NH₄Cl formation in Hydrotreating reactor effluents

Introduction

This article demonstrates how OLI Flowsheet: ESP can be used to model the formation of ammonium chloride (NH₄Cl) salts in the effluent from a hydrotreating reactor. Using OLI’s thermodynamic framework and electrolyte chemistry database, this example illustrates how temperature, water content, and hydrogen chloride concentration affect NH₄Cl salt deposition in process gas streams.

Overview of NH₄Cl Formation in Hydrotreating Units

Hydrotreating reactors operate under high pressure and temperature to remove sulfur, nitrogen, and oxygen contaminants from petroleum feedstocks. In the reactor effluent, ammonia (NH₃) generated from denitrogenation reactions can react with hydrogen chloride (HCl), formed from chloride-containing feed impurities, to produce solid ammonium chloride (NH₄Cl).

\( \mathrm{NH_3(g) + HCl(g) \rightleftharpoons NH_4Cl(s)} \)

NH₄Cl can deposit when the gas stream conditions cross its sublimation equilibrium, which is governed by both temperature and the partial pressures of NH₃ and HCl. This can occur during cooling and/or pressure changes, leading to fouling, corrosion, and plugging in equipment such as heat exchangers and air coolers.

OLI’s Approach to Predicting Salt Formation

OLI Flowsheet: ESP enables rigorous modeling of vapor–liquid–liquid–solid (VLLS) equilibrium in reactive gas mixtures, allowing prediction of NH₄Cl formation under process conditions.

The software accounts for:

• Multi-component reactive equilibria (NH₃, HCl, H₂O), with full speciation generated from defined inflows
• Formation of solid NH₄Cl from the vapor phase as well as ionic species (NH₄⁺, Cl⁻) in aqueous or deliquesced phases
• Temperature- and pressure-dependent phase behavior, including condensation and deliquescence effects

By simulating process cooling and phase transitions, OLI identifies the conditions (temperature, composition, and partial pressures) at which NH₄Cl first forms, enabling evaluation of fouling risk in downstream equipment.

Example Case Process Overview

Feed Composition and Process Conditions

A simplified reactor effluent composition typical of hydrotreating units was modeled as follows:

• Temperature: 350 °C
• Pressure: 30 bar
• Cooling range: 350 °C → 80 °C

Gas Cooling and Condensation Zone

The gas stream was cooled gradually in OLI Flowsheet: ESP using a Heat Exchanger UnitOp, followed by a Separator to quantify vapor, liquid, and solid phases at each temperature interval.

The simulation tracks:

• Dew point of water
• Incipient temperature of NH₄Cl solid phase
• NH₄Cl mass fraction in the condensed phase

NH₄Cl Solid Formation Prediction

As shown in the sensitivity analysis, NH₄Cl solid precipitation begins at approximately 300 °C under the modeled conditions. Below this temperature, the equilibrium shifts toward the formation of solid NH₄Cl.

Key trends:

• Increasing HCl concentration increases the NH₄Cl formation temperature. With an increase from 0.002 to 0.004 HCl mole fraction, the NH₄Cl formation temperature increases from 300°C to 310°C.
  
   
• At high pressures, the NH₄Cl formation boundary shifts to higher temperatures. At 75 bar:
  

Monitoring NH₄Cl Formation Using Relative Humidity Calculations

To complement the thermodynamic prediction of NH₄Cl solid formation, OLI Flowsheet: ESP users can employ the Calculator Block to monitor Relative Humidity (RH) within the process gas stream.

This approach, described in the Calculator Functionality article support.olisystems.com, enables users to dynamically compute RH using gas-phase temperature, pressure, and water content. Tracking RH helps determine whether local condensation conditions may promote NH₄Cl deliquescence or deposition in downstream equipment.

A Calculator Block can:

• Compute local RH using OLI’s vapor pressure properties.
• Track RH variation during cooling.
• Serve as a monitored variable in a Sensitivity Analysis or control strategy.

This method provides deeper insight into RH thresholds known to cause "wet salts" when crossed and complements the equilibrium prediction of NH₄Cl solids, allowing engineers to design mitigation strategies that maintain RH below deliquescence thresholds.

Key Results and Discussion

The OLI simulation enables quantitative analysis of:

• Salt deposition zones along the process flow.
• Effect of water wash on NH₄Cl suppression.
• Sensitivity to NH₃/HCl ratios, helping predict safe operation limits.

Such results support corrosion management strategies and heat exchanger design improvements by identifying temperature regions prone to NH₄Cl deposition.

Conclusion

This example demonstrates how OLI Flowsheet: ESP can effectively predict ammonium chloride salt formation in hydrotreating reactor effluents. The results highlight the importance of understanding equilibrium chemistry for preventing fouling and corrosion in refinery gas-cooling systems.

By simulating reactive gas-phase chemistry and solid formation behavior, engineers can optimize temperature control and water injection strategies to mitigate salt-related issues.

Disclaimers

The results presented in this example are based on OLI Flowsheet: ESP Version 12.5. User interfaces and output may differ across software versions due to ongoing product updates.