# Theoretical and Experimental Studies of Permeate Fluxes in Double-Flow Direct-Contact Membrane Distillation (DCMD) Modules with Internal Recycle

## TL;DR

This paper introduces a new membrane distillation module design that improves permeate flux by using a dual-flow system with internal recycling.

## Contribution

The novel dual-flow DCMD module with internal recycle is shown to significantly enhance permeate flux by reducing heat-transfer resistance.

## Key findings

- The dual-flow configuration improved permeate flux by up to 40.77% compared to single-pass modules.
- Inserting an insulation barrier minimized temperature polarization and improved flow characteristics.
- Increased convective heat-transfer coefficients and residence time enhanced device performance.

## Abstract

A new DCMD module design that introduces an insulation barrier of negligible thickness to divide the open duct of the hot-saline feed into two subchannels for dual-flow operation was investigated. This configuration enables one subchannel to operate in a cocurrent-flow mode and the other in a countercurrent-flow recycling mode, thereby significantly enhancing the permeate flux. Theoretical and experimental investigations were conducted to develop modeling equations capable of predicting the permeate flux in DCMD modules. These studies demonstrated the technical feasibility of minimizing temperature polarization effects while improving flow characteristics to boost permeate flux. Results indicated that increasing both convective heat-transfer coefficients and residence time generally improved device performance. The dual-flow operation increased fluid velocity and extended residence time, leading to reduced heat-transfer resistance and enhanced heat-transfer efficiency. Theoretical predictions and experimental results consistently showed that the absorption flux improved by up to 40.77% under the double-flow operation with internal recycling configuration compared to a single-pass device of identical dimensions. The effects of inserting the insulation barrier on permeate flux enhancement, power consumption, and overall economic feasibility were also discussed.