Thermodynamic simulations of isobaric hydrate-forming operations: Formulation of computational scheme and its application to hydrate formation from a methane plus ethane plus propane mixture

This paper presents a computational scheme for predicting the sequential variations in the gas-phase composition and clathrate structure of hydrates formed in a reactor during a continuous or semibatch operation for producing hydrates from multiple guest substances, typically fuel-gas-composing hydrocarbons plus a large-molecule guest substance (LMGS), which provides guest molecules to fit into the 51168 cages of a structure-H hydrate. The scheme is based on the thermodynamic modeling of the hydrate-forming operations such that the entire contents of the reactor are defined as a thermodynamic system and that the transient hydrate-forming process, which the system is to follow during each operation, is assumed to be a series of a numerous number of equilibrium states each slightly deviating from the preceding state. For the computational scheme, each equilibrium state of the system is specified with the aid of a proper phase-equilibrium calculation program (e.g., CSMHYD), and the system is forced to advance from one state to another by removing from the system in the former state a prescribed number of gas molecules fixed in a hydrate product and, at the same time, adding the same number of feed-gas molecules to the system, thereby maintaining a constant system pressure. Repeating such a state-to-state transition, thereby continually renewing the three- or four-phase system inside the reactor, we can simulate the entire process of each hydrate-forming operation. The paper exemplifies the application of this scheme to the simulations of the continuous and semibatch operations for forming hydrates from a methane + ethane + propane mixture with or without an LMGS. This scheme can be a useful tool for estimating the evolution of the crystallographic structure and composition of hydrates formed during industrial operations for processing gas mixtures such as natural gas and biogases.