Tray columns operation region

The limiting condition occurs at minimum reflux ration, when an infinite number of trays will be required to effect separation. Most columns are designed to operate between 1.2 to 1.5 times the minimum reflux ratio because this is approximately the region of minimum operating costs (more reflux means higher reboiler duty). 

The number of independent variables required to define the operation of an absorber or stripper may also be determined by applying the description rule, stated in Section 5.2.1. The number of trays or the column height is set by construction and may, in the design phase, be used as design variables. Since, by definition, the feeds are introduced at the top and bottom of the column, the feed locations are not variable. The feed compositions and thermal conditions are set outside the column region and are therefore beyond the operator s control. The operator can, however, control the valves on the two feeds and the two products. One of these four valves, usually the bottoms product valve, cannot be controlled independently since it must be set at steady state such as to maintain the required liquid level in the bottom of the column. The overhead valve is usually used to control the column pressure. The two feed valves may be controlled independently one controls the main process stream rate and the other controls the solvent or stripping gas flow rate. 

In contrast to other esterifications, a significant extent of reaction can be reached even without a catalyst though the reaction equilibrium constant is approximately one. A compilation of the major physical property data can be found elsewhere [25, 87]. Fig. 10.2 shows the residue curve map in transformed coordinates as introduced by Doherty and coworkers [108] at a pressure of 1.013 bar. Due to a single maximum azeotrope, there are two distillation regions. The concentration profile in a single feed, two product lab-scale column with 45 bubble cap trays is also displayed in Fig. 10.2. The column is fed with a stoichiometric feed of formic acid and methanol and operated at a reflux ratio of 5. Water and the desired methyl formate are recovered at purities of about 97 % molar concentration in the bottoms and at the top, respectively. 

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