Modeling a Crude Distillation Unit (CDU) Overhead Example in OLI Flowsheet: ESP

Introduction

In crude distillation units (CDUs), the overhead system is highly susceptible to salt point corrosion, also known as ionic dew point corrosion. This occurs when ammonium chloride and amine hydrochloride salts form and deposit in the overhead piping and exchangers, leading to localized corrosion. Understanding and predicting the conditions under which these salts form is essential for mitigating damage and optimizing neutralizing amine selection and injection strategies.

OLI Systems offers robust modeling capabilities to accurately predict ionic dew points, assess corrosion risks, and optimize chemical selection and injection strategies. This article demonstrates how to model CDU overhead line corrosion using OLI Flowsheet: ESP and OLI Corrosion Analyzer.

Overview of Ionic Dew Point Corrosion

Salt point and ionic dew point corrosion occurs when ammonium chloride and amine hydrochloride salts condense out of the vapor phase at specific temperatures, leading to localized corrosion. Key contributing factors include:

• High chloride concentrations in the overhead system.
• Presence of amines (e.g., neutralizing, tramp, and steam amines) that react with HCl to form salts.
• High ammonia concentration, often introduced via slop streams or wild naphtha processing.
• Condensation of acidic aqueous phases, which accelerates corrosion.
• Incorrect amine injection rates, leading to either insufficient neutralization or excess salt formation.
• Poor tramp amines control, originating from both internal sources (e.g., amine units, boiler feedwater) and external feedstock contaminants (e.g., crude oils treated with triazine-based H₂S scavengers).

Accurate prediction of the ionic dew point temperature of various amine hydrochloride salts is essential. This enables informed selection and validation of neutralizing amine blends and helps fine-tune injection rates, both of which are vital to minimizing corrosion risk and maintaining asset integrity in corrosive service environments.

OLI’s Solution for Predicting and Mitigating Corrosion

OLI’s Mixed Solvent Electrolyte (MSE) thermodynamic framework accurately calculates ionic dew points for 23 neutralizing amine hydrochloride salts, both as single compounds and in mixed systems. Additionally, OLI Flowsheet: ESP provides the following capabilities:

• Reconstructs the overhead vapor and liquid composition (hydrocarbon vs aqueous phase).
• Determines required wash water rates to dissolve deposited salts.
• Assesses conditions for NH₄Cl (ammonium chloride) salt formation
• Estimates dew point pH, 1 wt% and 5 wt% condensed water pH.
• Calculates ionic dew points.
• Calculates vapor stream relative humidity (RH to see if NH₄Cl will get wet, triggering the localized corrosion risk)

These tools enable data-driven decision-making to prevent overhead line corrosion while maintaining operational efficiency.

Example Case Process Overview

Process Conditions

For this case study, we model a CDU overhead system with the following conditions:

Naphtha Flow and Characterization

• Flow rate (naphtha product + reflux): 74.0297 tonne/hr
• Assay method: ASTM D86
• Specific gravity (SG): 0.7084

Water Flow and Characterization

• Total water in (steam + BS&W): 3577.07 kg/hr
• Chloride (Cl⁻): 17.4 ppm & 25 ppm
• Ammonia (NH₃): 5.2 ppm & 25 ppm
• Sulfides: 5 ppm
• pH: 4.3 (25 ppm Cl⁻, 25 ppm NH₃, no neutralizer)

Offgas Flow and Composition

• Total flow: 187.814 kgmol/hr

Example Video

Below is a video of setting up the CDU Overhead Example base case.

Timestamps:
0:13 Set units
0:42 Specify components
1:01 Add the naphtha assay
1:20 Add the streams and blocks
1:42 Specify streams
3:33 Specify initial blocks
4:07 Running the initial streams and blocks
4:27 Adding the remaining blocks and streams
5:37 Setting overhead conditions
5:48 Adding some neutralizer
6:50 Adding the condenser and accumulator drum
7:56 Running the simulation
8:08 Adding a callout
9:44 Adding manipulators and controllers to match drum water contaminant ppm
17:41 Adding a manipulator and controller for the neutralizer

Water Wash Calculation

A key operational benchmark for effective water wash is ensuring that a minimum of 25% by weight of the injected wash water condenses into the free aqueous phase (Liquid-1) downstream of the injection point. This threshold is considered best practice for mitigating corrosion and salt deposition risks. To assess compliance with this criterion, a Calculator Block is implemented within the simulation environment to compute the ratio of the condensed water phase mass (Liquid-1) to the total mass of wash water introduced into the system.

WashWaterMassIn:

WaterMassOut:

A sensitivity analysis is used to vary the wash water in to see how much water condenses.

It shows that ~143 L/min are required for 25 wt% water to condense. This is the result we get when using  feedback and feed-forward controllers to determine the required water wash.

Water wash feedback controller specifications:

Water wash feed-forward controller specifications:

Water wash calculation result:

NH₄Cl Salt Formation Temperature

A sensitivity analysis can be done on the overhead steam upstream of the neutralizer to determine the salt formation temperature of ammonium chloride.

The survey shows that NH₄Cl starts forming ~96°C.

A feedback controller can help narrow down at which temperature a small amount (1E-5 kgmol/hr) of the ammonium chloride salt forms.

Condensed Water pH (1 wt% water dew point T and 5 wt% water dew point T)

Feedback and feed-forward controllers can help us determine the 1% and 5% water dew point temperatures and pHs.

For 1% water dew point:

For 5% water dew point:

We see that the 1% water dew point temperature is 85.1°C and pH is 4.8. The 5% water dew point temperature is 84.6°C and pH is 5.1.

Ionic Dew Point Temperature (MEA Example)

A feedback controller and a sensitivity analysis can be used to help us determine at which temperature we may start forming the first droplets of a highly concentrated ionic liquid phase (OLI nomenclature: Liq-1) as we cool down the overhead vapors. This will be the ionic dew point for the MEA.HCl salt as this liquid-1 concentration is primarily the MEA (60+%) and chloride (35+%). It is important to note that this ionic liquid (molten salt) is not to be regarded as a pure substance, as it may incorporate a variety of water-soluble acid-base components.

The feedback controller is specified to adjust the temperature until it achieves a small amount (0.1 kg/hr) of liquid-1.

It shows the temperature is 153°C.

The controller can be tightened to find a slightly higher temperature when a smaller amount of liquid-1 forms, such as 0.001 kg/hr.

A sensitivity analysis can also be used; it shows us the liquid-1 starts appearing as the stream cools to 157°C.

The component compositions need to be specified in the Monitored Variables tab.

We can then view this in sensitivity analysis plot.

Further Analyses and Considerations

• A calculator block can be set up to calculate the relative humidity of any stream, such as a vapor stream, to see if NH₄Cl will get wet
• Identify risk of amine chloride stress corrosion cracking (ACSCC)
• Identify risk of chloride stress corrosion cracking (ClSCC)
• OLI Studio: Corrosion Analyzer can be used to calculate corrosion rates for any fully developed aqueous phases against carbon steel and other alloys.

Conclusion

OLI’s modeling tools provide critical insights into CDU overhead system corrosion. By leveraging OLI Flowsheet: ESP and OLI Studio: Corrosion Analyzer, engineers can:

• Accurately predict ionic dew points, salt formation temperatures, and water dew points and pHs.
• Optimize water wash and amine injection strategies.
• Assess corrosion risks to extend asset life.

This data-driven approach enables proactive corrosion management, reducing unplanned downtime and improving refinery reliability.

For further assistance, contact OLI Systems support or refer to our knowledge base.

More information regarding ionic dew points and refinery modeling solutions can be found here:

• Minimizing corrosion with ionic dew point process modeling to optimize downstream refinery operations - blog
• OLI System’s corrosion tools enhance reliability of assets - blog
• Modeling the Chemistry of Amines and Amine Hydrochlorides for Predicting Corrosion in Refinery Overheads - blog
• Challenging Historical Safety Factors in CDU Overhead Operations with MSE Automated Ionic Modeling - blog
• Refinery overheads salt point using OLI software - spotlight video
• Downstream refinery consulting services