Multi-stage internally-cooled membrane-based liquid desiccant dehumidifiers: Driving-force based insights into structural improvement

Abstract

Membrane-based liquid desiccant dehumidification (MLDD), as a promising technology for temperature and humidity control, suffers from a degradation of effectiveness when handling large air flows in industrial sectors. Existing methods such as multi-stage dehumidifying and internally cooling provide a solution to solve this issue, but their implementation in the large-scale systems is somehow challenging due to the difficulty in sealing ducts, and their mechanisms for enhancing effectiveness have not yet been clarified in depth. This paper aims to apply a tractable multi-stage internally-cooled structure for improving the MLDD effectiveness at an acceptable cost and to uncover the underlying mechanisms from the perspective of the driving forces for heat and mass transfer. A dimensionless finite-difference model is first developed to capture the physical fields of the multi-stage internally-cooled membrane-based liquid desiccant dehumidifier (MI-MLDD). The MI-MLDD is then compared with the single-stage adiabatic one (SA-MLDD) in terms of the effectiveness, maldistribution and thermodynamic limits of driving forces. Four structural improvement methods including increasing stages and layers in the unlimited-size and fixed-size schemes are proposed to further improve the MI-MLDD effectiveness. Their enhancing mechanism is explained by introducing six dimensionless parameters that denote the intensity, distribution and maximum driving forces of heat and mass transfer respectively. Besides, the specific cooling capacity is defined to evaluate the energy efficiency variation of dehumidifiers caused by different effectiveness improvement methods. Compared to the SA-MLDD, the MI-MLDD increases the sensible and latent effectiveness by up to 64% and 18% due to the mitigated maldistribution and the increased minimum local driving forces, even as the air-to-solution flow ratio exceeds 2.4. The unlimited-size scheme improves the MI-MLDD effectiveness more significantly than the fixed-size one due to a linear increase in the number of heat and mass transfer units. The specific cooling capacity could reach the maximum value at the condition where the MI-MLDD operates without unfunctional contact areas. These findings highlight the potential of MI-MLDD for handling the large flow air with the improved performance.