Effect of operating variables on composition of overhead vapors and residue from a continuous flash distillation for the system methanol-water at atmospheric and sub-atmospheric pressures

Although distillation studies have been made for many types of rectifying columns under various conditions of operation, no systematic study has ever been made of the process that is the basis of rectification, e.g., flash distillation. It was the purpose of this investigation to study the action of the system methanol-water under-going flash distillation under various conditions of temperature, agitation, and rate of feed at a total pressure of 500 millimeters of mercury, absolute. The distillation took place in a nine inch diameter, electrically-heated, controlled temperature and pressure, cylindrical still 14 inches high. The liquid phase was agitated by the rotation of an externally driven 1-1/2-inch square paddle. An aqueous 25 weight per cent methanol solution was fed to the still in which the solution was subjected to flash distillation under conditions of a liquid hold-up of 445 cubic inches; vaporizing temperatures of 172 °F, 166 °F, and 162 °F; stirrer speeds of zero, 500, 1000, 1500, and 2000 revolutions per minute; and feed rates varying from 0.01 to 0.50 pounds per minute. From the results obtained it was concluded that the fraction vaporized was a function of the feed rate as follows: Z K₁(R₁ - R₂)^-b) Where: Z fraction vaporized K₁ fraction vaporized when the term (R₁ - R₂) was equal to one R₁ feed rate, pounds per minute R₂ feed rate when residue rate was equal to zero, pounds per minute B a constant for a given temperature and stirrer speed. The fraction vaporized was directly proportional to the stirrer speed and the vaporizing temperature. The vale of the fraction vaporized varied from 1.0 to 0.04. The composition of the distillate was an exponential function of the feed rate as follows: x_d K₂(R₁ - R₂)^a where: x_d distillate composition, weight per cent methanol K₂ distillate composition when the term (R₁ - R₂) was equal to one, weight per cent methanol. R₁ feed rate, pounds per minute R₂ feed rate when the residue rate was equal to one, pounds per minute a a constant for a given temperature and stirrer speed. The greatest degree of separation, as measured by the difference in distillate and residue compositions, occurred at stirrer speeds of zero, 1000, and 500 revolutions per minute at vaporizing temperatures of 162 °F, 166 °F, and 172 °F respectively. Some dynamic values of the distillate composition exceeded the static or thermodynamic values, for example, the theoretical value of the distillate composition of 68.5 weight per cent methanol at 162 °F was exceeded at zero and 500 revolutions per minute for feed rates in excess of 0.12 and 0.16 pounds per minute respectively. At 172 °F and 166 °F, the heat input as Btu per pound of distillate increased with increasing feed rates for feed rates from 0.022 to 0.122 pounds per minute and decreased with increasing feed rate for feed rates from 0.122 to 0.450 pounds per minute all at constant stirrer speed. An average value of the heat input of 1500 Btu per pound of distillate was obtained at 172 °F for stirrer speeds from zero to 1000 revolutions per minute and at 166 °F for stirrer speeds from zero to 2000 revolutions per minute. At 162 °F the heat input as Btu per pound of distillate increased with feed rate at all feed rates from 0.01 to 0.50 pounds per minute with an average value of 2000 Btu per pound of distillate being obtained. The dynamic equilibrium ratio was related to the product of the stirrer speed and the hold-up time as follows: N x_D/x_s K₃(n t_h)^d where: N dynamic equilibrium ratio x_D distillate composition, weight per cent methanol x_s composition of the liquid in the still at equilibrium, weight per cent methanol n stirrer speed, revolutions per minute t_h hold-up time, minutes K₃ a constant for a given temperature d a constant for a given temperature

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Chemical Engineering

Department dc:contributor.department

Chemical Engineering

Grantor dc:publisher

Virginia Polytechnic Institute

Year dc:date.issued

1953

Author dc:creator

• Littlejohn, Charles Edward

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/10919/118899

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/118899