Apr-2007

Olefins plant cracker gas compressor fouling

Monitoring and treating process gas compressors to prevent corrosion and fouling. Several examples of improved compressor performance through the utilisation of antifoulants are provided

Sheri Snider
Nalco Company

Article Summary

Fouling in a cracked gas compressor can negatively affect plant economics. Initial efficiency loss does not increase costs significantly if there is adequate turbine capacity; increasing the turbine speed prevents lost production. However, once the turbine is limited, any additional loss in efficiency results in reduced throughput and a dramatic increase in the total cost of operation (TCO). In addition to energy cost and production loss, a fouled compressor may require an unscheduled shutdown. Even before shutting down, a fouled machine is usually operated at higher-than-desired suction pressure, in order to maintain the desired discharge pressure. This affects unit economics; increasing the first-stage suction pressure increases the furnace pressure and lowers selectivity towards ethylene.

Compressor fouling locations
Ompressor fouling can occur in the inlet guide vanes, wheels, diffusers and balance line. Figure 1 reveals how fouling and wear in the labyrinth seals between the wheels can reduce stage efficiency. Labyrinth seals prevent gas from leaking from a higher-pressure wheel to a lower-pressure wheel. The teeth create turbulence and resistance, and as a result the gas is slowed. As the teeth become fouled or damaged, there is less resistance, so gas leakage increases. From a monitoring standpoint, this is observed as a loss in efficiency.

The stage intercoolers are typically designed with the cracked gas on the shell side where polymer may collect on the bundle at the inlet to the exchanger. Once polymer starts to collect at the inlet, the exchanger starts to act like a filter. Polymer deposition creates a pressure drop that lowers the overall performance of the compressor. This is typically observed in our monitoring as an increase in intercooler pressure drop (dP).

It is important to understand that fouling in a compressor may affect vessels upstream or downstream of the compressor (Figure 2). The liquids that are knocked out in the early stages typically are routed back to the quench system. Polymers may contribute to emulsions or fouling in the quench system. The liquids knocked out in the later stages are often routed to the light ends fractionation section, potentially increasing polymer deposition in these towers.

Process gas compressor fouling mechanisms – chemical mechanisms
Many papers discuss fouling mechanisms occurring in process gas compressors. Most agree on three mechanisms: free radical polymerisation, Diels-Alder condensation and the thermal degradation to coke.

In free radical polymerisation, monomers with reactive double bonds, such as butadiene, styrene, isoprene and vinyl acetylene react to make polymer. In a compressor, the reactive monomers from the gas phase dissolve (or diffuse) into liquid hydrocarbons that condense during compression. Once in the liquid phase, the reactive monomers may undergo free radical polymerisation.

Free radical polymerisation begins with the initiation step, in which a radical is formed via hydroperoxide decomposition (if there is an oxygen source, heat or a metal catalyst) or heat. Once the unstable radical forms, it will quickly react with a monomer, generating a new radical. The new radical continues to react with monomer (propagation). As the polymer chain grows, the molecular weight of the polymer increases until the polymer becomes insoluble. The polymer then lies on piping and process equipment (compressor wheel, discharge piping, etc), as discussed previously.

Fouling kinetics is beyond the scope of this discussion, but it is important to emphasise the relationship between temperature and rate of polymerisation. The relative rate constant of peroxide-initiated free-radical polymerisation increases exponentially with temperature, indicating a greater potential for compressor fouling at higher discharge temperatures.

Whether the polymer is formed via free radical mechanism or via Diels-Alder mechanism, over time the precipitated hydrocarbons will reduce to a coke-like (monomer) substance. The coke-like substance’s cyclic monomers continue to react with other cyclic, dienic or acetylenic monomers to form polynuclear aromatic (PNA) material. The PNA will dehydrogenate over time to a form a coke-like substance commonly found when compressors are opened for inspection.

In addition to these polymerisation reactions, there may be physical sources of fouling in the compressor. Many ethylene producers use wash oil to mitigate fouling in a compressor; but poor-quality wash oils may introduce impurities that may deposit in the compressor. If there is foaming or entrainment in the caustic tower, caustic may get carried into the next stage of compression, causing salt deposits. Corrosion can also affect compressor fouling. Byproducts of corrosion, such as iron oxide and iron sulphide, can deposit in the compressor and related piping and equipment.

Fouling severity matrix
Once the compressor fouling mechanisms are fully understood, it is easy to predict how changes in ethylene plant design or operation will affect compressor fouling. For instance, high cracking severity in the furnaces produces more fouling precursors, such as butadiene and styrene. Operating at production rates above design capacity results in higher compressor operating temperatures and therefore higher fouling rates.

In addition, the compressor is often an ideal place to return various recycle streams from polymer plants and refineries. However, these recycle streams may contain fouling precursors, oxygen or peroxides, or other components that can adversely impact compressor operation and fouling.

Nalco created a fouling severity matrix (Figure 3) to incorporate several factors contributing to process gas compressor fouling. As one may expect, four-stage process gas compressors in gas crackers tend to have the highest fouling potential.