New hydrate risk evaluation for processing and transport of natural gas containing water and impurities

Abstract

The growth of natural gas transport by pipeline has led to the establishment of a large network of pipelines throughout the world [1]. Transport of natural gas through a network of pipelines is a challenging issue, mainly due to the hydrate risk associated with it. Hydrates are crystalline compounds in which hydrogen bonded water enclatherate one or more type of `guest' molecules inside cavities. Natural gas hydrates can be formed both in pipelines and process equipment's, as a result of abnormalities in the flow operation [2].I.e. when pressure drop occurs across a restriction or turbine causing the water to expand and eventually drop out as liquid and during emergency shut downs due to failure of a component in the system for e.g. compressor failure. Natural gas hydrates can form in pipelines as a result of vapor deposition and or splashing of water with subsequent conversions [2]. In view of the above, the maximum allowable content of water that can be permitted in a gas stream, [in this project methane (CH4) with impurities carbon dioxide (CO2) and hydrogen sulphide (H2S)] under transportation without risking the hydrate formation is a very complex issue [3]. Very complex because generally the system cannot reach equilibrium and many different competing phase transitions occur which leads to hydrate formation and phase transitions that can lead to re-dissociation. The dynamic situation further complicates the picture, which due to 1st and 2nd laws of thermodynamics may lead to a range of different hydrates depending on which phase the hydrate former is coming from. This is because chemical potentials for guest molecules in different phases are not the same in non- equilibrium situations. Yet another reason is that the most stable hydrates will form first even with liquid water and hydrate former from fluid phase mixture. In this MSc project three different routes to hydrate formation have been studied and analyzed. This also includes impact of solid surfaces that can induce phase transitions. For transport in pipelines this is essentially rusty surfaces of the pipe-wall. The main aim of the study is to bring more clarity to the most important and possible events that can lead to hydrate formation and as such will dominate the hydrate risk evaluation. The first of these routes to hydrate formation involves water condensing out as liquid and then subsequently forming hydrate together with hydrate formers from the gas. This sequence of events is the most used basis for hydrate risk analysis in industry today, i.e. hydrate risk evaluation based on dew-point calculations. A second possibility is that hydrate forms directly from water dissolved in the gas. A third route involves the possibility that water can adsorb on the rusty walls and then hydrate can be formed from water which is slightly (1 nm) outside the wall, by extracting hydrate formers from the gas. Since the systems [water, gas, adsorbed phase and hydrate] generally cannot reach equilibrium (Gibbs phase rule) and the combination of 1st and 2nd law of thermodynamics will always direct the system to have lowest free energy possible [3]. Hence a primary tool for comparison will be free energy analysis. The natural choice of method in free energy analysis is classical thermodynamics using residual thermodynamics (ideal gas as a reference state). The software developed for this purpose is developed by Professor Bjørn Kvamme. In addition, external software from DTU (Danish Technical University) has also been utilized to determine thermodynamic properties of the gas mixture. These codes are in FORTRAN language and Microsoft developer has been used as compiler. Water available for hydrate formation can exist in three different states. The first being able to form CH4 hydrate directly from water dissolved in CH4 (mass transport is a limitation though), the second being hydrate formed with water either condensing out, to create a liquid water phase and the third being hydrate formation from water adsorbed on solid surfaces (hematite in our case) [4]. In routes involving liquid water drop-out the impurities like CO2 and H2S may follow water from the first condensation. Sensitivity analysis was made and the maximum allowable concentration of water, before the water drop out was analyzed for different systems. The findings were that with the presence of H2S in a system with CH4and CO2 the hydrate formation process could be faster i.e. the maximum allowable water concentration which can be permitted in a gas stream (CH4) during transportation of gas decreased with increasing pressures as well as for the concentration of H2S as compared to a system with only CH4and CO2. Hence it can be observed from the results how even a small amount of H2S is able to affect the limit of water concentration in the gas stream.