Distillation thermosyphon

Make a preliminary design for a vertical thermosyphon for a column distilling crude aniline. The column will operate at atmospheric pressure and a vaporisation rate of 6000 kg/h is required. Steam is available at 22 bar (300 psig). Take the column bottom pressure as 1.2 bar. 

A distillation operation separating a low-viscosity hydrocarbon mixture requires three shell-and-tube heat exchangers. The liquid feed is to be preheated to saturated liquid by heat recovery from another low-viscosity hydrocarbon stream. The reboiler is to be a vertical thermosyphon using steam heating. 

Figure 6.4 shows a distillation tower served by a circulating thermosyphon reboiler. To some unknown extent, some foam will always be found in the bottom of such vessels. Not sometimes, but always. Why ... 

Downstream of the reaction zone, the lower static pressure permits the reactor content to boil and applies the thermosyphon effect for circulation. EDC vapor leaves the horizontal vessel and either enters the reboiler of a column (e.g., reboiler of high-boil-heads and/or vacuum column) or a heat exchanger, which condenses the EDC vapor. The reaction heat is transferred to the column indirectly. A fraction of the condensed EDC is fed back to the reactor and the rest is directly sent to the EDC cracker without further distillation. 

Also such factors as allowable tensile strengths, weld efficiencies, and possible inaccuracies of formulas used to calculate shell and head thicknesses may be pertinent—that is, the relative uncertainty or error in the function is related hnearly to the fractional uncertainties of the independent variables. For example, take the case of a steam-heated thermosyphon reboiler on a distillation column for which the heat transfer equation is... 

Kettle reboilers have five connections, two on the tube side and three on the shell side. Steam or hot oil flows through the tube side and provides the heat source. Flow rate is carefully controlled and frequently linked to the bottom temperature control system. The higher the flow rate, the hotter the bottom product. The shell side has three nozzles one liquid-product feed line, one vapor-return line to the column, and one heavy-liquid-out product line. A kettle reboiler can be used to (1) control the liquid level on the bottom of the column, (2) control the temperature of the column, and (3) help control product purity in the bottom of the column. Figure 6-5 shows what a kettle reboiler looks like, and Figure 6-6 shows two thermosyphon and one kettle reboiler arrangements on a distillation column. 

Reboilers are energy-balance devices attached to distillation columns to help control the temperature. Reboilers have two basic designs thermosyphon and kettle. 

I worked as a process design engineer for Amoco Oil in Chicago until 1980. Likely, I designed about 50 distillation columns, 90 percent of which had horizontal, natural thermosyphon circulation reboilers. 

A total condenser is used to produce liquid reflux and distillate product. The reboiler is a partial reboiler (vapor is boiled off a liquid pool). The composition of this liquid pool is the same as the bottom product ODmposition. Thermosyphon, ketde, internal, and forced-circulation reboilers ate aU usually partial reboilers. 

Forced-circulation reboilers are widely used for vacuum columns because of their lower pressure drop, for applications where a thermosyphon would be expected to foul, and for applications where thermally sensitive materials are being distilled. 

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