Research Paper Simulation study of flat-sheet air gap membrane distillation modules coupled with an evaporative crystallizer for zero liquid discharge water desalination Hanfei Guo ⇑ , Hafiz Muhammad Ali, Ali Hassanzadeh School of Engineering, University of California, Merced, CA 95343, USA highlights  A model of AGMD modules coupled with an evaporative crystallizer is developed.  Optimal values of operating and dimensional parameters were determined.  NaCl mass fraction in retentate stream influences system heat duty significantly. article info Article history: Received 10 May 2016 Revised 2 July 2016 Accepted 18 July 2016 Available online 19 July 2016 Keywords: Flat sheet AGMD Zero liquid discharge Desalination Evaporative crystallizer MDC abstract A flat sheet air gap membrane distillation (AGMD) model and an evaporative crystallizer model were developed for design and optimization of the lab-scale zero liquid discharge (ZLD) water desalination experimental plant. The models were validated by comparing with published experimental data. Univariate analysis was utilized to investigate the influences of thirteen operating and dimensional parameters of single stage and multi-stage AGMD modules on the permeate flux, evaporative efficiency, water recovery, and gained output ratio (GOR). Optimization of the parameters were conducted aiming to maximize the permeate flux, water recovery, and GOR of the AGMD module. Membrane distillation and crystallization (MDC) process was then altogether modeled in Aspen Plus software based on the param- eter studies of the single and multi-stage AGMD model. The effects of water removal ratio in the crystal- lizer and NaCl mass fraction of the MD retentate stream on the heat duty of the system were analyzed. The operating condition with the minimum input energy for the current MDC design was determined, and the input energy is 1651.5 kJ/kg-H 2 O. The process can be further optimized to tremendously reduce the required input energy when the heat stored in the evaporated vapor from the crystallizer is recovered. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Water scarcity problem becomes severer in recent years as the world population increases. In order to meet the increasing requirement of fresh water, many water treatment strategies have been proposed, among which desalination method is increasingly popular and has drawn greater attention. The desalination plants operated worldwide produced 78.4 million cubic meters water per day by the year 2013, and the number continues increasing [1]. Sea water or salt water desalination is a process that separates the saline water into two streams: a fresh water stream with low salt content, and a concentrated brine stream. Currently, the most popular commercially used desalination approach is reverse osmo- sis (RO) modules which can produce high-quality potable water [2]. However, the concentrated brine stream emitted by the desali- nation method will adversely affect the ecosystem if it is dis- charged directly to the rivers, lakes, or oceans. Therefore, zero liquid discharge (ZLD) becomes essential to mitigate the negative influence of desalination on the environment. The ZLD method can be integrated with the existing desalination approach to estab- lish a hybrid desalination technology. Membrane distillation (MD) coupled with crystallization (MDC) process is an important ZLD method which has attracted the inter- est of many scholars and institutes [3–9]. Membrane distillation modules are able to enrich the brine to a higher concentration than RO modules. Hydrophobic porous membrane is the key part of the membrane distillation module which acts as the media to allow http://dx.doi.org/10.1016/j.applthermaleng.2016.07.131 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: guohf07@yahoo.com (H. Guo). Applied Thermal Engineering 108 (2016) 486–501 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng