Thermodynamic Simulations of Isobaric Hydrate-Forming Operations:Formulation of Computational Scheme and Its Application to Hydrate Formation from a Methane + Ethane + Propane Mixture

摘要

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 5~(12)6~8 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.