Processing lower cost crudes with greater confidence and improved reliability
For more than 100 years, it has been understood in the petroleum refining industry that certain crude oils or, more accurately, crude oil fractions contain sulphur (S) species and levels of organic acids that may be very corrosive to equipment and piping in crude distillation and downstream units
Gerrit Buchheim, Win Robbins and Frank Sapienza
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Economic pressures on the refining industry are forcing many refiners to look at lower-priced high acid or opportunity crudes to improve margins. The challenge for the integrity management community is how to evaluate the effects of a crude on the equipment metallurgy installed and subsequent impact on equipment reliability. The benefits of having a more accurate crude corrosion model are large in that it allows a refinery to potentially process cheaper crudes for increased profitability with greater confidence and better anticipation and understanding of the potential damage to the equipment/piping.
Some operating companies have focused research on this subject individually or through several joint industry programs (JIPs), but current methods available to most refiners still struggle to accurately predict corrosion behavior in refinery streams on a consistent basis.
This article presents a new simultaneous S/TAN model combined with a superior flow model (SNAPS-TAC) to better predict the corrosivity of hot crude oil streams. Fundamentally, the model relies on a thin barrier layer between the iron(Fe)-based metal and the hot oil fluid. The competing reactions of barrier layer formations due to naphthenic acid (forming a magnetite/Fe3O4 scale) and S (forming an iron sulphide FeS scale) and their destruction by turbulence and acid species are at the core of the new model. Thermodynamic and kinetic factors were derived from literature published over the past 60 years.
Common industry rules of thumb are 1 or 1.5 to 1 ratio of S/TAN to minimise acid corrosion. However, the quantities of S (wt%) differ so much from the mg KOH/g used to neutralise the acid (total acid number/TAN) that such values are arbitrary. The new model can explain why two crude oils with similar S/TAN values can corrode quite differently at the same temperature.
The new SNAPS-TAC model, resulting from the combined work and experience of numerous Becht SMEs, can help:
● Set integrity operating windows (e.g., crude and site stream TAN, S/TAN ratio, flow velocity)
● Predict corrosion for RBI
● Evaluate corrosion rate of crude blends
● Address turbulent flow issues
● Estimate time to restore protective barrier layers
● Establish TAN or S processing limits with given equipment
● Estimate optimum aggressive crude slate to reach turnaround (or other controlled shutdown) within remaining life
● Provide guidance on use of commercially available inhibitors to mitigate corrosion when running corrosive crudes
● Prioritise the circuits to upgrade for a stepwise investment strategy
● Identify which circuits or parts of circuits should be monitored more thoroughly
● Identify spot crudes for feed blending for a given period of time
● Determine blend limits on opportunity crudes without excessive upgrading or replacement in kind
● Calculate crude blending requirements to reach non-corrosive levels
● Evaluate alternating high TAN/high S block operations
● Estimate barrier layer persistence
● Compare block operation with blending opportunity crudes
● Provide much needed information for proactive decision making to maintain or improve equipment reliability when running opportunity crudes
In addition to all of these offline uses, the new model can be linked to the refinery’s DCS/Historian system and track the expected cumulative metal loss over time depending on actual crudes/blends processed and the operating conditions.
Understanding the SNAPS-TAC Model
Without going too deep into scientific detail, the following paragraphs provide an overview of the SNAPS-TAC model that should be sufficient to demonstrate its ability to affect better decisions for refinery operations. This model combines sulfidic and naphthenic acid (NAP) attack and flow effects, based on well-established, published principles that were benchmarked against published lab and plant data. These benchmarking exercises are never easy, because all of the information is rarely provided and test results need to be conditioned for the way the testing was performed.
In the range of 450-750°F, reactive S compounds cause sulfidic corrosion (sulfidation) of carbon, chrome, or stainless steels in crude distillation units or in the front ends of other downstream units. Sulfidation, in the absence of added hydrogen, is often treated with the modified McConomy curves, which do not explicitly consider flow effects. The original McConomy curves were developed from a broad survey relating corrosion rate with alloys, temperature, and a factor of 0.6%S for concentration of the total sulphur (%S). The original curves were later modified to be less conservative. Although useful for alloy comparisons, scattering in the survey data limits these curves to broad guidelines, as outlined in API RP 939.
Naturally occurring carboxylic acids in crude oils, e.g., NAP, are very corrosive in the same temperature range.[3,4,5,6] However, NAP-containing crudes are not S free and are often co-distilled with S-bearing crudes so that simultaneous naphthenic acid and sulfidation (SNAPS) corrosion is applicable to crude unit operations.[7,8,9] Research and experience yield conflicting observations based on the %S/TAN ratio. In the absence of a generally accepted model, industry practice for SNAPS control relies on rules of thumb or on the %S/TAN tables found in the appendices of API RP 581. No validation data was provided for the API 581 table values and the guidelines advise caution in their application, especially for high-flow locations.
In general, the industry has modelled S and TAN independently and then used proprietary algorithms to combine them with wall shear stress (WSS) to capture the effect of refinery flow turbulence.
The authors have taken an alternate approach that is designed to connect the lab chemistry with refinery flow turbulence that is based upon diffusion mass transport. Research at the Institute for Corrosion and Multiphase Technology (ICMT) at Ohio University has developed a two step “pre-treat and challenge” protocol to follow the formation and depletion of scale formation. The published results show that scale formation and structure are a function of S/TAN ratio and that diffusion (mass transport) through the scale varied in part due to the presence of magnetite in the scale.[10,11,12]
A review of the underlying reactions led to the development of a SNAPS corrosion model. In this model, the primary, product, and interaction reactions are incorporated into the equations (Figure 1).
The combined kinetics of these laboratory reactions is linked with refinery application based on turbulence-driven mass transport kinetics (Figure 2). As discussed below, the flow factor, or turbulent acceleration coefficient (TAC), is calculated from flow conditions and fluid properties at reaction temperature. In this model, both reactive S and NAP acids react simultaneously, forming and depleting an amorphous nano-porous “barrier layer.” Mass transport affects the delivery of reactants to and through the barrier layer by a combination of fluid and solid phase diffusion mechanisms. In the model (which has parallels with an approach taken by the nuclear power industry for flow-assisted corrosion), mass transport characteristics of the fluid are calculated separately and then used as an input in addition to TAN, %S, and alloy. As the kinetics are a function of barrier layer growth over time, the algorithms can rationalise the effect of duration.[13,14,15]
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