Page 85 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

It might be possible to absorb or scrub the methane from Streams 8 and 17 into a hydrocarbon liquid. In
order to determine which liquids, if any, are suitable for this process, we must compare the solubility
parameters for both methane and hydrogen in the different liquids. This information is available in Walas
[11]. Because of the low boiling point of methane, it would require a low temperature and high pressure
for effective absorption.

Pressure-Swing Adsorption

The affinity of a molecule to adhere (either chemically or physically) to a solid material is the basis of
adsorption. In pressure-swing adsorption, the preferential adsorption of one species from the gas phase
occurs at a given pressure, and the desorption of the adsorbed species is facilitated by reducing the
pressure and allowing the solid to “de-gas.” Two (or more) beds operate in parallel, with one bed
adsorbing and the other desorbing. The separation and purification of hydrogen contained in gaseous
hydrocarbon streams could be carried out using pressure-swing adsorption. In this case, the methane

would be preferentially adsorbed on to the surface of a sorbent, and the stream leaving the unit would
contain a higher proportion of hydrogen than the feed. This separation could be applied to the HDA
process.

Membrane Separation

Commercial membrane processes are available to purify hydrogen from hydrocarbon streams. This
separation is facilitated because hydrogen passes more readily through certain membranes than does
methane. This process occurs at moderate pressures, consistent with the operation of the HDA process.
However, the hydrogen is recovered at a fairly low pressure and would have to be recompressed prior to
recycling. This separation could be applied to the HDA process.

From Example 2.3, we see that pressure-swing adsorption and membrane separation of the gas stream
should be considered as viable process alternatives, but for the preliminary PFD for this process, no
separation of hydrogen was attempted. In Example 2.4, the separation of toluene from a mixture of
benzene and toluene is considered.

Example 2.4

What process should be used in the separation of toluene and benzene?

Distillation

Normal boiling point of benzene = 79.8°C

Normal boiling point of toluene = 110°C

Separation should be easy using distillation, and neither excessive temperatures nor pressures will be