Case Studies: Mitigating Pressure Buildup in Spent Sulfuric Acid

Sulfuric acid is widely used in industrial applications like alkylation, petroleum refining and chemical manufacturing. Its stability plays a critical role in ensuring efficient and safe operations, especially when dealing with regenerated or spent acid. Factors like temperature, pressure and contaminants can impact the acid’s chemical stability, leading to unwanted byproducts. Understanding how these factors affect the acid is key to improving process efficiency and avoiding potential process safety risks [1].

Two case studies are presented to help mitigate risks. The first focuses on evaluating the thermal stability of both clean and spent sulfuric acid, with particular emphasis on determining the onset temperature of the reaction and the extraction of red oil. The second case investigates the efficacy of commercially available adsorbents in mitigating pressure buildup in spent acid within a closed vessel, simulating conditions encountered during storage and transportation. This research builds on previous works, including a study on red oil formation in spent acid conducted by American Oil Co. researchers in 1963[3]. By revisiting and expanding these earlier findings, we provide a more comprehensive perspective on how time, temperature and red oil content influence the stability of sulfuric acid.

Setting the Stage: Experimental Detail

Materials and Reagents

The specific origins of the two spent acid types from the refineries were intentionally omitted to maintain confidentiality. Instead, they will be referred to as spent sulfuric acids from Refinery A and Refinery B.

The baseline sulfuric acid used in this study was 90% reagent-grade, while the spent sulfuric acid samples were collected from railcars at the two refineries mentioned above. Conjunct polymers [3], or red oil, were extracted from spent sulfuric acid for further analysis. An aging study was conducted using a 600 mL Hastelloy [2] C stirred reactor, designed to simulate storage and transport conditions. The reactor is an adiabatic design, and all other chemicals used in this study were of analytical grade and were used without further purification.

Adsorbent and Ion Exchange Resins

The adsorbent used in this study is a commercially available, styrenic crosslinked polymer resin. This industrial-grade material consists of light tan, spherical, macroporous beads engineered to be insoluble in strong acids, strong bases and most organic solvents — properties that make it particularly well-suited for use in sulfuric acid environments. Depending on the process requirements, the resin can be regenerated using various methods.

The ion exchange resin evaluated in this study is also a commercially available, strongly acidic cation exchange material, widely used in the chemical industry for acid catalysis and purification processes. It is functionalized with sulfonic acid groups (–SO₃H), which facilitate efficient cation exchange by interacting with positively charged species in solution. The resin consists of gray, opaque, macroporous spherical beads designed to withstand the harsh conditions of concentrated sulfuric acid environments.

Red Oil Extraction

Red oil was extracted from spent sulfuric acid through a liquid-liquid separation method. The acid was diluted with an equal volume of deionized cold water and stirred in an ice bath for 10 minutes to reduce viscosity and facilitate extraction. The diluted content was extracted in an Erlenmeyer flask using n-pentane as the organic solvent. The solution was left undisturbed for phase separation, allowing the red oil to collect in the organic phase. The organic phase was isolated and subjected to a 5% caustic solution wash in another Erlenmeyer flask. The neutralized organic extract was then gently warmed and purged with a nitrogen stream to remove the n-pentane, gradually isolating the red oil [3].

Reactor Setup for Aging Study

The reaction kinetics between conjunct polymers and sulfuric acid were studied using a 600 mL vertical reactor constructed from Hastelloy C alloy. The reactor was equipped with a temperature control system and a mechanical stirrer to ensure uniform mixing. Samples of spent sulfuric acid, containing varying amounts of red oil and water, were heated to 110°F for five days, followed by 140°F on the sixth day to simulate summer storage conditions (e.g., in tanks or railcars) and extreme environmental temperatures. The reactor’s design enabled precise monitoring of both pressure and temperature.

Differential Scanning Calorimeter (DSC) 

DSC analysis was used to determine the onset temperature of spent acid under various experimental conditions. The onset temperature is the point at which the reaction accelerates, possibly generating excessive heat or becoming uncontrolled, thereby posing potential safety hazards [4].

NMR Analysis

A 600 MHz nuclear magnetic resonance (NMR) spectrometer was used to study the molecular characteristics of the red oil extracted from spent sulfuric acid. Samples were prepared by dissolving a small amount of the red oil in deuterated chloroform (CDCl₃) and placing it in NMR tubes. Both proton (¹H) and carbon (¹³C) NMR spectra were obtained to determine the chemical information of the conjunct polymers.

The NMR spectra provided detailed insights into the molecular structure, focusing on the presence of olefinic bonds, aromatic and aliphatic (saturates) functional groups. The results were used to assess the stability of the spent acid and the rate of conjunct polymer breakdown during storage.

Case study 1: Experimental results and discussions

Table 1 below compares the spent sulfuric acid (H₂SO₄) properties from Refinery A and Refinery B, focusing on water content, acidity strength and onset temperature. Additionally, it includes data for spent acid from Refinery A that was reduced to 82% acidity strength. Both Refinery A and Refinery B acids have the same acidity strength (89 wt%), indicating similar concentrations of sulfuric acid. The synthesis of 82 wt% acid involved plant acid reacting with a 60/40 mixture of C4 olefins and isobutane. The resulting acid has an increased water content (8%) and reduced thermal stability (143°F).

Spent acids from Refinery A and B, as depicted in Table 2, underwent solvent extraction to isolate their red oils. Comparative analysis revealed that the red oil content, onset temperatures and carbon-to-hydrogen ratios were remarkably similar across both refinery locations. These consistent values suggest that the conjunct polymers in the spent acids share comparable structures, indicating a similar chemical composition and reaction behavior. Reducing the acidity strength of spent sulfuric acid from Refinery A to 82% through the alkylation process resulted in notable compositional and thermal changes. The carbon content decreased to 75%, while the hydrogen content dropped to 8.3%. Additionally, red oil extraction increased significantly to 4.9wt%, with a rise in water content to 8%. The onset temperature for thermal reactivity declined sharply to 143°F, indicating a marked reduction in thermal stability.

In Table 3, NMR characterization reveals consistent aromatic content across all samples, with 0.1 wt% aromatic carbon. However, reducing the acidity strength of Refinery A spent acid to 82% significantly affects the olefinic and aliphatic carbon content. The olefinic carbon content increases from 8.7 wt% to 12.8 wt%, indicating enhanced olefin formation or concentration. Conversely, the aliphatic carbon content decreases from 68.0 wt% to 64.5 wt%, suggesting a potential shift in the molecular composition or degradation of aliphatic structures. Refinery B spent acid closely resembles the original Refinery A acid, with 8.3 wt% olefinic and 68.6 wt% aliphatic carbon, showing minimal variation. Using NMR to determine red oil's characteristics is not a novel approach. It was successfully applied in 1997 by John C. Edwards and Paul J. Giammatteo [5] of Texaco, Inc., as part of an online application study.

Figure 1 illustrates the pressure generated by various acids in a reactor when heated to 110°F for five days and raised to 140°F on the sixth day to simulate summer storage conditions (i.e., tank or railcar). Key observations include:

• 90% Reagent Grade H₂SO₄ (red line): Maintains the lowest pressure, remaining nearly flat throughout the experiment, indicating high stability under the given conditions (0 psig generated).
• Refinery A Acid at 89% Strength (green line): This is a standard strength spent acid for most refineries. The graph shows a moderate and gradual increase in pressure, reaching a plateau, suggesting controlled reactivity with minimal pressure escalation (22 psig generated).
• Synthetic Acid 82% strength (purple line): It exhibits a rapid and significant pressure increase within the first five hours, surpassing the others, before eventually plateauing. This sharp rise indicates reduced stability due to decreased acidity strength, which aligns with reports of railcar pressure buildup when spent acid is left in the hot sun for extended periods (reaching 37 psig).

Lowering the acid strength from 89% to 82% significantly impacts the pressure buildup, particularly under increasing temperature. This highlights the importance of maintaining higher acid strength to ensure safer storage and handling conditions.

Case study 2: Evaluation of adsorbent and ion exchange resins in mitigating pressure buildup.

The next series of tests focuses on identifying a solution to minimize pressure buildup in spent sulfuric acid during storage. A low-strength spent acid (i.e., an unstable acid) was synthesized. This synthetic spent acid was produced by reacting a 60/40 mixture of C4 olefins and normal/isobutane with typical plant-grade spent acid for two hours. The resulting solution exhibits the characteristics shown in Table 4.

A significant pressure buildup was observed over seven days during the aging study of this synthetic acid in the lab bench reactor. NMR results indicated that the presence of olefins within the conjunct polymers was the primary contributor to this pressure increase. These olefins likely underwent slow polymerization or decomposition reactions, releasing gaseous by-products, including sulfur dioxide and light hydrocarbon gases. Additionally, residual water in the spent acid appeared to catalyze further reactions, exacerbating the pressure rise.

In the context of our experimental study, activated charcoal and ion exchange resins were explored as potential materials for removing impurities from the reaction mixture. Despite their differences, these materials share several key attributes that help inhibit the reactive species. Activated charcoal relies on physical absorption, utilizing its large surface area to capture organic molecules and volatile contaminants. In contrast, the ion exchange resin functions as an adsorbent by attracting and binding substances to their surfaces. Figure 2 shows the seven-day aging study results of these different materials. 

Key experimental observations:

• Pressure Buildup in Synthetic Acid (Neat): The red line indicates that the neat synthetic acid (without any adsorbent) shows a rapid and significant pressure increase over time, reaching over 25 psig by the end of the experiment. This suggests that the acid experiences severe pressure buildup without mitigation due to ongoing reactions or gas formation.
• Effectiveness of Adsorbent: The purple line shows that a styrenic adsorbent effectively suppresses pressure buildup, with the pressure remaining below 5 psig throughout the experiment. This indicates it is highly effective in mitigating gas formation in spent acid storage.
• Ion Exchange Performance: The green line, representing an ion exchange resin, shows moderate pressure buildup, staying under 10 psig but slightly higher than styrenic adsorbent. It effectively reduces pressure compared to neat acid but is less efficient than adsorbent resin.
• Activated Charcoal’s Moderate Performance: The blue line for activated charcoal shows a steady but gradual increase in pressure, suggesting it provides some mitigation. However, it is less effective than the above resins.

Lessons Learned

This study demonstrates that the use of a commercially available, industrial-grade adsorbent can effectively mitigate pressure buildup in spent sulfuric acid, due to its porosity and surface area. Through controlled experiments in a benchtop reactor, the styrenic crosslinked polymer resin was shown to significantly reduce pressure by adsorbing reactive intermediates responsible for gas formation, particularly red oil. The data indicate that this material outperforms other tested resins, offering a reliable solution for enhancing the stability of spent sulfuric acid during storage. While ion exchange resins and activated charcoal showed some effectiveness, the styrenic adsorbent emerged as the most efficient option. It effectively inhibits certain olefins and other reactive species while remaining stable in strong acid environments. After adsorption, the material can be regenerated through thermal desorption or solvent extraction, enabling cost-effective reuse. Further engineering and feasibility studies are needed to evaluate the scalability of this solution and its integration into industrial applications. ⊕

Disclaimer

This article is intended solely for informational purposes and does not constitute professional advice or technical guidance. The author and PEMEX Deer Park disclaim any liability for actions taken or decisions made based on the content of this article and make no representations or guarantees regarding its accuracy or completeness. The results presented are based on specific conditions and should not be interpreted as a guarantee of future performance.