Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
lost with the desorbed carbon dioxide. At the lowest column pressure the purge is
initiated. Upgraded gas is blown through the column to empty it from all the carbon
dioxide that has desorbed from the column bed. The column is now regenerated
and can be repressurized, either with raw biogas or with upgraded gas, and the
cycle is complete (Grande 2011).
Pressure
(1) (2) (3) (4)
(1) (2) (3) (4)
Figure 7 Schematics of the four phases in the Skarstrom cycle and a pressure profile
of the cycle. Figure adapted from (Rege et al. 2001)
As this cycle consists of four phases, a common design for PSA units includes four
columns. Thus one of the columns is always engaged in adsorption while the other
three are in different phases of regeneration. To reduce the loss of methane from
the process the columns are usually interconnected so that the exiting gasflow
from one column during blowdown is used to pressurize another column in a pressure
equalization phase, which also reduces the energy consumption of the process.
A PSA column cycle is typically 2-10 min long (Spoorthi et al. 2010; Grande
2011).
Using several columns there are many ways of modifying the process cycle to
increase the yield of methane from raw biogas to upgraded gas, reduce the methane
loss and increase the energy efficiency of the process. The gas flow from
the blowdown phase can be recirculated together with the raw biogas, which can
increase the yield with up to five per cent. New advanced process cycles with nine
cycle phases have been proposed. According to simulations, a four column PSA
Time
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