Understanding Sulfur Content in Alkylate from Sulfuric Acid-Catalyzed

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Process Engineering

Understanding Sulfur Content in Alkylate from Sulfuric Acid-Catalyzed C/C Alkylations 3

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David L. Minnick, Rajkumar Kore, Christopher J Lyon, Bala Subramaniam, Mark B. Shiflett, and Aaron M Scurto Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04364 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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Revision Submitted January 31st, 2019 to Energy & Fuels:

Understanding Sulfur Content in Alkylate from Sulfuric Acid-Catalyzed C3/C4 Alkylations David L. Minnick†, Rajkumar R. Kore, Christopher J. Lyon, Bala Subramaniam, Mark B. Shiflett, and Aaron M. Scurto* Department of Chemical & Petroleum Engineering and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, KS 66045, USA.

Abstract: Alkylation of short-chain olefins with isobutane catalyzed by sulfuric acid is a common process for reformulated fuel. Here, pilot-plant and commercial C3 and C4 alkylates were examined for sulfur content, acid content, and emulsion formation. Even though the thermodynamic solubility of sulfuric acid in alkylate is negligible at process conditions, the C4 alkylate samples contained ~20 ppm sulfur mostly from very dilute emulsions with ~3 µm droplets of sulfuric acid and alkyl sulfates that were stable even after 6 months. The sulfur content and droplet size increased for propylene alkylation. However, no detectable emulsion or sulfur content could be generated synthetically by intense mixing with either 2,2,4trimethylpentane (a model alkylate) or a treated pilot-plant alkylate with concentrated or spent sulfuric acid over the course of several hours. Thus, the alkylate sulfur content is most likely created during the acidcatalyzed chemical reaction steps and not from high-shear mixing.

Keywords: alkylation, sulfuric acid, emulsions Introduction

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Alkylation of short-chain olefins (e.g. ethylene to amylene) with isobutane using acid catalysts yields iso-alkanes that produce vehicle fuels with high octane numbers (research octane number: RON; and motor octane number: MON).1-5 The olefins typically emanate from the fluid catalytic cracking (FCC) unit and isobutane from various units at a refinery. The process converts generally lower value three-carbon and four-carbon molecules to fuels of higher value. The alkylate is typically blended into other hydrocarbon fuel streams from the refinery process. The two most common commercial catalysts for the alkylation reaction are sulfuric acid and hydrogen fluoride (HF). Over the decades, the fraction of installed units has changed. In the 1980’s each catalyst represented about 50% of all commercial units.6 However, industrial accidents and tests in 1986 and 1987 involving HF7, which can form toxic and low-lying aerosol clouds, have resulted in the increased usage of sulfuric acid, which is now the most common catalyst.3 Both technologies require some amount of processing to remove either fluorides (using the HF technology) or sulfates (using sulfuric acid technology) from the alkylate. Since 2017, the U.S. Environmental Protection Agency’s Tier 3 emissions program limits gasoline to 10 ppm of sulfur on a per-year average and 80 ppm on a per-gallon basis8 which is similar to standards in Europe, Japan, South Korea, and several other countries.9 Other technologies have been explored to catalyze the alkylation reaction, including solid acids and ionic liquids. Solid acid catalysts have been investigated for over 50 years.10,

11

The main challenge

impeding commercialization of solid acid alkylation technologies is the relatively rapid deactivation of the catalyst. Packed bed, fluidized bed and moving bed reactors have been used to combine reaction/catalyst regeneration cycles to achieve continuous operation. The AlkyClean® process (Albermarle) and the Alkylene® process (UOP) have been demonstrated at pilot-plant scales but their capacities are still much lower than the sulfuric acid-based alkylation processes. Research into the use of ionic liquids as catalysts, co-catalysts, or modifiers for C4 alkylation began with Chauvin et al.12 in 1994 with a number of groups disclosing various advances with acidic ionic liquid phases. 13-21 Our group utilized mixtures of acidic and neutral ionic liquids and sulfuric acid or triflic acid to produce alkylates with high selectivity of


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trimethylpentanes and greater catalyst stability.22 More recently, Honeywell UOP and Chevron introduced a new liquid alkylation technology (ISOALKYTM) based on an ionic liquid catalyst.23, 24 This technology uses a proprietary chloroaluminate catalyst with HCl as a co-catalyst. The catalyst is claimed to possess tunable acidity to provide high quality alkylate while avoiding undesirable cracking, isomerization, and heavy byproduct formation.25 This technology has been demonstrated in a 10 BPD demonstration plant at Chevron’s Salt Lake City Refinery and requires separation of alkyl-chloride byproducts. Despite these efforts, a substantial majority of the global alkylate production capacity will continue to use sulfuric acidbased alkylation technology for the near future. As discussed above, progressively stringent regulations on sulfur content in gasoline fuel has generated interest in understanding the mechanism of sulfur entering alkylates produced from sulfuric acid-based technologies. Such an understanding is essential for developing rational process strategies for mitigating the sulfur content to acceptable levels. Accordingly, sulfur and acid contents in alkylates produced from either butene or propylene alkylation with isobutane using sulfuric acid as the catalyst have been characterized in this work. Samples from both pilot-plant and commercial scale operations were analyzed. These streams were investigated for the presence of sulfuric acid emulsions using dynamic light scattering (DLS) to understand the source and formation of sulfur content. Given that alkylates are produced in an intensely agitated reactor that promotes mixing between the hydrocarbon and acid phases, a sample of just 2,2,4-trimethylpentane (as an alkylate proxy) was intensely mixed with sulfuric acid and investigated for sulfur content to understand the role of intense mixing, if any, on sulfur content. Accordingly, the various experimental investigations are organized as follows: (a) sulfur content based on direct elemental analysis and indirect measurement of acid content; (b) acid emulsions present in the alkylate phase; (c) measurements of acid solubility and acid content in a model alkylate (2,2,4-trimethylpentane) with and without intense agitation. Results from these investigations shed new insights into the source and mode of sulfur content in the hydrocarbon effluent providing rational guidance for process development aimed at reducing sulfur content in fuels.


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Experimental Methodology Total Sulfur Analysis. A PAC Antek MultiTek sulfur analyzer was used to determine the total sulfur (TS) content of the alkylate samples using the ASTM D5453 method. The instrument was calibrated using sulfur standards that were prepared using n-dibutyl sulfide in 2,2,4-trimethylpentane at concentrations of 0.3, 5, 10, 15, 20, 50, 200, and 500 ppm elemental sulfur (1%). Concentrations in ppm are on a mass basis, e.g. g sulfur per 1 g sample. The standards were purchased from Alfa Aesar Specpure and used as received. The instrument was calibrated prior to each experimental trial and maintained a linear calibration curve between 0.3 to 200 ppm sulfur. The calibration curve became nonlinear at sulfur concentrations between 200 and 500 ppm and a second order polynomial equation was used to determine the sulfur concentration in this region when necessary. A calibration curve with a correlation coefficient R2 > 0.999 was achieved prior to running each batch of samples. Total Acid Number. A Metrohm 809 Titrando titrator equipped with a Solovtrode potentiometric electrode was used to conduct the total acid number experiments according to a modified ASTM D664 method. The titrant was prepared at a concentration of ~0.01 M potassium hydroxide (KOH) in isopropanol (IPA) and calibrated prior to each experiment using dried potassium hydrogen phthalate (KPH). The total acid number method is not selective to strong acid species (e.g. sulfuric acid) and instead measures the total quantity of acid species present in the sample (e.g. sulfuric acid, alkyl hydrogen sulfate, etc.). Untreated alkylate may contain sulfur dioxide which may impact total acid number measurements. Therefore, the alkylate samples were degassed using a small vacuum pump to approximately 100 mbar to remove any residual sulfur dioxide (SO2) that may or may not be present in the alkylate samples.26 Subsequently, 100200 grams of alkylate were vigorously contacted with approximately 30 grams of distilled water to extract any water-soluble sulfur compounds into the aqueous phase. The aqueous and hydrocarbon phases were separated in a separation flask. After extraction, the hydrocarbon and water phases were analyzed by total sulfur analysis and the water phase was analyzed by potentiometric titration. Multiple aqueous washings were performed on select alkylate samples to test the extraction efficiency. Quantitative extraction with


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one water wash was confirmed by analyzing the second wash for total sulfur, which was lower than the limit of detection (< ~0.3 ppm). The total acid number (TAN) in units of mg KOH/g sample was determined using the following equation:

TAN 

Vtitrant  Ctitrant  MWtitrant mg 1000 M sample g

where Vtitrant is the volume of titrant (here aq. KOH) in L, Ctitrant is the concentration of the titrant in molarity (mol/L), and MW is the titrant species molecular weight. The total acid number can be converted to composition of sulfuric acid ((TAN, in ppm mass)) by assuming that the only acidic species present in the water extract phase were from H2SO4 using the relation below:



1 g  1  1 mol H 2 SO4  6    MWH 2 SO4 10 1000 mg MW 2 mol KOH    KOH 

 TAN , ppm H 2 SO4   TAN  

where the molecular weight (MW) of KOH is 56.106 g/mol, and H2SO4 is 98.08 g/mol. In addition, the sulfuric acid composition can also be computed by the total sulfur (TS) content in the aqueous extract by assuming that all sulfur species are in the form of H2SO4. This sulfuric acid composition ((TS, ppm mass)) is:

 TS , ppm H 2 SO4   TS 

MWH 2 SO4 MWSulfur

Acid Strength. The acid strength is determined by a gravimetric titration with an aqueous sodium hydroxide solution and a pH meter. Approximately 0.5g of the acid sample is placed in 25 mL of deionized water with a phenolphthalein indicator and a stir bar. The sodium hydroxide solution is then added dropwise until the solution achieves a neutral pH. The measurement is repeated in triplicate and the average acid strength is then reported in mass percent.


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Dynamic Light Scattering (DLS). A Brookhaven NanoBrook Omni dynamic light scattering (DLS) instrument was used to detect the presence of emulsified sulfuric acid droplets within the alkylate hydrocarbon samples. The DLS experiments were conducted with the instrument in backscattering mode (173° scattering angle), which is optimal for working with dilute samples. The analysis was conducted by adding approximately 2 mL of alkylate to a cuvette which had a 10 mm path length. Sample acquisition times ranged from 30 seconds to 5 minutes depending on concentration of droplets in the sample. For samples with low droplet concentrations, extended acquisition times were provided to allow the droplets to pass through the laser with enough frequency to obtain a reliable measurement. DLS experiments are traditionally run on samples, which contain a concentrated number of droplets or particles in solution. In this regard, experiments on the alkylate samples were different due to the dilute nature of the impurity in solution. However, the technique was verified by analyzing samples of pure 2,2,4trimethylpentane (TMP) which have no droplets or particles and samples containing a known emulsion. When the pure TMP was analyzed in the DLS, only uncorrelated scatter appeared. However, for systems with emulsions, the correlated signal was orders of magnitudes higher than baseline scatter. DLS is typically used in other fields for determining particle or drop diameters often at volume fractions orders of magnitude higher than in these present studies. However, we have confirmed the ability of the DLS to determine if droplets are present or not; and we believe the measured droplet diameters to be at least semiquantitative. Moreover, optical microscopy (see below) was used to confirm the presence (or absence) of droplets and to confirm the approximate droplet diameters. An example of a strong DLS correlation from an emulsion is shown in Figure 1. The sample was synthetically prepared by vigorously mixing sulfuric acid and 2,2,4-trimethylpentane with immediate sampling (no settling time) and measurement. In general, DLS measurements are typically performed on solutions that contain concentrated quantities of the immiscible/insoluble liquid/solid phase. The large quantity of droplets in solution yields a strong DLS correlation similar to what is shown in Figure 1. However, if there are no detectable droplets, then the DLS plot will just contain baseline scatter.


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0.50 0.45 0.40 0.35 0.30

C(τ)

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0.25 0.20 0.15 0.10 0.05 0.00 1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

τ(µs)

Figure 1: Typical DLS correlation profile for a sample containing concentrated droplets 0.9 0.6 m.

Optical Microscopy. A BioTek Cytation 5 inverted optical microscope was used to visually analyze droplets within the alkylate samples. The instrument is equipped with three magnifications (5x, 10x, and 20x). The primary advantage of this instrument is the covered well plate where approximately 200 L of sample can be pipetted into a single well for analysis. Given the volatile and low viscosity nature of the alkylate samples, using traditional glass slides proved difficult as the sample would run off the slide and evaporate prior to analysis. In this regard, the Cytation instrument and well plate allowed a larger sample to be added and minimized evaporation and run-off issues by using the well plate. For select samples, vacuum evaporation was utilized to increase the volume fraction of the dispersed acid phase. Approximately 300 mL of untreated alkylate was added to a round bottom flask and connected to a rotary evaporator. The temperature was maintained at 40 oC with a maximum vacuum pressure of ~20 mbar. Since the alkylate vapor pressure is several orders of magnitude higher than that of sulfuric acid and alkylsulfates, the more volatile alkylate was removed, increasing the concentration of emulsified droplets in the residual alkylate phase. Approximately 90% of the initial alkylate was removed by evaporation. The distillate phase was analyzed for sulfur and acid content. Both measurements consistently provided non-


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detectable readings indicating that the acid and sulfur species remained in the bottoms phase. Optical microscopy of the distillate phase additionally showed no droplets present. The bottoms sample contained approximately ten times (10x) the concentration of sulfur and acid compared to the initial sample and the mass balance around the process, with respect to sulfur, consistently closed within ±5%. Additionally, the number of droplets visible by optical microscopy increased proportionally due to the removal of excess alkylate. Apparent Solubility of Sulfuric Acid in 2,2,4-Trimethylpentane. The apparent solubility and reactive interactions between trimethylpentane and sulfuric acid were investigated as a function of temperature and contact time. Approximately 1 g of 98.6 wt.% sulfuric acid, ~5 g of 99+% pure 2,2,4-trimethylpentane, and a small stir bar were added to a glass vessel and placed in a thermostatic water bath. The acid and hydrocarbon phases were gently contacted using the stir bar, which perturbed the phases without forming any dispersed droplets. Alkylate samples were obtained at specified times and analyzed for sulfur content using the total sulfur analyzer. Mixing and Settling Studies.

Intense mixing studies between acid and hydrocarbon phases were

accomplished using a 50 mL Autoclave Engineers Hastelloy reactor. The reaction vessel was equipped with a cooling jacket and connected to a thermostatic water bath to maintain temperature control. Agitation of the acid and hydrocarbon phases was accomplished using a 6-blade impeller that had a diameter of 10 mm. The reactor was capable of mixing at speeds up to 5000 rpm and the mixing rate was monitored by a digital tachometer. A typical contact study was run at 5000 rpm using approximately 15 g of acid and 25 g of hydrocarbon with the reaction vessel being ~90% full by volume. Generation of Pilot Plant Alkylate, Spent Sulfuric Acid, and Synthetic Spent Sulfuric Acid. The alkylate samples were produced at the DuPont-Stratco pilot-plant in Kansas City, KS. Here, either butenes or propylene was alkylated with isobutane by vigorously stirring with concentrated sulfuric acid in a temperature-controlled pressure vessel that simulates the commercial Stratco reactor under commercial alkylation conditions. The acid/hydrocarbon emulsion was separated in a gravity settler followed by partial


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debutanization of the hydrocarbon phase to remove most C3 and C4 hydrocarbons. The alkylate (remaining hydrocarbon phase) was then transported in plastic bottles to the University of Kansas for analysis. The sulfuric acid phase was considered as “spent” sulfuric acid and had a sulfuric acid content of at least 90%. “Neat” Sulfuric acid was obtained from DuPont Stratco at a titrated concentration of 98.6 wt.% (1.4 wt.% water). The “synthetic” spent sulfuric acid is created by bubbling only the C4 olefin stream used in the alkylation reaction in neat sulfuric acid without the presence of isobutane at typical alkylation conditions. The resulting mixture is then degassed and analyzed for purity.

Only samples with a purity of

approximately 90% are used. Materials. Reagent ACS Spectro Grade 2,2,4-trimethylpentane (99.9+ wt.% pure) was obtained from Acros Organics. 0.1 ± 0.0005 N potassium hydroxide in isopropanol was obtained from Lab Chem. HPLC grade water, ACS Grade potassium hydrogen phthalate (≥ 98.5 wt.% pure), Certified ACS grade toluene (≥99.5 wt.% pure), and Certified ACS Grade isopropanol (≥99.5 wt.% pure) were obtained from Fisher Scientific. Spec Pure standards of sulfur in isooctane were obtained from Alfa Aesar at concentrations of 0.3, 5, 10, 15, 20, 25, 50, 200, and 500 ppm (± 1% of specified concentration).

Results and Discussion The purpose of these studies was to quantify the amount of sulfuric acid and sulfur-containing derivatives found in typical sulfuric acid-catalyzed C4 alkylation reactions of industrial relevance. Fundamental studies on model systems were then performed to elucidate how sulfur enters and remains in the hydrocarbon phase. The effects of the sulfuric acid concentration/type and mixing were investigated for a model system (2,2,4-trimethylpentane), a series of pilot plant alkylate samples, and one commercial alkylate sample. Characterization of Alkylate from Pilot Plant and Commercial Alkylation Units


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The initial goal was to quantify the concentration of sulfur (e.g. sulfur-containing species) in typical C4 Table 1: Total sulfur content of untreated alkylation reactions using typical isobutane and butene olefin feedstocks. Eleven different alkylate samples were obtained from DuPont Stratco, Inc. from various pilot plant runs and one sample from a commercial alkylation unit at an oil refinery. These were analyzed for total sulfur concentration, total acid number (TAN), and for the presence of emulsions. The effect of C3 (propylene) feedstock was also

alkylate from pilot plant (PP) and commercial (CM) alkylation units. Sample ID PP1 PP2 PP3 PP4 PP5 PP6 PP7 PP8

Total Sulfur Contenta (ppm) 26 ± 2 16 ± 2 (19 ppm b) 50 ± 3 56 ± 4 49 ± 1 (56 ppm b) 9±1 11 ± 1 11 ± 1

investigated. Total Sulfur Content of Alkylate Phase The total sulfur content of each sample was measured and listed in Table 1. The nine samples consisted of pilot plant (PP) runs from a C4 olefin feedstock (labeled PP1 to PP9); one pilot plant alkylate from a C3 feedstock (PPC3); and one commercial sample (CM1). The results indicate that the majority of the C4 alkylate samples contained 10 to 50 ppm sulfur, most likely in the form of sulfuric acid, alkyl sulfate species, etc.2 These samples are taken from the effluent of the pilot plant reactor and settler. Except for sample PP1, the other samples did not undergo any treatment to reduce sulfur impurities as would be done commercially. An untreated sample (CM1) taken from the effluent of a commercial reactor at a refinery was also analyzed and found to contain 21 ppm of total sulfur. Thus, the pilot plant samples are similar in sulfur content to larger-scale commercial units. While 10 to 50 ppm levels are relatively low, they are still finite and represent a level that must be decreased even for an alkylate product that will ultimately be blended with other low-sulfur refinery streams.


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The accuracy of the total sulfur method was confirmed by an independent third-party testing laboratory (Magellan, Inc.) for alkylate samples PP2 and PP5. The results agreed within 5 ppm for the PP2

PP9 10 ± 1 CM1 c 21 ± 2 d PPC3 126 ± 2 a Based on elemental sulfur; b third-party testing; c commercial refinery untreated alkylate; d alkylate using a propylene-rich olefin feedstock.

alkylate sample (KU: 16 ± 2 ppm vs. Magellan: 19 ppm) and within 10 ppm for the PP5 alkylate sample (KU: 49 ± 1 ppm vs. Magellan 56 ppm) verifying the reliability of our analytical method. The effect of aging of the samples over time was evaluated since some of the samples were received over the course of 6+ months. If the samples contain unstable/reactive sulfur-containing species, or weak emulsions of sulfur-containing species, then the sulfur contents in such samples may change significantly due to phase separation. The samples were stored at room temperature (~20°C) in plastic containers. The sulfur content of the first four samples was measured upon receipt and then 6 months later. The average total sulfur numbers of these first four samples received were within  4% over 6 months indicating that the emulsions were stable. This is well within the average standard deviation of all measurements ( 7%) at these low ppm concentration ranges (~10-100 ppm). Thus, change in the sulfur content in the alkylate samples was not statistically significant over the course of these investigations. The PPC3 alkylate sample was derived from a propylene-rich olefin feedstock rather than the traditional butene feedstock. This sample contained significantly more sulfur (126 ppm) compared to other alkylate from a butene feedstock. This is consistent with previous studies that propylene-derived alkylate typically contains more alkyl sulfate impurities and acid species compared to C4 alkylate.27 Isopropyl hydrogen sulfate and di-isopropyl sulfate are more thermally stable than sec-butyl sulfates.28

Total Acid Content of Alkylate Phase Total acid number (TAN) titrations were performed on several samples to assess the relative quantity of acidic species within each sample. TAN measurements were conducted by first extracting acidic water-


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soluble species from the alkylate with a water wash step. As shown in Table 2 for sample PP5, this analytical method also helps discern the relative fractions of water-soluble vis-à-vis water insoluble sulfur species in the alkylation sample. The sulfur material balance for a 307.8 g alkylate sample contacted with 50.4 g water is shown in Table 2 based on the following measurements: the initial sulfur content of the alkylate sample, the residual sulfur in the alkylate after the water extraction, and the sulfur species that are extracted into the aqueous phase wash. The absolute mass of sulfur extracted (soluble sulfur species) along with the sulfur concentrations (in ppm) normalized to 1 gram of alkylate sample are reported. Importantly, the mass balance on sulfur for the liquid-liquid extraction process closed to within 2.9%, indicating the ability of this method to reliably quantify the amount of water-soluble sulfur species present in the alkylate samples. Acidic species within the aqueous extract phase were analyzed by titration with a dilute potassium hydroxide solution according to the total acid number (TAN) method. The TAN method provides an analytical result in units of mg of KOH per g of sample that is converted to ppm of sulfuric acid species present in the sample using a stoichiometric ratio of the molecular weights (see experimental details in the Methodology section).

Importantly, the entire aqueous extract phase was analyzed in the titration

experiment and therefore, the quantity of acidic species in the alkylate phase was computed using the quantities of alkylate and water used in the liquid-liquid extraction process.

Table 2: Sulfur mass balance using water wash technique for alkylate sample PP5.

Total Sulfur in Bulk Alkylate before wash Residual Sulfur in Alkylate after wash

Absolute Massa (µg)

Mass Fraction based on 1 g of Alkylate sample (ppm)

14,619 13,695

47.5 ± 1 44.5 ± 1


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Aqueous Soluble Sulfur Species in wash 1,355 Mass Balance Deficit: 2.9% a 307.8g of alkylate sample contacted with 50.4 g of water.

4.4 ± 0.5 2.9%

The sulfuric acid content is also estimated based on the total sulfur analysis of the aqueous phase extract and normalized to 1 gram of the alkylate using the molecular weight ratio of sulfuric acid and sulfur and is shown in Table 3. If only H2SO4 is extracted into the aqueous phase, then the estimates of sulfur concentrations based on TAN and elemental sulfur analysis should be identical within experimental accuracy.

Table 3: Total sulfur and acid content in the untreated alkylate from pilot plant alkylation units. Sulfuric Acid Equivalent Alkylate Sample using Total Aq. Sulfurd (ppm) f PP5 49 ± 1 0.019 ± 0.002 9±3 3 ± 0.7 10 ± 2 PPC3e 126 ± 2 0.446 ± 0.007 210 ± 5 55 ± 1 170 ± 2 a On elemental sulfur basis; b assumes that all acidic sulfur species to be H SO ; c solubility of 2 4 water-soluble sulfur species in ppm elemental sulfur in alkylate (g S/g alkylate); d assuming that all water-soluble sulfur species being H2SO4 only; e alkylate using a propylene-rich olefin feedstock. Total Sulfur Contenta (ppm)

Total Acid Number (mg KOH/g sample)

Sulfuric Acid Equivalent using TANb (ppm)

Total Sulfur of Water Soluble Speciesc (ppm)

The sulfur content determined from TAN and elemental sulfur analyses are comparable when assuming that all water-soluble sulfur species are derived from sulfuric acid. The sulfur species extracted into the aqueous extract phase are likely derived from a combination of sulfuric acid and short chain water-soluble alkyl hydrogen sulfate species. However, regardless of this assumption, our analysis clearly indicates that significant quantities of residual sulfuric acid are present within the alkylate samples and that the quantity of acid present is significantly greater than the measured solubility limits of H2SO4 in TMP (see the following section). In other words, the acidic species present in the untreated alkylate samples are likely stabilized as emulsified droplets containing sulfuric acid and alkyl sulfate species.


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For the propylene-based alkylate product, an important observation from the liquid-liquid extraction data is that approximately 44% of the sulfur species within the C3 alkylate are water soluble compared to a value of only 9% for the C4 alkylate sample. In conjunction with the total elemental sulfur measurements shown in Table 1, the results indicate that the C3 alkylate contains more stable sulfuric-acid containing droplets than C4 alkylate. These results agree with previous studies that show propylene’s propensity to form alkyl hydrogen sulfate and dialkyl sulfate species more readily than C4, C5 and higher olefins.29, 30 These sulfates presumably act as surfactants for sulfuric acid and stabilize the emulsified droplets within the hydrocarbon phase. Emulsions in the Alkylate Phase The alkylate samples were analyzed by dynamic light scattering (DLS) and optical microscopy to identify the presence of suspended droplets/emulsions in the hydrocarbon phase. As summarized in Table 3, the C4 alkylate samples typically contain sulfuric acid on the order of 10 ppm or greater. In sulfuric acid/TMP mixtures created by mixing neat H2SO4 with TMP (a model alkylate), the apparent sulfur content in the alkylate phase is much less than 10 ppm (see studies below). Thus, the higher sulfur content found in the alkylates formed during reaction may arise from some level of emulsion formation in the alkylate phase. DLS analysis can identify when droplets are present in the alkylate (even when sulfuric acid is present in the ppm range) and lack thereof. For example, Figure 2 displays a comparison of DLS correlations for the PP1 treated alkylate sample (treated for acidic species, left) as well as the untreated PP2 alkylate sample (right). The treated alkylate did not produce a response in the DLS scan, e.g. no correlation (Figure 2a), similar to pure 2,2,4-trimethylpentane. Following production, the treated sample was subject to water washing and adsorption purification in the pilot plant, steps that were meant to reduce the sulfur content. In contrast, the untreated alkylate sample demonstrated a quantifiable scatter, presumably due to the presence of emulsion (Figure 2b). This explanation is consistent with the earlier hypothesis based on measurement of sulfur content (Table 3).


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Figure 2: DLS correlations for the treated PP1 alkylate and untreated PP2 alkylate samples run in triplicate.

The raw DLS data in Figure 2 are scattered for the treated PP1 alkylate sample indicating that no emulsified droplets were present in this sample. This result was confirmed by the total acid number titration method that showed no measurable acidity in the sample as well as optical microscopy, which did not display any visible droplets in solution (see below). In sharp contrast, a strong correlation is observed for the PP2 alkylate sample, suggesting the presence of emulsified droplets within the untreated hydrocarbon phase. This was also confirmed by an optical spectroscopy method discussed below. As expected, when similar DLS experiments were conducted with 2,2,4-trimethylpentane (model alkylate), there was no evidence of the presence of such emulsions. For the PP2 alkylate sample, shown above, the DLS data was used to obtain a size range of the droplets in solution. The Gaussian distribution of droplet sizes within the sample is shown in Figure 3. For the PP2 alkylate sample, the average droplet size was approximately 3 microns with a size distribution of approximately 2-5 m as listed in Table 4. The accuracy of the DLS droplet size determination was checked against optical microscopy measurements conducted on this sample as shown in Figure 4. Importantly, the microscopy image shows two different types of species in solution; individual droplets with an approximate diameter of 3 m and clusters of droplets with a larger diameter closer to 5 m. These values agree with the DLS measurements.


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Droplets in the 3 m range with a relatively low polydispersity indicate a macroemulsion that is, theoretically, thermodynamically unstable and will eventually

separate.31, 32

However, for

Table 4: Characteristics of emulsion for various alkylate samples.

sample PP2, the droplet size was measured over 9 weeks with an average of 4.51.5m. For the

Sample ID

Emulsions Detected?

PP2 sample, the emulsion fraction was PP1 estimated as 0.002% by volume, based on the PP2 PP5 droplet diameters and the assumption that the PPC3 d

NO YES YES YES

Avg. Droplet Diameter DLS (m) 31 53 154

Microscopy (m) 3 1 2 1 10

droplets have the density of pure sulfuric acid. Other samples yielded results similar in magnitude. Although these are very dilute emulsions, they are evidently quite kinetically stable.

120 100

C(d)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0 0

1

2

3

4

5

6

Droplet Diameter (um)

Figure 3: DLS size distribution of droplets within the untreated PP2 alkylate sample. Three replicates are shown.


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Figure 4: Optical microscopy results for the untreated PP2 alkylate sample (20x Magnification). Circles included to aid identification of the droplets in this dilute emulsion. While the DLS instrument is capable of detecting droplets within the bulk untreated alkylate samples, concentrating the samples by approximately 10-fold using vacuum distillation increased the concentration of droplets in solution and strength of the DLS correlation. An example of this is evident when examining experimental results from the PP5 alkylate sample. A 250 g sample of PP5 alkylate was vacuum distilled at 40oC and 20 millibar for approximately 30 minutes. The resulting (lower volatility) bottoms sample was concentrated to 25.1 g (by approximately 10-fold). The distillate and bottoms samples were collected and analyzed by DLS and optical microscopy for the presence of droplets in solution. Since sulfuric acid and alkyl sulfate species have low vapor pressures it was hypothesized that these components would remain in the bottoms phase. This was confirmed by total sulfur analysis that illustrated that the distillate fraction contained only 2.5 ppm sulfur while the bottoms fraction contained more than 300 ppm sulfur. The resulting concentrated samples displayed an increased quantity of droplets in solution that resulted in stronger DLS correlations and more droplets visible in the microscopy images. Microscopy images of bulk and concentrated PP5 alkylate samples are shown in Figure 5. When concentrating the alkylate samples, droplets tended to agglomerate as shown in Figure 6 for the PP5 alkylate. Smaller aggregates of


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droplets were observed in the bulk alkylate samples as well. The larger clusters were approximately 50 m and were not observed in the non-concentrated sample and seem to be an artifact of the distillation concentration method. The clusters were broken up into individual droplets by briefly sonicating the samples indicating that the equilibrium droplet size in solution is 2-5 m in size, as in the case of the nonconcentrated sample.


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a.

b. Figure 5: Comparison of optical microscopy results on a bulk PP5 sample (a) and concentrated PP5 sample (b) (20x Magnification)


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a.

b. Figure 6: Comparison of optical microscopy results for a concentrated PP5 sample (a) showing agglomerated droplets due to evaporation process and a sonicated sample (b) where the clustered droplets had been broken apart. (20x magnification)


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The concentrated PP5 alkylate sample was investigated by DLS before and after passing the sample through a 0.2 µm syringe filter. The results indicate that approximately 5 µm droplets were initially present in the bulk alkylate and that immediately after passing the sample through the syringe filter the average droplet size was reduced to approximately 0.8 µm. However, after resting the sample for 18 hours the droplet size increased to ~3 µm. These results indicate that any impurities in the alkylate exist as liquid, as they were able to pass through the 0.2-µm syringe filter and that the droplets coalesced over time to form larger aggregates, indicating the relative stable nature of the ~3 µm size droplets of the emulsion in the hydrocarbon alkylate phase. This indicates that the acidic emulsions form during the alkylation reaction and not because of the agitation. A sample of propylene C3 alkylate (PPC3) was also analyzed by DLS and optical microscopy. Importantly, the C3 alkylate contained a significantly greater quantity of sulfur species (126 ppm), of which 44% were water soluble and presumed to be derived from sulfuric acid. Given the high concentration of sulfur species in the mixture, it was hypothesized that this sample would have a micro-emulsion of sulfuric acid. As shown in Figure 7, a clear correlation was indeed observed, indicating droplets in the propylenebased alkylate.

Figure 7: DLS correlations for the propylene-based PPC3 alkylate sample.


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As the sample (PPC3) was very dilute in the emulsified droplets, vacuum evaporation was utilized to concentrate the droplets. Microscopy images obtained on the C3 alkylate sample (Figure 8) showed the presence of droplets in solution, confirming the DLS results. Larger clusters were initially observed, probably as an artifact of the concentration procedure. The C3 alkylate sample was sonicated for a few seconds prior to optical microscopy analysis to disrupt aggregated clusters of droplets with the goal of measuring the individual droplet size. DLS measurements were also conducted on the C3 alkylate without sonication. Average droplet size, as measured by DLS, for the concentrated C3 alkylate sample was approximately 154 µm.

Figure 8: Optical microscopy image of concentrated C3 (PPC3) alkylate. (20x magnification) Qualitative Analysis of Sulfur Compounds in the Alkylate Two alkylate samples were analyzed by mass spectrometry (PP5 C4 alkylate and PPC3 C3 alkylate) in a preliminary investigation into the sulfur species in the alkylate. The primary goal was to identify the types of alkyl sulfate species present in the untreated alkylate samples. The prevailing theory is that the organic sulfur ester species present in the alkylate phase are branched dialkyl sulfate complexes (diisobutyl sulfate, diisopropyl sulfate, etc.).2,

3

Negative ion hexane chemical ionization (CI-HEX) was used to


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analyze the PP5 C4 alkylate sample. The results indicated that the sample contained sec-butyl hydrogen sulfate. This was somewhat unexpected as the di-i-alkyl-sulfate would have a higher solubility in the alkylate phase than the mono-alkyl-sulfate.33, 34 The CI-HEX method is a soft ionization technique, which identifies the exact mass of species within the sample followed by fragmentation to identify what functional groups are present within the parent molecule. Error! Reference source not found. (Supplemental Data) shows the spectrum results for the PP5 alkylate, specifically targeting a species in the mixture with a molecular weight of 153.02 Da, which corresponds to mono sec-butyl hydrogen sulfate. The peaks in Error! Reference source not found. visible at 139.01 Da and 96.96 Da correspond to the iso-propyl hydrogen sulfate and sulfate fragments of the decomposed butyl hydrogen sulfate parent molecule respectively. The presence of the alkyl hydrogen sulfates lends credence to the hypothesis that such species can function as surfactants to form stable acid emulsions in the hydrocarbon phase. A similar analysis was performed on the PPC3 C3 alkylate sample. The results indicated the presence of mono i-propyl hydrogen sulfate with a molecular weight of 139.00 g/mol as shown in Error! Reference source not found..

Solubility of Concentrated Sulfuric Acid in Model Alkylate, TMP, in Low Shear Environment To differentiate the thermodynamic (molecular) solubility of sulfuric acid in an alkylate phase from that of an emulsion/suspension, a set of fundamental experiments were performed to determine the phase behavior of sulfuric acid (98.6 wt%) in a model alkylate, 2,2,4-trimethylpentane (TMP). The literature generally indicates that sulfuric acid is immeasurably insoluble in the hydrocarbon phase.35,

36

With

advances in analytical techniques, it was believed that this solubility could be quantified down to the ppm level. Here, the acid and hydrocarbon phases were gently contacted using a stir bar that perturbed the phases without dispersing droplets in an isothermal bath. Initially, the solubility of sulfuric acid in TMP at 25°C as quantified by sulfur content was measured after 24 hours of contacting. The results are shown in Table 5. To assess the interphase mass transfer rate in this relatively quiescent system, a series experiments allowing various equilibration times were conducted at 10°C and 25°C.


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The experimental results displayed in Table 5 indicate two important findings. First, the results show that the solubility of sulfuric acid in TMP at the alkylation reaction temperature (10oC) is less than the detection limit of 0.3 ppm sulfur after 48 hours of contact time. This is equivalent to

Understanding Sulfur Content in Alkylate from Sulfuric Acid-Catalyzed

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