D11.3 Membrane-assisted CO2 liquefaction for CO2 capture from cement plants

Membrane-assisted CO2 liquefaction is a hybrid, two-stage separation process for capturing CO2
from flue gas. The first separation stage consists of a CO2-selective polymeric membrane unit
separating the bulk of CO2 from the flue gas. The resulting permeate on the vacuum side of the
membrane is a crude CO2 product, still containing a considerable fraction of diluents such as
nitrogen, oxygen and water. Before entering the second CO2 separation stage, the permeate is
compressed and dehydrated before it is cooled to around -54°C by recuperative and auxiliary
refrigeration. In two separation stages the CO2 is liquefied and purified for transport and storage.
The gaseous separation product is recycled to the inlet of the membrane unit.
Six different cases have been evaluated. Two different cement plant flue gas compositions have
been considered (with 18 mol% and 22 mol% CO2 concentration respectively), and two main
CO2 capture ratios have been targeted (90 % and 60 %).
A multicomponent membrane model is used to simulate the membrane separation process, and
this has been integrated into the interface of the commercial process simulator Aspen HYSYS.
Global process models for the hybrid membrane-assisted CO2 liquefaction process have been
used in the case studies. The assumed membrane material has a selectivity of 50 for CO2 over
nitrogen, and the total membrane surface areas used in simulations are 228 000 m3 and
152 000 m3 for 90 % and 60 % capture ratio, respectively.
The net electric power requirement for the hybrid CO2 capture process varies between
1066 kJ/kgCO2 for 60 % CO2 capture ratio from the flue gas with 22 mol% CO2 concentration,
and 1458 kJ/kgCO2 for 90 % CO2 capture ratio from the flue gas with 18 mol% CO2 concentration.
The power requirement is in all cases caused mainly by flue gas compression, vacuum pumping
and compression of crude CO2 permeate, and auxiliary refrigeration.
Compared to other end-of-pipe capture technologies, the performance of membrane-based
processes is highly sensitive to CO2 concentration in the flue gas stream. Therefore, in addition
to optimising membrane materials and configurations, it is equally important to reduce the air
ingress to increase CO2 concentrations in the flue gas. This will contribute to improving the
performance of membrane-assisted CO2 liquefaction.