Dario Palacios - PDVSA
A summary of the findings from the examination of 17 different materials tested for 16 months in an atmospheric distillation of a crude upgrader complex located in Venezuela, which started in 2002 and was designed to produce 180.000 bpd of upgraded crude 32 API and 0.2 wt % S.
The feed to this crude upgrader is called diluted crude oil (DCO) consisting of a blend of extra heavy oil from the Orinoco Belt reservoir which is 8 API, 4.3 wt% S, 3.2 TAN and naphtha. The atmospheric distillation column (where the majority of the naphtha is extracted and sent back to the oil fields) has had some corrosion problems since the startup of the unit.
The top packing of the column has been severely damaged several times; two times when it was built with 410 Stainless Steel and another when the material of construction was AL6-XN. The investigations have revealed two main damage mechanisms competing: Salts Deposition and shock condensation (Hydrogen Chloride corrosion, Wet H2S and Light Organic Acids).
The 17 materials evaluated in this investigation were tested using 3 specially designed test plates. These test plates were located in 3 different positions inside the atmospheric distillation column in order to establish a relation between the column temperature profile and the location of the different damage mechanisms that were taking place into the column. The samples were visually examined visually examined by optical microscopy (OM) and by means of the scanning electron microscopy (SEM). Also, the samples were evaluated with respect to general thinning and to other non aged related damage mechanisms such as stress corrosion cracking.
The effect of process parameter variations on the different alloys tested is also discussed. It is believed that sharing the conclusions based on this crude upgrader experience will be of benefit for the material selection process in revamps or new projects associated to the overhead systems of the atmospheric distillation units operating for upgrading extra heavy crude oils worldwide.