Simultaneous heat and water recovery from flue gas by membrane condensation: experimental investigation

A tubular ceramic membrane is investigated as the condenser for simultaneous heat and water recovery from flue gas. The effects of the operational parameters, such as fluid (gas and water) flow rates, temperatures of flue gas and coolant water, and flue gas humidity on the process performance in terms of mass and heat transfer across the membrane are studied. Particularly, the overall heat transfer coefficient is also evaluated. As the gas flow rate increases, water and heat transfer efficiencies and recoveries decline due to the reduced residence time. Increasing the water flow rate or lowing the coolant temperature can effectively improve mass and heat transfer efficiencies and recoveries. Increasing the temperature of the inlet gas can enhance water and heat fluxes and recoveries, but does not improve the overall heat transfer efficiency. The rise in flue gas humidity can dramatically improve water and heat transfer rates and the overall heat transfer coefficient, but has little effect on water and heat recoveries. These results offer a general guideline in optimising the operational parameters in low-grade heat recovery with membrane heat exchangers, and it may greatly advance the development of membrane condensation technology for practical low-grade heat recovery

Osmotic pressure versus swelling pressure : comment on “Bifunctional polymer hydros forward osmosis draw agents for continuous production of fresh water using solar energy”

Recently Razmjou et al. proposed a smart method of using bilayer polymer hydrogels as draw agents to absorb water from saline streams and using solar energy to dewater the hydrated hydrogels. The authors thought that the principle of absorbing water of the hydrogels was based on forward osmosis, that is, the hydrogel particles (2−25 μm) provided the osmotic pressure. The authors may misunderstand the swelling pressure of hydrogels as the osmotic pressure. The following discussion aims to clarify two confused concepts (i.e., osmotic pressure and swelling pressure) and to differentiate the principles of forward osmosis and of hydrogel absorbing water for both readers and the authors. It is believed that the clarification is very necessary because a number of published papers are based on these misunderstood concepts.2 page(s

Relating solution physicochemical properties to internal concentration polarization in forward osmosis

Recently forward osmosis (FO) has attracted growing attention on many potential applications such as wastewater treatment, desalination and power generation. FO performance is primarily limited by the presence of internal concentration polarization (ICP), which significantly reduces the permeate flux. This study explores the relationship between the physicochemical properties of the solution against the membrane support layer and ICP by incorporating constrictivity. Four solutions with different diffusivities, ion/molecule sizes and viscosities were systematically investigated using a bench-scale FO system. It is found that ICP in the support layer is strongly dependent on the physicochemical properties of the solution facing the support layer. When the solution against the membrane support layer has a lower aqueous diffusivity but larger ion/molecule size and higher viscosity, The ICP phenomenon will be more severe, resulting in lower water flux. The identical diffusion direction of the feed solute and the water flux may reduce the effective diffusivity of the solute in the support layer when the feed solution facing the membrane support layer, resulting in high concentrative ICP. These findings have significant implications for the development of new draw solutes, the pretreatment of the feed solution and the selection of the membrane orientation.9 page(s

Erratum to "Effects of working temperature on separation performance, membrane scaling and cleaning in forward osmosis desalination" [Desalination 278 (2011) 157-164]

The publisher regrets that during the publication of the above mentioned article, Eqs. (5), (6), (7) were incorrect

Effects of working temperature on separation performance, membrane scaling and cleaning in forward osmosis desalination

Recently forward osmosis (FO) has drawn increasing attention in wastewater treatment, brackish water/seawater desalination and power generation. It is supposed that FO has many advantages over other pressure-driven processes, such as low energy consumption, low fouling tendency, high water recovery and thus minimizing brine volume. In FO process, many parameters like osmotic pressure, fluid viscosity, mass transfer and mineral solubility are temperature dependent. In the current study, the effects of working temperature on separation performance (e.g. water fluxes and recoveries), membrane scaling and cleaning were systematically investigated through a bench-scale FO system. Both real and simulated brackish water were used as the feed solution in FO at temperatures of 25, 35 and 45 °C. Bench-scale FO experiments showed that higher temperature would afford higher initial fluxes, higher water recoveries and higher concentration factors, but also caused more adverse effects on membrane scaling and cleaning.8 page(s

Brackish water desalination by a hybrid forward osmosis–nanofiltration system using divalent draw solute

In the last decade osmotically driven membrane processes have attracted growing interest in wastewater treatment, desalination and power generation. A hybrid forward osmosis–nanofiltration (FO–NF) system designed for brackish water desalination was systematically investigated in this study. The hybrid FO–NF process was also compared with a stand-alone reverse osmosis (RO) process in brackish water desalination. It is found that the hybrid FO–NF process has many advantages over the stand-alone RO process in brackish water desalination, such as lower hydraulic pressure, less flux decline caused by membrane fouling and higher flux recovery after cleaning. Other benefits like no pre-treatment requirement, higher permeate quality and no need for chemical cleaning in brackish water desalination can also be achieved by the hybrid FO–NF system. Therefore, the proposed FO–NF system using a divalent draw solute (e.g. Na₂SO₄ or MgSO₄) can be a useful alternative to stand-alone RO processes in brackish water desalination.7 page(s

Effects of membrane orientation on process performance in forward osmosis applications

Forward osmosis (FO) has attracted growing attention for its great promise in desalination, wastewater treatment, liquid food processing and power generation. However, there is no clear agreement on the selection of membrane orientation in these applications. This study investigates the effects of membrane orientation on FO performance in saline water desalination without fouling, and with inorganic or organic fouling. The results show that the feed solution component and the concentration degree could influence the selection of membrane orientation. When severe membrane fouling or scaling occurs, the isoflux point occurs relatively early and FO mode (active layer facing the feed) provides a more stable and higher water flux than that in the alternative membrane orientation, i.e. pressure retarded osmosis (PRO) mode (support layer facing the feed). Additionally, lower fouling but higher cleaning efficiency is observed in FO mode compared with PRO mode. Therefore, in the applications of treating feed solutions with higher fouling/scaling tendencies (e.g. wastewater treatment) or treating higher salinity water (e.g. seawater desalination), FO mode is more favourable. While PRO mode is preferred when using the solutions with lower fouling/scaling tendencies as the feed (e.g. brackish water desalination), or where intensive concentration is unnecessary (e.g. power generation).8 page(s