Thermodynamic and Process Modelling of Gas Hydrate Systems in CO2 Capture Processes

Peter Jørgensen Herslund

Research output: Book/ReportPh.D. thesis

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Abstract

A novel gas separation technique based on gas hydrate formation (solid precipitation) is investigated by means of thermodynamic modeling and experimental investigations. This process has previously been proposed for application in post-combustion carbon dioxide capture from power station flue gases. Prior to this study it has been shown that formation of gas hydrates from mixtures of carbon dioxide and nitrogen may create a solid hydrate phase in which the carbon dioxide content is higher than the original concentration in the gas phase. Separation of carbon dioxide from flue gases by hydrate formation may be performed at pressures of approximately 20 MPa and temperatures below 280 K. Thermodynamic promoters are needed, to reduce the pressure requirement of the process, thereby making it competitive to existing capture technologies.
A literature study is presented focusing mainly on thermodynamic gas hydrate promotion by hydrate formers stabilising the classical gas clathrate hydrate structures (sI, sII and sH) at low to moderate pressures. Much literature is available on this subject. Both experimental and theoretical studies presented in the literature have pointed out cyclopentane and tetrahydrofuran as the two most efficient pressure reducing additives in classical hydrate forming systems.
The thermodynamic promoting effects reported in the literature for the two classical sII hydrate formers, tetrahydrofuran and cyclopentane are experimentally confirmed in the present work. Data presented in this work compares well with other data available in the literature for similar systems.
It is shown experimentally that the addition of tetrahydrofuran to the ternary system of water-cyclopentane-carbon dioxide provides an enhanced thermodynamic promotion of the gas hydrate phase. Hydrate equilibrium pressures are reduced by approximately 20 percent compared to the cyclopentane promoted system. The mixed promoter system thereby represents a new state-ofthe-art within thermodynamic promotion of gas hydrates in the framework of the classical hydrate structures.
A thermodynamic model based on the Cubic-Plus-Association equation of state and the van der Waals-Platteeuw hydrate model is presented. This model enables the performance of a thermodynamic evaluation of gas hydrate forming systems relevant for post-combustion carbon dioxide capture. All model details and complete lists of model parameters are provided. Three simplified carbon dioxide capture processes are simulated by use of the model. Three to four capture stages are needed in all processes to obtain a product stream richer than 95 mole percent in terms of carbon dioxide.
The modeling results presented here are discouraging for the post-combustion carbon dioxide capture process under development. The present study points out several drawbacks of using tetrahydrofuran or cyclopentane as thermodynamic hydrate promoters, when applied in low-pressure processes. Due to their high volatilities (cyclopentane in particular), they readily transfer to the vapour phases. Furthermore, they lower the process selectivity towards carbon dioxide, compared to the unpromoted system.
Finally it is concluded that separation of carbon dioxide from nitrogen by gas hydrate formation at near-atmospheric pressure provides too low gas uptakes in the hydrate phase for this process to become feasible at these conditions.
Original languageEnglish
PublisherTechnical University of Denmark, Department of Chemical and Biochemical Engineering
Number of pages327
ISBN (Print)978-87-93054-54-7
Publication statusPublished - 2013

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