energy transfer associated to the hydrogen permeation through a Pd-based membrane. The heat required by the reaction that takes place in a tubular MR is distributed in a larger reactor length when compared to the annular MR; therefore, the heat fluxes from the oven to the reaction side is lower in a tubular MR. Outlet MR conversion is an increasing function of the temperature, sweep factor and overall heat transfer coefficient. An annular MR at 600°C reaches the maximum conversion at a reactor length lower than 1 cm. A much higher reactor length of a tubular MR is necessary to achieve the same conversion. An annular MR presents a better thermal performance and a higher conversion at a reactor length characteristic of a lab scale MR, and also its reaction path is nearer to the optimal behaviour.

Equilibrium conversion for a Pd-based membrane reactor. Dependence on the temperature and pressure

Abstract A thermodynamic tool based on the ‘reactor in series method’ was used to evaluate the equilibrium conversion of dehydrogenation reactions such as methane steam reforming (MSR) and water gas shift (WGS) in a Pd-based membrane reactor (MR). The permeation equilibrium, expressed by the equality of the H 2 partial pressure on reaction and permeate sides, was imposed as the further constrain for MR. The equilibrium conversion shift is an increasing function of the sweep factor, which is an index of the extractive capacity of the membrane system. The equilibrium conversion of a MR was analysed as a function of temperature and pressure. It shows the same trend vs. temperature for MR and traditional reactor (TR). On the contrary, pressure play a very important role because it has a different influence on the equilibrium of MR with respect to a TR. In particular, the positive effect on thermodynamic conversion was shown also for the MSR reaction characterised by Δ ν >0.

Sieverts law empirical exponent for Pd-based membranes: critical analysis in pure H2 permeation.

In this paper, the physical meaning of the Sieverts-type driving force exponent n is analyzed for hydrogen permeation through Pd-based membranes by considering a complex model involving several elementary permeation steps (adsorption on the membrane surface on the feed side, desorption from the surface on the permeate side, diffusion through the metal lattice, and the two transition phenomena surface-to-bulk and bulk-to-surface). First, the characteristic driving force of each step is evaluated, showing that adsorption and desorption singularly considered and the adsorption and desorption considered at the same time are characterized by driving forces depending on the ratio of feed and permeate hydrogen pressure. On the contrary, the diffusion step is found to present a driving force that is composed of two terms, one which corresponds to the original Sieverts law (with an exponent of 0.5) and the other which is the product of the pressure difference and a temperature-dependent factor. Then, the characteristic n is evaluated by applying the multistep model to two different membranes from the literature in several cases, (a) considering each permeation step as the only limiting one and (b) considering the overall effect of all steps. The results of the analysis show that for a low temperature and thin membrane thickness, the effect of the surface phenomena is, in general, a decrease of the overall exponent n toward values lower than 0.5, even though, under particular operating conditions, the n theoretical value of the surface phenomena is equal to unity. At a higher temperature and thickness (diffusion-controlled permeation), n tends to 0.5, even though the rapidity of this tendency depends strictly on the membrane diffusional parameters. In this frame, the expression developed for the diffusion step provides a theoretical reason why n values higher than 0.5 are found even for thick membranes and high temperature, where diffusion is the only rate-determining step.

Process intensification strategies and membrane engineering

An important contribution to the realization of industrial sustainable development can be given by “green process engineering”. Based on the principles of the Process intensification strategy it can lead to the development and the re-design of new processes more compact and efficient that allow the better exploitation of raw materials, a lower energy consumption and a reduced plant volume. Membrane technology contributes to the pursuit of these principles and, in the last few years, the potentialities of membrane operations have been widely recognized. In this work, an overview of membrane application and their perspectives in the field of hydrogen production and distillation will be analysed considering membrane reactors and membrane distillation as case studies. The scope is to show how the redesign as membrane systems of traditional operations might contribute to the realization of the goals of process intensification and green chemistry by a new “green process engineering”.

H2 Separation From H2/N2 and H2/CO Mixtures with Co-Polyimide Hollow Fiber Module

A lab-scale membrane module, packed with more than 150 hollow fibers of P84® co-polyimide, was used for the separation of hydrogen mixtures. The ideal membrane performance was analysed with pure gases (H2, N2, CO, CO2, CH4) and H2/N2 and H2/CO mixtures at 50°C and up to 6 bar. Significant differences were observed between ideal selectivities and separation factors of mixtures. In gas mixture experiments, no variation of hydrogen flux was observed among the different feeds, whereas the permeance of the less permeating species, i.e., N2 and CO, was significantly higher than that measured with pure gases. A linear dependence of H2 recovery on the stage cut was observed in the whole feed pressure range investigated. No differences in the behavior of the membrane versus the two different mixtures were observed. A higher separation factor was obtained when H2 was mixed with N2 rather than CO, in agreement with the trend followed by ideal selectivity values, since the one of H2/N2 was 78, a bit higher than that of H2/CO (60). However, the hydrogen concentration in the permeate also reached 90% molar. The performances of a membrane system were compared with PSA and cryogenic, considering two metrics of process intensification.

Simulation of CO2 hydrogenation with CH3OH removal in a zeolite membrane reactor

A thermodynamic analysis of the CO2 hydrogenation to methanol where competitive reactions take place is presented for a membrane reactor (MR) where methanol was selectively removed. A non-isothermal mathematical model was written to simulate a micro-porous MR. Zeolite membranes with different values of the CH3OH and H2O permeances were considered in the MR modelling. The effect of temperature, pressure and species permeation on the conversion, selectivity and yield was analysed. A higher CO2 conversion and CH3OH selectivity can be reached by the use of an MR. An increased CH3OH yield allows to reduce the consumption of reactant and also to operate at lower pressures and higher temperatures, a fact, which favours the kinetics reducing the residence time and the reactor volume. The MR with the highest CH3OH/H2O permeance ratio resulted in better selectivity and yield of CH3OH with respect to the other MR characterised by a higher conversion.

Pd-based membrane reactors for one-stage process of water gas shift

The water gas shift (WGS) reaction is the upgrading stage in the cycles of hydrogen production by, for example, steam reforming of light hydrocarbons from fossil or renewable sources. This is a thermodynamics limited reaction and CO conversion is furthermore depleted by the presence of products, such as hydrogen as often/always happens in industrial applications. WGS industrial processes consist of two reactors in series: the first one operates at high temperature (300–400 °C), exploiting the advantages offered by a fast kinetics; the second one works in the low temperature range (200–300 °C) to the benefit of the higher thermodynamic conversion. This work proposes the use of one Pd-based membrane reactor (MR) operating at the same temperature range as the high temperature WGS reactor as a suitable alternative to the whole traditional reactor (TR) process. The hydrogen permeation allows the increase of the equilibrium conversion close to the total value and thus to operate in the higher temperature range exploiting the greater kinetics offered by Fe–Cr based catalysts. The values of gas hourly space velocity (GHSV), temperature, H2O/CO feed molar ratio, feed composition, etc. used in the simulations are those typical of an industrial application of a WGS upgrading stage. A reference value of 15 bar of feed pressure was assumed since this is the strength limit of the self supported Pd–Ag membrane considered in the simulations. However, a feed pressure of 30 bar was also considered as that used in industrial processes. The pressure proves to be one of the most interesting variables of MR processing. The proposed analysis demonstrates how only one stage based on a Pd-alloy MR can replace the two reactors of the traditional process, which also gives better performance for, e.g., CO conversion, pure hydrogen recovered on the permeate stream, etc. An intensified process with a smaller reaction volume, higher conversion and GHSV, etc. is the outcome.

Membrane air separation for intensification of coal gasification process

Abstract High-ash and other low-quality coals are available in huge quantities in Russia and in other East European countries. Similar solid fuels can also be obtained as by-product of the enrichment process of coal. The aim of this work is the analysis of the possibility to use such low-quality coals as alternative energy sources in fluidised bed gasification process. In order to intensify gasification process in conditions where possible ash melting is avoided, it is proposed to use oxygen-enriched air (OEA) instead of air as a blow and gasification agent. The supply of OEA as fluidisation medium can be beneficial in other respects: it allows to burn at least a part of fly ash before they leave the furnace, thus improving energy efficiency and ecological impacts of the process. In this paper, the first successful example of combination of membrane air separation and coal gasification process is given. This paper summarises the performance of coal gasification in the presence of different oxygen/nitrogen mixtures as a blow for processing of low-quality (high moisture content) coal. The optimal content of O 2 in the blow is in the range 27–33% v/v. The heating value of synthesis gas obtained in optimal conditions was in the range 3.5–4.7 MJ/m 3 (STP). In second part of the paper, the performance of several membranes available for cheap and efficient air separation is analysed and, after comparing them, recommendations on appropriate selection of membrane type and module needed are given.