Novel Tri Hybrid Desalination Plants
Authors: Mabrouk Abdel-Nasser , Darwish Mohamed 
Published: July 2019

Hybrid desalination (thermal/membrane) process is a viable solution in Gulf Cooperation countries (GCC). In GCC, large thermal desalination plant, e.g. Multi Stage Flash (MSF) and Multi-Effect-Distillation (MED), exist and will stay in operation through their expected life, (10’s of years) while new large capacity reverse osmosis (RO) plants should be applied to lower the water specific consumed energy and cost. The thermal desalination technology was the prevailing technology in the GCC, where capacity per unit is large (up tom 20 MIGD), low energy cost was assumed, seawater salinity is high, and combining desalting plants with power plants provides low cost steam extracted from turbines to desalting plants at low pressures (e.g. 1–3 bars). However, in recent years, membrane systems, namely seawater reverse osmosis (SWRO), become the prevalent desalting worldwide as a result of much lower specific energy consumption (compared to MSF and MED systems), development in RO membranes properties and energy recovery devices, and low electric load in winter forcing some power plant units to cease operation, and thus unavailability of steam extracted from turbines. In this case, the boiler generated steam is wastefully throttled to the conditions required by desalting units. Realizing the fact that large MSF and MED are existing and operating, while large SWRO have to be implemented, combining the operation of both technologies can be beneficial and a need to have optimum configurations exist in terms of reduced total desalted water cost. The synergy of the present commercial hybrid thermal/membrane desalting plants is limited by using common intakes and outfalls facilities, while are running independently at the same site. Product water of both membrane and thermal plants are usually blended to meet the international standards water quality specifications.

Moreover, the use of forward osmosis (FO) as a pre-treatment to the MSF units was suggested to remove the divalent ions, such as MgSO4 and CaSO4 in order to increase the top brine temperature (TBT) beyond 110°C was suggested. This increases the MSF unit capacity and gain ratio. In this arrangement, the MSF brine (of high concentration) is used as draw solution to the incoming seawater to the FO membrane. The brine is diluted by drawing pure water from seawater feed, and is circulated back to the MSF unit where distillate is recovered. The main advantage of this suggestion is the use of brine as draw solution without the need to external solution and the MSF process is utilized as recovery process. However, the relatively low osmotic pressure of the brines necessitates high required FO membrane area, which is expensive.

In the present work, a new configuration of tri RO-FO-MSF hybrid desalination process is proposed. The aim of the proposed configuration is reduce the specific thermal energy consumption by producing the desalted seawater by both MSF and RO systems. The cooling water leaving the MSF heat rejection sections is used as a feed to RO to improve the RO productivity (i.e. recovery ratio). The RO brine and MSF blow-down mixture is used a draw solution to the FO process, and the diluted solution is directed to the MSF unit. This lead to higher productivity and low chemical consumptions compared to separate systems.

A flexible and powerful tool of Visual Design and Simulation program (VDS) is used to evaluate this novel tri hybrid RO-FO-MSF. Typical desalting plants are simulated to verify and the wide scope and high capability of the developed package. In this work, the scope of the VDS program is extended to develop novel tri hybrid RO-FO-MSF and compare it against MSF-NF, MSF-FO and RO-MSF configurations. In order to compare among different configuration, the reference RO plant (33 MIGD) in UAE and reference MSF (15 MIGD) unit in Qatar are considered as a base of comparison.

For fare comparison between thermal and membrane in terms of energy consumption, the equivalent mechanical energy of thermal energy associated with heating steam supplied to the brine heater of the MSF is calculated using exergy method. This equivalent energy is equal to the power loss in steam power plant due to combined with thermal desalination plant. The equivalent energy varies according the Gain Output ratio (GOR) of the thermal system. The total energy consumption in MSF unit is a summation of the equivalent of the thermal energy and pumping energy.

Comparison between different hybrid configurations will be based on total consumed energy per unit production at different recovery ratio of the system. In the present methodology, a fixed value of input seawater feed flow rate is specified and then total energy consumption is calculated at varying recovery ratio for each hybrid system.

Simulation results showed that the consumed energy of the hybrid FO-MSF is 10 % lower than NF-MSF and 29 % higher in the recovery ratio. The hybrid RO-MSF in series is 20 % higher in the recovery ratio and 60 % lower in consumed energy compared to the traditional MSF. The hybrid RO-MSF consumes the same energy of standalone RO however the recovery ratio is 23 % higher than RO.

The recovery ratio for tri-hybrid RO-FO-MSF configuration is higher 30 % higher, when compare with the traditional MSF and standalone RO. The consumed specific energy in the tri hybrid configuration is 60 % lower than MSF and consume the same energy of the RO process.

The tri hybrid RO-FO-MSF, when compared with other hybrid configurations, has superior lower consumed energy, as well as with standalone MSF and RO processes. The novel

(RO-FO-MSF) configuration provides a solution of the limited recovery ratio of the standalone RO and MSF.

The product blend of the MSF distillate and RO permeate would enable using single pass RO and excluding the second pass which will reduce the capital investment cost. Also the product blend will resolve the borne issue rose up in standalone RO plant since the permeate of the RO is diluted by the MSF distillate.