Paper No. 586 APPLICATION OF IONIC EQUILIBRIA PROCESS SIMULATION FOR ATMOSPHERIC DISTILLATION OVERHEAD SYSTEMS G.G. Duggan and R.G. Rffihtien Baker Petrolite 1600 Industrial Blvd. Sugarland, TX 77478 ABSTRACT The use of organic neutralizing amines in distillation overhead systems is a common refinery corrosion control practice. Two problems are often encountered in these amine applications: underdeposit corrosion attack resulting from the deposition of amine-hydrochloride salts and aggressive acid corrosion in the vicinity of the aqueous dewpoint. Often it is the misapplication of neutralizers that creates these problems. To address these neutralizer concerns, the fundamental data required to accurately predict the behavior of these amines and their hydrochloride salts has been developed. These data were incorporated into commercially available process simulators to aid in the design and application of amine neutralizers. This new technology has been applied to a wide variety of refinery distillation overhead systems. Examples of its use in improving corrosion control are presented here. Keywords: underdeposit corrosion, organic amines, amine-hydrochloride salts, simulation, acidic corrosion, dewpoint INTRODUCTION Corrosion in the overhead condensing system of atmospheric distillation units is a common occurrence in refineries worldwide. Corrosion inhibitors and organic amine neutralizers are routinely applied to combat the problem, with generally good results. There remain, however, many instances where the corrosion control program is not effective in preventing equipment failures. In the majority of such cases, inspection of corroded equipment reveals that the failures occurred in the vicinity of the aqueous dewpoint. The two most frequently cited failure mechanisms are acid corrosion due to low pH at the dewpoint and underdeposit corrosion beneath neutralizer hydrochloride salts ahead of the dewpoint. The improper use of organic amine neutralizers has been identified as the underlying cause of both failure mechanisms. The neutralization of acids in the overhead system using ammonia and organic amines is a common practice, but a contlict arises. Underuse of neutralizer can lead to severe acid corrosion. Overuse. of neutralizer may result in deposition of Copyright 01998 by NACE International. Requestsforpermission to publish this manuscript in any form, I” part or in whole must be made in writing to NACE International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.
Transcript
Paper No.
586
APPLICATION OF IONIC EQUILIBRIA PROCESS SIMULATION FOR ATMOSPHERIC DISTILLATION OVERHEAD SYSTEMS
G.G. Duggan and R.G. Rffihtien Baker Petrolite
1600 Industrial Blvd. Sugarland, TX 77478
ABSTRACT
The use of organic neutralizing amines in distillation overhead systems is a common refinery corrosion control practice. Two problems are often encountered in these amine applications: underdeposit corrosion attack resulting from the deposition of amine-hydrochloride salts and aggressive acid corrosion in the vicinity of the aqueous dewpoint. Often it is the misapplication of neutralizers that creates these problems.
To address these neutralizer concerns, the fundamental data required to accurately predict the behavior of these amines and their hydrochloride salts has been developed. These data were incorporated into commercially available process simulators to aid in the design and application of amine neutralizers. This new technology has been applied to a wide variety of refinery distillation overhead systems. Examples of its use in improving corrosion control are presented here.
Corrosion in the overhead condensing system of atmospheric distillation units is a common occurrence in refineries worldwide. Corrosion inhibitors and organic amine neutralizers are routinely applied to combat the problem, with generally good results. There remain, however, many instances where the corrosion control program is not effective in preventing equipment failures. In the majority of such cases, inspection of corroded equipment reveals that the failures occurred in the vicinity of the aqueous dewpoint. The two most frequently cited failure mechanisms are acid corrosion due to low pH at the dewpoint and underdeposit corrosion beneath neutralizer hydrochloride salts ahead of the dewpoint. The improper use of organic amine neutralizers has been identified as the underlying cause of both failure mechanisms.
The neutralization of acids in the overhead system using ammonia and organic amines is a common practice, but a contlict arises. Underuse of neutralizer can lead to severe acid corrosion. Overuse. of neutralizer may result in deposition of
Copyright 01998 by NACE International. Requestsforpermission to publish this manuscript in any form, I” part or in whole must be made in writing to NACE International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.
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corrosive amine-hydrochloride salts. The difficulty in measuring the pH of the water at the most corrosive point - the aqueous dewpoint - further amplifies the acid corrosion problem. Also, the absence of thermodynamic data for amines reacting with HCl makes it impossible to predict where salts will form. Success at treating overhead corrosion has, thus, been a hit or miss proposition.
New, breakthough technology has overcome these weaknesses in corrosion control. Shell Oil Products Company has developed thermodynamic data that allows accurate determination of both pH and salt formation temperature for systems using organic amine neutralizers. Baker Petrolite has been awarded sole licensing rights to this technology. The data has been incorporated into process flowsheeb’simulation programs for ease of application. Intelligent application of neutralizers is now possible, controlling pH throughout the aqueous phase while avoiding the deposition of corrosive salts. The new technology, called Ionic Equilibria Modeling (IEM), has seen widespread application during the past two years. Corrosion problems that had previously gone unsolved can now be successfully addressed.
NEUTRALIZATION OF ACIDS WITH AMINES
Impurities in the crude oil charge contribute acids which distill into the overhead system. For example, the hydrolysis of magnesium chloride in the crude unit heater generates HCl by the following reaction:
MgCl, + 2H,O + 2HC1 + M&OH), (1)
Corrosion occurs as a result of the oxidation of the base metal by acid in the presence of water.
Fe + 2H + 2CI. + Fe’* + 2Cl. + Hz (2)
The rate of the corrosion reaction described in equation (2) is a function of pH. The pH is lowest (and corrosion rate highest) at the aqueous dewpoint. A sample pH profile for an overhead system is shown in Figure 1.
Organic amines are applied to elevate the pH of the aqueous phase in overhead systems. The rate of amine addition to a system is, in most cases, controlled to meet a target pH at the accumulator. While it would seem more sensible to apply amine neutralizers to control the pH at the dewpoint, the dewpoint pH cannot be directly measured without extensive effort’. The standard application approach, therefore, exposes the system to the possibility of dangerously low pH at the dewpoint. Clearly, the inability to control pH at the aqueous dewpoint represents a significant weakness in the battle to control crude unit corrosion.
DEPOSITION AND CORROSIVITY OF AMMONIA- OR AMINE-HYDROCHLORIDE SALTS
To control acid corrosion at the dewpoint, accumulator pH is often maintained at higher levels than needed, in an attempt to increase dewpoint pH. Higher pH targets necessitate higher amine addition rates. However, these higher amine rates introduce another threat: amine reaction with HCI in the vapor phase. The reaction product is a highly corrosive amine-hydrochloride salt.
The reaction between acid (HCI) and base (ammonia or amine) to form a salt is dependent on the concentration of the two reactants in the vapor phase and the temperature of the system. A typical reaction between acid and base would be witten as follows:
HCI (g) + RNH,(g) c) RNH,Cl (solid/melt)
Note, the salts formed can exist either in a solid or a molten state, depending on the salt and the system conditions. Once formed, the solid or molten salt will deposit from the vapor phase onto equipment surfaces. If salt formation does not occur until after the aqueous dewpoint, the salts will dissolve in the water, presenting minimal risk of corrosion. If, on the other hand, deposition occurs prior to the aqueous dewpoint, a serious corrosion problem can result. The resulting corrosion is a function of several characteristics of the salts. First, conjugate acid-base theory indicates that the salt of a weak base and a strong acid will itself be a weak acid. Second, these salts are hygroscopic and will absorb moisture from the vapor even in the absence of condensed water. This absorbed water provides the electrolyte necessary for the acid to oxidize (corrode) the metal surfaces. The third key factor is that corrosion inhibitors are unable to penetrate the salt to interact with the metal surface. The combination of these factors exposes the metal under the salt to severe corrosion rates, resulting in shortened equipment life’.
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For ammonia and amines, a phase diagram, such as the one shown in Figure 2, can be used to determine whether salt deposition will occur. At a given set of operating conditions (HCI and amine partial pressures and system temperature), the phase of the amine and HCI (salt or vapor) is provided by the phase diagram. If the phase diagram for a given amine were available, the salt formation temperature could be easily determined. Though the phase behavior for ammonium chloride has been thoroughly studied’, the thermodynamic data that defines the phase behavior for commonly used organic amines was not available. A rigorous research effort was undertaken to generate these vital amine salt data”.
THERMODYNAMIC DATA
To provide an effective corrosion control program, adequate pH levels must be maintained while avoiding deposition of corrosive salts. Figure 3 depicts a series of reactions which take place in a typical overhead system. Both pH and salt deposition are governed by these reactions. For simplicity, only the reactions involving HCI and an amine are shown, but there are numerous other ionizing species (e.g., sulfur oxides, carbonates, sulfides, ammonia and carboxylic acids) present in a typical overhead system for which analogous reactions are not shown. All of these species have an impact on the corrosivity of the system.
Two reaction steps are of critical importance in corrosion control. The dissociation/protonization reaction dictates the pH of the aqueous phase. Likewise, the salt deposition/sublimation reaction dictates the tendency to form salts. Determining the pH involves simultaneous solution of the reaction equilibria for all ionizable species present. Similarly, the deposition/sublimation reaction must be solved in order to quantify the salt formation tendency. Solving these reaction equilibria requires that thermodynamic data (e.g., AG of reaction, dissociation constants) be known for all reactions in Figure 3. These thermodynamic data are the heart of the IEM technology.
Thermodynamic data for many of the ionizable species of interest are. contained in commercially available process simulation sottware. However, the data for the most commonly used organic amines are not available in these programs or in the literature. An exhaustive research project was undertaken to develop the missing thermodynamic data. These resultant proprietary data have been incorporated into a framework accessible by process simulation software and are now available via IEM.
APPLICATION
When applying this technology to an overhead system, a process flowsheet is created in the simulation program. This flowsheet contains all unit operations, process data and measured stream compositions. The pH profile and salt formation tendency at all relevant points are then determined using the simulation model. The pH is calculated at all points where a free water phase is present. The salting tendency is quantified by comparing the salt formation temperature (calculated by IEM) to the temperature at key points in the system. This comparison is referred to as the Salt AT.
Salt AT = T,,, - TrwltfomMion
Figure 4 shows the Salt AT graphically as it relates to the salt formation phase diagram. By definition, the Salt AT is interpreted as follows:
Salt AT > 0 - Salts will not form Salt AT = 0 - Point of incipient salt formation Salt AT < 0 - Salts will form
Because amine hydrochloride salts are water-soluble, salt formation is not a concern once the aqueous dewpoint is reached. Therefore, the Salt AT is not calculated at temperatures below the dewpoint temperature.
Initially, a “base case” model of the system is created to represent typical operating conditions. Once this base case has been created, various perturbations of the base case model can be generated to identify the effect of changes to the system. Examples of perturbation cases include: neutralizer substitution; varying levels of contaminant species; and use of a recirculated water wash injection.
It is worth noting that the underlying philosophy of this modeling effort is to identify the best corrosion control program for the given system conditions. The emphasis here is on choosing the neutralizer which provides acceptable pH
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elevation at the dewpoint without the potential for salt formation. Because every overhead system is unique in its operating parameters and contaminant levels, one single neutralizer (or even, one neutralizer blend) will not address all possible treating scenarios. For example, amines which have high neutralizing capacity provide good pH control but tend to readily form salts. On the other hand, very weak amines have low salting tendency but do not elevate pH sufftciently, especially at the dewpoint. The entire overhead system must be rigorously modeled to arrive at the proper neutralizer choice for each set of conditions.
pH CONTROL STRATEGY
The traditional method of pH control is to adjust neutralizer usage based on a pH target at the accumulator. This method suffers from a critical shortcoming: the pH at the aqueous dewpoint is only indirectly related to the pH at the accumulator (refer again to Figure 1). It is possible that, even though the accumulator pH is in an acceptable range (5.5 6.5), the dewpoint pH may be unacceptably low (e.g., less than 4.0 for carbon steel with inhibitor). Because it allows for rigorous pH calculations, IEM yields the proper neutralizer usage for achieving an acceptable pH profile across the entire overhead system. Specifically, the minimum amount of neutralizer needed to attain a desired dewpoint pH can be determined as a function of the chloride and ammonia levels in the accumulator.
This relationship is shown graphically in the sample Neutralizer Nomograph (Figure 5). For a given level of both ammonia and chlorides in the accumulator, the minimum injection rate of organic neutralizer is provided by the nomograph (y-axis). This minimum neutralizer rate will produce the desired dewpoint pH. An example is given in Figure 5, where chloride level is 15 ppm and ammonia level is 20 ppm. For this example, the neutralizer rate needed to maintain the desired dewpoint pH is 50 liters per day.
Another important feature of the nomograph is the “Salt Deposition Region”. The boundary of this region represents the maximum neutralizer injection rate for a given chloride concentration. For 35 ppm chloride on the sample nomograph, the maximum rate of neutralizer which can be applied without forming salts is 35 liters per day. The use of this nomograph provides improved pH control while avoiding salt deposition, thus ensuring increased equipment life and enhanced system reliability.
CASE STUDIES
Regardless of its physical configuration, contaminant levels, crude slate or additive program, any overhead system can be accurately simulated using IEM. Table 1 is a partial list of systems for which IEM has been employed and improved corrosion control strategies implemented. A more detailed description of two applications is provided below.
Case # 1: Acid Corrosion in Second Stage
IEM was performed on a multi-stage overhead system which was experiencing corrosion problems in the second condensing section (carbon steel metallurgy throughout). Figure 6 gives a process schematic of the system. Neither inhibitor nor neutralizing amine was fed to the second stage. “Tramp” ammonia was the primary source of acid neutralization for this section. Until recently, corrosion probe readings at the tin fan outlet were 0.13 mm/y. However, over the course of several months, these probes began to show increased corrosion activity in the range of 0.5 1 - 0.64 mm/y.
The base case model results are shown in Figure 7. Note that the aqueous dewpoint occurs at the inlet of the tin fans. At the ammonia level modeled, the pH’s at both the tin fan inlet and outlet were unacceptable for uninhibited carbon steel equipment. Ammonium chloride deposition was not predicted ahead of the dewpoint.
A survey of historical operating data revealed that, during the earlier period of low corrosion rates, the ammonia levels in the overhead were twice the current levels. To examine the effect of increased ammonia, the IEM was run with double the ammonia concentration of the base case. The IEM results revealed that the pH profile in the second stage is strongly a function of the ammonia concentration, as shown in Figure 8. The higher ammonia levels increased the pH at the fin fan outlets considerably (from 4.9 to 6.3). Clearly, the measured corrosion rate at the tin fan outlet is inversely proportional to the ammonia levels in the system.
While corrosion at the tin fan outlet diminishes with higher ammonia levels, low pH (and corrosion) at the dewpoint persists. Several perturbation cases were run to identify a neutralizer which would elevate the dewpoint pH to the refiner’s specification level (pH=4.5) without salt formation at the fin fan inlet. Figure 9 provides the results of the recommended treatment program which also included the use of an oil-soluble tilming inhibitor for added protection. The
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recommended program provides an acceptable pH at the dewpoint, without the formation of ammonium chloride (Salt AT = + 17 “C) or amine hydrochloride salt (Salt AT = + 4 “C).
The additive program detailed in Figure 9 was successfUlly implemented, with the corrosion activity on the probes reduced to 0.13 mm/y or less.
case # 2: Salt Deposition and Acid Corrosion Near Dewpoiot
This crude unit had operated with few corrosion related problems in recent years, but a change in crude source (to a heavy, sour, foreign crude) one year earlier rendered past corrosion history invalid. IEM was undertaken to determine if severe corrosion problems should be anticipated while processing the new crude slate. A simplified flow drawing of the overhead condensing system is shown in Figure 10. The condensers are designed with a “J” type shell, where the flow enters the top bundle in the middle of the shell and divides, half flowing to the front end, the rest flowing to the back end. The bottom bundle has flow entering the shell at each end, combining and exiting from a single nozzle located in the middle of the shell. The tube metallurgy is carbon steel. Neutralizer is injected independently to each of the three passes, with injection quills located about one foot above the shell inlet nozzles on the top exchanger.
The base case IEM results are summarized in Figure 11. Key results to note are the Salt AT and the pH profile. The results indicated that neither ammonia- nor amine-hydrochloride salt deposition would occur under the conditions modeled. However, the pH profile indicated an unacceptable dewpoint pH for carbon steel @H=3.6). Though the bulk phase aqueous dewpoint occurs in the bottom bundle, tubeskin temperatures are cold enough to promote a localized dewpoint in the top bundle. Therefore, both top and bottom bundles are subject to acid attack.
Sensitivity studies were performed at varying neutralizer rates to address the low pH concern. Figures 12 and 13 reflect model results for no neutralizer and for double the neutralizer dose, respectively. Note that the dewpoint pH in the no neutralizer case shows that a somewhat more acidic condition is expected than in the base case (pH=3.4 vs. 3.6). On the other hand, double the neutralizer rate improves the dewpoint pH (pH = 3.8), but introduces a salt deposition problem.
As the modeling effort was underway, a failure occurred in the middle (C/D) bank of exchangers. The bundles were isolated and removed for inspection. A tube thickness scan revealed an interesting and most revealing corrosion pattern. The corrosion was found to be more severe in the front end of the bundle than in the back end. The front end of the bundle contained numerous tubes with corrosion rates of 0.75 to 1.0 mm/y (30-40 mpy), with the failed tubes having corrosion rates exceeding 1.26 mm/y (50 mpy). In contrast, the thickness scan showed corrosion rates ranging from 0.25 to 0.60 mm/y (10 to 24 mpy) in the back end of the bundle. Figure 14 is a schematic showing the area where the corrosion was most severe.
Closer examination of the system led to a key finding: the neutralizer injection nozzle on the middle bank was improperly installed. The nozzle design called for injecting the neutralizer co-current with the overhead flow, as shown in Figure 15. However, as installed, the injection nozzle was oriented toward the pipe wall, as shown in Figure 16. Given the short distance from injection to shell inlet, most, if not all, of the neutralizer was flowing to the front end of the bundle (effectively doubling the neutralizer dose).
The injection nozzle orientation promoted two types of corrosion attack in the bundles. In the front end, salt deposition resulted in underdeposit corrosion. In the back end, low pH led to acid corrosion. Note that the underdeposit corrosion was the source of the failure.
Armed with the unit experience and IEM results, recommendations have been made to overcome the salt deposition problem and to deal with the acid corrosion at the aqueous dewpoint. Implementation of these recommendations is currently underway.
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CONCLUSIONS
l Traditional corrosion control programs are often ineffective. l Misapplication of neutralizing amines is often a root cause of corrosion failures. l The type of amine and its usage rate are both important factors in proper amine application. l Knowledge of the pH at all points in the system (including tbe aqueous dewpoint) allows for improved corrosion
control. l Phase diagrams can be used to identify salt formation tendency. l Critical thermodynamic data to determine both pH profile and salt formation tendency has been developed. l Application of Ionic Equilibria Modeling has resulted in improved corrosion control.
ACKNOWLEDGMENTS
The authors wish to express gratitude to Ashok K. Dewan and Diego P. Valenzuela of Shell Oil Products Company for their invaluable assistance in the application of this new technology.
REFERENCES
1. K. Kato, T. Kondo, “Corrosion Prevention in Overhead Lines and Condensers of Crude Oil Fractionators,” Fifth International Congress on Metallic Corrosion, p.967, May (1972).
2. H. Morinaga, et.al., “Corrosion Control of Crude Distillation Units by Using Inhibitors,” Asian Symposium on Corrosion and Protection in Oil and Gas Operations, Oil Refineries, and Petrochemical Industries, paper no. A-302, (Osaka, Japan: The Corrosion Engineering Division and The Society of Materials Science, 1994).
3. Y. Wu, Oil and Gas Journal, p. 38, January 3, 1994. 4. D. Valenzuela, A. Dewan,“Corrosion Control in Crude Column Overheads - A Closer Look at Amine Neutralizer
Thermodynamics and Phase Behavior,” Twentieth OLI User Conference, paper no. 5 , (Morris Plains, NJ: OLI, 1997).
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TABLE 1 IEM APPLICATION SUMMARY
Location
Midwest Refinery
CklllSdi~ Refinery
Northern Refinery
Midwest Refinery
Northern Refinery
Western Refinery
Northern Refinery
System Configuration 2 stages, no water wash
1 Stage, no reflux, column has top pumparound
2 stage, no water wash
1 stage, intermittent water wash
1 stage, continuous water wash 1 stage, continuous water wash
1 stage, continuous water wash to multiple exchanger banks
Corrosion History
1 st stage condensers suffer tube leaks yearly, bundles replaced every 3
Overhead condensers fail yearly. Severe corrosion damage in top 4 trays of column and in top pumparound
1 st stage bundles average 2 year life
Pressure drop limits throughput, condensers fail every 2 years
Average bundle life >7 years
Corrosion failures every 15-20 months
Corrosion failures every 20-24 months
EM Results
Amine and ammonia hydrochloride salt deposition in first stage condensers
deposition continuously in overhead, intermittently in top section of column
NH&l deposition problem severe
&position , insufficient pressure to intermittently wash bundles Current recirculating water wash effective means of control Insufficient water wash rate combined with excessively high pH leading to FeS deposition, underdeposit corrosion Unbalanced water wash rate created high pH areas, leading to FeS deposition, underdeposit corrosion
IEM Corrosion Control Strategy Change to new amine program, increased caustic to reduce chloride levels
Change from ammonia neutralizer to amine program, implemented tighter control over unit operations to minimize chloride and ammonia excursions Implemented continuous water wash with organic amine neutralizer Functional intermittent water wash and new amine program implemented. Pressure drop problems resolved No changes to existing corrosion control program Continuous water wash rate raised to safe level, pH target range decreased to limit FeS deposition
Water wash rates to each bank were balanced to give lower pH and decreased FeS deposition
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System Temperature
Aqueons newpoint Accumulator
FIGURE 1: SAMPLE pH PROFILE
Salt
Vapor Species
Temperature -
FIGURE 2: SAMPLE PHASE DIAGRAM FOR AMINE-HCl SALT FORMATION
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