Key difference: Chemical Engineering is the science of applying chemistry to industry. Petrochemical Engineering is the science of dealing with the chemical products extracted from petroleum.

According to Wikipedia, Engineering is the application of scientific, economic, social and practical knowledge in order to design, build and maintain structures, machines, devices, materials and processes. It, like science, is a broad field and has many different aspects to deal with. It can be divided into many sub categories, chemical and petrochemical being one of them. This article helps differentiate the two.

Wikipedia defines Chemical Engineering, as the application of physics, chemistry and engineering principles on the chemical process required in commercial industries. It is used in producing materials; modern chemical engineering also concerns with producing valuable new materials and techniques - such as nanotechnology, fuel cells and biomedical engineering. George E. Davis developed the concept of unit operations, a main principle in the science of chemical engineering and helped lay a solid foundation for it.

Chemical engineering involves application of many principles. Key concepts being: chemical reaction engineering, involves managing of plant processes and conditions to ensure optimal plant operation. Plant design, is the creation of plans and specification and income projection of plants in industries. Process design is the managing of unit operations. Unit operations are used to prepare reactant or purifying products. Chemical engineers develop economic ways of using materials and energy. They use chemistry and engineering to turn raw materials into usable products, such as medicine, petrochemicals and plastics for industries. They are also concerned with waste management and research.

Petrochemicals are chemicals which are obtained from petroleum. This process is called Petrochemical Engineering. Few chemical compounds obtained from petroleum are also found in fossils such as coal or natural gas, or renewable resource such as sugarcane. Petrochemical can be classified into two classes, olefins and aromatics. Oil refineries produce olefins and aromatics by catalytic cracking of petroleum. Olefins are produced by steam cracking of natural gas in chemical plants, whereas aromatics are obtained by catalytic reforming of naphtha. Olefins and aromatics are the building-blocks for a wide range of materials such as solvents, detergents, and adhesives.

Petrochemicals are divided depending on their chemical structures:

• Olefins - in which are included ethylene, propylene and butadiene.
• Aromatics – which include benzene, toluene, and xylenes. Benzene is a raw material for dyes and synthetic detergents.
• Synthesis gas - it is a mixture of carbon monoxide and hydrogen, which is used to make ammonia and methanol.

Comparison between Chemical and Petrochemical Engineering:

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