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Evaluate Ammonium Chloride Corrosion Potential with Water Partial Pressure

In conjunction with the water dew point line, a simplified ammonium chloride corrosion chart has been developed to evaluate ammonium chloride corrosion potential at different temperatures with different water partial pressures.

Product Number: 51317--8960-SG
ISBN: 8960 2017 CP
Author: Huang Lin
Publication Date: 2017
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Ammonium chloride (NH4Cl) salt because of its hygroscopic characteristic can be very corrosive and often leads to heavy pitting and localized corrosion. Even in the absence of free liquid water at a certain high relative humidity ammonium chloride salts start absorbing water from the vapor phase and forming a concentrated NH4Cl aqueous phase. This concentrated liquid NH4Cl solution has a low pH and can be very corrosive causing pitting even in stainless steels and some nickel alloys. Hydroprocessing and catalytic reforming reactor effluent streams FCCU and coker fractionator overheads and crude tower overheads are typically subject to severe NH4Cl corrosion especially when the crude slates have high chloride contents.The potential of NH4Cl salt deposition has been related to NH3 and HCl partial pressure. With high amounts of NH3 and chlorides in the vapor phase a large amount of NH4Cl salt can form and might foul equipment. However this does not necessarily lead to corrosion because dry NH4Cl is not corrosive. The potential and severity of NH4Cl corrosion depends heavily on water availability. This paper attempts to answer the question how dry does the process need to be to avoid NH4Cl corrosion.At a certain temperature the threshold relative humidity for formation of concentrated NH4Cl solution is defined as critical relative humidity (CRH). Currently the CRHs of NH4Cl at elevated temperatures are not readily available in literature. Based on the solubility data of NH4Cl at elevated temperatures CRH of NH4Cl from 90 to 400 F is thermodynamically calculated and presented in this paper. For the convenient application in refinery environments CRH is converted to critical water partial pressure (CWP). Above CWP severe NH4Cl corrosion is expected to occur when NH4Cl salt forms. Based on water availability a simplified CWP chart is presented to evaluate the likelihood of NH4Cl corrosion at a temperature up to 400 F.

Key words: Ammonium Chloride Corrosion Chart, Ferrous Chloride, Deliquescence

Ammonium chloride (NH4Cl) salt because of its hygroscopic characteristic can be very corrosive and often leads to heavy pitting and localized corrosion. Even in the absence of free liquid water at a certain high relative humidity ammonium chloride salts start absorbing water from the vapor phase and forming a concentrated NH4Cl aqueous phase. This concentrated liquid NH4Cl solution has a low pH and can be very corrosive causing pitting even in stainless steels and some nickel alloys. Hydroprocessing and catalytic reforming reactor effluent streams FCCU and coker fractionator overheads and crude tower overheads are typically subject to severe NH4Cl corrosion especially when the crude slates have high chloride contents.The potential of NH4Cl salt deposition has been related to NH3 and HCl partial pressure. With high amounts of NH3 and chlorides in the vapor phase a large amount of NH4Cl salt can form and might foul equipment. However this does not necessarily lead to corrosion because dry NH4Cl is not corrosive. The potential and severity of NH4Cl corrosion depends heavily on water availability. This paper attempts to answer the question how dry does the process need to be to avoid NH4Cl corrosion.At a certain temperature the threshold relative humidity for formation of concentrated NH4Cl solution is defined as critical relative humidity (CRH). Currently the CRHs of NH4Cl at elevated temperatures are not readily available in literature. Based on the solubility data of NH4Cl at elevated temperatures CRH of NH4Cl from 90 to 400 F is thermodynamically calculated and presented in this paper. For the convenient application in refinery environments CRH is converted to critical water partial pressure (CWP). Above CWP severe NH4Cl corrosion is expected to occur when NH4Cl salt forms. Based on water availability a simplified CWP chart is presented to evaluate the likelihood of NH4Cl corrosion at a temperature up to 400 F.

Key words: Ammonium Chloride Corrosion Chart, Ferrous Chloride, Deliquescence

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