Energy Aspects in Osmotic Processes

Abstract

ab s t r ac tWater, energy and environmental issues are on the top list of the world problems. Energy is needed for augmenting our water resources. Renewable energies are hardly the answer since innovative techniques based on biofuels and biodiesel consume an incredible amount of water. The modern desalination techniques in use consume different energy levels from different sources. Thermodynamics sets the absolute minimum limit of the work energy required to separate water from a salt solution. Unavoidable irreversibilities augment the actual energy consumption. Modern desalination techniques have succeeded in narrowing considerably the gap between actual and minimum energy levels. The implication of this small gap is that only marginal energy reductions are possible. Energy consumption of different desalination processes are reviewed. Forward osmosis is shown to be a high energy consumption process. It offers, however, advanced cost effective backwash techniques. The limitations of power generation by osmotic processes is discussed. Sidney Loeb together with Sourirajan were the pioneers that opened the door and introduced the RO process that allows us to desalinate seawater and brackish water at affordable energy consumption. Loeb continued during his last years to develop the osmotic energy machine, and even tried to develop an air condition system based on water evaporating from tubular membranes.

Energy consumption in desalinating produced water from shale oil and gas extraction

“…The recovered pressures (P rec1 and P rec2 ) in stages 1 and 2 are determined by modeling the PX as an isentropic depressurization-pressurization process, derated by the PX efficiency, η P X [30] (Eqs. (38) and (39)). …”

“…Models exist for the energy consumption of FO seawater desalination with thermal regeneration [38,39], but a more general model is needed to extend these predictions to produced water. The model presented here is developed for a system similar to that used in [36], with the same ammonia-carbon dioxide draw solution and with regeneration efficiency benchmarked against data from the pilot plant.…”

“…Circulation pumps are neglected in the model because of their minimal contribution to energy consumption. Not modeled are any additional steps that may be needed to remove aqueous ammonia from the permeate [38].…”

Raising forward osmosis brine concentration efficiency through flow rate optimization

“…[14], membrane distillation (MD) [15], and multi-stage flash (MSF) [16]. Thermally-regenerated FO has also been modeled by Semiat et al [17]. An FO pilot system with RO regeneration (FO-RO) was used to concentrate low-salinity O&G wastewater [2,18].…”

On the potential of forward osmosis to energetically outperform reverse osmosis desalination

“…This is largely because the lower the temperature of the heat source, the lower its exergy, and, consequently, the larger the amount of heat required and the higher the heat exchanger costs. For example, if we consider a draw solution regeneration process requiring E regen =13 kWh of exergy per m 3 of product water desalinated (the electrical energy requirement computed by Semiat et al for a thermally regenerated seawater forward osmosis process [34]), we can compute the heat exchanger size, p HX [in kW t /(m 3 /day)] theoretically required for the process to be thermally driven by a heat source at temperature T source :…”