Locating hydrate deposition in multiphase compressed natural gas flow lines using computational fluid dynamics techniques

Jassim, Esam Ismaeel (2008) Locating hydrate deposition in multiphase compressed natural gas flow lines using computational fluid dynamics techniques. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The main aim of this research is to develop a comprehensive flow model capable of predicting the nucleation process, the growth rate, and the deposition potential of hydrate particles. The model is developed for applications in two-phase fluid flow through flow lines with flow restriction geometries using Computational Fluid Dynamic (CFD) approach. -- The primary objective of the research is to predict the risk hazards involved in the marine transportation of compressed natural gas. However the proposed model can be equally used for other applications including production and transportation of natural gas in any high pressure flow line. -- The influence of the major parameters affecting the hydrate formation rate and deposition are addressed. The background knowledge concerning the development of the proposed model has been reviewed through literature survey. From the existing literature, it was found that the fundamental concepts related to the nucleation phenomena and growth theory are well established and ready to be integrated. A number of comprehensive models have also been successfully developed to predict hydrate formation and accumulation. These models however were not specifically developed to predict the most probable location for hydrate deposition under conditions where natural gas flows through restrictions in pipeline systems. -- The proposed model employs the following three main components to approach the problem: (a) computational fluid dynamics (CFD) technique is used to configure the flow field; (b) nucleation and growth are incorporated in the simulation to predict the incipient hydrate particles size and growth rate; and finally(c) the novel approach of the migration and deposition of the particle is used to determine how particles deposit and adhere to the flow conduit wall. These components are integrated in the proposed model to locate the hydrate deposition in flow lines. Experimental tests are also established to assess the agreement of the proposed model. Further, the influence of pipe size and flow rate on the distance of deposition is also studied. The results predicted by the model simulation show that the distance of the deposition decreases as the particle size increases. However, after certain size of particle, there is no effect on the distance of deposition. Such size has been called "deposition critical size." This behaviour can be returned to the fact that small particles are influenced by the main fluid velocity but for relatively large particles such effect diminishes as a result of the high particle inertia. -- The experimental tests that performed in the Centre for Marine Compressed natural Gas Inc. are shown to be in good agreement with the model prediction in term of the following criteria: 1) Formation of hydrate particles are observed to be polydisperse since different sizes of particles are formed. This observation is matched with the analytical correlation of the Particle Growth and Distribution proposed in the study; 2) Studying the influence of flow Reynolds number and pipe diameter, the deposition distance is found to be linearly responded to the Reynolds number and pipe size.

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