Open-source industrial-scale module simulation: Paving the way towards the right configuration choice for membrane distillation
Graphical abstract
Introduction
Recent years have witnessed an increasing interest in membrane distillation (MD) for desalination – an innovative concept that utilises membrane to recover pure water from seawater or brackish water. Many advantages have been identified for MD desalination, for instance it can utilise low-grade heat such as solar-thermal and geo-thermal energy. Furthermore, unlike reverse osmosis (RO) process for desalination, the capacity of MD is not limited by osmotic pressure, indicating its potential use in the area where the salinity is too high to be processed by RO [[1], [2], [3], [4], [5]]. In response to the increased industrial attention on MD process, innovative MD development has become a topical research area, with most efforts being devoted to material chemistry. Meanwhile, investigation on MD configuration and module design, particularly in industrial-scale, has received much less attention, despite the efficiency and efficacy of full-scale MD desalination processes being strongly dependent upon an appropriate choice of MD configuration and an optimal module design [6].
Typical MD configurations include direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD), and vacuum membrane distillation (VMD), which can be further divided into cross-flow VMD (X-VMD) and submerged VMD (S-VMD). Many studies can be found in open literature on the performance evaluation and optimisation of individual configurations. However, comparison across different configurations has been barely broached, with only few publications available to date.
Among the limited number of publications on comparing mass and heat transfer behaviours across different configurations, most stated that the VMD offers the highest water flux due to its negligible conductive heat loss [[7], [8], [9], [10], [11], [12], [13], [14]]. However, this conclusion may only valid for certain VMD configurations with forced convection and cannot be extended to S-VMD with natural convection. S-VMD is a less well-known configuration but has some potential benefits in terms of overcoming heat pinch and solids handling by suspending the membrane in the feed tank. This aspect will be elaborated further in the following sections. Following VMD in performance is the DCMD, and its lower water flux is mainly attributed to the conductive heat loss [[15], [16], [17], [18]]. At the other end of the performance spectrum are the SGMD and AGMD. The low water flux provided by these two configurations is mainly due to the extra mass transfer resistance imposed by the air film in the permeate side [19,20]. Furthermore, the stagnant air film of AGMD further amplifies the mass transfer resistance, resulting in AGMD having the lowest flux [20]. Apart from water flux as one obvious performance measure, the choice of configuration should also consider thermal efficiency. For this criterion, VMD, once again, appears to be the best due to its negligible conductive heat loss, and thus the highest thermal efficiency. Similar to VMD, AGMD and SGMD also provide good thermal efficiency as a result of the negligible conductive heat loss [21]. Whereas in the case of DCMD, the direct contact of membrane with the cold liquid leads to additional heat loss, and therefore, a lower thermal efficiency. In addition to the aforementioned factors, other issues in terms of equipment and operating complexity need to be taken into account. For instance, the design and operation of DCMD is considered the simplest, due to the fact that condensation takes place inside the membrane module [3,22]. As for VMD, which has the highest water flux and thermal efficiency, it also has improved mass transfer inside the membrane due to the removal of air in the membrane pores [23,24]. However, despite its many advantages over other configurations, the potential of undesired pore wetting for VMD is higher than other configurations due to the vacuum applied on the permeate side [25,26]. In addition, costs of vacuum systems are also a potential concern, and the need for an external condenser outside the membrane module complicates the design and operation of the VMD system but may offer additional opportunities for heat recovery and compact module configurations [3].
Although the aforementioned studies seem to provide a ranking for the selection of MD configurations, these works were only conducted in lab-scale MD settings with small membrane areas. Many conclusions drawn from these works cannot be extended to industrial-scale MD modules with membrane areas that are orders of magnitude larger. In these large modules, the driving force could diminish over module length because of the heat transfer across the membrane (heat pinch effect). Such phenomenon are difficult to observe in lab-scale settings due to the small membrane size and negligible heat loss along the module length [3]. Our previous simulation study demonstrated that, for desalination with DCMD, when scaling up two vastly different membranes from lab- to industrial-scale, the performance gap between the two membranes quickly closed as the result of this heat pinch phenomenon, with the supposedly “good” membrane only showing marginally better performance than the “bad” membrane at full industrial-size [27]. The significantly different performance between the lab- and industrial-scale DCMD modules suggests that many aforementioned conclusions based on the lab-scale configurations do not address significant heat and mass transfer issues with industrial-size modules. However, to compare the MD performances in industrial-scale for the selection of MD configurations is a task difficult to achieve in most laboratory settings. Fortunately, such a challenge can be tackled by a computer-aided simulation, which can predict mass and heat transfer behaviour inside large MD modules, and thus, providing critical information to supplement MD configuration selection. In this vein, several modelling efforts have been made in the past aiming to predict mass and heat transfer in the MD processes. However, few constraints were found in these studies: (i) most were developed for guiding membrane development in lab-scale (e.g., predicting how the thickness or porosity of a small size membrane affect lab-scale performance), and thus, unsuitable for large-scale module design; (ii) most focused on specific MD configurations and cannot be used for comparison across different configurations [3]; (iii) many were developed based on expensive commercial software such as Aspen HYSYS or ANSYS that are inaccessible to many membrane researchers, and (iv) all these simulators are unavailable to public, which has severely limited the impact of these works within membrane research.
In this context, also considering that the DCMD and VMD are widely recognised as the most promising configurations for desalination [28], the current work aims to study the mass and heat transfer behaviour inside industrial-size DCMD, S-VMD, and X-VMD modules with the aid of computer simulation. These MATLAB-based simulators were developed based on an algorithm that couples finite difference method and black box method and were capable of predicting industrial-scale MD module performance using lab-scale experimental results. Using the results from these simulators, we were able to reveal the relationship between the lab-scale MD performance and industrial-scale MD module design, and therefore, bridging the gap between academic membrane research and industrial MD design. Furthermore, the comparisons across configurations allowed us to examine the effect of undesired heat pinch phenomenon on each configuration, and thus revealing the least affected configuration which should also give the highest pure water productivity. In addition, these simulators are open-source, which allows researchers to use these tools to develop specific scale-up strategies for their own MD membranes — a critical step towards commercialisation.
Section snippets
Simulator development
Similar to our previous work on DCMD simulation [27], all the simulators developed in the current study take lab-scale experimental results as inputs to compute the membrane water permeation coefficient (kg·m−2·Pa−1·s−1) — an intrinsic membrane property that is only affected by the structural and thermo-physical properties of the membrane itself. Subsequently, the simulators take this coefficient as input to predict the large-scale MD performance. The following assumptions were made during the
Experimental
The experiments carried out in this study were to validate the two in-house developed simulators (S-VMD and X-VMD). Note that the validation of DCMD simulator was reported in our previous work [27]. To study the performance response of different membrane types on module scale-up, two types of hollow fibre membranes were used to provide lab-scale simulation inputs, and they are: polypropylene (PP) and polyvinylidene fluoride (PVDF) hydrophobic hollow fibre membranes. The structural and
Simulator validation
The lab-scale validation was done by comparing the experimental and simulation results of two hollow fibre membranes (PP and PVDF) in three configurations (DCMD, S-VMD, and X-VMD). Note that for lab-scale S-VMD validation, no agitation was applied. This decision was made considering (i) the magnetic stirrer used in lab-scale set-up gives different hydrodynamic behaviour from blade paddle or Rushton turbine found in large-scale tank agitation; and (ii) stirring in lab-scale single-fibre setting
Conclusions
Three open-source simulators were developed on the Matlab GUI platform for the performance prediction of industrial-scale DCMD, S-VMD, and X-VMD. The developed simulators were subsequently used to demonstrate selection considerations for the appropriate configurations for industrial-scale MD desalination. By means of a thorough assessment including a wide range of criteria, the following conclusions were drawn.
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Significant heat loss in radial direction is a key concern for industrial-scale
List of symbols
A Membrane area m2 Cm Membrane water permeation coefficient kg·m−2·Pa−1·s−1 cp Specific heat capacity J·kg−1·K−1 d Diameter of the agitator m h Heat transfer coefficient W·m−2·K−1 J Water flux across the membrane kg·s−1 k Thermal conductivity W·m−1·K−1 L Effective fibre length m N Agitator speed s−1 n Number of fibres in one module p Vapour pressure Pa po Vacuum pressure Pa qc Conductive heat transfer rate W qv Vaporisation latent heat W r Hollow fibre radius m T/t Temperature K/°C Greek letters β Thermal expansion coefficient δ
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