ABSTRACT

In a Refinery Condensate Splitting Unit, the lighter Feed hydrocarbon is distillated into Liquified Petroleum Gas, Naphtha, Kerosene, Light & Heavy Gas Oil and Atmospheric Residue. Operating philosophy of this unit is similar to that of Crude Distillation unit but without a desalter. The Pre-flash Column has two stage overhead system, and the Main Fractionator column has Single stage overhead system. Perennial corrosion was experienced in the Main fractionator Overhead fin fan cooler tubes in the form of thinning/pitting due to HCl & Amine/Ammonium chloride salt corrosion. Ionic Equilibria modelling of the overhead systems was carried out to estimate the Ionic Dew point, Water Dew point and Salt point of the overhead systems. The modelling helped to identify that improper neutralizing amine (NA) injection location in Pre-flash overhead system, inadequate wash water flow, incompatible amine formulation and feed quality as the major contributor for the corrosion in the overhead system. Based on this study location of NA injection was modified, Wash water injection rate was optimized, and amine formulation was changed to have lower salting point. Finally, an Integrity Operating Window [IOW] was established to increase the reliability of the overhead systems in this unit.

INTRODUCTION

This twin train Condensate Splitting Unit (CSU) is basically an atmospheric distillation unit processing predominantly Middle Eastern condensate along with other non-system condensate. The products from this unit are Light/Heavy Naphtha, Kerosene, Light/Heavy Gas Oil and Atmospheric residue. There is no desalter in this unit and preheated feed is flashed in Pre-flash column to separate out light and medium naphtha. Subsequent distillation is achieved through Main fractionator column [C-002] along with side strippers.

Corrosion mechanism in CSU overhead systems is same as crude distillation unit and it starts primarily from the presence of hydrogen chloride (HCl) vapor present from the hydrolysis of salts in the condensate feed. The most common source of HCl is the hydrolysis of calcium chloride and magnesium chloride salt at temperatures exceeding 120°C (240°F) though the HCl may also come from decomposition of organic chloride species1. HCl evolution takes place primarily in preheat trains and furnaces. Dry hydrogen chloride, especially in the presence of large amounts of hydrocarbon vapor or liquid, is not corrosive. But HCl being a light volatile gas, moves in the crude unit overhead condensing systems where it is readily absorbed into condensing water. Stripping steam added to the bottom of the crude column to facilitate fractionation condense in the overhead system and dilutes the HCl. However, severe aqueous chloride corrosion of carbons steel components can occur, as per Eq.1, upon cooling at temperatures below the initial water dewpoint. The pH at initial dewpoint can drop below 2.

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