Crystallization Behavior of Ammonium Chloride in High-Pressure Heat Exchanger of Hydrotreating Unit

Abstract

In view of the corrosion failure of a high-pressure heat exchanger in a diesel hydrogenation unit, the formation mechanism of ammonium chloride in a multiphase flow system is investigated in this article. Numerical simulation is carried out by user defined function (UDF) on the process of adding source of mass transfer in computational fluid dynamics (CFD) solvers. The distribution characteristics of ammonium chloride are illustrated by the parameters including crystallization temperature of ammonium chloride, volume fraction of ammonium chloride, and mass transfer rates of NH₃ and HCl, and the causes of corrosion cracking in the U-shaped bend of the heat exchanger are discussed. The results show that there is a great risk of ammonium chloride deposition in the heat exchanger from 4.5 m away from the outlet of the second pass. The crystallization area in the tube gradually expands from the wall to the center along the flow direction, and the crystallization rate is higher near the tube wall. The field sampling test results show that the corrosion cracking is hydrogen-induced cracking, which is due to the existence of large amount of hydrogen, high impacting force, excessive flow rate, and the risk of ammonium chloride particle erosion at the U-bend. In order to alleviate the corrosion of ammonium chloride deposition, some improvement measures are put forward, such as raising the inlet temperature of the tube side to 215 °C and increasing the water injection by 30%, which play an important role in decreasing the formation of ammonium chloride in the heat exchange system.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The petrochemical industry is one of the pillar industries for national economy and security, and the hydrogenation device is the core equipment of oil refining industry. With the increasingly diversified, heavy, and inferior sources of international crude oil, the content of sulfur, nitrogen, and chlorine in the raw oil is increasing. In the procedure of crude oil processing, due to the fluid-containing corrosive substance and the variations in the operating conditions, the heat exchangers are prone to form ammonium salt, causing tubes corrosion thinning and perforation failure. Moreover, there are also tubes explosion accidents, which seriously affect the safety of production, making it a common problem in the chemical industry [1–3].

The problem of ammonium salt crystallization generally exists in the process devices of the hydrogenation unit, especially in the heat exchange system. It is reported frequently that there appear various erosion-corrosion of ammonium chloride in the high-pressure heat exchanger in the diesel hydrogenation unit. However, the understanding of the ammonium chloride crystallization process is really poor, which is mainly reflected in the following aspects. Ammonium chloride crystallization and salt formation are two different concepts. Crystallization is an extremely tiny process: the precipitation process of NH₄Cl particles in crystal form generated by the gaseous reaction of HCl and NH₃ under operation conditions. Salt formation is the accumulation of a large amount of NH₄Cl crystals into a visible white or yellow solid [4–6]. At present, no research has been performed on the internal mechanism of ammonium chloride crystallization, nor on where to crystallize first inside. Most of the research studies on the corrosion failure analysis of hydrogenation equipment is to carry out corrosion classification on the surface, which is far clear away from the mechanism of the crystallization behavior, and there is no real in-depth study on the ammonium chloride crystallization behavior. To prevent the deposition of ammonium chloride, water washing systems are commonly used in hydrogenation units. However, the water injection method and the amount of water injection are disconnected with ammonium chloride corrosion, which does not essentially indicate that water injection can alleviate the corrosion process [7, 8].

The erosion-corrosion phenomenon in the multiphase flow is mainly divided into two types: gas-solid two-phase erosion-corrosion and liquid-solid two-phase erosion-corrosion [9–11]. As for the crystallization behavior of ammonium chloride, the gas phase is concentrated in the tubes located at the upper part of the heat exchanger. And the formation of ammonium chloride is a rapid gas-phase reaction, which is first formed in the upper region within the tubes. Therefore, the gas phase and ammonium chloride particles filled in the upper layer of the tubes are typical gas-solid two-phase erosion-corrosion. When ammonium chloride continuously spreads in the upper tubes and falls to the bottom of the tubes, it forms a typical liquid-solid erosion-corrosion with the liquid phase in the lower layer. The crystallization process of ammonium chloride includes both gas-solid erosion-corrosion and liquid-solid erosion-corrosion. In addition, since the crystallization reaction of ammonium chloride is affected by complicated coupling of temperature field, flow field, and composition field, the transformation mechanism is more complex and difficult [12]. The researchers mainly qualitatively describe the failure mechanism through corrosion characterization parameters combined with experimental simulation. There are few quantitative studies on the crystallization process of ammonium chloride in the heat exchange system, and there are few studies on the behavior of ammonium chloride in the tubes [13–17].

In this article, the corrosion mechanism and mathematical prediction model of the corrosion site of the tubes are constructed based on the actual process to investigate the problem of crystallization and corrosion of high-pressure heat exchangers. As for the formation process of ammonium chloride, the transformation mechanism of ammonium chloride in a multiphase flow system is proposed. Based on the thermodynamic phase diagram of ammonium chloride crystallization, the user defined function (UDF) is compiled into numerical simulation software Fluent. The formation conditions and distribution characteristics of ammonium chloride are studied, and the corrosion-erosion mechanism of the heat exchanger is analyzed by using the crystallization temperature, volume fraction, and formation rate of ammonium chloride as characteristic parameters. The flow field, temperature field, and composition distribution in the high-pressure heat exchanger are explored. Combined with the on-site corrosion situation and sampling test results, the causes of U-bend corrosion cracking are analyzed. Further on, specific improvement measures are put forward to decrease the crystallization of ammonium chloride and reduce the occurrence of ammonium chloride corrosion failures.

2. Failure Analysis

2.1. The Process Flow

The diesel hydrogenation process involves seven high-pressure heat exchangers and air coolers, as shown in Figure 1. The raw oil of the device is catalytic diesel oil and straight-run diesel oil, with the processing capacity of 150 t/h. The content of sulfur, nitrogen, and chlorine in the raw oil is 301.01 ppm, 640.95 ppm, and 1.46 ppm, respectively. The reactants flow into R1101 for diesel hydrogenation reaction, and the reaction outflow flows into three heat exchangers E1103 A/B/C through E1101 and E1102 heat exchangers and then flows out of the air cooler A1101. The flow in the tubes of the heat exchanger enters from the top and flows out from the bottom. The shell side flows in the opposite direction. To prevent the formation and deposition of ammonium chloride, two intermittent water injection points are set: one is injected before heat exchanger E1103 A and the other is injected before air cooler A1101. The operating temperature and pressure of tube and shell sides of heat exchanger E1103 B are shown in Table 1.

[figure omitted; refer to PDF]

Table 1

Operating conditions of heat exchanger E1103 B.

	Inlet temperature/°C	Outlet temperature/°C	Outlet pressure/MPa
Tube side	176	147.3	7.1
Shell side	122.5	153	8.9

During the operation, the high-pressure heat exchanger E1103 B suddenly suffered internal leakage. E1103 B heat exchanger is a typical horizontal two-way U-shaped tube-and-shell heat exchanger, its model is DFU1300–7.9/9.42–598–6.4/19–2/2I, the material of the tube is S32205, and the shell is 15CrMoR with surfacing welding.

2.2. Field Failure Analysis

After the internal leakage of high-pressure heat exchanger E1103 B, the tubes were inspected and it was found that there were circumferential fractures at the U-shaped bend, white scaling at the outlet of the tubes, and different small cracks on the tube sheet. The U-bend fracture, salt crystallization in the tubes, and tube sheet crack are shown in Figures 2–4. The heat exchanger tubes were checked with an endoscope, and it was found that significant deposits of ammonium chloride appeared in some tubes.

[figure omitted; refer to PDF]

The crack analysis of the fracture site of the heat exchange tube was carried out. The metallographic analysis found that the metallographic structure was austenite, martensite, and ferrite, and the outer wall structure was coarser than the inner wall. There were microcracks that penetrate and propagate outward along the thickness of the tube wall. There were smooth and straight microcracks on the tube surface, and there were partial bifurcations, which were typical hydrogen-induced cracking characteristics, accompanied by stress corrosion cracking characteristics [18]. The analysis by scanning electroscope and energy spectrum found that there existed many corrosion products on the surface of the break, of which Cl, O, and S contents were higher. The fracture in the interrupt area was quasi-solution fracture characteristic with claw torn edges. The spectrum analysis showed that the content of Ni and Ti in cracked heat exchange tubes was lower than the technical requirements of GB/t13296-2007. In conclusion, the heat exchanger produced a large amount of hydrogen-induced martensite in the acidic environment, and the microstructure deterioration, and hydrogen-induced cracking [19, 20]. Part of the white scale was analyzed, showing that was a large amount of ammonium salt scale compounds. It can be concluded that the main composition of the white scale deposited in the tubes of the heat exchanger was ammonium chloride.

2.3. Crystallization Analysis of Ammonium Chloride

2.3.1. Ammonium Chloride Salt Transformation Mechanism

The heat transfer process and the composition transfer and transformation mechanism in the multiphase flow system are extremely complex. With the heat transfer process, the temperature distribution in the heat transfer system changes, and as the change of the temperature field, the transformation of the composition in the gas phase of the multiphase flow system will be triggered. The raw oil contains a large amount of nitrogen, chlorine, and other elements, which will be converted into the corrosive media HCl and NH₃ during the hydrogenation reaction. In the continuous heat exchange process, due to the large specific heat capacity of the gas, the upper temperature of the tubes first decreases to the crystallization temperature of NH₄Cl, and then, rapid gas reaction will occur between HCl and NH₃ under a certain pressure to generate NH₄Cl and precipitate out. The equation is as follows:$$\begin{array}{}\text{(1)}& {\text{NH}}_3\text{g}+\text{HCl}\text{g}⟶{\text{NH}}_4\text{Cl}\text{s}\end{array}$$

Under the action of gravity, buoyancy, and drag force, some of the generated NH₄Cl particles will fall down and deposit at the bottom of the tube, whereas others will attach to the top of the tube and continue to spread and block the tube. The formation process of ammonium salt in multiphase flow system is shown in Figure 5. Because the chlorinated ammonium salts are hydraulics and easily to absorb water, API 571 states that small amounts of water can cause severe corrosion. Ammonium chloride is highly water-soluble and corrosive, forming acid solution when mixing with water and forming acid corrosion environment [18]. The deposited ammonium chloride is attached to the inner wall of the tube, which will gradually corrode and destroy the metal oxide film, resulting in corrosion thinning [21, 22]. The deposited ammonium chloride is usually dissolved into the solution by the injection of water. There are two ways to fill the water continuously and intermittently, but the amount of water injection still needs to be carefully discussed.

[figure omitted; refer to PDF]

According to the composition distribution of heat exchanger E1103 B under actual operating conditions, the thermal decomposition constant of ammonium chloride at the inlet of tubes is calculated and the crystallization equilibrium temperature of ammonium chloride is 196.07 °C. However, the inlet temperature of 176 °C is lower than the crystallization temperature. Under this condition, ammonium chloride is produced when the fluid flows into tubes of the E1103 B heat exchanger.

3. Numerical Simulation

3.1. Governing Equations

The flow process of the fluid medium in tubes of the Heat exchanger obeys the law of continuity equation, momentum equation, the equation of conservation of energy, and the equation of conservation of compositions [24]:

(1) Continuity equation$$\begin{array}{}\text{(3)}& \frac{\partial \rho }{\partial t}+\frac{\partial \rho u}{\partial x}+\frac{\partial \rho v}{\partial y}+\frac{\partial \rho \omega }{\partial z}=0,\end{array}$$where $\rho$, $t$ is the density and flow time of the fluid, and $u$, $v$, and $\omega$ are vector velocity values in three directions x, y, and z, respectively.

(2) Momentum equation$$\begin{array}{c}\text{(4)}& \frac{\partial \rho u}{\partial t}+\text{div}\rho uu=-\frac{\partial p}{\partial x}+\frac{\partial {\tau }_{xx}}{\partial x}+\frac{\partial {\tau }_{yx}}{\partial y}+\frac{\partial {\tau }_{zx}}{\partial z}+F_x,\frac{\partial \rho \nu }{\partial t}+\text{div}\rho u\nu =-\frac{\partial p}{\partial y}+\frac{\partial {\tau }_{xx}}{\partial x}+\frac{\partial {\tau }_{yx}}{\partial y}+\frac{\partial {\tau }_{zx}}{\partial z}+F_y,\\ \frac{\partial \rho \omega }{\partial t}+\text{div}\rho u\omega =-\frac{\partial p}{\partial z}+\frac{\partial {\tau }_{xx}}{\partial x}+\frac{\partial {\tau }_{yx}}{\partial y}+\frac{\partial {\tau }_{zx}}{\partial z}+F_z,\end{array}$$where ${\tau }_{xx}$, ${\tau }_{yx}$, and ${\tau }_{zx}$ are the values of the viscous stress components in different directions, $P$ is the pressure in the multiphase flow medium, and $F_x$, $F_y$, and $F_z$ are the forces on the micro-elements of the medium, respectively.

(3) Energy conservation equation$$\begin{array}{}\text{(5)}& \frac{\partial }{\partial t}{\rho }_m{h}_m+\nabla ·{\rho }_m{h}_m\stackrel{⟶}{V_m}+\nabla ·P\stackrel{⟶}{V_m}=\nabla ·k_m\nabla T,\end{array}$$where $\hspace{0.17em}{\rho }_m$ is the density of the mixture, $h_m$ is the heat of the mixture, $k_m$ is the thermal conductivity of the mixture, $V_m$ is the velocity of the mixture, and $P$ is the pressure of the fluid [25].

(4) Composition conservation equation$$\begin{array}{}\text{(6)}& \frac{\partial }{\partial t}\rho m_1+div\rho u{m}_1+J_1=R_1,\end{array}$$where $m_1$ is the composition of chemical component 1, $R_1$ is the generation rate of chemical composition 1 at unit volume, and $J_1$ is the diffusion flow density. The diffusion flow density $J_1$ is expressed by Fick's diffusion law:$$\begin{array}{}\text{(7)}& J_1=-{\Gamma }_1\text{grad}{m}_1,\end{array}$$

  where Γ is the diffusion coefficient. The two formulas are combined as follows:$$\begin{array}{}\text{(8)}& \frac{\partial }{\partial t}\rho m_1+\text{div}\rho u{m}_1=\text{div}\Gamma \text{grad}{m}_1+R_1.\end{array}$$

3.2. Geometric Models and Meshing

A three-dimensional geometric model is constructed according to the actual structure of heat exchanger E1103 B. In the light of the symmetry of the structure, half of the heat exchanger is selected as the calculation domain. The mesh is based on a hexagonal fit mesh, combining a triangular mesh with a mixed mesh model of a four-sided mesh. The grid scale reaches 14.91 million after merging the grids of each part with HYPERMESH. The accuracy of this grid is tested, and the obtained tube and shell side outlet temperatures are compared with the actual temperatures. The error is 1.03% shown in Table 2, which meets the requirements of calculation accuracy. The geometric model and mesh division of heat exchanger E1103 B are shown in Figures 7 and 8.

Table 2

The grid accuracy test.

	Inlet temperature/°C	Calculate outlet temperature/°C	Actual outlet temperature/°C	Error/%
Tube side	176	146.9	147.3	0.27
Shell side	122.5	154.6	153	1.03

[figure omitted; refer to PDF][figure omitted; refer to PDF]

3.3. The Boundary Conditions

Since the internal fluid medium of heat exchanger E1103 B tubes contains multiphase, gas phase, oil phase, water phase, and ammonium chloride phase, interspersed between composition, and has a composition transfer and transformation mechanism, and the Mixture model is used for numerical simulation [26]. Wherein the gas phase is set to the main phase, the others are the secondary phase in turn. The turbulence model adopts shear-stress transport (SST) k-Omega model [27]. Considering interphase mass transfer, the UDF of the reaction of HCl and NH₃ in the gas phase to NH₄Cl in the solid phase is loaded. No slip boundary condition is adopted for the wall surface, pressure and velocity coupling is phase-coupled simply, and a first-order upwind solver is used for momentum, volume, turbulent kinetic energy, and turbulent dissipation rate [28]. Mass flow inlet and pressure outflow are imposed on the inlet and outlet boundary conditions of the heat exchanger, respectively. And the physical parameters of the medium are shown in Table 3 and 4.

Table 3

Physical parameters of the tube-side medium.

	Density/(kg/m3)	Viscosity/(Pa s)	Thermal conductivity/(W/m k)	Specific heat/(J/kg K)
Diesel	873.7	0.006566	0.1237	2135
Gas-phase mixture	Ideal gas model	1.16E-05	0.254	14569

Table 4

Physical parameters of the shell-side medium.

	Density/(kg/m3)	Viscosity/(Pa s)	Thermal conductivity/(W/m k)	Specific heat/(J/kg K)
Diesel	873.7	0.006566	0.1237	2135
Hydrogen	Ideal gas model	1.16E-05	0.254	14569

4. Results and Discussions

In order to explore the crystallization behavior of the ammonium chloride in the tubes of the heat exchanger and the cause of U-bend cracking, the mechanism of ammonium chloride crystallization will be discussed, and the high-risk corrosion-erosion location of the heat exchanger will be analyzed.

The overall temperature distribution along the tube of the heat exchanger is shown in Figure 9. The distributions of different phases in tubes of the heat exchanger, such as H₂ mass fraction, HCl mass fraction, NH₃ mass fraction, water volume fraction, and oil volume fraction, are shown in Figure 10. As can be seen from Figure 9, the temperature of the multiphase fluid in tubes decreases gradually along the flow direction. When the fluid flows out of the tube outlet, the temperature drops from 176 °C to 146.9 °C. The heat transfer process between tubes is far different from each other. Generally, the temperature outside the tube is lower, and the temperature in the center area is higher. The area with lower temperature is concentrated in the second pass, and the risk of ammonium chloride crystallization is higher in the second pass of the heat exchanger.

[figure omitted; refer to PDF]

Due to the cooling effect of the shell-side fluid in the tube side, the temperature near the inner wall of tubes is low. If there are HCl and NH₃ gas compositions, it is possible to form ammonium chloride. In order to explore the variation in the ammonium chloride crystallization region in a single tube, a tube with a temperature of 166 °C on the inlet of the tube side is selected for analysis. Figure 14 show the variation of isotherm at 166 °C along the flow direction in the selected tube. It can be seen that the 166 °C isotherm gradually shrinks to the center of the tube along the flow direction. It means that the low-temperature area gradually increases, and the crystallizing area becomes larger. When the crystallization area extends to the center of the tube, the tube will be blocked.

[figure omitted; refer to PDF]

In order to carefully investigate whether the ammonium chloride generated can be dissolved by the injected water with the improvement measures, the amount of ammonium chloride that can be dissolved is deduced according to the solubility of ammonium chloride in water and then compared with the real amount of ammonium chloride generated, to see whether the improvement measures are effective. As shown in Figure 18, when increased by 19 °C, the amount of ammonium chloride generated in the first pass is less than that which can be dissolved. The amount of ammonium chloride generated in the first pass can be dissolved by water. The amount of ammonium chloride generated in the second pass is greater than the amount dissolved; that is, the amount of ammonium chloride generated in the second pass cannot be completely dissolved.

[figure omitted; refer to PDF]

After increasing the injection amount by 30%, the amount of ammonium chloride generated and dissolved along the flow direction of the tube at the lowest temperature is shown in Figure 21. As can be seen from Figure 21, the amount of ammonium chloride produced in the first and second passes is lower than the amount dissolved. When the water injection is increased by 30%, the ammonium chloride will be dissolved completely.

[figure omitted; refer to PDF]

The volume fraction of ammonium chloride generated by the lowest temperature tube after the improved measures is compared with that under standard conditions: when increasing the inlet temperature to 195°C, the amount of ammonium chloride decreased to 1.06%; when heating up to 215°C, its ammonium chloride volume fraction is reduced to 0.94%; and when the injection water increased by 30%, ammonium chloride volume fraction reduces to 1.16%. By comparison, the best effect is to increase the inlet temperature to 215 °C, which greatly reduces the production of ammonium chloride and ensures the continuous and safe operation of the heat exchanger.

6. Conclusions

(1 )Aiming at the problem of ammonium salt crystallization of the high-pressure heat exchanger in hydrogenation unit, the potential corrosion risk position under the practical flow condition is studied. Through computational fluid dynamics simulation analysis, the causes of multifactor corrosion are determined and propose the methods of corrosion risk analysis, prediction, prevention, and control.

(2) The internal temperature field and concentration distribution in the high-pressure heat exchanger are simulated numerically, and the crystal corrosion of ammonium chloride is calculated by using UDF. The ammonium chloride corrosion of heat exchanger tubes is predicted by using the key parameters such as ammonium chloride crystallization temperature, ammonium chloride volume fraction, and mass transfer rate of NH₃ and HCl. The risk of ammonium chloride deposition is higher after 1.7 m of the second pass.

(3) The crystallization behavior of ammonium chloride in a single tube is explored: along the flow direction, the crystallization area gradually expands from the inner wall of the tube to the center, and the inner wall of the tube has a higher crystallization rate than other locations due to low temperature.

(4) For the U-bend corrosion cracking, the results of field inspection and sampling analysis show that the corrosion cracking was hydrogen-induced cracking. Through the analysis of the flow structure of the multiphase body and the distribution of gas and liquid in the heat exchanger, the reasons for the cracking are as follows: the excessive hydrogen content in the bend, high impact force, excessive flow velocity, and erosion risk of ammonium chloride particles.

(5) In order to reduce the phenomenon of ammonium salt crystallization after water injection under actual production conditions, the improvement measures of raising the temperature to 195 °C and 215 °C and increasing the water injection volume by 30% are put forward. By comparison, the effect of increasing the inlet temperature of the tube to 215 °C is more obvious, and the amount of crystallization is reduced the most.

Acknowledgments

The authors acknowledge the financial support of the National Key Technology R & D Program of China (2015BAK39B02) and National Key Science and Technology R & D Plan (2021YFB3300133).

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