column (see Table 1) but not more than two thirds of the capacity of the flask. Add some pieces of glass or porcelain to promote even boiling.

A1.6.2 Connect the flask to the pressure drop manometer as shown in Fig. A1.1.

A1.6.3 Circulate water at ambient temperature in the condenser.

A1.6.4 Apply heat to the flask until the test mixture boils, then increase the heat progressively up to the flood point. This will be noted by visible slugs of liquid in the packing or on the plates or by liquid filling the neck of the condenser and a sudden increase in pressure drop. Reduce heat to allow the flooding to subside. Increase the heat gradually to just below the flood point. Record boilup and DP measurements just below the flood point. This is considered the maximum rate.

A1.6.5 Adjust the heat input until the column is refluxing at a steady rate corresponding to about 200 mL/h times the cross-sectional area of the column in cm2.

A1.6.6 Maintain the column under total reflux for 1 h. Record the pressure drop and determine the boilup rate by timing the filling of the calibrated bulb.

A1.6.7 Take rapidly, and almost simultaneously, a sample of the flask liquid and reflux in sufficient quantity for the determination of the refractive indexes at 20°C but not more than 0.5 mL each. The duration of withdrawal of the reflux sample must not exceed 2 s.

A1.6.8 Measure refractive indexes of reflux and flask samples.

A1.6.9 Repeat A1.6.7 and A1.6.8 at intervals from 12 to 30 min until refractive index measurements indicate that steady conditions of maximum efficiency have been attained.

A1.6.10 Check the ultraviolet absorption of the flask sample at 260 and 270 nm. If an absorption is detected, aromatics contamination is present and the efficiency determination will be in error. The apparatus must be cleaned and the test mixture repercolated through fresh silica gel before starting over.

A1.6.11 Repeat successively the series of operations A1.6.5 to A1.6.10, without preflooding, at four or more other boilup rates approximately equally spaced between (200 mL/ h) X cm2 and the maximum rate. One of these should be at a rate that is above 90 % of maximum.

A1.7 Calculation

A1.7.1 For each pair of samples taken, determine the molar composition of each sample by comparison of its refractive index with a curve of refractive index versus molar composition of n-heptane drawn from the following data:

Molar fraction of n-heptane 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Refractive Index 1.4231 1.4191 1.4151 1.4113 1.4076 1.4040 1.4005 1.3971 1.3939 1.3907 1.3876

A1.7.2 Calculate the number of theoretical plates in the column by means of the Fenske equation7 [Eq A1.1]:

logT2XD ~ log T2XO

log a



Was this article helpful?

0 0

Post a comment