Distillation and Separation

Start with the vessel and the product. If one product leaves overhead and the rest stays in the bottoms, the distillation pages apply. If the column makes several products from side draws, or splits a recovered solvent into cuts, go to fractionation. If the condenser sits directly on top of a reactor, evaporator, pot still, column or tank and the liquid returns without a pump, begin with the knock-back pages, since that arrangement brings sizing limits of its own.

Every duty in this section returns some condensate, and the way it returns changes the condenser. A drum and pump let the condenser sit anywhere and let reflux be metered precisely, at the cost of extra equipment and a subcooled return. Gravity return from a condenser on the vessel removes that equipment but ties reflux to coolant control and puts a flooding limit on vapor velocity in the tubes. Knowing which arrangement the plant intends is the first thing to settle.

Many separations run under vacuum to lower boiling points and protect heat-sensitive products. Under vacuum the vapor volume is large, and every fraction of a psi lost in the condenser raises the column or vessel pressure and, with it, the bottoms temperature. Vacuum condensers therefore come out short and wide, with large nozzles and many tubes, and the vacuum pump downstream depends on the condenser removing nearly all condensable vapor and venting inerts cleanly.

Vertical and horizontal Fixed Tubesheet shell and tube condensers cover most of this section. U-Tube bundles fit where thermal expansion is large and the process side is clean. Floating Head bundles suit fouling hydrocarbon overheads that need mechanical cleaning. Spiral Exchangers handle dirty or wide-glide streams in a compact footprint, and Horizontal Fin Fan air coolers take overhead duty where cooling water is scarce. The individual pages explain which construction suits each duty and which does not.

In separation work the condenser decides how much of the separated product is actually captured. Vapor escaping through the vent is product lost and a load on the scrubber, carbon bed or oxidizer downstream. Condensing closer to coolant temperature, venting inerts from the cold end and holding pressure drop low all raise recovery. Solvent recovery columns depend on this most, since the value of the recovered solvent is the reason the column exists.