Figure 3. Effects on overall energy demand for
CO2 capture (kWh/tonneCO2 normalized,
where base case is 403.1 kWh/tonneCO2) and other
relevant process conditions while varying (a) separator feed pressure,
(b) separator operating temperature (blue: net heat removal process
requirements), (c) separator vacuum pressure, and (d) liquefaction
pressure (blue: minimum temperature of liquefaction column). All
variations are with respect to the base case listed in Table 1.
Two significant physical phenomena must be considered when considering
operating a PSA system for this process, the heat of adsorption and the
pressure drop. Structured sorbent contactors can help address the
challenges associated with these phenomena. Structured contactors can
reduce pressure drop greatly32,60 and also fully
manage or at least limit thermal effects in the
bed.31-33,61 Using a fiber morphology, similar to
those used in membranes, but with the polymer skeleton filled with
sorbent particles, we can accomplish the same sorts of very low pressure
drops experienced in hollow fiber membranes.32,60 To
manage the heat of adsorption, which can be significant in high
productivity beds with high operating capacities, the fiber sorbent bed
can also incorporate microencapsulated phase change materials
(μPCM).31 While fiber sorbent contactors have not yet
been used industrially, their manufacturing process mimics that of both
textile fibers and fiber membranes, so it is reasonable to expect their
transition to application in adsorptive separations is feasible if they
bring significant advantages.
The cost of fiber sorbent contactors containing phase change materials
has not previously been considered in detail. Fiber beds were estimated
to be made up of a multitude of individual fibers (diameter 650 µm,
length 1 m). The weight ratio of phase change material to sorbent was
taken at 1.25:1.The cost of the fibers was broken down into three parts,
the price of the sorbent (base case economics take this value as
$30/kg), the price of μPCM ($10/kg, based on an estimate from an
industrial manufacturer), and the cost of the manufacture of the fibers
and fixing them in flow modules ($20/m2 fiber
external surface area). The manufacturing cost was estimated from the
price of industrial hollow fiber membranes, which are also produced
using dry-jet wet-quench techniques. Figure 4 shows the estimated
breakdown in the cost of manufacturing the PSA unit containing fibers
with phase change materials for our baseline case as a function of
sorbent price. Across all considered sorbent costs, the price of
manufacturing the fibers and potting them in modules dominates. Even in
cases where the sorbent price is high ($100/kg), the device
manufacturing costs rather than the material costs still make up the
majority of the total cost. The effects of fiber diameter, the relative
loading of phase change materials and sorbent loading on the overall
cost of the separator is discussed in greater detail in S2.2 of the
Supporting Information.