Ethylene cracking

Ethylene is a basic chemical raw material for synthetic fibres, synthetic rubber, synthetic plastics (polyethylene and polyvinyl chloride) and synthetic ethanol (alcohol). It is also used in the manufacture of vinyl chloride, styrene, ethylene oxide, acetic acid, acetaldehyde and explosives, amongst others. Furthermore, it can be used as a ripening agent for fruit and vegetables and is a proven plant hormone.

Ethylene is one of the world’s most widely produced chemical products; the ethylene industry forms the core of the petrochemical sector, with ethylene products accounting for over 75 per cent of petrochemical output, thereby occupying a significant position in the national economy. Globally, ethylene production is regarded as one of the key indicators of a country’s level of development in the petrochemical sector.

Its physiological effects include: the triple reaction; promoting fruit ripening; promoting leaf senescence; inducing the formation of adventitious roots and root hairs; breaking seed and bud dormancy; inhibiting flowering in many plants (though it can induce and promote flowering in pineapples and plants of the same genus); and, in dioecious plants, altering the direction of floral sex differentiation during early flower development.

Ethylene serves as a raw material for many industrial products and is itself an important petrochemical product; it is produced by the cracking of hydrocarbons under high-temperature, low-pressure conditions.

The production of ethylene involves the steam cracking of hydrocarbons at extremely high temperatures.

The use of extreme heat during processing ensures a high conversion rate of feedstock to ethylene. However, temperatures (reaching 1,000 °C) also increase the rate of coking, leading to frequent furnace shutdowns.

To minimise furnace downtime and thereby maximise operating time, the ethylene furnace must be maintained as close as possible to its optimal setpoint.

The key to control lies in the compositional analysis of the reactor feed and effluent streams. To accomplish this task, real-time data on the reactor feed and effluent streams is essential.

As cracking occurs within a matter of seconds, the speed at which data is acquired is also critical.

Industrial process mass spectrometers analyse multiple pairs of reactor inlet and outlet streams, with each stream taking only a few seconds to analyse.

Manufacturers can use the real-time data analysis provided by online instruments to bring the operating setpoint closer to the optimum setpoint. This enhanced process control capability results in higher ethylene yields.

The typical feedstock for ethylene cracking is naphtha; the cracker gas produced by the cracker consists of non-condensable gases such as hydrogen, oxygen, nitrogen, carbon monoxide and carbon dioxide, as well as hydrocarbons ranging from C1 to C6.

The analysis of cracker gas components is of paramount importance for the control and adjustment of process parameters. Among these, ethylene yield serves as a key indicator of cracking depth, capacity and efficiency; its accurate measurement is therefore of great significance.

In ethylene plant retrofit projects, mass spectrometers can be used in the cracking unit to replace industrial gas chromatographs, primarily for the analysis of methane, ethylene, ethane, propane and other components.

The use of online mass spectrometers has improved ethylene yield and extended coke-cleaning cycles, thereby enhancing economic benefits; it also plays a similar role in the dehydrogenation of ethane and propane, as well as in the cracking of long-chain alkanes.