Air-gap diffusion distillation: Theory and experiment
Introduction
Distillation is one of conventional separation technologies, which is widely applied in many industrial fields such as desalination, water treatment, solution concentration etc. [[1], [2], [3], [4]]. There are two categories in distillation technologies: thermal distillation (TD) and membrane distillation (MD). For TD, multi-stage flash (MF) and multi-effect distillation (MED) are the most popular technologies applied in practical engineering [5,6], since low-grade heat (LGH) can be repeatedly utilized to reduce the energy consumption and enhance separation efficiency [[7], [8], [9]]. However, a relatively high manufacturing cost of them is necessary due to their relative rigor and complex manufacturing technique [3,[10], [11], [12]].
Other hand, MD is a relative new separation technology. In this category, air-gap membrane distillation (AGMD) and direct contact membrane distillation (DCMD) have been fully studied and applied in practice [[13], [14], [15], [16]]. Compared with the traditional TD, MD has advantages in compact structure, simple device and convenient operation [6,17]. However, the main drawbacks of MD are low membrane mass transfer flux and high resistances of heat and mass transfer [6,[17], [18], [19], [20], [21]], which leads to the difficulty in distillation heat recovery [22,23].
To adopt the advantages of two distillation technologies and overcome their shortcomings, a novel distillation technology named Air-Gap Diffusion Distillation (AGDD) was proposed by us [24]. Fig. 1 illustrates the principle of AGDD system. In AGDD, cold and hot streams flow through each individual channel in counter-current arrangement. The cold stream flows from bottom to top by a solution pump and the hot stream flows from top to bottom by gravity. A strong hydrophilic porous medium replacing the feed channel and permeable membranes is utilized as hot stream channel to control flow of the hot stream. Since the solvent (water) evaporates and diffuses directly from the porous medium surface to air gap, the resistances of heat and mass transfer can be reduced largely compared with MD. Due to temperature difference existing between cold and hot sides, the vapor of solvent can diffuse from hot interface to cold interface through air-gap. Finally, the vapor condenses on the outside surface of cold stream channel and the condensation heat is absorbed by cold stream in the channel, which makes the distillation heat consumption decrease.
As shown in Fig. 1, the AGDD consists of alternately arranged cold and hot stream channels, air-gaps and two end plates. Since it operates at normal atmospheric pressure, the components of AGDD can be made of metal or non-metal materials and the structure is simple, compact and dismountable. Therefore, the manufacturing cost of AGDD is low, and the relevant components are able to cleaned or replaced by new ones when they fail.
In this paper, the performance of AGDD will be investigated theoretically and experimentally. Firstly, a set of mathematical models is developed to explore the heat and mass transfer features of AGDD. Then, these models are verified through experimental investigation. Moreover, the coupling effects of structure and operation parameters are also investigated and discussed. The results obtained by this work are useful for the practical application of AGDD in the future.
Section snippets
Mathematical models
Due to the symmetrical and parallel arrangement of the cold and hot stream channels in AGDD, the heat and mass transfer characteristics in each air-gap are the same. Therefore, an arbitrarily typical air-gap combination with a pair of cold and hot stream half channels (shown as the red dotted box in Fig. 1) can be taken as the research physical model shown in Fig. 2. The distributed parameter method is adopted here to develop the mathematical models of heat and mass transfer.
Experimental system
For verifying the model developed above, an experimental system for AGDD has been set, shown as Fig. 4. In the system, the structure of a key facility, air-gap diffusion distiller or air-gap combination, is shown as Fig. 5. The structure parameters and materials of the distiller are listed in Table 1. Fig. 6 is a photo of actual experimental system.
As shown in Fig. 5, the parts from cold side to hot side are cold side end plate, cold stream channel plate, air-gap plate, hot stream channel plate
Performance parameters
The performances of AGDD will change if the structure and operation parameters of AGDD vary. Some indicators can be used to present the performances. In this investigation, water productive rate or water productive flux, GOR, heat loss rate and desalination rate were introduced as performance indicators.
Results and discussion
Actually, the structure and operation parameters of AGDD can be changed in practical application. The structure parameters, air-gap height and thickness, can be changed during designing process to meet the requirement of its operation. The operation parameters, inlet temperatures of cold and hot stream, will also vary when the temperature of heat resource or season changes. Similarly, the flow rate of cold stream can be adjusted by operation requirement. All these parameter variations will
Conclusions
In this paper, a novel AGDD technology was proposed and its working principle was introduced. The operation characteristics of AGDD with one air-gap combination were investigated and analyzed theoretically and experimentally. Conclusions can be drawn as following.
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In AGDD, the cold and hot streams flowed through each channel in counter-current arrangement, and the condensation heat could be recovered by cold stream, which could reduce the heat consumption of distillation and get a high GOR.
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The
Acknowledgements
This project is financially supported by the National Natural Science Foundations of China (NSFC), No.51776029, No.51606024 and No.51876023.
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