Modeling Growth Kinetics of Methane Hydrate in Stirred Tank Batch Reactors

Abstract

Hydrate formation could be looked upon as multicomponent and multiphase reaction which is heavily dependent on mass transfer and heat transfer limitations even under favorable thermodynamic conditions. Gas uptake measurement is one of the easiest ways to understand the kinetics of hydrate growth. In a typical gas uptake measurement, one could easily observe three phases of hydrate formation: in phase-I, hydrate forming gas dissolves in the liquid phase which leads to hydrate nucleation; in phase-II, fast hydrate growth is observed; and in phase-III, hydrate grows slowly for relatively longer time periods. In a batch reactor, a slow down in hydrate growth rate as seen in phase-III is either due to a drop in the pressure of the reactor during hydrate growth and/or reduced mass transfer due to hydrate accumulation at the interface. In this work, a model is developed to predict phase-II events. The model proposed is based on an earlier model available in the literature which captured the intrinsic kinetics of gas hydrate growth for a semibatch reactor. Model discussed in the current study works for batch and semibatch reactor, it captures the kinetics for different stirrer speeds, water to gas ratios and different thermodynamic conditions. Experimental validation was done in a batch reactor at 274 K and 6 MPa with methane as the hydrate forming gas. A batch reactor with two different stirrer arrangements, different water-to-gas ratios, and different stirrer speeds were considered, and the mass transfer limitations for both the reactor configurations were studied. Further, a comparison study with the existing model and the modified model (current study) showed that the current model can be extended to other reactor types.

Significance of Low Stirring Modes on the Kinetics of Methane Hydrate Formation

“…Moreover, the CP layer is expected to adversely affect hydrate formation. A number of studies ,− reported that the formation of hydrate was prolonged as a result of the aforementioned hindering effect. A longer induction time is undesirable in a practical water desalination process using gas hydrate formation.…”

“…Nevertheless, “stirring mode” is the frequently used terminology in the literature indicating the mechanical stirring speed/rate. In hydrate research, the effect of stirring speed on the hydrate formation process has been reported in publications where the range of tested stirring speeds was between 200 and 1000 rpm, and it has been established that no improvements in hydrate formation are observed using speeds higher than 600 rpm. ,, In addition, most studies on methane hydrate formation kinetics have used higher driving forces, such as pressures of about 60 to 80 bar, and a minimum stirring speed of about 300–500 rpm. Therefore, in this work, we considered low stirring mode as below 200 rpm.…”

Review of Gas Hydrate Research in India: Status and Future Directions

“…Gas hydrate formation typically consisted of three phases: phase I, which involved dissolution of gas in liquid; phase II, wherein rapid hydrate formation/growth was observed; and phase III, wherein a slow hydrate growth rate was evident due to the pressure drop in the reactor and reduced mass transfer. Gootam et al 185 developed a model to predict the hydrate growth during phase II events in a batch/semi-batch reactor based on the earlier model that captured intrinsic kinetics of hydrate growth in a semi-batch reactor. The model predicted the rapid hydrate growth in phase II through a moments-based approach in solving the population balance model.…”

Toward Economical Seawater-Based Methane Hydrate Formation at Ambient Temperature: A Combined Experimental and Computational Study

“…Thus, for example, Linga et al found that fixed bed silica sand performed better than the stirred reactor 18 . Recent studies such as Gootam et al showed that a top-mounted stirrer can improve the rate constant due to more efficient mixing 19 . However, scale-up studies that involved both heat and mass transfer analysis showed that the stirred-reactor configuration is not favorable for any significant economic scale-up 20 .…”

“…18 Recent studies such as that of Gootam et al showed that a topmounted stirrer can improve the rate constant due to more efficient mixing. 19 However, scale-up studies that involved both heat and mass transfer analysis showed that the stirred-reactor configuration is not favorable for any significant economical scale-up. 20 This conclusion can be explained by the lower hydrate mass in water (≤5 wt %), agitation energy cost as the slurry thickens, and postprocessing cost of filtration and packing.…”