Distillation temperature correction

The results are presented as a distillation curve showing the boiling temperature (corrected to atmospheric pressure) as a function of the distilled volume. 

The solution of the mass balances and the temperature correction has been heavily studied, and is now quite rapid. In distillation columns, the correction of flows through solution of the stage heat-balances is also simple and well developed. Even for complicated columns, distillation processes are readily solved by the present iteration methods, convergence often being reached after only a few iterations. 

The barometer is an 800-mm or longer tube which is sealed at the upper end, filled with mercury (Fig. 3-la), and inserted into a pool of mercury. If it has been filled in such a way as to avoid having air trapped in the upper end (e.g., by distilling in the mercury under a good vacuum), the height of the mercury column above the pool gives the atmospheric pressure. A temperature correction is usually applied, so that the readings correspond to that of a mercury column at 0°C. The open-end manometer (Fig. 3-1/ ) is closely related to the... 

With pipe diameters of this size, the molecules do not collide with each other, but solely with the walls of equipment and tubes. Therefore, different flow laws apply here than at higher pressures where the molecules collide mostly with each other. With vapours that are sensitive to heat, the collision numbers at distillation temperatures are in the hundreds of thousands. This has a negative impact on the gentle handling of the substance to be distilled. All these factors have to be taken into account for the correct evaluation of the type and size of the vacuum pumps, the tubes and the other distillation equipment. 

Use a clean medicine dropper to transfer the liquid and obtain the refractive index of the distillate. Apply the temperature correction determined by the water measurement and record the value. 

Bordas and Touplain (1930) have defined the fundamental terms used in alco-holometry and recommended the use of the apparent density in commerce rather than the absolute density. Bordas and Roelens (1930) also prepared some useful tables on the temperature corrections at below-sero temperatures. The influence of acetic acid on the determination of the specific gravity of the alcoholic distillate has been calculated by Errichelli (1960). The true specific gravity equals p where d is... 

The temperature during the distillation may be monitored either with a thermometer or a stainless steel temperature probe. If a stainless steel probe is used, it must be used in conjunction with either a digital thermometer or one of the Vernier devices (see Technique 13, Section 13.4, and Technique 14, Figure 14.12). Your instructor will provide instructions about the method that you will use. Assemble the appropriate distillation apparatus (see figures). Carefully notice the position of the thermometer in these figures. The bulb of the thermometer or the end of the stainless steel probe must be placed well below the sidearm or they will not read the temperature correctly. If a thermometer is used, it is held in place with a thermometer adapter. If a temperature probe is used, it is held in place with a rubber septum (see Technique 14, Figure 14.12). 

Because the volume of distillate is small, it is somewhat difficult to measure the distillation temperature (boiling point) accurately at the microscale level. An approximate method is to determine the temperature of the heat source at the point when distillation occurs. This temperature is about 20 °C or so above the distillation temperature, so it is necessary to make the corresponding correction in the boiling point recorded for the distillate. Alternatively, a thermometer can be inserted through the condenser attached to the stillhead so the thermometer bulb is in the lower neck of the stillhead. A more accurate measurement of the boiling point of the distillate is possible using the technique of microscale boiling point determination (Sec. 2.8). 

When the percentage distilled at a certain temperature is required and the barometric pressure varies from 760 mm Hg by more than 100 mm, use the temperature corrected to 760 mm Hg prior to the distillation. 

As a starting point for identifying candidate solvents, all compounds having boiling points below that of any component in the mixture to be separated should be eliminated. This is necessary to yield the correct residue curve map for extractive distillation, but this process implicitly rules out other forms of homogeneous azeotropic distillation. In fact, compounds which boil as much as 50°C or more above the mixture have been recommended (68) in order to minimize the likelihood of azeotrope formation. On the other hand, the solvent should not bod so high that excessive temperatures are required in the solvent recovery column. 

Two heat-sensitive organic liquids of an average molecular mass of 155 kg/kmol are to be separated by vacuum distillation in a 100 mm diameter column packed with 6 mm stoneware Raschig rings. The number of theoretical plates required is 16 and it has been found that the HETP is 150 mm. If the product rate is 5 g/s at a reflux ratio of 8, calculate the pressure in the condenser so that the temperature in the still does not exceed 395 K (equivalent to a pressure of 8 kN/m2). It may be assumed that a = 800 m2/m3, /x = 0.02 mN s/m2, e = 0.72 and that the temperature changes and the correction for liquid flow may be neglected. 

Sources of Heat.—Liquids which do not boil above 80° are heated in the water bath (enamelled jar or beaker) the temperature of the bath should be about 20° above the boiling point of the substance. The maintenance of the correct heating temperature is of the greatest importance, since if it is raised too much the boiling point of the distillate will be found too high in consequence of superheating. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

The idea is best explained with an example. Suppose the base level in a distillation column is normally held by bottoms product withdrawal as shown in Fig. 8.4a. A temperature in the stripping section is held by steam to the rcboiler. Situations can arise where the base level continues to drop even with the bottoms flow at zero (vapor boilup is greater than the liquid rate from tray 1). if no corrective action is taken, the reboiler may boil dry (which could foul the tubes) and the bottoms pump could lose suction. 

---