Handbook of Solvents - George Wypych - ChemTech - Ventech!

1534 Klaus-Dirk Henning
vent condensation and back to the blower V-2. At the end of the adsorption step the bed contains
10-30% solvent and 1-2% water.
There are various ways to remove the water separately and to recover a solvent containing
between 0.1 and 1.5 % water (Table 22.1.10).
Table 22.1.10. Process conditions and water content in the solvent (After references
32,33)
Water in the
raw solvent
Investment
cost, %
Heat needed,
kWh/kg S
Power,
kWh/kg S
Basic process with molsieve beds 0.1 100 2.2 1.0
Chiller instead of molsieve 1.5 91 1.5 1.0
Chiller and 1 carbon bed 0.4 - 1.0 93 1.1 1.0
Chiller, and 1 carbon bed molsieve 0.1 96 1.1 1.0
Using molecular sieves
Beds of molecular sieves are used to recover a solvent with 0.1%. This process requires
around 2-2.5 kWh heat/kg of recovered solvent. Regeneration loop pressure drops
are high, because of the additional molsieve beds and valves.
Separate condensation of water
Initially, when regenerative heating of the bed starts, very little solvent is desorbed,
but much of the water (about 87%) is desorbed. The circulating gas is first passed through a
cooler and then a separate chiller for the condensation or freezing of the water. Subsequently
the gas is passed to the chiller where the water is condensed or frozen. From 1 to
1.8% water is recovered. This process is simpler than the molecular sieves process.
Two separate chillers
Two separate chillers are used in another adsorption process which offers further substantial
improvements. A third activated carbon bed is added so that while two of the beds
are being regenerated one of these is being cooled and transferring its heat to the other bed
which is being heated. This not only gives excellent heat recovery but also provides a means
of reducing the water content of the recovered solvent. The cooled bed absorbs the water vapor
from the gas stream coming from chiller. This dry gas stream is transported to the bed
which is being heated and the desorbed solvent from the heated bed remains dry. Solvents
recovered at this stage contain only 0.4 to 0.5% water. Solvent which is removed from the
bed being cooled is readsorbed on the bed being heated.
The process, which is patented, brings important benefits of regeneration gas flow
rates. Such low flow rates mean that the process requires less heat, less refrigeration and a
lower cost regeneration loop. The overall efficiency is high because the low residual solvent
loading of the regenerated beds leads to increased solvent recovery.
Two separate chillers plus molecular sieve bed
A two chiller system with a molecular sieve bed has to be regenerated only about once
per week, since most of the water is removed in the chiller and readsorbed on the cooled carbon
bed. This process offers a high solvent recovery rate giving a solvent with only 0.1 %
water at greatly reduced heat consumption, lower investment, and higher solvent recovery
rate.