Abstract:
Hollow fibre membrane contactors (HFMC) are a gas-liquid contacting technology
suggested as a successor to existing gas-liquid absorption columns for the selective
separation of carbon dioxide (CO₂) from biogas i.e. biogas upgrading, which in the United
Kingdom (UK) is a rapidly expanding sector for renewable heat production. Current
incentivisation schemes also encourage process innovation to reduce cost, and those
that enable the revaluation of waste. In response to these drivers, this thesis firstly
describes the implementability of HFMC as a successor technology to packed columns
for biogas upgrading, due to its capability for process intensification, and subsequently
introduces how to employ environmentally sourced ammonia to drive chemical
absorption in HFMC, thereby extending process intensification, whilst also reducing
aeration costs and through a unique contribution of the membrane, enables the
crystallisation of ammonium bicarbonate which can increase value as a new product.
This thesis has introduced an assessment of mass transfer in multi-module
configurations to further intensify the process and demonstrated that when producing a
high purity methane product, a simplified mass transfer model, based on the overall mass
transfer coefficient, can be used to determine gas product quality, process scale and
membrane configuration. Mass transfer behaviour in commercially favoured transverse
flow HFMC modules was compared to parallel flow HFMC modules, typically used in
laboratory investigation which are known to suffer from maldistribution, to enable the
reconciliation of maldistribution with a description of parallel flow and the translation of
the overall mass transfer coefficient across module scale. The resilience of HFMC to
industrial conditions, including gas-phase contaminants such as particulates, was
assessed at a WWTW, demonstrating the primary mechanism of fouling to arise from
the absorbent, in particular biological adsorption and clogging of the shell-side, which is
readily reversible through chemical cleaning. Integration of an NH₃ chemically reactive
absorbent for the co-production of a high purity methane product and recovery of
ammonium bicarbonate demonstrates that the reduction in specific nucleation rate and
preferential crystal growth in HFMC protects the system from blocking by the reaction
product, in contrast the high specific nucleation rate and subsequent agglomeration of
the reaction product induces process blocking during column operation.