Transcript
Process Flow Diagram: 1, 2
Technical procedures:
Calculate the mole fractions and densities of methanol and water.
Measure the empty pycnometer and the empty distillated receiver flask.
Clean up and empty the boiling pot if solution is found.
Remove the stopper and add sample solution into boiling pot
Turn on the Bunsen burner. (should be caution the frame)
Record the boiling point of the sample solution.
Place the distillated receiver flask under the condenser.
Measure the density of sample from the pot and distillated flask
Return the distillated liquid into the boiling pot for 3 times.
Measure the density of sample from the pot and distillated flask each time.
Laboratory Safety concern:
Gloves, safety glasses, and long pants should always be worn when working in the lab.
The water lines connected to the condensers should be secured and inspected before each experiment.
Never start and run without cooling water.
This experiment involves highly flammable liquids. Fire extinguishers must be checked and placed in accessible locations.
Figure: 1: Equilibrium relationship between gaseous and liquid alcohol-water mixtures (atmospheric pressure).
At equilibrium, the relationship between the compositions of species i in the vapor and liquid phases is often deined by a K-value, or equilibrium ratio: 1
(1)
where xi and yi are the mole fractins of species i in the liquid and vapor phases, respectively. Assuming ideal conditions near ambient temperature and pressure, the K-value may be described according to Henrys and Raoults laws for dilute and concentrated solutions respectively: 1
(2)
(3)
Where Hi is an empirical constant, Psi is the saturation vapor pressure of pure component i, and P is the total pressure of the equilibrium vapor mixture assuming Daltons Law:
(4)
where Pi is the partial pressure of species i in the vapor phase.
A common correction to equation (3) for non-ideal liquid mixtures at ambient pressures is1
(5)
where ?i is the activity coefficient for species i in the liquid mixture (sometimes referred to as the modified Raoult’s law). Note that equation (5) still assumes ideal gas behavior which is generally reasonable under conditions of ambient pressure and temperature. 1, 2
Several empirical and semi-theoretical equations exist for correlating liquid-phase activity coefficients of binary mixtures. Perhaps the simplest form is the one-constant Margules equation, given here for a binary system:
(6)
where A is an empirical constant. Equation (6) assumes symmetric curves for the activity coefficients of the two species. 1
Some other equations for correlating activity coefficients are the two-constant Margules, van Laar, and Wilson equations. The most commonly used form is the two-constant van Laar equation: 1
(7)
where the constants may be obtained by solving equation (7) directly:
(8)
One approach to modeling the vapor-liquid equilibrium may consist of several models combined for different solution conditions. For example, at low concentrations, Henrys law may apply. At high concentrations, Raoults law may be satisfactory. A model such as Margules may be appropriate for the intermediate compositions where neither Henrys law nor Raoults law are adequate.1, 2
Reference:
_http://www.d.umn.edu/~dlong/exvle.pdf#search=%22othmer%20still%20experimental%20procedure%22_
_http://www.scs.uiuc.edu/chem_eng/course/374/manuals/DistillationManual.pdf_
_http://lorien.ncl.ac.uk/ming/distil/distilvle.htm_
