CO 2 Removal from Contaminated Natural Gas Mixtures by Hydrate Formation Mark van Denderen, Erik Ineke, and Michael Golombok* Shell International Exploration and Production, Kessler Park 1, 2288 GS Rijswijk, The Netherlands We analyzed the formation rates of gas hydrates from methane/CO 2 mixtures associated with contaminated well gas streams, which results in a methane-enriched gas product. The initial pressure affects both the final clean gas composition and the time taken to achieve it. Although promoters have been described, we note that this term often refers to interstitial materials that stabilize equilibrium hydrate structures rather than classical chemical rate promoters (i.e., catalysts). We also show that previously reported hydrate rate improvers are irrelevant for commercial purposes because they merely overcome inadequate mixing systems. We identify the key process requiring rate acceleration and show the potential for reducing the time for this process to make selective CO 2 hydrate formation commercially attractive. We also examine the effect of salinity on the selective formation of CO 2 hydrates. The phase boundary pressures increase with increasing salt concentration, and the rate of formation decreases. Formation times are on the order of 1 h, so catalysts are required to make this process commercially viable. 1. Introduction The formation of hydrates as a way of removing CO 2 from gas streams has recently been a subject of increasing interest. 1-7 The relevant gas streams can be split into two types: Best known are the exhaust gas streams resulting from the combustion of fossil fuels. These consist principally of nitrogen and CO 2 . The phase boundary curves for pure nitrogen and CO 2 are well separated from one another, so one would expect the driving force, represented by the supersaturation overpressure of each component with respect to its phase boundary pressure at a particular temperature, to selectively favor more rapid CO 2 hydrate formation. 8,9 The other CO 2 -containing gas mixtures of interest are those with methane (i.e., natural gas streams) 10 containing high (>15%) concentrations of CO 2 or H 2 S. Although mixed hydrates (i.e., N 2 /CO 2 or CH 4 /CO 2 ) might not be sufficiently selective thermodynamically to enable gas cleanup, they can be kinetically selective to the uptake of CO 2 even though the phase boundary line of methane is much closer to CO 2 than that for nitrogen. 11-13 We have recently shown 14 that, for appropriately selected pressure and temperatures, the kinetics of CO 2 uptake proceed much more quickly than those of methane uptake. This is true despite the fact that the phase boundaries for the pure compo- nents are relatively close. Although some aspects of hydrate kinetics have been studied in detail, such efforts have been mostly focused on methane and other paraffinic hydrates. 15-18 The reasons for this are logical and practical: The “upstream” utilization of methane hydrates as a gas reservoir is determined by the decomposition kinetics, and the “downstream” growth of paraffinic hydrates in pipelines is determined by the formation kinetics. Another intensive area of study has been in inhibiting the formation of light hydrocarbon (C2-C4) hydrates in gas pipelines. 19,20 However it is only relatively recently that attention has focused on nonhydrocarbon hydrates. Our interest here is in the formation of CO 2 hydrates. In this study, we examined whether methane can be purified from a mixed stream using selective formation of CO 2 hydrates. Section 2 considers some thermodynamic concepts and reviews the expression of these ideas in previous literature. Section 3 describes our experimental apparatus, and section 4 describes the results, with a discussion focused on accelerating the process to make its application in industry viable. 2. Background The hydrate phase boundaries for methane and CO 2 are well- known (Figure 1). There is ambiguity in the literature about the region between the methane and CO 2 lines. 21 Thermody- namic principles and some experimental evidence indicate a phase boundary for a gas mixture intermediate between the two pure-component lines. 22 This is to be expected given that, as in mixed crystal structures, there will be interstices in the methane hydrate structure that can be filled by CO 2 molecules and similarly methane molecules can stabilize “holes” in the CO 2 structure. Such interstitial stabilization is known from the use of hydrate “promoters” and “inhibitors”. 20 Although, in chemical parlance, these terms are utilized to denote increased or decreased rate kinetics, respectively, it appears that, in hydrate studies, these * To whom correspondence should be addressed. E-mail: michael. golombok@shell.com. Also with Department of Mechanical Engineer- ing, Technische Universiteit Eindhoven, PO Box 513, 5600 MB Eindhoven, The Netherlands. Figure 1. Phase diagram showing hydrate formation boundary for pure methane and CO 2 with stoiochiometric water. Ind. Eng. Chem. Res. 2009, 48, 5802–5807 5802 10.1021/ie8017065 CCC: $40.75  2009 American Chemical Society Published on Web 05/18/2009