Ethylene oxide/Ethylene glycol

Ethylene oxide (EO) is the second most important organic chemical product derived from the ethylene industry, second only to polyethylene. In addition to being used in part for the manufacture of non-ionic surfactants, amino alcohols, ethylene glycol ethers, speciality additives and other types of fine chemical products, it is also primarily used to produce ethylene glycol, which is the main raw material for the manufacture of polyester resins and is also widely used as an antifreeze agent.

Due to the rapid development of the domestic polyester industry in recent years, demand for ethylene glycol has risen sharply, injecting new vitality into the ethylene glycol and ethylene oxide sectors, whilst also driving the development of silver catalysts for the direct oxidation of ethylene to produce ethylene oxide.

Ethylene glycol plants primarily consist of two sections: ethylene oxide (EO) and ethylene glycol (EG).

The ethylene oxide process is complex and represents a key focus of process control. It predominantly utilises the ethylene-oxygen oxidation method, carried out in the presence of a silver catalyst (the micro-reaction evaluation unit for the silver catalyst requires analysis of the gas at the reactor inlet and outlet; by comparing the set target concentration of ethylene oxide in the outlet gas with the actual analysis results, the reactor heating temperature is adjusted to ensure a stable concentration of ethylene oxide in the outlet gas). , in which ethylene and oxygen are directly oxidised to form ethylene oxide:

C₂H₄ + 0.5O₂ → C₂H₄O ethylene oxide (EO)

Side reactions occur simultaneously with this chemical reaction, as shown by the following equations:

C₂H₄ + 3O₂ = 2CO₂ + 2H₂O

C₂H₄O + 2.5O₂ = 2CO₂ + 2H₂O

The carbon dioxide produced by the side reaction is absorbed by a carbonate solution and removed from the recirculating gas. The reaction is as follows:

CO₂ + K₂CO₃ + H₂O → 2KHCO₃

During the above reaction process, methane is added to the recirculating gas as a stabiliser to raise the lower explosive limit and the permissible concentration of oxygen in the reactor feed. Furthermore, to achieve the optimum yield of ethylene oxide, the inhibitor dichloroethane is added to prevent the complete oxidation of ethylene to carbon dioxide. The ethylene glycol process utilises the hydration method, whereby ethylene oxide is converted to ethylene glycol through pressurised hydration:

C₂H₄O (ethylene oxide) + H₂O →→ 2C₂H₄(OH)₂ (ethylene glycol)

The ethylene oxide reactor is a key component of the entire plant and is also where the online analytical instruments (mass spectrometers) are concentrated.

The primary components in the reactor and the recirculating gas stream are C₂H₄, O₂, EO, as well as stabilisers such as CH₄, N₂ and the reaction-generated CO_₂.

EO oxidation is an extremely rapid reaction process; the mixture of ethylene and oxygen is flammable and explosive. Oxygen determines the selectivity of the catalyst, whilst EO is a chemical medium with a wide explosive limit. It is therefore necessary to maximise EO yield whilst ensuring concentrations remain below the lower explosive limit. Consequently, controlling the concentrations of C₂H₄, O₂ and EO is crucial for reactor safety and performance.

The oxygen concentration in the oxidation reactor is a crucial monitoring parameter. If the oxygen concentration is too low, the ethylene conversion rate decreases; if it is too high, although the ethylene conversion rate increases, the selectivity for ethylene oxide does not improve significantly; if the oxygen concentration exceeds the explosive limits, the plant faces serious safety risks.

Consequently, it is vital to analyse the oxygen concentration at the reactor inlet and outlet rapidly and accurately. The oxygen concentration in the oxidation reactor is generally required to be maintained at around 8.5 per cent; a concentration of 9 per cent reaches the lower explosive limit, making an explosion highly likely, whilst a concentration below 8 per cent impedes the normal progress of the reaction.

When the oxygen concentration exceeds the control limit, the interlock system activates an ultra-fast shut-off valve to cut off the oxygen feed line; the shut-off time is less than 0.2 seconds.

The ethylene oxide process in ethylene glycol production requires dynamic monitoring and involves multiple process points in closed-loop control; online analysers must therefore be fast, accurate and reliable.

Some manufacturers utilise 2–4 industrial chromatographs to analyse the concentrations of various components in the mixed gas at the reactor inlet and outlet, thereby calculating and controlling parameters such as ethylene oxide selectivity, ethylene conversion rate and explosion limits;

Some utilise 3–5 magneto-mechanical oxygen analysers with a response time of 6–10 seconds; three of these form a ‘take-two-out-of-three’ redundant system, with the analysis results fed into the DCS and ESD to implement emergency shutdown control in the event of process anomalies;

Two to four infrared gas analysers are used to monitor the ethylene content in the gas streams at the reactor inlet and outlet.

During operation, due to the limitations of the aforementioned analysers, not only is the maintenance workload for the sample handling systems and instruments substantial, but each maintenance procedure requires prior notification of process operators to deactivate the interlocks. This not only complicates routine maintenance but also poses a threat to the safe operation of the unit.

Process mass spectrometers can be used to accurately measure the components in the reactor inlet stream (feed plus recirculation) and multiple reactor outlet streams, rapidly analysing the inlet and outlet streams to calculate oxygen and carbon balances, ethylene conversion rates and efficiency, and provide data to maintain catalyst activity and prevent bed overheating.

The instrument analyses the concentrations of various components in the mixed gas at the inlet and outlet of the EO reactor, with a response time measured in seconds. After sample extraction, the sample must undergo filtration, dehydration, and pressure and temperature regulation before being fed into the mass spectrometer for analysis; the sample transfer line must be heated and insulated throughout the entire process.

The analysis results are transmitted to the Distributed Control System (DCS) for the calculation and control of ethylene oxide selectivity, ethylene conversion rate and explosion limits. By calculating and controlling multiple process parameters, the process mass spectrometer can maximise ethylene oxide (EO) yield.