• Energy Research

Abstract The environmental impacts of brine disposal from seawater desalination plants and wastewater treatment plants represent a subject of growing concern; thus, determining the potential applicability of zero liquid discharge (ZLD) for water treatment is crucial. Membrane-based technologies are a potentially attractive strategy that can be used to reach this goal. Recent studies have highlighted that integrating a series of membrane processes is a viable approach to achieving ZLD for industrial use. However, a relatively limited number of reports have been published on the challenging problems encountered with ZLD approaches. Here, we provide a review of membrane processes that may be used in ZLD approaches and describe their problems as well as potential solutions and innovative technologies for improving their performance. Furthermore, the energy consumption of the different approaches is calculated and analyzed because it represents a major contributor to the total cost, and investments in innovative technologies are discussed. Finally, the prospects for membrane-based ZLD and further research are highlighted.

Aim of the present paper is to investigate and compare the performance of three different possible membrane condenser configurations in terms of amount of recovered liquid water and energy consumption. Membrane condenser is an innovative unit operation utilized for the recovery of evaporated waste water from industrial gases. In the first proposed configuration, the fed waste gas is cooled by cooling water before entering the membrane module; in the second configuration the cooling is obtained inside the membrane module through a cold sweeping gas; the third configuration is in between the two previous ones: the fed waste gas is first partially cooled via an external medium and then a sweeping gas is used for the final cooling of the stream. The achieved results indicate that configuration 2 has the lowest energy consumption, and configuration 3 allows achieving the highest water recovery whereas its energy consumption is in between configuration 1 and 2.

An integrated membrane process for the treatment of wastewaters from a flue gas desulfurization (FGD) plant was implemented on a laboratory scale to reduce their salt content and to produce a water stream to be recycled in the power industry. The process is based on a preliminary pretreatment of FGD wastewaters, which includes chemical softening and ultrafiltration (UF) to remove Ca2+ and Mg2+ ions as well as organic compounds. The pretreated wastewaters were submitted to a reverse osmosis (RO) step to separate salts from water. The RO retentate was finally submitted to a membrane distillation (MD) step to extract more water, thus increasing the total water recovery factor while producing a high-purity permeate stream. The performance of RO and MD membranes was evaluated by calculating salts rejection, permeate flux, fouling index, and water recovery. The investigated integrated system allowed a total recovery factor of about 94% to be reached, with a consequent reduction of the volume of FGD wastewater to be disposed, and an MD permeate stream with an electrical conductivity of 80 μS/cm, able to be reused in the power plant, with a saving in fresh water demand.

This paper presents a study of energy requirements of membrane distillation (MD) for different lab-made flat module designs of 40 cm2 membrane area. The MD runs were carried out in direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) mode with a 0.2 micron polypropylene membrane and in all tests pure water was fed to the system. In DCMD, the effect of the operating temperatures and streams flow rates on the flux, the evaporation efficiency and the energy consumption, was studied, whereas in VMD the parameters analyzed were the feed flow rate, the feed temperature and the vacuum applied at the permeate side. The VMD performed better than the DCMD and the cross-flow module resulted to be the most efficient design for obtaining high fluxes with moderate energy consumptions. The highest flux (56.2 kg/m2 h) was achieved with the cross-flow module working in VMD at a feed flow rate of 235 L/h, feed temperature of 59.2 oC and a permeate pressure of 10 mbar. The lowest values of energy consumption/permeate flow rate ratios obtained were 3.55 kW/(kg h-1) (longitudinal-flow membrane module) and 1.1 kW/(kg h-1) (cross-flow membrane module) for DCMD and VMD tests, respectively.

Modified poly(ether ether ketone) (PEEKWC) membranes with different poly(vinyl pyrrolidone) (PVP) content were used to separate ethanol/cyclohexane (EtOH/CHx) azeotropic mixture in a pervaporation lab scale apparatus. Eight blends were prepared with the PVP weight percent up to 24%. Several characterization tests were carried out to investigate mechanical, morphological and operational properties of the membranes. These tests include FTIR-ATR, tensile strength, and water contact angle measurements. Furthermore, swelling and pervaporation tests are reported. All the membranes were EtOH selective. The highest separation factor was observed at 21 wt% of PVP. Permeation total flux was continuously increased from 3.75 × 10-3 to 14.81 × 10-3 kg/m2 h as the PVP content increased from 0 to 24 wt%. The structure of the blends changed to a more porous form at higher PVP percents as shown in the SEM images, resulting in higher fluxes. Furthermore, it increased the tendency of EtOH uptake which caused the separation factor to increase simultaneously. Swelling results at different concentrations confirmed the same affinity to EtOH with respect to the PVP amount. As the result of the mechanical tests, Young's modulus and maximum stress values decreased in the contrary of elongation of the membranes at higher PVP contents and EtOH concentrations. Since EtOH molecules compared to CHx molecules have higher interactions with the polymer chains, the mechanical strength of the membranes is expected to decline at higher EtOH concentrations, particularly at higher PVP amounts.

The aim of this work is to discuss the role of membrane operations for re-designing industrial productions in the logic of the process intensification. In particular, new metrics for comparing membrane performance with that of traditional operations are tentatively proposed. The comparison is performed in terms of the following: productivity/size ratio; productivity/weight ratio; flexibility; modularity. Referring to the flexibility of the plant, two different aspects are considered: the ability to handle variations that might occur during the life of the plant (such as variation of the pressure, the temperature, or the feed composition) and the ability to be applied in different cycles of production. The modularity takes into account the changes of the plant size related to variations of the plant productivity. As a case study, the new metrics are applied to the sparkling water production. Although the new metrics have been defined for membrane operations, the proposed approach can be more generally applied for evaluating the "degree of intensification" achievable with any other process of interest.

The always increasing necessity arose in the last decennia in redesign the industrial processes with new unit operations more compact, efficient and, thus, able to address various environmental concerns, has led to the definition of new unit operations able to overcome the limitations imposed by conventional units. Among these the use of membrane reactors for hydrogen production is becoming more and more a reality with various pilot installations all over the world. However, to make the use of a new technology more attractive, it is fundamental to define a new way of analysing its performance and highlighting its potentialities with respect to the well consolidated traditional technologies. Hand in hand with the redesign of new processes comes, thus, the identification of new indexes, so-called, metrics, that together with the traditional parameters usually used to analyse a process can supply additional and important information to decide on the type of operation and the identification of the operating condition windows that makes a process more profitable. In this work, some sustainability indexes, mass and energy intensities [1,2], volume and conversion indexes were used in a no conventional evaluation of the up-grading stage in hydrogen production, i.e. the water gas shift reactor, by means of membrane reactors. Defined as the ratio between the total inlet mass and total energy involved in the reactor, with respect to the hydrogen fed and produced by the reactor, mass and energy intensity provide useful information about the material exploitation and the energy efficiency of this new technology. On the other side, Volume and conversion index provide an indication on the gain offered by membrane reactor in terms of catalyst volume required and reactants conversion achieved, at the same operating conditions of a traditional unit. The comparative study of membrane reactor performance with respect to the conventional reactor was analysed as function of the main process variables, such as temperature, pressure, feed molar ratio and space velocity. In the comparison between the two unit operations, the membrane reactor resulted always more material and energy intensive than a traditional reactor. The advantage offered by a membrane reactor was also quantified in terms of ratios referred to the equilibrium condition of the traditional reactor. The membrane reactor resulted always more intensified than a traditional reactor operated in similar conditions and exceeded also the ideal performance achievable by a traditional reactor, at a temperature higher than 350°C. At the highest temperature (450°C) and gas hourly space velocity (40000 h-1) the indexes for the membrane reactor were quite the same of the ideal values (the ones at equilibrium) of the traditional reactor; the membrane reactor can, thus, be operated with different combinations of operating conditions, achieving the same performance in terms of material exploitation and energy efficiency (Figure 1). Mass and energy intensities demonstrated, in line with the process intensification strategy, the assets of the membrane reactor technology also in terms of better exploitation of raw materials (reduction up to 40%) and higher energy efficiency (up to 35%). In addition, the catalyst volume required by membrane reactor was more than three times lower than the one of traditional unit, for achieving the same conversion [3,4].

Enzymatic membrane bioreactors (MBR) have been studied for very different applications since many years. Submerged MBR has also been successfully used for treatment of wastewater. In the existing submerged configuration, the membrane works as the separation unit operation while the bioconversion is carried out by microorganisms suspended in the tank reactor. In the present work, a novel approach that combines the concept of biocatalyticmembranes and submerged modules is proposed for the treatment of biomass. Lipase enzyme from Candida rugosa has been immobilized in polyethersulphone hollow fiber (PES HF) membrane in order to develop atwoseparatephasebiocatalyticsubmergedmembranereactor in which the membrane works as both catalytic and separation unit. Furthermore, the submergedbiocatalyticmembranereactor is intended for production of valuable components from waste biomass, and different physical, chemical and fluid dynamics has been optimized. Response surface methodology (RSM) has been used to model the operating parameters and Box-Behnken method has been applied to maximize the fattyacidsproduction and optimization. At TMP 80 ± 2 kPa, pH 7.40 ± 0.1, temperature 35 ± 0.5 °C with an axial velocity of 0.07 ± 0.01 ms-1 and organic stirring 89.01 rad s-1 the system showed the global examined value within the experimental scope. The proof of principle using fried cooked oils has been performed in later period.