1. Introduction - Firenze University Press

Investigations were carried out in batch conditions. Therefore, at the end of each experiment, gas is
vented out from gas outlet port. Flange is opened both for visual observations and for taking hydrate
samples out.
In fact, several samples are taken directly out from the reactor. Hydrate storage capacity is
determined putting hydrate samples inside a custom built dissociation vessel. It is a cylindrical AISI
304 stainless steel vessel with a volume of 1.4 lt. It was designed and built to carry out the
dissociation of samples of gas hydrate formed.
After sealing the vessel, the dissociation starts and gas pressure and temperature after dissociation
are measured. To calculate number of gas moles Eq.(1) was used:
P V = Z n R T (1)
where P is the gas partial pressure in the vessel at the end of dissociation, V is the volume of gas in
the vessel, n is number of the gas moles, T is the temperature in K at the end of dissociation, R is
the universal gas constant, and Z is the compressibility factor, which can be calculated using
Benedict-Webb-Rubin equation of state.
As the number of gas moles is calculated, hydrate storage capacity, measured both in %wt of CO2
and in Nm 3 /m 3 , can be determined, since hydrate density is known.
In the calculation of hydrate storage capacity, the contribute of CO2 solubility in water was also
taken into account.
3. Results and discussion
A first set of experimental runs were carried out for CO2 hydrate production. Effects of additives,
such as THF and SDS, were tested. The amount of additives was chosen according to the optimal
ranges of concentration found in literature [15, 18, 19].
Typical profiles of internal pressure and temperature for an experimental run of 15 minutes are
shown in Fig. 3. Those profiles are for experimental run 2 in Table 1.
In particular, internal temperature is calculated as the average of the two temperature values
measured by two thermocouples in two different positions.
All experimental runs were carried out with an internal pressure of 3 MPa and with initial
temperature values of ca. 3 °C. With an experimental pressure of 3 MPa, the equilibrium
temperature of carbon dioxide hydrates is ca. 280 K [27], therefore experiments were carried out
with a not negligible subcooling as a driving force for the process.
Before starting aqueous solution recirculation and spraying, a slight decrease in pressure values was
observed and ascribed to carbon dioxide solubility. Therefore, only when constant values of
temperature and pressure were reached, recirculation started and continued for 15 minutes (runs
1,2,3 in Table 1) or 30 minutes (Runs 4,5 in Table 1).
As a result of the hydrate formation, which is an exothermic process, internal temperature increases
after ca. four minutes. Heat removal and temperature control is an issue, especially for applications
in scaled-up systems, in which constancy and uniformity of internal temperature are difficult to
achieve. With the improvements brought to the temperature control system, variations were kept
within 1 °C, as shown in the temperature profile.
Moreover, internal and external heat exchangers of the apparatus allow to achieve also relatively
uniform values of temperature inside the entire internal volume.
In Fig.4 it can be noted that internal pressure is constant for the first minute and then decreases
smoothly for 6-7 minutes. This can be ascribed to gas consumption due to hydrate formation. The
following four peaks result from gas injection for re-establishing the fixed experimental pressure.
After each peak, a rapid decrease in internal pressure, due to formation of gas hydrates, is
observable.
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