Novel multistage flash reversal Concept: Modelling and analysis

Highlights

• New MSF based process, termed as MSF reversal, is proposed and modelled.

• MSF reversal is particularly suitable for low-cost thermal energy sources.

• MSF reversal with brine mixing with two cooling options is modelled and analysed.

• MSF reversal has lower energy demand than MSF Once through but higher surface area.

Abstract

In this study, a rigorous modeling and simulation of a novel multi-stage flash (MSF) configuration consisting of reversing the brine circulation, termed MSF reversal (MSF-RV), is developed. Its performance is theoretically investigated and compared with conventional MSF Once Through (MSF-OT) with and without brine mixing. The MSF-RV concept is suitable for treating geothermal streams and can be driven by low grade thermal energy such as solar and geothermal energy and waste heat for direct seawater desalination. Hence, two options of MSF-RV are proposed, i) driven by a direct hot stream (MSF-RVc), and ii) powered by external heat to treat raw seawater (MSF-RVh). The analysis showed that the temperature distribution throughout the stages plays a significant role in the thermal efficiency and heat transfer area requirements for both configurations. Hence, careful selection of the design parameters is necessary to achieve the best performance. For the same recovery ratio, the MSF-RVc was found superior to MSF-OT in terms of gain output ratio (GOR) and specific energy consumption (SEC) by 52% and 60%, respectively. However, the specific area (sA) requirement of MSF-RVc is higher than that of MSF-OT by 50%. Brine mixing by recycling the rejected brine enhances the recovery ratio, GOR, and SEC for both structures. Conversely, the sA requirement increases with brine mixing but marginally for MSF-RV and remarkably for MSF-OT. Moreover, the design parameters of MSF-RVc such as the coolant inlet temperature, the temperature drop on the coolant side, and the coolant to brine ratio affect the overall performance. However, a trade-off between the thermal efficiency (GOR, SEC) and surface area requirement is still observed.

Introduction

Supplying drinkable water has become a very challenging task around the globe. It is more pressing in the Middle East and North Africa due to water scarcity [1]. In reality, only 2.53 % of the global water resources are potable out of which merely 0.36 % are easily accessible by mankind [2]. It is estimated that 57 % of the world population will encounter acute water shortages by 2050 [3]. Hitherto saline water purification is the main method to meet the escalating demand for fresh water worldwide [4]. Eke et al. [5] reported that up to 2020 the deployed desalination capacity is 97.2 × 10⁶ m³ per day while the collective production capacity is 114.9 × 10⁶ m³ per day. This global drinkable water production is supplied by a total of 20,971 commercial desalination plants. The conventional desalination technologies are multi-stage flash (MSF), multi-effect evaporation (MEE), and reverse osmosis (RO). The most challenging issue regarding conventional desalination technologies is energy consumption [6]. For example, the combined thermal and electrical energy consumption of the MEE lies between 5.5 and 9 kWh/m³ which is less than that of the MSF (10 ∼ 16 kWh/m³) [7]. On the other hand, RO plants consume high grade electrical energy (3 ∼ 4.5 kWh/m³) [8]. According to Askari et al. [2], the RO global share of installed production capacity is 69 % while the combined capacity share of MSF and MEE is 25 %. However, the rapidly growing energy cost, environmental regulations, and water and energy demand impose serious challenges on all desalination technologies irrespective of their specific structures and features. To achieve sustainability, numerous investigations were reported dealing with retrofitting conventional technologies or inventing new methods [9].

Despite the global dominance of RO and MEE, MSF remains popular because of its interesting features, such as when combined with a power plant resulting in cogeneration of electricity and water structure, its robustness to challenging feeds, and its flexibility to be integrated with various sustainable energy sources, such as solar and geothermal sources [10], [11]. Besides, it tolerates high feed salinity and temperature, and long-life [12]. Moreover, MSF still dominates the desalination market in the Middle East which comprises 86.7 %∼94 % of the total installed desalination capacity [13], [14]. Once-through MSF (MSF-OT) is a simplified generation of the conventional MSF as the rejection section is eliminated. This simple layout leads to lower capital and operational cost [15]. However, it has limited industrial applications because it suffers from low thermal performance and low recovery ratio (PR and RR, respectively). Hence it is limited to regions with low annual temperature variation [16]. Nevertheless, the RR and thermal performance can be improved by reject brine recirculation [16]. Roy et al. [13] studied the effect of top brine temperature (TBT) on the design and performance of MSF-OT using a mathematical model. They concluded that the effect of increasing TBT on PR is positive while its effect on a specific area (sA) may be positive or negative depending on the state of the other operating conditions. In the same fashion, Hanshik et al. [17] studied the effect of TBT on the MSF-OT desalination performance. It is found that increasing TBT will boost the production rate and decrease the energy consumption per mass of distillate. But at the same time, it will increase the required surface area for heat transfer. Baig et al. [15] studied the effect of various design parameters on the performance of MSF-OT. They concluded that the most influential parameters are the TBT, outlet and inlet brine temperature, and number of stages, Similarly, Mussati et al. [18] have investigated the influence of various parameters including heat consumption and modes of recycles on the performance of MSF-OT. They concluded that design parameters have a great effect on the optimal structure of the evaporator. Abduljawad and Ezzeghni [19] optimized the operation of MSF-OT to maximize the gain output ratio (GOR). They used the TBT as the main design parameter. They obtained different optimal operating conditions for various production capacities. The optimal conditions are found to be insensitive to the variation of the seawater temperature. Alhazmy [20] proposed a modified structure of the MSF process with brine mixing by adding a cooler to maintain a low feed-brine mixture temperature. He reported that the proposed configuration can operate independently of the seasonal seawater temperature which results in maintaining a uniform production rate around the year. The results show in particular that for every 1 °C reduction in the plant bottom temperature an enhancement of about 1.4 % of the yield can be achieved compared with standard equivalent MSF systems. In a subsequent paper, Alhazmy included exergy and cost analysis showing the merits of cooling the feed at the MSF plant intake [21].

The integration of MSF with solar energy has attracted increasing attention in the last few decades. This interest is justified by the urgent need to reduce the environmental impacts associated with the conventional energy sources powering the desalination technologies but also to find means to reduce the energy consumed by those technologies [22], [23]. Novel designs and structures have been proposed and assessed [5], [23]. Al-Othman et al. [24] investigated the feasibility of coupling parabolic trough collectors (PTC) and a solar pond with MSF system to produce 1880 m³/day of desalinated water required for 5000 population of a small community in Sharjah, UAE. The results show that two collectors of a total aperture area of 3160 m² can provide 76 % of the total MSF energy needs while a solar pond of a 4 m depth and 0.53 km² surface area can supply the remaining 24 %. A new design for a solar driven MSF desalination plant utilizing two separate thermal storage tanks was recently proposed by AlSehli et al. [25]. A judicious adjustment of the brine circulation between the MSF and the two storage tanks in order to maintain the required TBT value and ensure continuous MSF operation was proposed and its effectiveness was evaluated using a dynamic model of heat and mass transfers. The results show that an average daily distillate production of 53 kg per square meter of solar collection area can be achieved resulting in total production of 2230 m³/day at 2.72 $/m³. Other attempts to power MSF plants with other advanced sources of energy such as nuclear power, microbial cells, and retarded osmosis are also reported in the literature [26], [27], [28].

It is obvious from previous studies that the design and operating parameters of MSF still need further investigation to maximize the production capacity and reduce the energy consumption and hence the operating cost. This means there is considerable room for further development of such a process [7], [15], [19], [22].

To further the development of MSF-based configurations for improved operation and performance, it is of interest to investigate through rigorous modeling and simulation a novel design modification of MSF by using brine reversed flow, termed MSF reversal (MSF-RV) [29]. Unlike, standard MSF-OT, the feed seawater is fed in the first brine flashing chamber, instead of the condenser, after being heated by a low-grade thermal energy source. Alternatively, the reversed MSF structure can treat directly naturally hot saline water such as hot springs, geothermal water, or industrial utility water. The brine exiting the last flashing stage will be cooled further and fed to the condenser tubes as a condenser stream. In this modified structure, heating of the concentrated brine by steam (brine heater) is eliminated and the process can be driven by sensible heating. Therefore, the energy requirement of the process is reduced substantially. This will boost thermal efficiency and reduce capital and operation costs.

Usually, thermal desalination technologies especially MSF are prone to high energy consumption and hence high operational costs. To reduce such costs, integrating waste or renewable energy sources into thermal desalination systems might be of interest [30]. For this purpose, the proposed novel MSF-RV structure is more suitable to be powered by waste heat. It is worthy to mention that MSF-RV configuration can easily be hybridized with other desalination technologies either thermal or membrane ones. For example, it can be integrated with the Membrane Distillation (MD) process which would contribute to improving the overall performance of the hybrid MSF-RV/MD by increasing its recovery ratio and energy efficiency. On another side, MSF-RV can treat first the hot brackish water from geothermal sources before being supplied to RO plants for further desalination. This hybrid MSF-RV and RO option would avoid wasting geothermal energy which is in general damped in atmospheric air using cooling towers before entering RO plants [31].

A similar reversal concept was studied by Lee et al. [32] where a multi-stage air gap membrane distillation (AGMD) reversal was proposed and examined. They found that GOR can be increased sixfold because of the use of free natural cooling sources. Thereby, the objective of this work is to develop the model for MSF-RV, analyze its operation, and compare its performance to the MSF-OT. Analysis of the effect of design parameters as well as brine mixing on the performance of the proposed MSF structure is also presented. In addition, the proposed formulation and analysis will serve as the basis for further studies and developments of this modified MSF structure.