April 2021

Water Management

TOTAL refineries improve overhead systems corrosion and salting with amine-neutralizing technology

Weak organic amines are commonly used in crude unit overhead systems to prevent acidic corrosion from chlorides and other acidic contaminants via a neutralization reaction.

Brun, L. A., Majorel, C., Thoret-Bauchet, J. P., Total; Pothuaud, A., Cross, C., SUEZ Water Technologies & Solutions

Weak organic amines are commonly used in crude unit overhead systems to prevent acidic corrosion from chlorides and other acidic contaminants via a neutralization reaction. While using commodity amines, some refineries experience salt-induced fouling and corrosion issues, which may prompt them to move to non-salting, lower strength amines. TOTAL decided to test this approach to improve reliability for its overhead systems in crude units. This was done through partnership with an industry-leading water technology and process providera to use their proprietary amine-neutralizing technologyb. This technology is based on a set of principles surrounding the blending of a variety of targeted amines to achieve a balanced neutralization profile, which minimizes salting potential for a given operating envelope. This article describes the route used by TOTAL and the service provider to develop individualized selection and implementation programs for two of their crude units. Each program was based on comprehensive analysis used to choose the best product, define the specific application strategy and define the subsequent benefits of the program changes, according to unit characteristics.

Purpose

The neutralizers are designed to improve both pH control and chloride salt precipitation potential in overhead systems vs. classical amine neutralizer programs. The unique properties of the amine strength and favorable water/hydrocarbon partitioning reinforces these abilities.

The following studies were driven by TOTAL and the service provider to validate amine speciation results between water and hydrocarbon flows over time in two of TOTAL’s European refineries. Results from the studies reveal the complex partitioning and recycle behaviors by which these products work and show how detailed water analysis helps to achieve optimization of both neutralizer consumption and corrosion mitigation. The benefits of the analytical campaigns and the overall knowledge of amine behaviors were not possible without a wide collaboration and involvement of all TOTAL teams in developing a systematic and exhaustive monitoring program around the overhead system.

Part of a modern overhead corrosion control program calls for the speciation of amines in a wider variety of flows and computation of detailed salt point calculations on a more frequent basis than are traditionally performed. The advantage is that a refinery can know in a much more granular fashion the precise amount of amine in the overhead circuit and surrounding flows and how they change over time. This allows better control and understanding of the prevailing salt point, which changes over time and is influenced by many complex, dynamic factors that are linked with operations and crude diet. In such a program, amine salt point calculations should not be based on the rate of amine injected into the overhead vapor line, but should be based on the measured amount of amine present in the overhead system receiver at a given time.

Such a program can be important for optimal results because the concentration of amines in the system—and the prevailing salt points that depend on them—is key to ensuring that the transition to a neutralizer will benefit the refinery in terms of its overall corrosion treatment cost.

For both the initiation and ongoing control of the new programs, it was decided that both TOTAL and the service provider would work together to optimize the balance between ongoing neutralization needs and salt point deposition constraints and to then quantitatively validate the specific benefits gained by crude units adopting the new programs.

Amine properties

Commodity amines have been used for decades to protect overhead condensing systems in both atmospheric and vacuum fractionation units. One of the foremost challenges toward the control of corrosion rates in overhead systems is avoiding deposition due to amine hydrochloride neutralization salts.

Traditional primary amines, like monoethanolamine (MEA) and methoxypropylamine (MOPA), have a high polarity and relatively large base strengths (pKa), causing them to react readily with acidic species at relatively low injection rates. However, these same properties also make it difficult to control pH in the typical target range of 5.5–6.5 (blue line in FIG. 1). In addition, the neutralization salts formed have higher than desired salt point temperatures that increase the potential for salt-induced corrosion and fouling.

FIG. 1. Comparative neutralization strength and needs.

Conversely, the neutralizers used in this study are generally composed of blends of both secondary and tertiary amines. As such, they have a lower polarity and smaller base strength than classical amines. This results in better controllability within the target pH range (red line in FIG. 1), as well as neutralization salts characterized by lower salt point temperatures and improved tendencies for salt formation downstream of water condensation to better avoid salt laydown. Using a mix of amines also decreases relevant partial pressures, which lower salt deposition risk. While these same properties might, at first, lead to the expectation of higher injection rates needed to elevate the pH to target, in practice, additional complex behaviors cause the amines in these blends to significantly partition to oil phases in a beneficial and targeted way when oil and water mix under typical operating conditions. As this article will examine, this phenomenon significantly reduces practical injection rate requirements and makes these programs cost effective, with better controllability and salt property benefits.

The partitioning effect outlined is highly pH dependent and is unique for each amine species used in an amine neutralizing product. This strongly impacts the amount of amine in both the hydrocarbon phase of the overhead circuit and in the desalter through the addition of desalter wash water returned from topping.

FIG. 2 shows theoretical partitioning curves for three amines used in the service provider’s neutralizer programs outlined here. It shows that some amines start to partition to the hydrocarbon as pH rises above 8, while others can start partitioning as pH rises above 5. Traditional primary amines generally have effectively no hydrocarbon partitioning potential in the range of desired boot water (BW) pH control. This property was confirmed by the detailed analytical plan developed in cooperation between TOTAL and the service provider.

FIG. 2. Effect of pH on amine partitioning in hydrocarbon phase.

Amine partitioning can cause amines to recirculate when water containing amines is contacted with a hydrocarbon stream, which will be reinjected to the column through the desalter. These recycle contributions can result in higher levels of amines in the overhead line than would be expected based only on the mass flowrate of amines injected in the overhead vapor line as a neutralizer. Due to a low salt point temperature, it has no expected impact in the column, as it is important to avoid any salt-induced corrosion in the lower side draw of the crude unit.

Amine analysis in the different streams of a crude unit can be very important and will lead to a better knowledge of concentrations for various amines inside the tower and the overhead system. This is important because the information can be used to better monitor real-time amine salt points and practical corrosion control tactics. This is especially true for the amines, where, due to the partitioning and recycle phenomenon previously described, simple mass balance calculations tend to overestimate the amount of neutralizer required to meet ongoing demand.

Analytical plan

The service provider developed a new method for amine speciation in overhead water sample by ion chromatography mass spectrometry (IC-MS), with a detection limit as low as 0.1 ppm for each amine and that avoids the interferences and coelution issues usually seen between amines of the same family (FIG. 3). Comprehensive and detailed amine analysis using this method is economical, precise, accurate and make the analysis of a wide variety of key amines available in overhead water samples.

FIG. 3. Amine speciation device.

Conversely, for hydrocarbon samples, it is necessary to first extract the amines from the hydrocarbon phase with acidified water. This works well for light hydrocarbon cuts like naphtha, but reliable results are more difficult in crude oil due to numerous interferences and the potential for column contamination by long eluting compounds. Therefore, the amines in desalted crude oil are calculated by material balance using the wash water and brine, respectively entering and leaving the desalter.

Case 1: Simple crude unit

The first unit where TOTAL and the service provider transitioned to the amine-neutralizing technology is in one of TOTAL’s European refineries, where the crude unit contains both a desalter followed directly by a furnace and then a main fractionator (MF). The neutralizer was injected into the MF overhead to control BW pH. As shown in FIG. 4, amine recirculation primarily takes place in two different circuits of the crude unit. These include:

FIG. 4. Amine recycle paths.
  1. In the overhead naphtha reflux
  2. With overhead water used as wash water to the desalter and around the desalter.

However, it is important to note that in some refineries, there can be several other significant recirculation routes impacting the prevailing steady-state overhead concentrations, which will not be discussed here.

Using the new method for amine speciation, amines from the program were examined in several streams around the crude unit to determine their respective concentrations in the overhead system, as compared to the injected quantity.

Samples were analyzed by TOTAL and the service provider from the following streams:

  • Water samples
    • BW
    • Desalter wash water (WW)
    • Desalter brine (DB)
  • Hydrocarbon samples
    • Reflux naphtha (RN).

In addition, amines were calculated for the following streams:

  • Crude oil: Calculated by difference (CO = WW – DB)
  • Injected neutralizer (IN): Based on injection rate and amine concentration in product.

TOTAL and the service provider agreed to perform a comprehensive set of analysis on a frequent and regular basis to continuously assess the prevailing amine concentration ratio and associated salt points in the overhead because pH dependence and other complex factors can often cause the values to be very dynamical. One example of results—shown in grams of amine—is presented in FIG. 5.

FIG. 5. Analytical results.

If there were no amine recycle, then the BW analysis would match calculated amines from the IN. However, the measured level of BW amines is 40% higher than that of the IN amines. This demonstrates that there is significant amine recycle; therefore, calculations for salt point temperatures based on injected amine quantity will result in salt point temperatures that are lower than actual. When controlling tower operations to maintain a certain safety factor surrounding salt deposition, the difference between amine-partial pressures assumed to be derived from amine “as injected” and the partial pressures computed based on actual measured circulating amine can result in unexpected issues with salt-induced corrosion and fouling. The additional streams shown in FIG. 5 were then examined to determine the source of the amine recycle and close the mass balance. No amine was detected in the RN, as expected, due to a pH of around 6 in the BW. To restate, the amines in the DB were subtracted from those contained in the desalter WW to calculate the amines in the crude oil. As the desalter operates at a pH of about 8, as directionally expected from FIG. 1, the data treatment indicated that more than 50% of total amines were entrained with desalted crude oil.

The quantity of amines entrained in the desalted crude oil, combined with those injected into the overhead system IN, agrees with the amount analyzed in the BW, with a 2% difference, which is quite good.

To better illustrate the typical propagated error expected between these two methods, salt points are compared. These were obtained first by once-through mass balance calculations based on amine injection rate (IN) and then by computing them using the measured BW concentrations.

As shown in FIG. 6, the difference in computed salt points for the two methods is 3.6°C, which is significant. This difference would be especially important regarding the mitigation of corrosion in cases where the salt point and the water dewpoint are close to one another. In such cases, the evaluation of amine recycle factors would be necessary to accurately evaluate system salt points. The overall concentration ratio factor for amines in this overhead system (BW/IN) was shown to be 1.4 in FIG. 5.

FIG. 6. Salt point temperatures.

This exercise was conducted on a regular basis and confirmed over time that the BW/IN ratio varied between 1.1 and 1.4, mainly as a function of desalter pH and unit operating conditions. It also confirmed that the neutralizer is always present in the atmospheric tower through desalter recycling. Defining a standard recycle ratio (BW/IN) is needed to properly and practically evaluate daily salt points based on the amine injection rate. This is especially important because the amine speciation methodology requires time to obtain results subsequently used to perform the amine balance. To have enough safety margin, TOTAL and the service provider agreed to use an ongoing recycle ratio of 1.5 and calculate, on a daily basis, the amine salt point based on both the injected amount only and then a second salt point based on amine expected by including the recycle factor of 1.5.

Because of the recycle ratio factor and its impact on salt point computation, it is important to choose the right neutralizing amine and to exploit its recycle behavior to ensure that no salt fouling and associated corrosion occurs in the tower or in the overhead condenser system.

A second benefit from the recycle factor is the global injection rate of the amine-neutralizing programs, which is very often less than when using classical amines at a constant pH target range; therefore, reducing treatment program costs, while maintaining the benefits of better corrosion control and system performance. In this case, when moving from a commodity amine to a neutralizing amine, the injection rate based on the use of a lower neutralizer strength amine (FIG. 1) was proposed to be 20% higher. However, due to the recycling advantage of the amine, the actual injection rate represents about 80% of the previous injection rate, which confirmed this program to be very cost effective.

Case 2: Crude unit with preflash (PF)

After this first success, TOTAL and the service provider decided to apply the same technology and approach in a more complex unit with a PF column upstream from the furnace and the MF column. The behavior of amines injected into the overhead of both the PF and the MF is different than in the case previously presented, with a more complex recycle scenario. Because of the different configuration of the unit, it was decided to once again implement a similar type of analytical plan to evaluate the recycling effect and its consequences, with respect to computed salt deposition potentials before adopting the amine-neutralizing program.

This was critical because overhead water acidity is a key driver of amine selection. Light organic acids (e.g., acetic acid) are more likely to condense in the overhead of a PF than in the downstream fractionator overhead. This behavior can then drive a higher neutralization demand. This is true even in the absence of problematical chloride levels. Conversely, chlorides are more likely to condense in the overhead of a MF because hydrolyzing chlorides from mineral salts in crude oil need the additional time and temperature provided by passage through the crude furnace.

As represented in FIG. 7, when Amine 2 is injected into the overhead of the MF, it can recycle with BW through the desalter as wash water and route to the PF overhead system rather than to the MF one where it was originally injected.

FIG. 7. Recirculation of service provider amine in a PF column.

Similarly, Amine 1 injected into the overhead of the PF recycles through the PF overhead but does not end up in the MF overhead. Unlike Case 1, this causes preferential cycle-up of oil soluble amines in the PF but not generally in the MF. This difference—caused by recycle loop behavior—should drive program design with the goal to manage salt points in both overheads simultaneously. To achieve these goals, granular and frequent amine speciation with following salt point calculations are of prime importance.

By using an amine-neutralizing productb (Amine 2 in the MF overhead FIG. 7), operators can manage salt points in the MF overhead and it plays a role in PF overhead pH control through recycling. Recycling reduces the required injection rate of Amine 1 in the overhead of the PF (even possibly down to 0 in certain operating cases). It also allows the use of a classical primary amine because fewer condensed salts are expected due to overall lower hydrochloric acid condensation. This behavior depends on the bottom temperature of the PF column and on the amine boiling point.

TOTAL and the service provider decided to adopt a similar analytical plan as used in Case 1 to evaluate the recycle of amines for Case 2. The following streams were sampled with the same type of analysis as before:

  • Water samples
    • MF BW
    • PF BW
    • Desalter WW
    • Desalter effluent
  • Hydrocarbon samples
    • MF RN
    • PF RN.

Two different neutralizer products were chosen for the two injection points. These included:

  • The neutralizer (Amine 2 in FIG. 7) for the MF overhead system and chosen to avoid any salt deposition there by chloride ranging between 5 ppm–20 ppm
  • A MEA-based neutralizer in the PF overhead (Amine 1 in FIG. 7) and chosen because chloride levels were consistently below 1 ppm, so salt precipitation was unlikely.

First, no amine was analyzed in the NR of both the PF and the MF. This confirmed what was analyzed in Case 1: a BW pH range of 5.5–6.5 prevents amines to be recycled with NR. It was reinforced by a pH target in PF in the range of 5.5–6 as acidity is more expected to be organic than chlorhydric. The pH and the choice of a conventional primary amine, which has more affinity with water, explains why there is no recycling of Amine 1 injected in PF via the desalter. The second confirmation is that the Amine 2 injected in the MF overhead was recycled into the PF. The data showed that approximately 30% of neutralizing amine injected into the MF was recycled into the PF (FIG. 8), with the variation driven primarily by changes in the desalter pH. This phenomenon helped to reduce the Amine 1 injection rate to a minimum level, which greatly reduced the risk for salt deposition, while effectively protecting against dewpoint acid attack.

FIG. 8. Amine 2 recycled in PF.

This second case confirmed the ability of the amine-neutralizing solution to recycle in the system and to reinforce corrosion protection, while reducing salt precipitation risk. At the end, there was no more risk of salt precipitation in the MF, and neutralization needs were reduced in the PF. This situation enabled TOTAL to better mitigate corrosion in both columns without additional costs.

Takeaways

The amine-neutralizing technology can be a very cost-effective program compared with classical neutralizing amine treatments. As this article has shown, this is due to the complexities involved with the use of secondary and tertiary amines, which show pH-dependent partitioning and recycle behavior vs. traditional primary amines. The large economic and reliability benefits imparted by using such amines typically justify the extra complexity and attention needed to control and optimize them.

The ability for the amines to partition strongly into hydrocarbons and concentrate in the overhead system helps to reduce neutralizing injection and chemical needs, while also reducing detrimental pH fluctuations and salt precipitation tendencies. When corrosion is not optimally mitigated, it can often result in loss of production and increased maintenance cost, which greatly increases total cost of ownership. If used properly and considering the total costs of overhead corrosion over time, the neutralizers can represent an optimal choice.

In summary, the amines helped TOTAL to better control pH, with minimal risk of salt deposition. Partitioning and recycle advantages illustrated in these two case studies allow these amines, in these two cases, to:

  • Concentrate in overhead of the atmospheric column with a typical recycle ratio of 1.4, used to calculate a more representative salt point in the overhead system with reduced global neutralizer demand
  • Recycle 30% of amine injected in a MF to a PF overhead to eliminate amine salt deposition in the MF overhead system, while reducing injection needs in PF.

Both the corrosion control program and the study were very successful towards maximizing corrosion mitigation, with minimal risk of salt deposition in both affected units. Thanks to the detailed ongoing analytical plan that was adopted, TOTAL and the service provider were able to document the improved corrosion control, salt deposition potential and chemicals costs. This work also helped to develop a more structured approach to implementing an overhead neutralizer program using the amine-neutralizing technology that considers the variation in crude unit design and its operations over time. HP

NOTES

          a SUEZ – Water Technologies & Solutions
          b Refers to SUEZ’s LoSALT technology

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