Article pubs.acs.org/EF
Investigation of the Influence of Hydroxyl Groups on Gas Hydrate Formation at Pipeline-Like Conditions Florian Stephan Merkel,*,† Carsten Schmuck,‡ and Heyko Jürgen Schultz† †
University of Applied Sciences Niederrhein, Krefeld 47805, Germany University Duisburg-Essen, Essen 47057, Germany
‡
ABSTRACT: Clathrates of natural gases, also called “gas hydrates”, are ice-like solids made up of water and gas molecules. They have become increasingly interesting in science and industry in the last decades, because of their potential as an energetic resource as well as because of their ability to block and damage pipelines (so-called “plugging”) under certain conditions. The mechanism of hydrate formation, however, is not fully explored yet, especially regarding the formation in the presence of substances other than gas and water molecules. Therefore, in this paper, the influence of substances with OH-molecular groups on methane hydrate formation has been thoroughly investigated in high-pressure experiments.
1. INTRODUCTION Due to the discovery of enormous gas hydrate reservoirs and the simultaneous shortage of conventional fossil fuels, research on hydrates of natural gases (gas hydrates) has become the focus of industry and the economy. Gas hydrates are solid inclusion compounds made up of water and guest molecules. They have an ice-like morphology and form at high pressures and low temperatures. But hydrates are not only seen as promising future energy carriers, they also cause massive problems in the oil and gas industry, mainly in gas-transporting pipelines. Because the conditions for hydrate formation are given in “cold” regions, hydrates can form inside pipelines in deep sea or permafrost regions and even completely block them (so-called plugging), especially after bends and valves.1−5 To better understand hydrate formation as well as methods to facilitate (promote) or suppress (inhibit) it, it is critical to investigate hydrate formation at “field-relevant”, in this work meaning “pipeline-like”, conditions. Pipeline-like means that the driving force (pressure) is not kept constant, but decreases over time during hydrate formation (transient conditions). Also, it is essential to investigate the influence of chemicals, especially with specific functional molecular groups, e.g., OHgroups, on the formation of gas hydrates. Hence, in this work, research on the formation of methane hydrate in the presence of molecules with an increasing number of OH-groups per molecular volume has been carried out. OHgroups were chosen because of their potential to interact with the hydrogen bonds that form the hydrate “cages” (see → Theoretical Background). Induction times as well as plug formation times (as a self-defined critical parameter for the evaluation of influencing factors) have been determined. The insights gained in this work will be used for evaluating the influence of OH-groups and to gain deeper insight into hydrate formation itself. Focus was on an increasing number of OH-groups per molecular volume; the goal of the investigations was to determine if there is an “optimal local concentration of OHgroups” in regards to inhibition of hydrate formation. The © XXXX American Chemical Society
underlying concept of the investigations is the idea to develop a functional coating, that shall be applied permanently in pipelines and inhibit gas hydrate formation, without the need to continuously add inhibitor to the pipeline. To evaluate the feasibility of this concept and to find suitable substances to apply as a coating, it is critical to determine if the abovementioned “optimal local concentration of functional groups” exists and what the exact concentration is.5,6
2. THEORETICAL BACKGROUND Gas hydrates belong to the substance group of “clathrates”. The word is derived from the Latin word “clatratus”, which means “barred”. Clathrates are inclusion compounds made up of “host” and “guest” molecules. They are called “true” clathrates when there is no chemical bond between host and guest, but solely stabilization via, e.g., van-der-Waals-forces. In the case of water clathrates, the host is water; therefore, they are also called “hydrates”.1−3,7−10 The formation of gas hydrates occurs by the scheme illustrated in eq 2.1 and is exothermic. Hydrate formation is favored by high pressures as well as low temperatures. gas + water ←⃗ gas hydrate; ΔHr < 0
(2.1)
Gas hydrates form different structures depending on the size and type of guest molecules as well as ambient/environmental conditions. According to Jeffrey and McMullan,11 possible guest molecules are sorted into 4 groups: 1. 2. 3. 4.
hydrophobic substances acid gases (water-soluble) polar substances (water-soluble) and ternary/quaternary alkylammonium salts (watersoluble).1
Received: July 22, 2016 Revised: September 20, 2016
A
DOI: 10.1021/acs.energyfuels.6b01795 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
act as so-called promoters, or they can suppress or delay hydrate formation and act as so-called inhibitors. Inhibitors are further divided into groups, based on their acting mechanism. The first group of inhibiting chemicals are the so-called thermodynamic hydrate inhibitors (THI). They operate by shifting the equilibrium of hydrate formation to higher pressures or lower temperatures and can be used to prevent hydrate formation as well as to “melt” existing hydrates. Commonly used THI are, e.g., methanol, ethylene glycol (which is investigated at lower-than-usual concentrations in this paper), or electrolytic solvents. They inhibit in a reliable and effective way. However, a permanent surveillance of the pipeline is required, since the THI can even act as promoters and accelerate hydrate formation, when their concentration is too low (“under-inhibition”). Their biggest disadvantage is that they have to be dosed in the pipeline at very high concentrations (“up to two barrels of THI per barrel of water”7), which causes high operating costs and also poses a high risk to the environment.1,2,4,5,12 The second group of inhibitors are the so-called Low dosage hydrate inhibitors (LDHI). These chemicals are used in very low concentrations (usually
