7.1 Industrial Preparation and Use of Alkenes

Industrial Preparation of Alkenes

Industrial preparation of ethylene and propylene

Steam cracking is the primary industrial method for producing ethylene and propylene. The process heats saturated hydrocarbons (typically naphtha or ethane) to very high temperatures (750–950°C) in the presence of steam. It occurs without oxygen to prevent combustion, and the reaction takes place in a pyrolysis furnace with an extremely short residence time (0.1–0.5 seconds). That short time window is deliberate: it maximizes the yield of desired alkenes and minimizes unwanted side reactions.

Mechanism of steam cracking:

1. High temperature causes homolytic cleavage of C–C bonds in the saturated hydrocarbon feedstock, splitting each bond so that one electron goes to each fragment.
2. This produces highly reactive free radicals.
3. These radicals then undergo chain reactions including dehydrogenation, isomerization, and decomposition, ultimately forming smaller unsaturated hydrocarbons like ethylene and propylene.

Factors affecting product distribution:

• Feedstock composition directly impacts the product mix. Ethane feedstocks produce mostly ethylene, while naphtha yields a broader range of products including propylene and butadiene.
• Cracking severity (higher temperature and/or longer residence time) favors lighter products like ethylene.
• Higher pressure reduces ethylene and propylene yields because cracking increases the total number of gas-phase molecules, and high pressure shifts equilibrium away from that expansion (Le Chatelier's principle).
• Steam as a diluent lowers the partial pressure of the hydrocarbons, which improves selectivity toward desired alkene products.

Other cracking methods

• Catalytic cracking uses zeolite or other catalysts to break down larger hydrocarbons at lower temperatures than steam cracking. It's widely used in oil refineries to produce gasoline-range molecules, though it also generates some alkenes.
• Thermal cracking uses heat and pressure alone to break down hydrocarbons, without catalysts or steam. It generally produces a less selective product distribution than steam cracking.

Importance of ethylene and propylene

Ethylene is the most widely produced organic compound in the world, and propylene is the second most important petrochemical feedstock. Both are classified as olefins (an older term for alkenes) and serve as building blocks for a huge range of industrial products.

Ethylene applications:

• Polyethylene (PE) plastics: High-density polyethylene (HDPE) is used for bottles, pipes, and automotive fuel tanks. Low-density polyethylene (LDPE) is used for plastic bags, packaging films, and squeeze bottles.
• Ethylene oxide: a precursor to antifreeze (ethylene glycol), polyester fibers, and surfactants.
• Ethylene dichloride: converted to polyvinyl chloride (PVC) plastics.
• Ethylbenzene: converted to styrene monomer, which is polymerized into polystyrene.

Propylene applications:

• Polypropylene (PP): used in packaging materials, textiles, and automotive parts.
• Propylene oxide: used to make polyurethane foams and propylene glycol.
• Acrylonitrile: used in acrylic fibers and ABS plastics.
• Cumene: an intermediate in the production of phenol and acetone.

Thermodynamics of steam cracking

Steam cracking is an endothermic process, meaning it absorbs heat. Breaking C–C bonds in the feedstock requires energy input, which is why such high temperatures (750–950°C) are needed to overcome the activation energy barrier.

Why high temperature makes cracking thermodynamically favorable:

Cracking reactions break one large molecule into multiple smaller ones, increasing the total number of molecules. This means the entropy change is positive ($\Delta S > 0$).

The Gibbs free energy equation ties this together:

$\Delta G = \Delta H - T\Delta S$

Since $\Delta H$ is positive (endothermic) and $\Delta S$ is also positive, the $T\Delta S$ term grows larger as temperature increases. At sufficiently high temperatures, $T\Delta S$ exceeds $\Delta H$, making $\Delta G$ negative and the reaction spontaneous.

This is also consistent with Le Chatelier's principle: raising the temperature shifts equilibrium toward the endothermic direction (toward products), favoring the formation of lighter alkenes over heavier hydrocarbons.

Feedstock and processing

Before cracking can occur, crude oil must first be separated into usable fractions. Fractional distillation separates crude oil components by boiling point, producing fractions like naphtha and ethane that serve as feedstocks for steam cracking. The choice of feedstock affects both the cracking conditions used and the distribution of alkene products obtained.