Article pubs.acs.org/EF
Compositional Analysis and Advanced Distillation Curve for Mixed Alcohols Produced via Syngas on a K‑CoMoSx Catalyst† Jesse E. Hensley,*,‡ Tara M. Lovestead,§ Earl Christensen,‡ Abhijit Dutta,‡ Thomas J. Bruno,§ and Robert McCormick‡ ‡
National Renewable Energy Laboratory (NREL), Golden, Colorado 80401, United States National Institute of Standards and Technology (NIST), Boulder, Colorado 80305, United States
§
S Supporting Information *
ABSTRACT: The distillation behavior of mixed alcohols was studied by use of the Advanced Distillation Curve (ADC) methodology. Crude mixed alcohols (oxygenates) were generated from syngas over a potassium-promoted cobalt−molybdenumsulfide catalyst and assayed for major and minor products. Distillation (boiling) curves were generated for the crude mixed oxygenate products and composition channel data were collected. The crude mixed alcohols consisted primarily of methanol with significant quantities of ethanol, 1-propanol, 1-butanol, methyl acetate, and ethyl acetate. These six species constitute 93.7%− 95.8% (mass/mass) of the total product. Ester, ether, and aldehyde impurities were identified, as well as thiols and organic sulfides. Considering just the alcohol products without impurities, these can be blended into gasoline at 8.5% (v/v) and meet the requirements of the Octamix waiver if an appropriate corrosion inhibitor were also included (the blend would contain 3.0%− 3.4% methanol, >2.5% higher alcohols (v/v), and a total oxygen content of 3.7% (mass/mass)). Distillation targeted at 50% methanol removal increased the volume of product that could be blended to over 9% (v/v). Methanol, aldehydes, and dimethyl sulfide were the first to vaporize from the mixture, and all C4+ alcohols remained within the last 20% of the distilled volume. Other products, including ethanol, propanols, esters, and organic sulfur species distilled over a range of boiling temperatures. ADCs suggest the presence of one or more azeotropes in the distillate, consistent with a large number of known binary azeotropes between components found in the mixed oxygenate product. Enthalpies of combustion were calculated for multiple distilled fractions and ranged from 890 kJ mol−1 in the first drop of distillate to 1150 kJ mol−1 in the first drop collected after distilling 80% of the original liquid volume. This energy density is low, compared to 91-octane gasoline at 3700 and 4940 kJ mol−1 in the first drop and at 80%, respectively. Comparisons of fractional distillation of the mixed oxygenate products showed directional agreement between experiment and simulation with Aspen Plus. This study provides useful insights into mixed oxygenate products derived from a sulfided catalyst, including considerations for process recycle, product constituents and their blending, and the applicability of distillation information from process simulators.
1. INTRODUCTION 1.1. Overview. In recent decades, research and development has focused on production of liquid transportation fuels from lignocellulosic biomass at a scale and cost that were competitive with petroleum fuels without consuming and/or competing with food crops. One approach to production of gasoline blend stocks is gasification of lignocellulosic biomass to syngas (carbon monoxide and hydrogen), followed by direct synthesis of mixed oxygenates on a potassium-promoted cobalt− molybdenum-sulfide (K-CoMoSx) catalyst, followed by fractionation of the products. These products include methanol and longer-chain alcohols (ethanol, 1-propanol, etc.), light nonalcohol oxygenates (aldehydes and esters), water, gaseous and liquid hydrocarbons, and sulfur-containing molecules (from catalyst sulfur loss). Ethanol is widely used in gasoline, and several of the long-chain alcohols may also be useful gasoline blend stocks.1 Blends of gasoline and the mixed oxygenate products may also be well-suited for use as gasoline if most of the methanol is removed, with the advantage of reduced purification cost in comparison to production of fuel-grade ethanol from this process. Under the Clean Air Act, transportation fuels used in the United States must be “substantially similar” to those that were © 2013 American Chemical Society
used to certify vehicles for compliance with emissions standards.2 The United States Environmental Protection Agency (USEPA) has ruled that blends of traditional gasoline, aliphatic alcohols, and/or ethers are substantially similar to gasoline when the oxygen content does not exceed 2.7% (mass/ mass).3 Under the Clean Air Act, the USEPA has the authority to grant waivers for use of other fuel blends or finished fuels with greater oxygen contents, once sufficient data and testing have confirmed that the new fuels will not harm emission control systems.4 The USEPA has granted two such waivers for ethanol, which can be blended to 10% (v/v) in gasoline5 (nominally 3.7% mass/mass oxygen), and more recently, 15% (v/v) (5.5% mass/mass oxygen) for use in model year 2001 and newer vehicles.6 Other waivers have been granted for the use of mixtures of alcohols; for instance, the “Octamix” waiver, which allows a blend of up to 5% (v/v) methanol and at least 2.5% (v/v) longer-chain alcohols (∼3.7% mass/mass oxygen), with a corrosion inhibitor, was developed explicitly for mixed alcohols produced from syngas.7 However, there are several Received: February 12, 2013 Revised: April 16, 2013 Published: April 19, 2013 3246
dx.doi.org/10.1021/ef400252x | Energy Fuels 2013, 27, 3246−3260
Energy & Fuels
Article
analysis, and corrosivity assessment of each DVF.37−40 Perhaps the most important advantage presented by the ADC metrology is the ability to sample the fluid during the course of the distillation. Sampling very small volumes of the distillate (5−50 μL) yields a composition-explicit data channel, with nearly instantaneous composition measurements. All inflections and slopes of the distillation curve are the result of the changing composition, and this feature provides a measurement of this changing composition. Thus, a significant advantage offered by the metrology discussed in this paper is the ability to develop a thermodynamic model of the distillation curve with an equation of state.41−46 The composition-explicit data channel also allows one to add thermochemical data to the distillation curve.47−51 The enthalpy of combustion is a well-known thermochemical quantity for a large number of compounds, tabulated in several reliable databases.52−54 Thus, for a mixture, knowledge of the identities of the chemical components and their relative molar concentrations allows access to the composite enthalpy of combustion of the mixture. In this way, the composition-explicit data channel of the ADC approach allows determination of the composite enthalpy of combustion of each distillate fraction.32,34,35,55 The theory and assumptions based on the calculation of enthalpy for fuel component mixtures are included in these publications, and are repeated in the Supporting Information (SI) for convenience.
other requirements that must also be met before a mixed alcohol blend stock can be used. If impurities are present, it must be demonstrated that these have an adequately low level of toxicity and will not harm engine fuel systems or fuel dispensing equipment. This may entail development of an American Society for Testing and Materials (ASTM) or similar standard for the blend stock that limits allowable levels of impurities. In addition, speciated emissions and health effects testing data would have to be submitted to the USEPA. There are several tools in the literature for would-be adopters of a thermochemical biomass-to-alcohols process. The National Renewable Energy Laboratory (NREL) has developed process designs for the production of ethanol or mixed oxygenates from woody or herbaceous biomass and has provided technoeconomic predictions and sensitivity analyses.8 The Dow Chemical Company characterized the volatility and combustion properties of model mixed alcohol blends.9 Several reviews have outlined options for gasification,10,11 syngas cleanup,12,13 and syngas to mixed oxygenate syntheses,14−16 and reaction kinetics have been at least partly characterized.17−19 Studies have confirmed the loss of sulfur from the sulfide catalyst and the need for supplemental feed of H2S to maintain catalytic activity.9,20,21 Despite these advances, detailed analyses of the crude mixed oxygenate products have yet to be reported. Information such as sulfur content, types and quantities of byproducts, and distillation properties are critical for advanced design of process equipment, confirmation of finished product composition, and certification of the finished product as an acceptable blend stock for gasoline. We present compositional analyses and measurements of the vapor−liquid equilibrium for crude mixed oxygenates produced from syngas on a K-CoMoSx catalyst operated at four processrelevant conditions. The measurements were performed using the composition-explicit or advanced distillation curve (ADC) method, which has been applied to numerous complex mixtures.22−24 We present results from a rudimentary fractional distillation of the crudes, compare these results to a simulated distillation with Aspen Plus using commercially relevant separation specifications, and comment on the implications for using these mixed oxygenates as a gasoline blend stock. 1.2. Advanced Distillation Curve Metrology. The distillation curve is a graphical depiction of the boiling temperature of a fluid or fluid mixture plotted against the volume fraction distilled.25−27 The most common presentation of the distillation curve is a plot of the boiling temperature (at ambient pressure) as a function of the distillate volume fraction (DVF). The standard test method, ASTM D86, provides the usual approach to measurement.28 The data obtained with ASTM D86 are the initial boiling point, the temperatures at various DVFs, and the final boiling point. The ASTM D86 test suffers from several drawbacks, including large uncertainties in temperature measurements and little theoretical significance.29 In an effort to remedy the shortcomings of the standard distillation method described above, an improved distillation method and apparatus were developed.29−36 Improvements to the traditional distillation apparatus include the following: (1) temperature measurements that are true thermodynamic state points that can be modeled with an equation of state; (2) temperature, volume, and pressure measurements of low uncertainty suitable for equation of state development; and (3), a compositionexplicit data channel for each distillate fraction (for both qualitative and quantitative analysis). In addition, the improved experiment allows an assessment of the energy content, trace chemical
2. EXPERIMENTAL SECTION 2.1. Synthesis of Mixed Oxygenate Crudes. A K-CoMoSx catalyst was prepared by coprecipitating cobalt acetate and ammonium tetrathiomolybdate from solution, followed by washing, calcination, and physical blending with potassium carbonate. Additional details can be found elsewhere.56 The resulting material consisted of 9/18.7/7.9/ 64.4 Co/Mo/K/S (at. %, O-free) per elemental analysis, with a BET surface area of 10−12 m2 g−1. Catalyst samples were handled in a nitrogen-purged glovebox containing
