Consider Preflash Separation (and Droplet Removal) for Purification of Wide-Boiling Range Mixtures

Conventional chemical distillation columns are developed to receive a feed at an intermediate tray or between packing sections. Preheating with or without bottoms interchange is often used to optimize distillation energy. Locating the best position of the feed point relative to theoretical stages can be determined by analysis and rules developed using McCabe-Thiele diagrams, for which the operating and the V-L equilibrium curves are determined for the key components. For systems with very high relative volatility between the key components, a relatively deep Preflash step can be combined with a stripping section or with a rectification section to provide a reliable, and simpler, yet optimized separation solution. Processes suited to this approach include: • Some processes with reactant or solvent removal and recycle • Purity requirements for top or bottom products that require distillation to low levels, ie. ppm levels; however, usually both top and bottom products do not require high purity. • Polymer-solvent systems with relatively lower MW polymers and lower viscosities, < about 100 cP. • Processes handling temperature or heat-history sensitive products • Processes with heavy by-product (residues) to be removed as a bottoms cut. One common example of this is the Crude Column system at the front end of an oil refinery. Preflash equipment can be designed to better resist fouling services, and with better turndown than distillation trays or packing. Also, preflash heat exchangers can likely be debottlenecked for lower investment than the revamp of an entire distillation column system. Examples in this paper are systems where the (NBP,heavykey – NBP,lightkey) ≥ 70 C and vacuum distillation is employed. Two case studies from recent Bayer Technology Services Americas experience illustrate the modification of Preflasher designs to allow very substantial increases in production rates. The first involved developing a novel droplet separator for preventing carryover of a critical component into a recycle stream. In the second case, rates were being stretched to the point where equipment suffered mechanical erosion-corrosion. Modifications were done with minimal capital to eliminate problematic entrainment of liquid droplet carryover. Applications and some general design parameters for falling film evaporators are also reviewed.