Catalysis

The topic of catalysis recurs throughout fuel chemistry. A catalyst increases the rate of a chemical reaction without itself being permanently altered by the reaction, or appearing among the products. The key word is rate. Catalysts affect reaction kinetics. A catalyst affects reaction rate by providing a different mechanism for the reaction, usually one that has a markedly lower activation energy than that of the non-catalyzed reaction. Catalysts do not change reaction thermodynamics; they do not alter the position of equilibrium [A], but they can help reach equilibrium much more quickly. And, they cannot cause a thermodynamically unfavorable reaction to occur.

Catalysts can be classified as homogeneous, in the same phase as the reactants and products, and heterogeneous, in a separate phase. Homogeneous catalysts mix intimately with the reactants. This good mixing often leads to enormous rate enhancements, in some cases by more than eight orders of magnitude. But, because they are in the same phase as the reactants and products, industrial use would require a separation operation for catalyst recovery downstream of the reaction, unless one were willing to throw away the catalyst (possibly allowing it to contaminate the products) as it passes through the reactor. For many catalytic processes, the catalyst costs much more than the reactants do, so loss of the catalyst would result in a significant economic penalty. Usually, heterogeneous catalysts have no major separation problems, thanks to their being in a separate phase from reactants and products. However, because of their being in a separate phase, mass-transfer limitations can hold up access of the reactants to the catalyst, or hold up departure of products. Heterogeneous catalysis can also be affected by various problems at the catalyst surface (discussed in Chapter 13). Large-scale industrial processing almost always favors use of heterogeneous catalysts, to avoid possibly difficult downstream separation issues. Nevertheless, steady progress is being made in finding ways to overcome separation problems with homogeneous catalysts, including, as examples, membrane separation, selective crystallization, and use of supercritical solvents.

Evidence that Earth is heating is incontrovertible. Glaciers and permafrost are melting. Sea level is rising. Deserts are spreading. Growing seasons are getting longer in far northern latitudes. Migratory species arrive at their summer breeding grounds earlier and remain later. Animals, including some of the less-pleasant snakes and disease-carrying insects, are increasing their ranges. Meteorological records show that the past decade has been the warmest on record. So many independent observations from different areas of science make an exceptionally strong case that a real effect is occurring.

Like any other system, temperatures on Earth are governed by a simple heat balance:

(Heat in)–(Heat out) = (Heat retained in system).

Several sources provide heat. These include incoming solar radiation, heat generated by human activity, and heat from decay of radioactive species in the Earth’s interior. Of these, solar radiation dominates, by far. It is estimated that the entire yearly energy needs of all of humankind could be met by capturing and converting all of the solar energy falling on Earth for about 45 minutes. Heat is lost primarily by radiative heat transfer back into space, much in the infrared. The balance between heat coming in, mainly solar energy, and heat going out, mainly infrared radiation to space, maintains the average global temperature. Any change in either term necessarily results in a change in the amount of heat retained, which in turn eventuates in a change in average global temperature. Because temperature has a major role in affecting climate, the net effect is a change in global climate.

Among the fossil fuels, the progression from natural gas to petroleum to coal is one of increasing complexity. Even a wet, sour gas has only a small number of possible components. Once the gas has been treated and purified for distribution to consumers, it typically contains >90% of a single compound, methane. Gas contains no inorganic impurities that might leave an ash residue on combustion. Petroleum usually is a homogeneous liquid with a narrow range of elemental composition – about 82–87% carbon, 12–15% hydrogen and the balance nitrogen, sulfur, and oxygen – with atomic H/C ratio of ≈1.5–1.8. On a molecular level, petroleum contains thousands of individual compounds, every one of which could be separated, at least in principle, using common techniques of the organic chemistry laboratory, and identified [A]. Inorganic ash-forming constituents are commonly less than 0.1%. In contrast, coals have an extremely wide range of composition, some 65–95% carbon, 2–6% hydrogen, up to about 30% oxygen, and possibly several percent each of sulfur and nitrogen. The H/C ratio is less than 1. Coals are opaque, heterogeneous solids. Coals cannot be distilled reversibly. Coals are not completely soluble in any solvent, and even the partial solubility in various solvents is an extraction of components rather than a true, reversible dissolution process. Coals have a macromolecular structure that varies from one coal to another and that has never been completely elucidated for any coal. Coals contain a variable, but appreciable, amount of inorganic material, so that burning a particular coal leaves an ash residue that represents anywhere from a few percent to over 25% of the original weight of the coal. Coals also contain some variable amount of water as they are mined from the Earth, from several percent to about 70%.

Despite the complexity of coals and the difficulties encountered in studying them, systems for classifying and describing coals are nevertheless needed. Such systems can provide a conceptual framework for organizing knowledge of coal composition and properties. In a very practical sense such systems provide the descriptions needed for legally binding buying and selling of coals.

Intermolecular interactions

Virtually all substances of interest in fuel chemistry consist of covalently bonded molecules. Many are hydrocarbons in the literal sense of the word – compounds containing only hydrogen and carbon atoms. Others contain one or more heteroatoms, i.e. atoms of oxygen, nitrogen, or sulfur. Physical properties of fuels have numerous important roles in fuel technology and utilization, e.g. boiling point, because distillation is commonly used for separations; density, because the amount of fuel that can be carried on vehicles or aircraft is limited by volume and not by mass; and viscosity, because we need fluids to flow, or to be pumped, from place to place. An understanding of how chemical composition and molecular structure influence physical properties shows that the properties of substances do not come about by some haphazard chance but rather because of fundamental links between composition, structure, and properties. Further, such links provide useful guidelines or rules of thumb for estimating expected properties from composition, or vice versa.

The most noticeable property of most substances is their physical state: solid, liquid, or gaseous. The first point of inquiry becomes that of why molecules form solids or liquids at all. Why isn't everything a gas? To exist in a condensed phase, i.e. as a liquid or solid, there must be attractive forces among molecules strong enough to hold them in proximity.

Fuel gas

Hydrogen and carbon monoxide are readily combustible with high calorific values (–286 and –283 kJ/mol, respectively). Although the name synthesis gas reflects its intended use in subsequent operations for production of other fuels or chemicals, no technical issues prevent use of products of gasification (or partial oxidation or steam reforming) directly as fuels. Such products can be used for domestic heating and cooking, for process heat, or for raising steam in industry. Many countries had, at one time, significant infrastructure for making and distributing fuel gases from coal. Indeed, until the development of syntheses based on CO and H2, in the early decades of the last century, the whole purpose of converting coal to gas was for domestic or industrial heating and illumination. Current interest focuses on use of synthesis gas as fuel in IGCC plants.

Water gas, town gas, illuminating gas, and related fuels made from early coal conversion processes, as well as the products of oxygen-blown gasifiers, have calorific values in the range 11–19 GJ/m3. Table 21.1 compares calorific values of some gases produced from coal with hydrogen, methane, and LPG.

A problem with some of these products is the high toxicity of carbon monoxide. Many gas-fired domestic appliances, such as water heaters or stoves, were equipped with pilot lights, in which a small quantity of gas was always being burned. Then, when the gas was turned on to the main burner or heater, it would be ignited by the pilot light, without a need to find matches. This bit of convenience for the householder, however, meant that if anything should cause the pilot light to go out, a small quantity of gas was now being emitted directly into the home.

In practical industrial processing, reactions must take place on time scales reasonably short from a human perspective – ideally in units of hours, at the most. Compared to natural geological processes, reaction times need to be reduced by up to ten orders of magnitude. Two approaches can do this. One is to increase reaction severity, usually increasing temperature. As a rough rule, reaction rate doubles for every 10 K increase in temperature. The highest temperature encountered in fuel formation is ≈225 °C, the closing of the gas window or the fourth coalification jump. Temperatures of fuel processing are often much higher, and reaction rates are correspondingly higher. The second approach is to use a catalyst to enhance reaction rate. Of course, in many situations both strategies are used together.

A catalyst changes the rate, outcome, or both, of a reaction without appearing in the net equation for the reaction (i.e. without being consumed in the reaction, or being permanently altered by the reaction). Although catalysts often find use to enhance rate, sometimes they are used to arrive at a different set of products. This is very important in, e.g., the production of high-quality gasoline (Chapter 14). As materials, catalysts are of extreme importance. Virtually all biochemical processes in living organisms are catalyzed by enzymes. About 90% of the fuels, synthetic chemicals, and plastics produced by the chemical industry have benefited from a catalyst in at least one of their processing steps.

Chapter 2 introduced the concept of catalysis, and focused on homogeneous catalysis. For large-scale production of commodities such as fuels, a homogeneous catalyst requires separation and recovery steps downstream of the reactor, unless the catalyst either is thrown away or is allowed to dilute or contaminate the product. This adds to the complexity and expense of a process. Heterogeneous catalysts are favored by industry, especially for production of commodities. In part, this derives from a very easy, even non-existent, separation from the process stream. Many heterogeneous catalysts can withstand more severe conditions of temperature and pressure than homogeneous catalysts, especially enzymes. Heterogeneous catalysts work well for gas-phase reactions, where it might be difficult to select a homogeneous catalyst [A].

Natural gas is a mixture of hydrocarbons with various quantities of non-hydrocarbons, which exists either in the gas phase or in solution with petroleum in natural underground reservoirs. The principal hydrocarbon component is methane. In most parts of the world, by the time the gas has been treated and distributed to consumers, it consists almost entirely of methane.

Gas produced during catagenesis usually migrates through porous rocks in the Earth's crust until it encounters a formation of non-porous rock. This non-porous rock prevents further migration of the gas, effectively trapping it in the porous rock below. The porous rock becomes a reservoir for the gas. The gas can be classified according to how it is found. Associated gas is found in conjunction with accumulated oil, either dissolved in the oil, called dissolved gas, or as a separate gaseous phase above the oil, gas-cap gas. Non-associated gas is found without accompanying oil. A reservoir of non-associated gas could arise from gas migrating to a different location than that to which oil migrated, or from formation of gas in the gas window, i.e. without oil. About 60% of the world's natural gas is non-associated.

Other sources of methane-rich gases occur in nature. Biogenic gas, produced during diagenesis, comes from the action of anaerobic bacteria on accumulated organic matter. Landfill gas is produced in the same way, but differs in that the feedstock is the organic residues of civilization, accumulated as solid waste in landfills. Excrement from humans or other animals also reacts in the same way, providing a useful source of fuel for farms or even domestic use [A]. Methane forms and accumulates in coal seams; its deliberate removal prior to mining provides another source, coalbed methane.

Gas clean-up

Gasification or partial oxidation is likely to be practiced with one of two purposes: to supply gaseous feed for an IGCC plant, or to use the carbon monoxide/hydrogen mixture as synthesis gas. Neither application uses raw gas with no downstream treatment. The desirable components of the product gas are carbon monoxide, hydrogen, methane, and other light hydrocarbons. If the gas were to be used in combustion applications on site, carbon dioxide and water vapor would be considered neutral, i.e. having neither a positive nor negative effect, except for their effect as diluents of the combustible gases. If the gas is to be upgraded, further processed, or shipped by pipeline, then both of these components are undesirable. Processing units would have to be larger to handle these “extra” components that make no contribution to the calorific value of the gas. Some heat and compression work would be wasted on diluents of the desired components of the gas. Components of the raw gas that are always undesirable include particulate matter, droplets of tar, hydrogen chloride, ammonia, and hydrogen sulfide and other sulfur-containing compounds.

A first treatment step involves removal of particulate matter, which may consist of fine particles of ash or of partially reacted feedstock. This can be accomplished using a cyclone separator. Cyclones can be designed to operate at temperatures to 1000 °C, and pressures to about 50 MPa. They work well for particle sizes above 5 µm. If it is necessary to remove finer particles, baghouses with fabric filters or electrostatic precipitators can be used.

Thermal cracking

In the progression through the oil window, thermally driven reactions break kerogen and larger hydrocarbon molecules into smaller ones. In a refinery, analogous processes take some of the heavier products and break them into smaller molecules. This shifts the molecular weight downward, increasing the amounts of relatively small molecules boiling in the gasoline range. To operate on a human rather than a geologic time scale requires running at much higher temperatures than are encountered in the oil window. Processes that rely entirely on heat for breaking down large petroleum molecules into smaller ones are called thermal cracking.

With the steady increase in use of automobiles and trucks in the early decades of the past century, market demand for gasoline exceeded what could be supplied by straight-run gasoline, even augmented with natural gasoline. Cracking processes can increase the relative proportion of molecules in the C5–C10 range, at the expense of larger molecules in products having a lower value than gasoline.

Refinery cracking processes are of two types, thermal (that rely entirely on temperature to drive the cracking reactions) or catalytic. Chapter 14 included a discussion of catalytic cracking reactions and processes. Thermal processes were developed starting around 1913. Numerous thermal cracking processes were developed in the early decades of the twentieth century. They helped meet the increasing demand for gasoline in the 1920s and 30s. The process developed by C.P. Dubbs [A] provides an example (see Figure 16.1).

Inspecting a collection of various kinds of wood would show considerable variability in physical properties such as color, density, and hardness. Densities range over an order of magnitude, from 110 kg/m3 for balsa to 1330 kg/m3 for lignum vitae. Colors vary from the nearly white of some varieties of maple to ebony wood, which lends its name as a synonym for black. A measure of hardness, the Janka test [A], shows again the extremes between lignum vitae, with a hardness of 20 000 N, and balsa, at 440 N. Some woods display almost no obvious grain structure, while others have structures so remarkable that they are prized for applications such as making fine furniture. Yet, despite the great range of diversity among woods, all varieties, regardless of botanical or geographical origin, also have many properties in common.

All woods have a cellular structure in which the cell walls are composed of a matrix of biopolymers, discussed later in this chapter. Wood is anisotropic, displaying different physical properties along the three major axes. Anisotropy results from the structure of cellulose in the cell walls, along with the shapes of wood cells and their orientation vis-à-vis the trunk. Wood gains and loses moisture (i.e. it is hygroscopic) as a result of changes in humidity and temperature. Because of the anisotropy, gain or loss of moisture results in unequal swelling or shrinkage along the three axes. Fungi, bacteria, and insects such as termites attack wood. The fact that wood is, as a result, biodegradable has both good and bad points. Obviously it is undesirable when, for example, a wooden structure rots. But, this behavior can also be turned to good use in chemical processing, when fungi or bacteria can be used deliberately to degrade the biopolymers in wood to produce useful chemical or fuel products.

Species in which carbon has a valence other than its customary four form as intermediates in reactions, and usually display high reactivity. Five kinds of reactive intermediate are known: carbonium ions [A], in which carbon has a valence of five, as in the methanonium ion, CH5+; carbocations, radicals, and carbanions, in which carbon has a valence of three; and the divalent carbenes. Of these, carbocations and radicals are the most important in fuel chemistry.

Bond formation and dissociation

When the ultimate effect of a chemical reaction is breaking of one or more bonds, the overall reaction will invariably be endothermic. When bond-breaking is one in a series of elementary steps in a reaction mechanism, that individual step is also invariably endothermic, even if the overall reaction is exothermic.

Suppose there are two generic atoms, A and B, that approach each other from an infinite distance apart (on an atomic scale, this “infinite” distance could be, say, a millimeter). At first, they have no interaction. As they approach to a distance at which they can interact, the potential energy of the A–B system becomes lower than that of the two separated A and B atoms. The potential continues to drop, until a bond has formed between them. Attempts to decrease the A–B distance even further cause the potential to shoot up rapidly, because of repulsion between electron clouds on the two atoms.

A plot of potential energy vs. interatomic distance, sometimes called a Morse curve [B], will look like Figure 7.1.

The lowest energy state, the ground state, is not exactly at the bottom of the “potential well.” The point at the very bottom is the zero-point energy, which is the energy the bond would retain at absolute zero to avoid disobeying the Heisenberg Uncertainty Principle [C]. Addition of energy to the bond, usually thermal energy, promotes the system to a higher state of vibrational energy, see Figure 7.2.

Analyzing petroleum samples collected from around the world would show that their elemental compositions vary over only a narrow range: 82–87% carbon, 11–15% hydrogen, with the balance being oxygen, nitrogen, and sulfur. Oxygen and nitrogen seldom exceed 1.5% each, but sulfur can amount to as much as 6% in extreme cases. Yet these samples could show remarkable diversity in physical characteristics, ranging from lightly colored, free-flowing liquid to dark, smelly, highly viscous material. A consequence of the wide variability of physical properties coupled with a very narrow range of composition is that carbon content cannot be used as a simple predictor of properties; in this respect, petroleum is very different from coal (Chapter 17). If these same samples were analyzed to determine the specific chemical compounds present in each, any particular sample would be found to contain some 105 individual components, varying in concentration from one sample to another. That is, on an elemental basis, most oils have about the same composition, but on a molecular basis no two are exactly alike. These seemingly disparate properties arise because most components of petroleum belong to a small number of homologous series of compounds, of which the composition, on a weight percent basis, varies only little, even over a long span of the series. For example, pentane, C5H12, is 83.3% C and 16.7% H on a weight basis; pentadecane, C15H32, is 84.9% C and 15.1% H. All possible isomers of all the alkanes between pentane and pentadecane amount to 7666 compounds, yet their elemental compositions change by only 1.6 percentage units. The range of boiling temperatures of the components spans at least 550 °C; pentane, the smallest alkane liquid at room temperature, boils at 36 °C, and some oils contain material that does not boil even at 600 °C.

Composition

Petroleum contains four classes of compound: alkanes, cycloalkanes, aromatics, and heteroatomic compounds with one or more atoms of nitrogen, sulfur, and/or oxygen. In petroleum chemistry and technology, alkanes are called paraffins; cycloalkanes, naphthenes; and the heteroatomic compounds are lumped together as NSOs. Cycloalkanes, aromatics, and NSOs can all have one or more alkyl side chains.

Biosynthesis of plant oils

Plants produce starch from glucose to store energy for future requirements. Plants and animals have a second energy-storage mechanism, synthesis of fats and oils. Plants may have evolved two mechanisms for energy storage because of differences in energy density. Heat of combustion values for starch are –17.5 MJ/kg or –26.2 MJ/l. Comparable values for peanut oil are –33.7 MJ/kg and –33.5 MJ/l. In the main body of the living plant, mass or volume are not usually crucial parameters, and storage of energy as starch, with relatively low energy density, does not become an issue. In storing energy for future generations, the amount of energy that can be packed into the limited volume available in nuts or seeds is critical. Here oils provide a significant advantage relative to starch. Fats and oils represent a subdivision of the broad category of biologically important compounds called lipids. Lipids are characterized as being insoluble in water, but generally soluble in common organic solvents, such as chloroform or diethyl ether. The family of lipids includes many diverse kinds of compound, including beeswax, cholesterol, and oil of turpentine.

Fats and oils are esters. In the special case of fats and oils, the alcohol fragment is 1,2,3-propanetriol, almost always known by its common name glycerol, or by the even older common name, glycerine. The acid fragments are long-chain aliphatic acids, collectively called fatty acids. These acids could be almost any aliphatic acid larger than butyric acid, but in biologically important materials they typically contain 12 or more carbon atoms. Esters of fatty acids with glycerol are called glycerides. Three families of glycerides exist, depending on whether one, two, or all three of the hydroxyl groups in glycerol have been esterified. These families would be known, respectively, as mono-, di-, or tri-glycerides. In plants, the overwhelmingly dominant form is the triglyceride [A]. Mono- and di-glycerides are important only in the digestion of fats. Simple triglycerides are ones in which all three fatty acid segments are identical; in mixed triglycerides, the –OH positions in the glycerol are esterified by different fatty acids. Mixed triglycerides dominate among plant products. Triglycerides solid at 20 °C are classified as fats; those liquid at this temperature are oils. Generally, fats contain saturated fatty acid chains, while oils contain unsaturated ones. Animals usually store energy as fats, while plants store energy as oils.