A new separation method using gas hydrate formation is proposed for separating HFC-134a from gas mixtures containing N2 and HFC-134a. The feasibility of this separation method was investigated from various points of view. First, to determine the mixed hydrate stability region, three-phase equilibria of hydrate (H), liquid water (Lw), and vapor (V) for HFC-134a + N2 + water mixtures with various HFC-134a vapor compositions were closely examined in the temperature and pressure ranges of 275-285 K and 0.1-2.7 MPa, respectively. Second, the compositions of the hydrate and vapor phases at a three-phase equilibrium state were analyzed for identical mixtures at 278.15 and 282.15 K to confirm the actual separation efficiency. Third, kinetic experiments were performed to monitor the composition change behavior of the vapor phase and to determine the time required for an equilibrium state to be reached. Furthermore, X-ray diffraction confirmed that the mixed HFC-134a + N2 hydrates were structure II. Through an overall investigation of the experimental results, it was verified that more than 99 mol % HFC-134a could be obtained from gas mixtures after hydrate formation and subsequent dissociation processes. Separation of HFC-134a using hydrate formation can be carried out at mild temperature and low-pressure ranges. No additive is needed to lower the hydrate formation pressure.
Formation of CO2 hydrate using a Kenics-type static mixer was studied experimentally. The
flows of liquid CO2 and water were mixed in the static mixer, and CO2 hydrate was formed
continuously from the two-phase flow. The patterns of hydrate formation were found to be
dependent on the flow velocities of liquid CO2 and water. The flow of agglomerated hydrate chunks
in water occurred under relatively CO2-rich conditions, while dispersed flow of tiny particles of
CO2 hydrate with small liquid CO2 drops was observed under relatively water-rich conditions.
These effects could be explained by two mechanisms occurring in the static mixer, namely,
continuous shedding of hydrate films from the interface between liquid CO2 and water induced
by the shearing force and breakup of the CO2 drops. The energy consumption by the static mixer
for the hydrate formation process was estimated, and it was significantly less than that for a
stirring vessel type reactor. A continuous hydrate formation process could be achieved using the
static mixer.
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