A polyisobutylene succinic acid ester intermediate intended for sour brine corrosion inhibition is most commonly prepared by the thermal ene reaction of high-reactivity polyisobutylene having a number-average molecular weight between 950 g/mol and 1,050 g/mol with maleic anhydride at 200–230 °C under a nitrogen blanket, followed by esterification of the resulting polyisobutylene succinic anhydride with a polyol such as trimethylolpropane, pentaerythritol, or sorbitan monooleate at 160–180 °C. The anhydride intermediate is normally controlled to an acid value of 80–110 mg KOH/g by ASTM D664, and incomplete esterification deliberately leaves a free carboxylic acid content of 35–60 mg KOH/g in the ester intermediate. In a 5,000 L glass-lined batch reactor equipped with a turbine agitator operating at 65–85 rpm, the esterification reaction is driven by continuous removal of water through a packed column and vacuum system at 200–250 mbar absolute. Industrial batch records indicate that the acid value can drift by as much as ±2.5 mg KOH/g when the initial polyol-to-anhydride mole ratio varies by 0.03 or when the reactor jacket temperature overshoots the set point by 8 °C during the first 3 h. The resulting intermediate is typically diluted with an aromatic or glycol ether solvent to a viscosity suitable for metering pumps, and its solubility in sour brine is limited until neutralization with an amine or amino alcohol produces a more water-dispersible corrosion inhibitor derivative.

Thermal Decomposition of Pentaerythritol-Based Ester Intermediates During Vacuum Distillation

Pentaerythritol esters of polyisobutylene succinic anhydride can undergo thermal decomposition when the vacuum distillation step used to strip solvent and unreacted maleic anhydride exceeds 210 °C for more than 45 min. The decomposition pathway appears to involve retro-esterification and release of maleic anhydride, followed by carbonization of the polyol core, producing a dark brown intermediate with acid value drift above 8 mg KOH/g and sediment levels exceeding 0.5 wt% as determined by gravimetric filtration through a 0.8 µm membrane. The presence of thermally degraded components reduces the interfacial activity of the final corrosion inhibitor in sour brine because the high-molecular-weight fraction aggregates in high-TDS solution and deposits on the walls of injection quills. Operations that use a wiped film evaporator with an internal surface temperature of 195 °C and residence time of 20–30 s have been observed to preserve the color and acid value within release specifications. Batch-to-batch variation in viscosity at 100 °C from 75 mm²/s to 160 mm²/s measured by ASTM D445 has been reported when the nitrogen purging rate is insufficient, causing oxidative coupling through unsaturated polyisobutylene terminal groups. In sour brine systems the operational consequences include poor pumpability at winter gathering-line temperatures and reduced film persistency under high shear.

In field evaluations of continuous corrosion inhibitor injection into sour crude gathering lines operating at 40–70 °C with hydrogen sulfide partial pressures of 0.01–0.08 bar and carbon dioxide partial pressures of 0.30–0.80 bar, a formulation containing a trimethylolpropane-based polyisobutylene succinic acid ester intermediate neutralized with diethylenetriamine was injected at active dosages between 50 ppm and 150 ppm based on total liquid throughput. Weight loss coupons of API 5L X65 carbon steel mounted in a 20 L bypass autoclave conforming to the general arrangement of NACE TM0172 displayed general corrosion rates of 2.0–4.5 mpy, while the uninhibited baseline reached 48–62 mpy, corresponding to inhibition efficiencies of 91–95%. Electrochemical linear polarization resistance measurements using a three-electrode arrangement under ASTM G59 showed polarization resistance values rising from 120 Ω·cm² to 1,400–1,800 Ω·cm² within 45 min of inhibitor application. The field data also demonstrated that the intermediate required a polar co-solvent concentration of 10–15 vol% ethylene glycol monobutyl ether to prevent phase separation in the chemical injection skid at ambient temperatures below −20 °C. These observations establish an operational boundary for cold-climate sour brine applications, where the fully formulated inhibitor must remain homogeneous through 50 µm in-line filters.

What Limits the Acid Value Reduction Efficiency in Single-Stage Esterification Reactors?

The conversion of polyisobutylene succinic anhydride to the ester intermediate in a single-stage batch reactor is limited by the equilibrium concentration of water, the steric hindrance of the branched polyisobutylene chain, and the progressive depletion of the polyol in the liquid phase. When a fixed charge of 2.2 mol of trimethylolpropane is reacted with 1.0 mol of polyisobutylene succinic anhydride having an anhydride number of 95 mg KOH/g, the acid value after 14 h at 170 °C typically plateaus at 40–48 mg KOH/g. Further reduction to 35 mg KOH/g requires a vacuum of 120 mbar or a second esterification step using an azeotropic solvent such as mixed xylenes. In 4,000–6,000 L reactor vessels, the mass-transfer limitation of water removal becomes the dominant kinetic barrier below a residual water content of 0.10 wt%, and the reaction rate becomes pseudo-zero-order with respect to acid value. The resulting acid ester retains a measurable free succinic acid group, which is deliberately preserved because subsequent imidazoline or amidoamine condensation with polyethyleneamines requires a stoichiometric balance between carboxylic acid and primary amine functionality. Complete esterification to an acid value below 5 mg KOH/g would consume the reactive acid sites and produce a neutral ester that cannot be converted to the final corrosion inhibitor by amine condensation. The acid value window of 35–60 mg KOH/g is therefore not an unreached target but a compositional control range for downstream reactivity.

Sour brine systems containing dissolved hydrogen sulfide above 0.01 bar partial pressure and chloride concentrations from 80,000 mg/L to 220,000 mg/L present a specific testing matrix for polyisobutylene succinic acid ester intermediates because both the interfacial film and the steel surface chemistry are affected by iron sulfide deposition. The intermediate itself is not generally considered an active corrosion inhibitor until it is condensed with a polyamine or an amino alcohol; however, its residual carboxylate groups can interact directly with iron sulfide scales and modify the wettability of the surface. Contact angle measurements on polished C1018 steel exposed to 3 wt% NaCl brine saturated with 0.05 bar H₂S at 60 °C have shown that the acid ester intermediate changes the water contact angle from 28° to 65–80°, indicating formation of a hydrophobic layer even in the absence of full neutralization. This wetting transition is accompanied by a reduction in the cathodic hydrogen evolution current under potentiodynamic scanning at 0.5 mV/s according to ASTM G5, but the protection is less durable than that of the fully condensed inhibitor because the free acid group is susceptible to desorption in high-salinity water. The distinction matters for sour gas wells where the inhibitor is first contacted with produced water containing 2.5 wt% CaCl₂; there the calcium ion can react with the free succinic acid to produce a sticky precipitate. Published data for the calcium salt formation constant of this specific polymeric succinate is limited, but operational experience recommends maintaining brine pH above 4.5 and limiting the acid value of the intermediate to 50 mg KOH/g or less to reduce the risk of filter blockage.

Salt Tolerance and Brine Partitioning in High-TDS Flowback Water

The salt tolerance of a polyisobutylene succinic acid ester intermediate in high-TDS flowback water is governed by the ratio of polar ester and acid groups to the nonpolar polyisobutylene backbone, the degree of maleation, and the solvency provided by the carrier solvent. A sorbitan monooleate-based ester with an acid value of 45 mg KOH/g and a kinematic viscosity of 180 mm²/s at 100 °C by ASTM D445 may remain clear in 10 wt% NaCl at 20 °C for 48 h but exhibits cloud points below 5 °C when the total dissolved solids exceed 150,000 mg/L. The addition of 8–12 wt% isopropanol or 15–20 wt% ethylene glycol monobutyl ether lowers the cloud point and stabilizes the microemulsion in brine, but high shear through a control valve can still produce a stable emulsion that complicates produced water separation. In sour brine systems at 70 °C, the partitioning coefficient between the brine phase and the hydrocarbon phase determines the concentration of inhibitor that reaches the steel surface. When the intermediate is formulated as a 25 wt% active solution in light aromatic naphtha and injected into a mixed flow of 20 vol% brine and 80 vol% sour crude, the acid ester partitions preferentially to the oil phase, with a measured brine-phase concentration of only 8–15% of the total active ingredient. This oil-phase preference is beneficial for protecting the upper walls of a low-velocity pipeline but insufficient for water-wet bottom sections, where localized pitting is typically initiated. The fully amine-neutralized derivative, by contrast, partitions more strongly into brine and can reduce pitting density on API 5L X65 coupons from 40 pits/cm² to 8 pits/cm² in flow-through tests. The intermediate must therefore be considered a precursor whose brine partitioning can be altered by the choice of neutralizing amine, with shorter-chain polyethyleneamines producing greater water solubility and longer-chain fatty amines preserving oil solubility.

For release of a polyisobutylene succinic acid ester intermediate into sour brine service, the conformity matrix below aligns the required properties with recognized test methods. The acceptance windows are based on typical supply specifications for intermediates used in downstream imidazoline and amidoamine synthesis.

Infrared spectroscopic monitoring of the esterification endpoint in polyisobutylene succinic acid ester intermediates uses the disappearance of the cyclic anhydride absorption band at 1,865 cm⁻¹ and the growth of the ester carbonyl band at 1,735 cm⁻¹; residual free acid is detected by a broad O-H stretch between 2,500 cm⁻¹ and 3,300 cm⁻¹. In a process laboratory using a Fourier transform infrared instrument equipped with a diamond attenuated total reflectance stage, a single scan of the reactor sample can differentiate a predominantly monoester intermediate from a diester or a hydrolyzed acid sample within 2 min. Gel permeation chromatography according to ISO 16014-1 reveals that the modal molecular weight of the ester intermediate increases by 200–400 g/mol relative to the starting polyisobutylene succinic anhydride, while the polydispersity index shifts from 1.4 for the parent polyisobutylene to 1.6–1.9 for the esterified product. The higher polydispersity is not generally detrimental to sour brine performance, but a low-molecular-weight shoulder below 700 g/mol indicates incomplete maleation or residual polyol esters that can contribute to foaming in separators. Hydrolysis of the ester intermediate in sour brine is another critical parameter. The ester linkage in a trimethylolpropane-based intermediate undergoes measurable hydrolysis in high-salinity brine at 80 °C and pH 4.0, releasing the polyol and generating additional free succinic acid. Static autoclave exposures of 5 wt% intermediate dispersed in 10 wt% NaCl brine with 0.05 bar H₂S and 0.50 bar CO₂ have shown an increase in acid value from 38 mg KOH/g to 52 mg KOH/g after 14 days, equivalent to a 25% loss of ester functionality. The hydrolysis rate is lower for pentaerythritol esters because the neopentyl structure provides steric shielding, and higher for sorbitan monooleate esters whose ester carbonyl is more accessible. This hydrolytic sensitivity means the intermediate should not be stored premixed with acidic brine for extended periods; it should be injected continuously or neutralized shortly before application. The exact hydrolysis half-life varies with brine composition and temperature, and published data for this specific polymeric ester system is limited, so short-term autoclave verification under field-representative conditions is required for each new well or flowline.

Upon injection of a formulated polyisobutylene succinic acid ester intermediate into sour brine systems that also contain commercial demulsifiers, scale inhibitors, and oxygen scavengers, compatibility must be verified before continuous application. The most common incompatibility is the formation of a viscous interfacial rag between the acidic ester intermediate and amine-neutralized demulsifiers or quaternary ammonium compounds. In a 500 mL graduated cylinder test using 80 vol% synthetic sour brine and 20 vol% light crude, a dosage of 100 ppm of the acid ester intermediate combined with 50 ppm of a standard polyoxyalkylene demulsifier produced an interfacial pad of 12–18 mm after 30 min of agitation, whereas the amine-neutralized derivative reduced the pad to 4–6 mm. The poor compatibility arises from the free carboxylic acid functionality of the intermediate, which protonates the basic nitrogen atoms in the demulsifier and reduces its surface activity. For this reason, the intermediate is generally not injected alone into sour brine systems that require simultaneous crude dehydration. Instead, the condensation product is used, or an amine is co-injected at a molar ratio of 0.8:1 to 1.0:1 relative to the acid value to convert the intermediate in situ to the active inhibitor and reduce the interfacial rag. The in-situ conversion approach has been applied on a 12,000 barrel/day production battery where the rag layer was eliminated within 8 h after amine addition, but the residual free amine concentration in the brine must be monitored to avoid amine-induced foaming in separators.

When the Inhibitor Intermediate Is Formulated with Glycol Ether Solvents in Cold-Climate Gathering Systems

When the polyisobutylene succinic acid ester intermediate is formulated with ethylene glycol monobutyl ether or propylene glycol methyl ether for winter sour gas gathering systems, the solvent concentration must be balanced against the risk of water stripping into the brine phase. Ethylene glycol monobutyl ether at 10–15 wt% in the final inhibitor formulation lowers the pour point to −25 °C by ASTM D97 and prevents gelling in chemical injection lines at −30 °C, but at concentrations above 20 wt% it partitions into the aqueous phase and reduces the film persistency of the neutralized inhibitor by displacing the hydrophobic tail from the steel surface. Field measurements using electrical resistance probes in a sour gas gathering line operating at 0.02 bar H₂S and 0.40 bar CO₂ indicated that the corrosion rate at 45 °C remained below 3.0 mpy when the glycol ether content was 12 wt%, but increased to 7.5 mpy when the same active dosage of 75 ppm was delivered with 22 wt% glycol ether. The mechanism is believed to involve competitive adsorption of the solvent at anodic sites on the carbon steel surface, which interrupts the formation of a compact iron sulfide/polymer complex. The use of methanol as a hydrate inhibitor further complicates the phase behavior of the acid ester intermediate. At methanol concentrations above 15 vol% in the produced water, the ester intermediate can precipitate as a tacky solid that accumulates on the downstream side of flow-control chokes, particularly where the pressure drop across the choke exceeds 20 bar. This accumulation correlates with localized adiabatic cooling of the gas phase and the associated reduction in solvency of the high-molecular-weight polyisobutylene tail. Operations that require methanol injection above this threshold should use a fully condensed inhibitor with a lower acid value and a higher hydrophilic-lipophilic balance, or pre-blend the intermediate with a mutual solvent such as butyl cellosolve at 25–30 wt% to prevent deposit formation. Published data for the solubility limit of polyisobutylene succinic acid esters in methanol-water mixtures is limited; therefore the safe operating window is typically verified by cloud point titrations in the intended produced water composition rather than extrapolated from simple solvent data.

The use of a polyisobutylene succinic acid ester intermediate in sour brine systems with high shear choke valves and multistage centrifugal pumps places specific demands on the molecular weight distribution and the residual free acid content. High shear can strip an adsorbed inhibitor film from the pipe wall, but the branched polyisobutylene backbone provides a comparatively compact hydrodynamic volume that reduces the rate of shear-induced desorption relative to linear carboxylic acid inhibitors. In a recirculating flow loop equipped with a 3.5 kW centrifugal pump producing wall shear stress above 80 Pa, the weight loss corrosion rate on carbon steel specimens in a sour brine containing 0.03 bar H₂S and 0.35 bar CO₂ was 5.0–6.5 mpy with the unneutralized intermediate at 100 ppm, but decreased to 1.8–2.6 mpy after neutralization with polyethylenepolyamine. Parallel electrochemical impedance spectroscopy showed the charge-transfer resistance of the neutralized system remained above 1,200 Ω·cm² after 24 h, whereas the unneutralized intermediate fell to 450 Ω·cm², indicating a less coherent surface film. These results demonstrate that the acid ester intermediate is a structural building block rather than a complete corrosion inhibitor for sour brine service, and that its performance is defined by the controlled introduction of polyisobutylene segments and pendant carboxylate groups into the final molecule.