BASIC SEPARATION THEORY
The remainder of the converter equipment, with the exception of the gas cooler, is designed to
direct the process gas flow inside the converter.
The Gas Cooler
The gas cooler's purpose is to remove the heat of compression from the process gas which has
just been discharged from a compressor. Certain type of stages use a gas cooler contained
within the converter as shown on Figure A-4. The gas coolers may be separate units placed
between the compressors and the converters.
In the early stages of converter design, it was recognized that barriers made in the form of flat
sheets and stacked in a container would offer the most barrier area per unit volume. For
practical reasons, the barrier had to be made in the form of tubes. More engineering knowledge
was available for tubular construction and the urgency of the problem did not allow any
unnecessary development time. The use of tubes also allowed more satisfactory replacement
of damaged barrier material. As a result, the barrier is made in the form of thin-walled tubes.
Process Gas Flow
We can now trace the process gas flow within the converter. In the AB cooled converter of
Figure A-4, the process gas enters one end of the converter at the center and is directed to the
outside of the shell by a series of baffles. It flows through the gas cooler and is directed to the
outer section, or pass, of the barrier tubes. All of the flow enters the outer, or first pass, and
flows through the tubes. Part of the flow diffuses through the barrier tube walls and the
remainder of the flow passes through the tubes and is directed by a crossover to the second pass
of tubes. This crossover is sometimes called a doughnut because of its shape. The process gas
flow which does not diffuse through the barrier tube walls of the second pass is directed by a
second crossover to the third pass. The first crossover is sometimes refereed to as the 1-2
crossover because it directs gas flow from the first pass to the second pass. Similarly, the
second crossover is called the 2-3 crossover. The undiffused process gas which passes through
the third pass is directed to the "B" outlet of the converter and flows to the stage below. The
process gas which was diffused through the barrier tubes in all three passes is collected and
flows to the "A" outlet and on to the stage above.
Since some of the process gas flowing into the first pass will diffuse through the tube walls, a
reduced amount of process gas flow will enter the second pass. In order to maintain the same
flow velocity, the second pass contains fewer tubes than the first pass. Similarly, the third pass
will contain still fewer tubes. The velocity of process gas through the tubes affects the flow
through the tube walls and thus affects the separation efficiency. The efficiency is highest when
the flow velocity is the same in all of the tubes.