From natural gas, crude oils, and other fossil materials such as coal, few intermediates are produced that are not hydrocarbon compounds. The important intermediates discussed here are hydrogen, sulfur, carbon black, and synthesis gas. Synthesis gas consists of a nonhydrocarbon mixture (H2,CO) obtain- able from more than one source. It is included in this chapter and is fur- ther noted in Chapter 5 in relation to methane as a major feedstock for this mixture. This chapter discusses the use of synthesis gas obtained from coal gasification and from different petroleum sources for produc- ing gaseous as well as liquid hydrocarbons (Fischer Tropsch synthesis). Naphthenic acids and cresylic acid, which are extracted from certain crude oil fractions, are briefly reviewed at the end of the chapter.
Hydrogen is the lightest known element. Although only found in the free state in trace amounts, it is the most abundant element in the uni- verse and is present in a combined form with other elements. Water, nat- ural gas, crude oils, hydrocarbons, and other organic fossil materials are major sources of hydrogen. Hydrogen has been of great use to theoretical investigation. The struc- ture of the atom developed by Bohr (Nobel Prize Winner 1922) was based on a model of the hydrogen atom. Chemically, hydrogen is a very reactive element. Obtaining hydrogen from its compounds is an energy- extensive process. To decompose water into hydrogen and oxygen, an energy input equal to an enthalpy change of +286 KJ/mol is required1:
H2O r H2 + 1/2O2 ∆H = +286 KJ/mol
Electrolysis, and thermochemical and photochemical decomposition of water followed by purification through diffusion methods are expensive processes to produce hydrogen. The most economical way to produce hydrogen is by steam reforming petroleum fractions and natural gas (Figure 4-1).2 In this process, two major sources of hydrogen (water and hydrocarbons) are reacted to pro- duce a mixture of carbon monoxide and hydrogen (synthesis gas). Hydrogen can then be separated from the mixture after shift converting carbon monoxide to carbon dioxide. Carbon oxides are removed by pass- ing the mixture through a pressure swing adsorption system. The shift conversion reaction is discussed in relation to ammonia synthesis in Chapter 5. The production of synthesis gas by steam reforming liquid hydrocarbons is noted later in this chapter. Recently, a new process has been developed to manufacture hydrogen by steam reforming methanol. In this process, an active catalyst is used to decompose methanol and shift convert carbon monoxide to carbon dioxide. The produced gas is cooled, and carbon dioxide is removed:
CH3OH(g) + H2O(g) r CO2(g) + 3 H2(g)
Figure 4.1. A process for producing hydrogen by steam reforming of hydrocar- bons:2 (1) reforming furnace (2,3) purification section, (4) shift converter, (5) pres- sure swing adsorption.
Nonhydrocarbon Intermediates 113
This process is used to produce relatively small quantities (0.18–1.8 MMscfd) of highly pure hydrogen when methanol is available at a rea- sonable price. In the petroleum refining industry, hydrogen is essentially obtained from catalytic naphtha reforming, where it is a coproduct with reformed gasoline. The use of hydrogen in the chemical and petroleum refining industries is of prime importance. Hydrogen is essentially a hydrogenating agent. For example, it is used with vegetable oils and fats to reduce unsaturated esters (triglycerides). It is also a reducing agent for sulfide ores such as zinc and iron sulfides (to get the metals from their ores). Hydrogen use in the petroleum refining includes many processing schemes such as hydrocracking, hydrofinishing of lube oils, hydrodealkyla- tion and hydrodesulfurization of petroleum fractions and residues. Hydro- cracking of petroleum resids is becoming more important to produce lighter petroleum distillates of low...