9.8 Introduction to nanofiltration 265

9.7 Reverse osmosis software

The brief introduction to reverse osmosis in the previous sections has provided just aglimpse into the complexities of this important membrane filtration process. Reverseosmosis is ultimately concerned with separating multi-component mixtures of ions bytaking advantage of the relative permeabilities of solutes through a polymeric membrane.In that sense, it is inherently more complicated than ultrafiltration, and understandingit requires a deep knowledge of the chemistry, including chemical thermodynamics, ofhighly complex ionic solutions. For that reason, real industrial design of RO systemsinvolves a mix of theoretical and empirical equations, the latter often being of a pro-prietary nature. Such equations are typically embedded in design software along withdatabases of physical and chemical property data [1]. Probably the best known of these isthe ROSA (Reverse Osmosis System Analysis) package produced by the Dow Corpora-tion. Routine use of software like ROSA makes RO design quite close in spirit to the moretraditional chemical engineering separation techniques, such as distillation and liquidliquid extraction. These separations involve multi-component systems and modern daycalculations are done with the aid of one or more of the many design and flowsheetingpackages with which chemical engineers are familiar. Just like reverse osmosis, doingdistillation and liquidliquid extraction calculations requires a deep knowledge of thephysical chemistry of multi-component systems, along with the availability of extensiveand accurate databases of physical and chemical property data.

Ultimately, therefore, the purpose of the calculations shown in the preceding sectionis to give the reader a feel for the sort of issues that are involved in RO design. In truth,reverse osmosis is a subject that requires deep study, incorporating as it does, manyaspects of membrane science, chemistry and chemical engineering.

9.8 Introduction to nanofiltration

Nanofiltration (NF), so named because the membrane pores are in the nanometre range, isone of those awkward processes that are sometimes encountered in chemical engineering.It exists in something of a transition zone between UF and RO, involving solutes in themolecular weight range 1001000 Da and operating at pressures up to about 40 bar. Ifyou have studied fluid mechanics, you will be aware of transitional flow, i.e., that flowthat lies between laminar and fully turbulent. Here, the flow contains both laminar andturbulent zones, making numerical predictions fraught with difficulty. The transitionalnature of NF makes it difficult to define precisely and sometimes causes people toquestion whether it is really a membrane filtration technique worthy of its own name.It could be argued, quite reasonably, that it is an especially leaky form of RO or aparticularly tight form of UF.

Nonetheless, membrane filtration in the nano range is gaining huge importance in avariety of applications. These include water treatment processes of all kinds, includingremoval of low levels of contaminants such as endocrine disruptors; food processes