Performance simulation of a multi-VMD desalination process including the recycle flow

In this paper, a performance evaluation of a multi-vacuum membrane distillation (VMD) module was conducted using a one-dimensional model. The mathematical model consisted of momentum, mass and energy balance equation using the water permeate flux model and heat flux model. The simulation results were in good agreement with the experimental results from previous literature. The validated VMD model was implemented into Aspen Plus. Then, a multi-VMD module with one-through flow was simulated. As a result, in the high velocity region, the hydraulic pressure was used as the constraint to determine the number of membrane modules. In the low velocity region, the number of membrane module was determined based on the feed temperature. To improve water recovery and thermal efficiency of the multi-VMD module, the recycle flow was considered and the waste heat included in the discharge brine was recovered. As a result, it was possible to achieve water recovery over 40%. In addition, as the recycle flow ratio increased, thermal efficiency also improved since heat duty and thermal consumption per unit water production decreased. © 2013 Elsevier B.V. All rights reserved.

In this study, a one-dimensional VMD model was developed and implemented in Aspen Plus. The use of Aspen Plus was helpful to investigate the complex process including the heat exchanger circuit and recycle flow. To determine the maximum number of membrane modules connected in series, variations in feed temperature and hydraulic pressure with respect to the number of membrane modules were analyzed using Aspen Plus. As a result, in the high feed velocity, it was appropriate to determine the number of membrane modules based on the outlet hydraulic pressure. In the low feed velocity region, the outlet temperature of the feed solution was proper for the determination of the number of membrane modules since the feed temperature decreased significantly and approached to the bubble point temperature. In the multi-VMD module with one-through flow, as the inlet feed velocity and the inlet feed temperature increased, the water production increased. However, the increase in the inlet feed velocity caused the decrease in the water recovery. As a result, it was difficult to achieve the water recovery over 8% although inlet velocity and inlet temperature of feed seawater were 0.4 m/s and 80 °C, respectively. Thus, a multi-VMD module including the recycle flow was considered in this work. In addition, the heat exchanger was added to recover the waste heat included in the discharge brine. As a result, when the feed velocity was 1.0 m/s, it was possible to achieve a water recovery over 48.3% at a recycle flow ratio of 6.0 without a significant decrease in water production. As the recycle flow ratio increased from 0.0 to 6.0, the heat duty gradually decreased from 8.15 MW to 6.98 MW. Especially, at the recycle flow ratio over 3.0, the thermal consumption per unit water production was slightly varied in the range of 2.37– 2.91 MJ/kg.