RO exergy simulation models

A significant challenge in RO exergy analysis is the simulation and modelling of proposed improvements to mitigate exergy destruction and increase system efficiency. The reason for this complexity is that the factors which influence the plant energy consumption also have a large bearing on water quality performance. This pertains to UPW where purity specifications are critical. The relationship between temperature, pressure, recovery and TDS/feed concentration on RO performance metrics was illustrated previously in Figure 2-9, and thus, in order to simulate suggested process improvements, a model that combines exergy calculations and the transport equations for RO is desirable.

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The transport equations for the calculation of permeate flux and percentage salt rejection (for specified water quality and permeate flux rates) can be modelled using membrane manufacturer’s software like Filmtec membranes ROSA software from Dow [163], or IMSDesign® from Hydranautics [164]. ROSA software, shown in Figure 2-12, facilitates the calculation of a specific energy value for various membrane choices. For example, low pressure membranes require less feed pressure and thus have a lower specific energy. This is a direct result of their higher permeability factors. However, there is a caveat, i.e. the lower pressure membranes may not have the same salt rejection capabilities as their higher pressure counterparts for various feed water characteristics. For UPW, this is critical and requires careful evaluation. The ROSA software calculates the specific energy for different membrane types, but importantly, it also calculates the relevant ionic salt rejection for that membrane. However, it does not directly link the exergy calculations to the transport equations. One possible way to overcome this limitation would be to assess the low pressure membranes with respect to the permeate quality, and then use the relevant temperatures, pressures, and concentrations in a different exergy calculation program.

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Figure 2-12: ROSA water specifications excerpt [163]

There are several publications relating to simulation/modelling programs, which aim to combine RO and exergy simulation. One program uses building blocks for simulation in an object oriented approach [152], and another develops process flow sheets from an icon library using Visual Basic language [153, 154]. Uche et al. [152] reported the development of a software program for the thermodynamic and thermoeconomic analysis of integrated power and desalination plants. The program enables the creation of flow sheets made up of process blocks that can be “parametrically described by means of some properties [152]”.

Description parameters include isentropic efficiency and specific energy. The software consists of process building blocks including RO, heat exchangers, valves, and pumps among others, and calculates heat and mass balances, including exergy balances.

Capabilities extend to a comprehensive thermoeconomic analysis. Unfortunately, the case

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study presented in the cited publication, considers a coupled power and MSF plant, and not RO. On this basis, it is difficult to assess the possible application of the Building Blocks Software for Water and Energy Systems (BBWES) to the high purity requirements of UPW RO systems.

A design and simulation package was developed by Nafey et al. [153, 154]. Figure 2-13 illustrates some of this tool’s calculation capabilities. One of the cited publications does contain a case study for seawater RO [153]. Reported results were compared with ROSA and show good correlation: a 2% (>ROSA) difference was reported in the permeate salinity calculations and a 7% (<ROSA) was found in the feed pressure calculations. The main benefit of the package over ROSA is that it does contain exergy calculation capabilities (including recovery pressure energy). Also, importantly, the exergy calculations used in the package take account of exergy losses in efficiency calculations. The exergy calculation model for the simulation package [154] is based on the ideal mixture model by Cerci [134, 136]. There are two concerns, (1) whether the package is accurate for high purity applications (seawater feed salinity used in the analysis was 45,000 ppm with a 30%

recovery), and (2) whether membrane substitution is straightforward, i.e. for the energy and exergy comparison of low pressure membranes.

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Figure 2-13: RO simulation package excerpt [153]

Mehdizadeh [7] developed a mathematical model combining the Drioli aqueous solution model equations with a multi-solute RO analytical model in an effort to model the changes in exergy with respect to different plant operating conditions for both and RO and an integrated NF and RO plant. The reported method divides the membrane module into a number of completely mixed cells; it is an iterative approach similar to a finite element type analysis. The concentration of the first cell retentate and permeate is calculated, the first cell retentate acts as the feed to the second cell and so on. The advantage of this model is that the rejection rate for each solute is calculated, similar to ROSA, and it is combined with exergy calculations. The disadvantage is that the RO transport equations, described in the work, are difficult to follow and assess because, (1) the nomenclature is incomplete, and (2) the concentration equations are not clearly explained. According to the reported results,

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the integrated NF/RO plant performed better than the RO plant, with higher permeate flux and lower exergy destruction. However, it should be noted that the plant feed water had high salinity (45,000 ppm). Considering just the RO process, it was found that the specific exergy destruction rate decreased, the recovery increased and the percentage salt rejection rate increased with increased operating pressures, all favourable outcomes due to higher operating pressures. It should also be noted, however, that the favourable outcomes resulted in an operating recovery of only 9%, low even by seawater standards. The model is interesting, if the equations can be assessed, and compared to ROSA, the exergy calculation capability makes the program an interesting alternative to the two previous simulation packages.

2.7 Discussion

This chapter has reviewed the literature on UPW production energy mitigation, which is rare. Questions have been posed regarding the necessity of such stringent UPW quality specifications and the suitability of energy recovery devices for UPW applications. These important questions merit further investigation. However, following an in-depth review of energy reduction in the desalination literature, exergy analysis has been chosen as the analytical tool to model and characterise UPW production processes.

To date, the application of exergy analysis to characterise a UPW production plant has not been reported in the literature. Before undertaking such an exergy analysis, the correct choice of exergy calculation model must be carefully considered. Based on the literature reviewed, it is evident that there is a variety of exergy models used in desalination exergy

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applications. Due to the fact that the merits and limitations of these models have not been previously assessed in the literature, it is unclear whether any, or all, of these models are suitable for the exergy analysis of a semiconductor UPW plant. It is also interesting that the reference environment models for calculating intrinsic chemical exergy, proposed by several key exergy researchers, have not been considered in specific desalination exergy analyses.

A methodology for the characterisation of UPW plants using exergy analysis is proposed.

The basis of this methodology is exergy analysis because, (1) it offers keen insight into the system exergy and energy flows, and (2) it provides a very suitable platform for process system benchmarking. The proposed methodology can be summarised as follows;

1. Choose analytical model – exergy analysis;

2. Measure the plant operating parameters of interest;

3. Apply exergy model;

4. Analyse results;

5. Assess potential improvement;

6. Model improvement opportunities/make recommendations to the system owner.

There is at least one major obstacle to the proposed methodology, i.e. which is the most appropriate model for the exergy analysis of a UPW plant? An initial investigation is outlined in the next chapter. Another important consideration is the choice of exergetic efficiency definition. Ideally, the exergetic efficiency should take account of the system

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function. The selection of the most suitable exergy model will most likely involve the synthesis of other models and approaches, or possibly, the development of a new model.

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