Maintain the vacuum
The vacuum-pump station keeps the vacuum applied to the condensation station at the selected setpoint.
Technical solution for stable vacuum distillation
Temperature and pressure control at the head of vacuum distillation columns
The process separates vacuum, head-temperature and head-pressure control into independent control loops. The vacuum at the condensation station is kept constant, while the column-head temperature is stabilised by recycled condensate and the head pressure by controlled injection of an inert gas.
Technical basis: DE 10 2017 010 783 A1Initial challenge
Changing pressure losses can alter the composition of the overhead product
Fouling, changing liquid loads and varying flow resistance across trays and heat exchangers affect the pressure profile of a vacuum distillation system. Even at a stable head temperature, the actual head pressure can fluctuate and change the composition of the withdrawn distillate.
The process therefore stabilises not only the vacuum applied at the condensation station, but also controls temperature and pressure directly at the column head as separate variables.
Vacuum distillation and rectification columns
Recovery of a light-boiling component as overhead product
Bioethanol and other temperature-sensitive mixtures
Plants with an available carbon-dioxide or nitrogen source
Process sequence
Four separate control variables for stable distillation
Energy input, vacuum, head temperature and head pressure are each influenced where the corresponding process variable can be controlled effectively.
Supply vaporised medium
Energy is introduced below the stripping trays using vaporised water, condensing steam or, preferably, externally heated bottom product that flashes on entering the column.
Control head temperature
Condensate from the condensation station is returned to the column head as required. Heating and re-evaporation remove energy from the head section.
Control head pressure
A fast control valve meters carbon dioxide or nitrogen into the column head and maintains the absolute pressure at its setpoint despite changing pressure losses.
Condense and split the distillate
The overhead product is condensed. Part of the condensate is used for temperature control, while the remaining product stream is withdrawn.
Control concept
Independent control loops instead of indirect corrections
Separating the manipulated variables reduces mutual interference: the vacuum pump establishes the base vacuum, condensate reflux influences head temperature, and inert-gas injection corrects head pressure.
Base vacuum: vacuum-pump station at the condensation station
Head temperature: controlled return of cooled condensate
Head pressure: controlled supply of carbon dioxide or nitrogen
Energy input: vaporised, preferably recirculated bottom product
Technical features
Integration of condensate, bottom-product and inert-gas circuits
External bottom-product circuit
Bottom product can be heated indirectly outside the column and flashed only as it re-enters. This can avoid large-volume vapour piping outside the column.
Condensate as control medium
The condensate temperature can be stabilised using the available cooling medium. The condensate is then immediately available for column-head cooling.
Use of fermentation CO₂
At bioethanol plants, carbon dioxide from the fermentation section may be used as the inert-gas source before or after CO₂ scrubbing, provided its quality, availability and safety are suitable.
Automatable operation
Separate measurement and control loops for vacuum, temperature and pressure provide a clear basis for automated column operation.
Client benefits
More stable product quality and energy-efficient process integration
Consistent distillate composition
Direct stabilisation of head temperature and head pressure reduces quality fluctuations that may result from changing pressure losses within the column.
Lower process temperatures
Vacuum operation enables evaporation at lower temperatures than atmospheric operation and can protect temperature-sensitive products.
Heat recovery
Feed, overhead product and bottom product can be thermally integrated through heat exchangers. Actual savings depend on the material system, temperature levels and plant concept.
Integration of existing CO₂ streams
In bioethanol plants, an existing carbon-dioxide stream can be used as the manipulated medium for pressure control and functionally integrated into the distillation process.
Example from the published application
Vacuum distillation column for producing highly concentrated bioethanol
The described example uses a column with eight stripping trays and 32 rectifying trays. A feed containing approximately 6.5 vol.% alcohol is preheated by heat recovery. Externally heated bottom product provides the process drive, recycled condensate controls head temperature, and carbon dioxide from fermentation stabilises head pressure.
The figures do not constitute a general performance guarantee. Separation performance, energy demand, product quality, control parameters and equipment design must be calculated and verified for the specific material system and project.
8 stripping trays in the example32 rectifying trays in the exampleapproximately 6.5 vol.% alcohol in the feedcarbon dioxide as inert gas in the example
Technical background
The process solution presented is based on German published patent application DE 10 2017 010 783 A1, “Process for temperature and pressure control at the head of vacuum distillation columns”. The column, heat exchangers, condensation and vacuum systems, measurement and control loops, inert-gas supply, materials and safety systems must be designed for the specific material system and project.
Technical project assessment