Process Analytics in Ethylene Production Plants

Case Study

Oil and Gas Industry

Process Analytics in Ethylene Production Plants

Ethylene is the largest volume industrially produced organic material and its majority is used in the production of polymers and derivatives. Between a variety of processes the thermal cracking of hydrocarbons in the presence of steam (steam cracker) is mostly used. Regardless of the process type, all plants require process analytical equipment to collect reliable and accurate process data for process control, product quality, and plant safety.

Siemens, a leader in process analytical instrumentation, has proven over decades its capability to plan, engineer, manufacture, implement and service analyzer systems for use in ethylene plants worldwide. This case study provides an overview of the steam cracking process and describes how Siemens, with its outstanding analyzer technology, application know-how and system integration expertise can provide remarkable user benefits.

Current worldwide production is about 95 MM tons/year and is projected to increase for the foreseeable future. A typical

modern plant produces in excess of 800,000 tons/year. Feedstock to ethylene plants ranges from light Ethane/Propane mix to

heavy naphta and vacuum gas oils. Most plants are designed with

raw material flexibility in mind. Majority of ethylene produced is used in the production of polymers and ethylene derivatives such as ethylene oxide and glycol. A typical ethylene plant also makes a number of other important chemicals such as propylene, butadiene and pyrolysis gasoline.

In the past years, Ethylene plants have evolved into highly integrated, highly flexible processing systems that can profitably adjust to changing raw material availability and market demands for Olefins products. Advanced process control technologies are used in Olefins plants and have greatly improved products quality, plant efficiency and resulted in quick payback of the investment.

Typical process features of an ethylene process are short residence time in the furnace, high selectivity, feedstock flexibility, operational reliability and safety, easy start-up, and energy efficiency.

Process analytics is a key issue for process control by online monitoring the various process streams in ethylene and propylene production. Process analytics maximizes yields and ensures product quality specifications.

Answers for industry.

Ethylene

Ethylene, H2C=CH2, is the lightest olefin. It is a colorless, flammable gas, which is produced mainly from petroleum-based

feedstocks by thermal cracking in the presence of steam. Ethylene has almost no direct end uses but acts almost exclusively as an intermediate in the manufacture of other chemicals, especially plastics.

Ethylene may be polymerized directly to produce polyethylene, the world’s most widely used plastic. Ethylene can also be chlorinated to produce1,2-dichloroethane, a precursor to the plastic polyvinyl chloride, or combined with benzene to produce ethylbenzene, which is used in the manufacture of polystyrene, another important plastic. Smaller amounts of ethylene are oxidized to produce chemicals including ethylene oxide, ethanol, and polyvinyl acetate.

Ethylene quality depends on users requirements in downstream processes. No single chemical grade ethylene exists, but ethylene content normally exceeds

99,7%. Sulfur, oxygen, acetylene, hydrogen, carbon monoxide and carbon dioxide are the most troublesome

impurities that must be controlled carefully.

Raw materials

Various feedstocks (liquid and gaseous) are used for the production of ethylen. The principal feedstocks are napthas, a mixture of hydrocarbons in the boiling range of 30 to 200°C. Depending on the origin, naphta composition and quality can vary over a wide range requiring quality control of the feed mixtures. Preferably in the US and the middle east light feedstocks (natural gas, ethane, propane, butan) are used. Gas oils (crude oil fractions) are also gaining importance as feedstocks in some areas of the world.

Chemical analysis of the feedstock is important to ensure the required product specification and even more when the production is based on varying feedstocks.

Production

The bulk of the worldwide production is based on thermal cracking with steam. The process is called pyrolysis or steam cracking. Production can be split into four sections (Fig. 1): The first three sections

are more or less identical for all commercial processes, with the exception that primary fractionation is required only in case of a liquid feedstock.

A large variety of process routes, however, exist for the hydrocarbon fractionation section.

A hydrocarbon feed stream is preheated, mixed with steam and further

heated to 500 to 700°C. The stream enters a fired tubular reactor (known as cracker, cracking heater), where under controlled conditions the feedstock is cracked at 800 to 850°C into smaller molecules within a residence time of

0.1 to 0.5 s. After leaving the radiant

coils of the furnace, the product mixtures are cooled down instantaneously in transfer line exchangers (TLE) to preserve the gas composition. This quenching time is a crucial measure for severity control of the final products.

The steam dilution lowers the hydrocarbon pressure, thereby enhancing the olefin yield and reducing the tendency to form and deposit coke in the tubes of the furnace and coolers. For details of the process steps, see Fig. 2 to 5. Cracking furnaces (capacity of modern units up to 150,000 tons/year) represent the largest energy consumer in

an ethylene plant.

Other processes for ethylene production besides conventional

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