Niu Luna; Han Lei; Chen Wenwu; Pan Long; Zhang Yanling; Qu Dingrong

(State Key Laboratory of Safety and Control for Chemicals, SINOPEC Research Institute of Safety Engineering, Qingdao 266101)

Abstract: In order to optimize the atmospheric tower overhead low-temperature system, the physical parameters, multiphase composition, aqueous dew point temperature, and ammonium salt crystallization temperature are simulated with process simulation software. The temperature distribution in overhead heat exchanger is calculated by heat transfer calculation. The special parts with elbows near the inlet and outlet of heat exchanger are studied by fluid field analysis. Results indicate that under current operating conditions, the aqueous dew point temperature and initial crystallization temperature of NH4Cl are 91 °C and 128 °C, respectively. Ammonium salt appears in the distillation tower and liquid water occurs in heat exchanger tubes, in which the dew point induced corrosion is the most direct factor for heat exchanger corrosion. In the heat exchanger,condensate water appearing in the area 2.7 meters away from the bundle inlet can give rise to corrosion risk under the moist NH4Cl and high concentration of acidic solution circumstance. For the pipes and elbows located near the inlet and the outlet of heat exchanger, the flow field presents an unsymmetrical distribution. High risk areas are mainly concentrated on the external bend of elbows where the liquid water concentration is higher. The coupling of simulation methods established thereby is approved as an effective way to evaluate the corrosion risk in the atmospheric column overhead system and can provide a scientific basis for corrosion control.

Key words: atmospheric tower overhead; simulation; corrosion; ammonium salt crystallization; fluid field

1 Introduction

In recent years, the deterioration of crude oil quality and harshness of operating conditions have become serious, which brings about great challenges to the safety and stability of the petroleum refining units[1-3]. As the first process of petroleum refining, impacts on crude distillation unit are even more severe[4]. Frequent failures,such as dew-point corrosion, erosion and ammonium salt blockage[5-6], which are particularly critical and difficult to control, always occur in overhead pipelines or heat exchanging systems. Compared with corrosion in high-temperature operating circumstances, the mechanism and interfering factors of atmospheric tower overhead system are more complicated[7-9]. Therefore,studying the corrosion and deposition characteristics of multicomponent and forecasting the corrosion risk by means of simulation model are suitable and feasible.

Current researches have been done on the anti-corrosion techniques for tower overhead low-temperature system.Wang[10]analyzed the mechanism and main factors of low temperature HCl dew point corrosion, and studied the corrosion laws of four typical materials with working conditions containing different HCl concentrations.By comparing the electrochemical corrosion rates and morphology of corroded S32101 duplex stainless steel and 410 martensitic stainless steel in specific concentrations of HCl acid, Roland[11]found that S32101 duplex steel displayed stronger resistance to corrosion,and the stable electrochemical behavior making it more suitable for application at crude distillation overhead system as compared to 410 martensitic stainless steel due to its higher percentage composition of chromium, nickel and other alloying elements. Han, et al.[12]established theoretical models for two main corrosion mechanisms of overhead system and calculated parameters such as the aqueous dew point, the water injection rate, and the ammonium chloride formation temperature. Zhang, et al.[13]analyzed the influencing factors of overhead system and optimized the reflux process, which improved the level of anti-corrosive control of the plant. Fu[14]studied the corrosion perforation problem at the finned tubes of atmospheric column air cooler. By analyzing the corrosion status and studying the electronic microscopic corrosion morphology, the failure reasons and control methods,such as rational allocation of crude oil, utilizing the spraying nozzle for water injection, material upgrading,and corrosion monitoring, were proposed. However, these researches are concerned with post-prevention and control based on mechanism analysis, corrosion monitoring,laboratory experiments or manual experience, which are lacking in intelligent prediction and scientific anticorrosion design.

Ou’s team[15-18]carried out a series of researches to study the corrosion characteristics by using numerical simulations, including the fluid flow model, the particles tracking model, and the heat transfer model. But they focused more on hydrogenation reaction effluent air cooler system, and the atmospheric distillation tower overhead system was rarely investigated comprehensively. Wang, et al.[19]developed a method for forecasting water dew point and pH value in tower overhead system based on ASPEN Plus simulation, which could provide a basis for corrosion prediction. Chen, et al.[20]studied the dew point corrosion of overhead pipeline by means of CFD software, which simulated the multiphase and multi-component state in pipeline and the distribution of pH value in condensation zone. Even though simulation methods were used to explore the dew point and ammonium salt deposition at a micro level, a combination of process models, flow fields and temperature fields aimed to analyze the flow corrosion mechanism of atmospheric tower overhead system in a complex multiphase environment and predict the high-risk failure zone of dew point corrosion and ammonium salt deposition was still lacking.

Therefore, based on the previous studies, a coupling of simulation models of overhead low-temperature system was established by means of ASPEN process simulation,together with FLUENT fluid dynamics simulation and HTRI heat transfer calculation to evaluate the corrosion risk and ensure the long-term operation of atmosphericvacuum distillation unit.

2 Models and Methods

2.1 Process simulation

Figure 1 displays the process simulation model established by ASPEN software to establish a process simulation model for a typical atmospheric tower overhead system. The annual processing capacity of atmospheric and vacuum distillation unit is 780 t/h. The content of sulfur, nitrogen and chloride in crude oil are 2.10%, 0.19% and 70.7 μg/g, respectively. The heated effluent is distilled from atmospheric tower overhead,which is followed by injection of corrosion inhibitor,water and neutralizing reagent. Then the effluent flow is condensed through heat exchangers and separated into gas, oil and sour water in the separator. The operating parameters of overhead system are shown in Table 1.In this case, the reaction effluent which shows phase changing is a typical multiphase flow system. Since it is difficult to get reaction effluent from tower overhead, the initial mixture composition was obtained by using mixer model in ASPEN according to the principle of material conservation. The thermo-dynamic Peng-Robinson (PR)equation, which was modified especially for petroleum refining environment, was chosen as the governing equation. The composition of oil, sour water and gas in the separator is presented in Table 2.

Through the process simulation, the phase distribution law, as well as the temperature of aqueous dew point,and the ammonium salt crystallization mechanism were studied. It also provides basic data for subsequent heat transfer calculation and flow field simulation.

2.2 Heat transfer calculation

Under the effect of temperature and flow, there is a high possibility of ammonium salt induced blocking and corrosion in heat exchanging system. The HTRI software was selected for carrying out heat transfer calculation of heat exchanger to obtain the temperature distribution of heat exchanger tube bundles, which could clarify the specific dangerous zones in heat exchanger.

Figure 1 Process model of atmospheric tower overhead system

Table 1 Operating parameters of overhead system

Table 2 Compositions of medium in separator

Fluid in the tube side and the shell side can transfer heat through heat conduction. The heat transfer equation is:

The different symbols involved are shown below:Qis the total heat transfer value (W);Kis the total heat transfer coefficient (W/m2·°C);Ais the effective heat exchange area (m2); Δtmis the mean temperature difference (°C).

The heat transfer models were composed of geometry model and process model, which were set according to structural specification and operating condition. The model is shown in Figure 2. The diameter of shell is 1400 mm, and the length of shell is 6 m. The number of passes and tubes are relatively 2 and 1280. The tube specification is Φ25 mm×1.65 mm. By importing the physical properties and parameters got by ASPEN into HTRI for heat transfer calculation, the mass flow in the tube side and shell side of the heat exchanger is 94632 kg/h and 517009 kg/h, respectively. Some detailed parameters are shown in Table 3.

2.3 Fluid flow simulation

Figure 2 Geometry model of heat exchanger

Table 3 Parameters of heat exchanger by simulation methods

The ammonium salt drifting motion, corrosion, and erosion are closely related to the multiphase flow, so FLUENT was applied to perform 3D simulation of the flow field and phase change for main parts in the overhead system. The multiphase flow can be described using the mixture model and realizable k-ε model[21-22]. Volumetric reaction was set in component transport model. The mass equation, energy equation, momentum equation, and turbulence equation are expressed as follows:

whereρis the density of mixed phase (kg/m3);υis the velocity (m/s);μis the kinematic viscosity (Pa·s);λis the coefficient of thermal conductivity [W/(m·K)];kis the turbulent kinetic energy (J);εis the turbulent dissipation;Tis the absolute temperature (K);pis the pressure (Pa);Cpis the specific heat capacity at constant pressure [J/(kg·K)];Gkis the turbulent kinetic energy caused by velocity gradient in laminar liquid flow[23].

By simulating the process of overhead system, the mass flow, which was set as the inlet type, was 21.86 kg/s for gas phase and 4.42 kg/s for the liquid phase. The outlet condition was set as the pressure outlet. Natural convection method was selected, considering the heat dissipation of pipe and air, and SIMPLE algorithm was used in the calculation process. The physical characteristics of inlet boundary are shown in Table 4. Energy equation was turned on and the gravity was considered in the study. The initial mesh type was mainly treated as tetrahedral mesh, and was then converted into polyhedral mesh in the flow simulation. The mesh was adaptively smoothed to improve its quality.

Table 4 Physical characteristics of inlet boundary

3 Results and Discussion

3.1 Process simulation results analysis

Figure 3 shows the relationship between phase fraction and temperature. It can be seen that during the fluid cooling process, along with liquefaction and dissolution there is a consistent change on the molar fraction of oil-gas-water phases. The gas phase continues to liquefy, leading to a decrease in gas flow rate, while the oil and water flow gradually increases. Oil begins to appear when temperature reaches 111 °C and liquid water appears at 91 °C, which can be regarded as the dew point temperature. The overhead heat exchanger inlet temperature is 95 °C and the outlet temperature is 83 °C, indicating that the dew point falls within the heat exchanger, and there is a risk of dew point corrosion in tube bundles[24]. Nitrogen compounds and chlorides contained in raw material can be converted into NH3and HCl. Generally, NH4Cl does not crystallize at higher temperatures. As temperature goes down, the product of partial pressure which represents theKpvalue of NH4Cl increases. WhenKpis greater than the equilibrium constantK, ammonium salt crystallization comes into being. Thus, the intersection ofKp-Tcurve and NH4Cl equilibrium curve obtained from API recommended practice 932-B is the NH4Cl crystallization temperature.Under current operating conditions, the initial crystallization temperature of NH4Cl is 128 °C, which indicates that the ammonium salt has already appeared in the tower. By monitoring the on-line salt washing process of tower overhead system, it is found that the content of Cl-and NH3-N in wastewater was high at the initial stage and then decreased gradually, which could also provide evidence for the presence of ammonium chloride salt deposit. The simulation result is consistent with the actual outcome of on-site investigation. However, only when liquid water appears in overhead system, NH4Cl can be dissolved in the water-producing wet circumstance,which will deposit on the inner wall of equipment and under-deposit corrosion will occur. It can be concluded that dew-point corrosion is the most direct factor which can influence the safe operation of heat exchanger, and it makes tube bundles a high priority to be focused on.

Figure 3 Relations between phase molar fraction and temperature

Figure 4 Result of the NH4Cl crystallization temperature

3.2 Heat transfer calculation results analysis

In order to clarify the specific dangerous zones of corrosion failure, HTRI is used to simulate the temperature distribution in overhead heat exchanger. The overhead oil vapor flows through two tube bundles and each row of tubes is divided into 22 monitoring points at equal intervals. Figure 5 shows the fluid temperature distribution in tube and shell. It can be found that the fluid temperature in tube bundles decreases as its distance from the inlet increases. The temperature at 2.7 meters away from the bundle inlet in the first tube bundles is about 91°C. Theoretically, the dew point of water condensation is 91 °C, so this location is the high risk area where dew point corrosion occurs. During the actual heat transfer process, temperature distribution is not uniform and there is a certain temperature gradient. The temperature at tube wall is lower than that in tube center, leading to the condensation of vapors near the inner wall. As a result,even though the temperature does not reach the aqueous dew point temperature, there is still a small amount of condensate in front of the area, which is 2.7 meters away from the bundle inlet.

The initial ammonium salt crystallization temperature is 128 °C, which is higher than the inlet temperature. Thus,NH4Cl appears before heat exchanger entrance. With a decreasing fluid temperature, a large amount of liquid water can weaken the dew point corrosion and wash the ammonium salt off the tube inner wall. So it is probable that the moist NH4Cl particles can deposit onto the tube wall between inlet and the dew point area, leading to risk of under-deposit corrosion and local erosion.

Figure 6 shows the corrosion morphology of tube wall. It can be seen that the local corrosion-induced wall thinning is extremely obvious and there are also some shallow pits of small size on the inner wall. The corrosion-induced thinning of tube wall is the result of multiple factors.HCl dissolves in the condensed water forming a highly acidic electrolyte solution, and then hydrogen evolution reaction occurs. The NH4Cl particles depositing onto the inner tube wall can cause the under-deposit corrosion.Meanwhile, some corrosion products and NH4Cl scale can be rushed away with the fluid, leading to a synergistic effect of corrosion and erosion.

3.3 Fluid flow simulation results analysis

The overhead fluid flows out from the volatile line and then enters the two parallel heat exchangers through the distribution pipe. Upon considering that there are often vortex areas where flow field changes, the heat exchanger’s inlet and outlet pipes together with elbows were selected to make an in-deepth analysis.

Figure 5 Fluid temperature distribution in heat exchanger

Figure 6 Morphology of tube corrosion

Figure 7 is the structural diagram of the distribution pipe before the heat exchanger inlet. The liquid volume fraction contours and velocity contour of fluid in heat exchanger inlet distribution pipe are presented in Figure 8 and Figure 9, respectively. The computational results show no symmetry liquid distribution in this distribution pipe, and there is a large difference in velocity between the sides 1 and 2 as labeled by the dotted lines in Figure 7. In the φ800 mm horizontal pipe, the liquid phase flows close to the bottom region because of the gravity,making the local velocity lower than that of center and top regions. Liquid phase is mainly distributed in the inner bend when passing through the elbow to the vertical part. Then the fluid enters the tee and it can be observed that the deviations of fluid at the tee are severe. Due to the effect of wall pressing weir flow, most liquid flows into side 1 and accumulates on the upper region of the φ500 mm pipe, contributing to high flow velocity. In addition, the fluid represents a swirling state near the inlet of heat exchanger of side 1. The average inner velocity can reach up to 18.29 m/s and 25.89 m/s,respectively. The temperature contour of the distribution pipe before heat exchanger inlet is also provided for a better comparison in Figure 9. It shows that the larger velocity regions correspond to a higher fluid temperature.This phenomenon can be ascribed to the shorter and insufficient residence duration in heat transfer promoted by high fluid velocity.

Figure 7 Structural diagram of the pipe before heat exchanger inlet

Figure 10 is the structural diagram of the pipe after heat exchanger outlet. Figure 11 is the liquid water volume fraction contour of different sections. Based on comparison, the multiphase flow stratifies because of the centrifugal force, and liquid water with greater gravity is mainly concentrated on the external bend of elbows A, B, and C. Under the interaction of elbow D and elbow E, liquid water volume fraction at the inner bend of elbow D also gradually increases. HCl dissolves into liquid water forming an acidic solution.With the process of convective mass transfer, the acidic solution interacts via an electrochemical corrosion reaction with the pipe surface. As a result, the corrosion of pipe inner wall is more severe at the place where the volume of liquid water is large. Thus, the corrosion degree is enhanced at the external bend of elbows A, B,and C. When it comes to the downstream of this pipe,at sections D and E, a small amount of liquid water is distributed to the inner bend that would increase the corrosion area.

Figure 8 Liquid volume fraction contours of the distribution pipe before heat exchanger inlet

Figure 9 Velocity contour of the pipe before heat exchanger inlet

The shear stress is related to fluid viscosity, flow characteristics, and pipe geometry. In Figure 12, the contours show that shear stress at the external bend is greater than that at the inner bend of elbow. The maximum shear stress is 4.94 Pa, which is far less than the allowable stress of pipe material, and its negative effect on pipe surface can be ignored when comparing with acidic corrosion.

In order to verify the simulation results, the actual situation was investigated. Figure 13 is the pipeline photos in a refinery. The elbows at outlet pipe of heat transfer are severely corroded and thinned. Signs of repair can be seen at these parts meaning that the accuracy of simulation is satisfactory.

Figure 10 Structural diagram of the pipe after heat exchanger outlet

4 Conclusions and Suggestions

The main reasons for the failure of equipment and pipelines in atmospheric tower overhead system are dew point corrosion and ammonium salt crystal deposition.

Figure 11 Liquid water volume fraction contours of pipe after heat exchanger outlet

Figure 12 Shear stress contours of pipe after heat exchanger outlet

This paper studies the corrosion and ammonium salt crystallization characteristics of multi-component fluid and establishes a coupling simulation method for forecasting comprehensively the corrosion risk of atmospheric tower overhead system. The conclusions and suggestions are listed below:

(1) Under current operating conditions, the dew point temperature and initial crystallization temperature of NH4Cl are respectively 91 °C and 128 °C, which indicates that ammonium salt has already appeared in the tower and liquid water can form in the heat exchanger tube.

(2) In the heat exchanger, condensate water appears in the area which is 2.7 meters away from the bundle inlet. It leads to corrosion risk in the presence of moist NH4Cl and high concentration of acidic solution.

(3) For the distribution pipes located near the inlet of heat exchanger and the elbows located near the outlet of heat exchanger, the flow field presents an unsymmetrical distribution. Refracted flow exacerbates the unevenness of heat transfer and intensifies the corrosion of one branch of heat exchanger. For elbows located after the heat exchanger outlet, under the effects of turbulence and inertia, liquid water is distributed more non-uniformly.The high risk areas are mainly concentrated on the external bend of elbows where the volume of liquid water fraction is larger.

(4) After analysis by combination of multiple simulation methods, the corrosion risk in atmospheric tower overhead system can be predicted, which can be regarded as the basis of corrosion control and the direction of structural optimization for the refinery.

Figure 13 Photos of actual pipe corrosion

（5）Improving the quality of crude oil to reduce the content of chloride and nitride is a fundamental ways to avoid corrosion problems in the atmospheric distillation tower overhead system. Additional crude oil pretreatment and alkali injection devices can be installed if necessary conditions are available. Optimizing the operating conditions is also beneficial to alleviating the risk of NH4Cl crystallization and dew point corrosion. The water injection rate is relatively low, which cannot guarantee a sufficient amount of liquid water for diluting the acidic solution and washing NH4Cl scale away. Considering the simulation results, it is recommended to increase the volume of water injected and add two injecting points at the entrance of heat exchangers.

Acknowledgment:This research was financially supported by the scientific research project through the SINOPEC Science and Technology Division (Contract No. 318021-8).