Corrosion control in ethylene plants
Treatment concepts for corrosion control in ethylene production by steam cracking of hydrocarbons
BERTHOLD OTZISK and ANDRE DE BACHE
Kurita Europe GmbH
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Ethylene is produced in very large quantities as the ‘building block’ for many petrochemical processes. Coke formation in the pyrolysis furnace and transfer line exchanger (TLX), or unwanted polymerisation reactions in the downstream parts of the plant, present serious problems. Fouling in distillation columns and reboilers and process gas compressors is common in most ethylene plants. Deposits are mainly formed by radical reactions of diolefins like butadiene or isoprene. Severe corrosion can also occur in various areas of the system, in addition to unwanted fouling. Powerful corrosion inhibitor programmes prevent or reduce the risk of acid attack, allowing for longer plant run times.
Erosion/corrosion can be observed in some areas where chemical treatment programmes unfortunately do not show much benefit. Areas affected by erosion/corrosion are the circulation lines from the oil quench, water quench, dilution steam generator (DSG) feed pump, process water stripper, and dilution steam condensate piping. Piping modifications or a good filtration system reduce the risk of erosion/corrosion. Aqueous corrosion caused by acidic components, on the other hand, can be treated very well with suitable chemical treatment programmes. Figure 1 shows typical areas of concern, where aqueous corrosion may damage equipment. Corrosion is usually caused by organic acids such as acetic acid, propionic acid, or carbonic acid, which are formed during cracking.
Particularly affected are the TLX, the primary fractionator overhead (oil quench system), water quench tower and DSG. Aqueous corrosion can be treated very well with suitable chemical treatment programmes. Oxygen scavengers, film-forming amines (FFA), and neutralising amines are commonly used for corrosion control. Oxygen scavengers bind the oxygen and prevent or reduce the risk of unwanted polymerisation reactions and corrosion attacks. Filming amines build up a very thin protective layer between the metal surface and acidic components, thus preventing or reducing the risk of corrosion. Neutralising amines (also called alkalising amines) neutralise the acids and raise the pH value to a higher level, thus counteracting acid attack.
Neutralising amines and filming amines are injected into the overhead system of oil quench towers. In water quench columns, mainly neutralising amines are needed. Neutralising amines and filming amines are dosed in the DSG system, depending on the corrosion conditions. Process conditions and the corrosive components present determine which neutralising amines are selected. Amine blends with different active materials and different boiling ranges are usually chosen for process water strippers so that the amines are active over a wide range at the bottom section and top section to react with the acidic components. In most cases, they are not inorganic neutralisers but blends of alkanolamines.
Kurita’s Cetamine Technology is used for corrosion protection on the water side. On the process side, additives from the Kurita CI series are used, which cover different neutralising amines or FFAs. The physical and chemical properties of the neutralising amines differ significantly and the choice of additive depends on the process conditions.
TLX and boiler system
On the water side of TLXs, high pressure steam is generated by recovering the energy of the cracking process characterised by high heat flux densities which makes the application of proper water and steam treatment essential (see Figure 2). Traditional treatment programmes, usually combining ammonia or alkalising amines with phosphate and dispersants, are frequently applied. Some plants additionally use reducing agents such as carbohydrazide (CHZ) or diethylhydroxylamine (DEHA) to keep the dissolved oxygen concentration in the lower ppb range to avoid any risk of oxygen corrosion.
Cetamine Technology, based on film-forming substances (FFS) combined with low volatile and high volatile alkalising amines, eliminates the need for reducing agents and inorganic treatment programmes such as phosphates or caustic. The FFS adsorbs onto metal/metal oxide surfaces to form a hydrophobic film or barrier, which prevents corrosion by stopping water and other corrosive agents from contacting the metal/metal oxide surface (see Figure 3). Furthermore, the thin film fosters the formation of a smooth and compact iron oxide layer.1
Once formed, the protective film remains intact in both wet and dry conditions, even after dosing has stopped. This offers significant potential benefits for the preservation of both drained and (partially) filled plants during a shutdown, especially for plants under a cycling mode of operation.
The technology of FFSs is now included in internationally accepted guidelines: The International Association for the Properties of Water and Steam (IAPWS) has published two Technical Guidance Documents on the three main FFA molecules that have been the subject of intensive research and where significant application experience is available.2,3
The active components of Cetamine products are volatile and protect the whole water/steam cycle. Investigations in a triple pressure heat recovery steam generator (HRSG) proved the presence of the protective Cetamine film on both water and steam touched surfaces, including high pressure evaporators, reheaters, and low pressure turbine cylinders.4 To demonstrate the presence of the protective Cetamine film on system surfaces, Kurita’s Cetamine Wipe Test can be applied on accessible system surfaces during routine plant inspections (see Figure 4). This is a semi-quantitative method to transfer the FFA molecules from a defined system surface into a liquid solution. Once the molecules have been transferred, they can be analysed with the Cetamine Photometric Method.
The high pressure steam system of an ethylene plant in The Netherlands was converted to a treatment programme based on film-forming amines in 2005, and finally converted to Cetamine Technology in early 2009. The former treatment programme that was carried out with hydrazine as a reducing agent and ammonia, as well as morpholine, has led to two main corrosion phenomena: flow-accelerated corrosion (FAC) in the pre-boiler and condensate system; and first condensate corrosion (FCC) in the condensate system. Both have led to the transport of iron oxides with subsequent fouling of the thermally highly loaded heat transfer surfaces in the TLXs with porous deposits. These have acted as concentration cells for non-volatile boiler water contaminants, resulting in boiler tube failures.
Since applying the FFA based treatment concept, the steam and condensate quality has significantly improved concerning cation conductivity and total iron concentration. Due to the improved condensate quality, the time between the regenerations of the cation exchangers in the condensate polishing unit (CPU) has almost doubled, leading to economical savings.5 The overall improved water and steam quality in the system, including decreased transport of iron oxides to heat transfer surfaces, has resulted in system adjustments. In the course of the FFA based treatment programme, the continuous blowdown of the high pressure steam cycle was reduced by 75%, representing significant water and energy savings.5 Inspections have shown TLXs and drums effectively protected against corrosion by a thin, uniform, adherent magnetite layer and turbines were exceptionally clean and free of corrosion.5
During treatment with FFAs, in addition to the standard parameters such as pH value, conductivity, cation conductivity, silica, and total iron, the active film-forming component needs to be monitored. This is carried out by a reliable photometer test in grab samples which is simple to apply on-site and is a suitable tool to avoid product overdosing.
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