you can hook up raman systems in line with industrial processes

I think on-line or at-line GC  is the best analytical technique to investigate composition of differnt destillation streams. If you use an unpolar column (100 % PDMS) GC separation is very similar to destillation processes, so this technique is a very good tool to check your destillation performance. 

Any sensibility analysis starts by the determination of the number of degrees of freedom. A continuous distillation column has two degrees of freedom when the pressure is fixed. These two variables can be manipulated to produce desired streams purity e.g. reflux and distillate flow rate. To determine the distillate flow rate influence using rigorous simulation, fixing the feed flow rate and composition, it is necessary to choose the values for the number of stages for the stripping and rectifying sections and for the reflux. If any of these values are smaller than its minimal values, there will not be solution and neither convergence. Infinite reflux and infinite number of stages are the simplifying hypotheses which have inspired the name of the  ∞/∞ analysis. This analysis was firstly proposed by Petlyuk and Avet’yan (1971), but without any significant impact on the scientific community. The feasibility of the stream molar compositions obtained by the mass balance is checked by the existence of a residue curve from distillate to bottom (hypothesis of infinite reflux). Attainable flow rates for each column must be inside the interval from xB = xF (B = F and D = 0) to xD = xF (D = F and B = 0) (Bekiaris et al., 1996). The ∞/∞ analysis allows an easy and fast checking of the interrelation of the system streams in a column sequence without any column design consideration. For more details you can go to the link:

http://core.ac.uk/download/pdf/12039670.pdf

I agree with Georg Dierkes and would recommend gas chromatography for the same reasons.

Thank you all for your valuable inputs