Page 168 - Design of Simple and Robust Process Plants

5.3 Combination of Process Functions 153
The increased use of recycle streams in process designs leads to a build-up of impu-
rities. This may lead to control problems on columns, as the traditional temperature
measurements for quality control are affected by impurity build-up. The solution is
found in the installation of purge streams at selected column locations.
Major product streams can also be taken as a side draw, though one drawback
might be that the side stream above the feed point contains lights. A provision with
a side stripper, as applied in refineries, might be helpful to achieve a higher enrich-
ment. Liquid side streams below the feed point always contain heavier components.
The solution might be found in a vapor side stream to minimize the heavies content
in the side stream or further purification of this vapor stream by a dephlegmator
(see Figure 5.2, fourth column). A vent stream on the overhead condenser is also to
be seen as a side stream. The purification of this vent stream is possible by installing
a knock-down vent condenser after the overhead condenser, or by the application of
a dephlegmator at the vent. Another applied technique is the installation of a top
section on a column (also called a ªpasteurization sectionº) to enrich the vent stream
and take the major product as a side stream from the column (Figure 5.3).

Vent at Vent at vent Vent at
condenser condenser dephlegmator

Pasteurization
section

Fig. 5.3. Different vent options for three component separation.

The clear advantage of side draws is that of low cost, while the disadvantages include
less purified streams, unless provisions are applied for side stripping or partial con-
densation of vapor side draws. Adequate control is required, although this would
also be a requirement if an additional column were to be installed. The side stripper
or rectifier can also be installed inside the column as shown in Figures 5.4 and 5.5,
as published by Emmrich et al. of Krupp Uhde 2000.