Minimisation of energy consumption via optimisation of a simple hybrid system of multi effect distillation and permeate reprocessing reverse osmosis processes for seawater desalination

Abstract

Multi Effect Distillation (MED) and Reverse Osmosis (RO) processes have been extensively used to produce freshwater from seawater resources. amongst many performance indicators, energy consumption of different configuration of hybrid system of MED and RO processes have been analysed in the past. Hybrid MED-RO system is energy intensive and use of fossil fuel can significantly increase the carbon footprint, unless stable renewable energy sources are used. In this work specific energy consumption of a simple hybrid MED-RO system with permeate reprocessing is minimised while optimising a number of operating decision variables using model based optimisation technique. A detailed process model developed earlier by the authors is embedded in the optimisation framework resulting in a constrained Non-linear Programming (NLP) problem. The minimum specific energy consumption achieved in this work is about 18% lower than what is reported in the literature resulting in a significant energy saving and thus carbon footprint.

Introduction

Due to a continuous growth for water demand in warm regions of the world, the market of seawater desalination technologies has experienced a remarkable increase to provide potable water (Elsaid et al., 2020). In this regard, the MED and RO systems have been selected as the favourable thermal and membrane processes, respectively, due to their stability, high quality of fresh water produced and product capacity (Al-Obaidi et al., 2019; Emamdoost et al., 2020). Specifically, MED process can produce salt free water from high salinity seawater. Moreover, RO process has been utilised for different water types including seawater, brackish water, and wastewater with strong potential. de Boer (2014) confirmed that both the RO and MED processes are the most energy demanding filtration processes because of the use of high-pressure pumps and steam generators, respectively. Therefore, the integration of MED with a variety of industrial processes such as concentrated solar power cogeneration scheme and RO process has been investigated (Cipollina et al., 2017; Filippini et al., 2018). From an economic perspective, Mahbub et al. (2009) observed that the low-temperature steam MED process is more suitable to be integrated with the RO process. Filippini et al. (2018) compared the performance of several designs of hybrid system of MED+ retentate reprocessing design of RO process via a detailed process model. The performance of the hybrid system was assessed, and results show an improved performance compared to individual processes of MED and thermal vapour compression (TVC) and RO.

Al-hotmani et al. (2019) suggested a novel design of a simple hybrid system of permeate reprocessing RO process integrated with MED_TVC process and using simulation technique compared the performance of this design with other alternative designs of hybrid systems of MED process and retentate reprocessing RO process presented by Filippini et al. (2018). The simple hybrid MED_TVC+ permeate reprocessing design of RO process was found to be a superior layout compared to others in terms of low energy consumption measured in kWh per m³ of freshwater (specific energy consumption).

The optimisation technique of any industrial process including MED and RO processes is one of the most prominent tools to attain the favourable objective functions such as the lowest freshwater production cost, lowest total annualised cost and lowest energy consumption wile fulfilling the design, operation and quality constraints. For example, Esfahani et al. (2012) optimised the MED-TVC desalination system using genetic algorithm based on artificial neural network model. In this respect, multi-objective optimisation was developed to minimise the total annualised cost and simultaneously maximise the gain output ratio (GOR). The result presented the optimal values of decision variables including the motive steam mass flow rate, temperature difference between the effects, and preheated seawater temperature. Furthermore, it was confirmed that MED_TVC system with 6 effects was the optimum compared to MED_TVC system with 3, 4, and 5 effects. Sadri et al. (2017) deployed multi objective function optimisation to forecast best trade-off between the exergetic efficiencies of MED and RO processes and therefore to attain the optimum conditions for the hybrid system. In this regard, they carried out the irreversibility analysis and chemical and physical exergy destruction. The study elaborated the highest exergy efficiencies of both MED and RO processes. Also, Al-Obaidi et al. (2019) used a single-objective non-linear optimisation framework to predict the lowest freshwater production cost of a simple design of hybrid system of MED process and retentate reprocessing RO process by discovering the optimal values of the main operating conditions. The optimisation results affirmed the economic viability of the hybrid seawater desalination plant.

He authors of this paper believe that there is still a capacity to reduce significant amount of the total consumption energy of the hybrid design of MED_TVC with permeate reprocessing RO process developed by Al-hotmani et al. (2019) which can be achieved via optimisation. Therefore, this study aims to resolve this challenge by embedding the model developed for this layout earlier within an optimisation framework. This is specifically targeted at lowering energy consumption (objective function) by operating the hybrid process at optimum conditions.