Article pubs.acs.org/IECR
Change of Hydrocarbon Structure Type in Lube Hydroprocessing and Correlation Model for Viscosity Index Kyungseok Noh,†,‡ Joohyun Shin,† and Jay H. Lee*,† †
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡ Global Technology, SK Innovation CO., Ltd., 325 Expo-ro, Yuseong-gu, Daejeon 305-712, Republic of Korea S Supporting Information *
ABSTRACT: Viscosity index (VI) is one of the most important properties that determine the quality grade of lube base oil and it strongly depends on the molecular structure of the hydrocarbon constitute. To produce group III base oil, which is the most valuable base oil type, through lube hydroprocessing, the VI of the final product should be over 120. However, rigorous studies and analyses on the compositional change in lube hydroprocessing and a practical correlation model for VI with respect to the chemical compositions are still lacking. In this study, pilot tests for lube hydroprocessing, which is composed of hydrotreating/ cracking followed by hydroisomerization, are implemented with three different types of feedstocks (paraffinic, intermediate, and naphthenic vacuum gas oils) and under different reaction conditions (catalyst and temperature). The fractional changes of the hydrocarbon components by the lube hydroprocessing reactions are quantitatively confirmed using a gas chromatography with mass selective detector. The resulting VI changes of the intermediate and final products are analyzed, and consequently the required reaction condition to manufacture the final lube base oil having a desired VI value is studied for each feedstock type. Finally, on the basis of the experimental data, two regression models for VI are developed: the one is purely databased using a stepwise regression method and the other one considers the physical meaning of the regressed component using the constrained linear regression.
1. INTRODUCTION A crude-oil refinery is an important production facility for refining and converting crude oils into valuable products. In a refinery, multiple products are produced by distillation and various subsequent downstream processes intended to satisfy their desired specifications, or to improve the product value. The value of a final product is determined by the quality and use of each product. Since the yield of each product depends on the compositions of the refined crude oil as well as the operating conditions of the subsequent processes, crude oil types, of which price and quality vary significantly, should be optimally selected with consideration of the overall processing unit network in order to maximize the refinery margin. Lube base oils are typically produced through upgrading processes from vacuum gas oil (VGO) or deasphalted oil (DAO) having relatively low value.1,2 In general, lube base oils are considered as higher value products than fuel-side products since they are used for the base material of the finished lubricants such as automotive engine oil, industrial oil, and marine oil, which are expensive. Lube base oils are divided into several grades according to each quality, and Table 1 shows the grade classification of the lube base oil provided by American Petroleum Institute (API).1 © XXXX American Chemical Society
Table 1. Grade Classification of Lube Base Oil Given by the American Petroleum Institute grade group group group group group
I II III IV V
saturates, wt %
sulfur, wt %
viscosity index (VI)
0.03 ≥90 ≤0.03 ≥90 ≤0.03 all poly-alpha-olefins all other base stocks not included in
80−119 80−119 120 min groups I to IV
Group I base oil is generally produced by solvent refining processes such as solvent extraction and solvent dewaxing with low yield and quality.3,4 On the other hand, group III base oils are produced by catalytic processes such as hydrotreating/ cracking and hydroisomerization; they are considered highquality lube base oils because of their high viscosity indices, high saturate contents, and low impurity levels. The current trend is to replace the conventional group I products with the Received: Revised: Accepted: Published: A
March 7, 2017 May 22, 2017 June 22, 2017 June 22, 2017 DOI: 10.1021/acs.iecr.7b00967 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
changes over the lube hydroprocessing and a practically useful correlation model for VI with respect to the chemical compositions are lacking thus far. In this study, compositions of the feeds and products are classified by their representative hydrocarbon structure types (e.g., n,i-paraffin, naphthene-rings and aromatic-rings structure), and the compositional changes in each lube hydroprocessing step are quantitatively analyzed by a gas chromatography with a mass selective detector (GC-MSD). To manufacture the final lube base oil having a desired VI value, the effect of each hydrocarbon structure component on the VI and the required level of cracking conversion are analyzed through a pilot study using three different types of feedstocks (paraffinic, intermediate, and naphthenic VGOs). Finally, a VI regression model is developed in which analytical data of hydrocarbon structure types are fitted by a stepwise regression method. Furthermore, to reduce the potential inaccuracy of the statistical method that uses data only, a modified regression model is introduced that can take into account the physical meaning of the effect of each component on the overall VI. Analysis of variance (ANOVA) tests are performed to validate the obtained two regression models, and the distributions and confidence regions of the estimated parameters are analyzed in terms of expected errors in the measurements of VI and compositions of the hydrocarbon components. Since the VI specification of lube base oil is one of the most important factors in crude oil selection and operation, it is expected that the developed VI prediction model will be useful for refinery optimization. The rest of the paper is organized as follows. Section 2 provides general backgrounds on VI in relation to the hydrocarbon type and lube hydroprocessing condition. In section 3, a lube hydroprocessing experiment conducted on a pilot plant is described including the technical methods used for the analysis of the feed and the products. In section 4, quantitative results of the experiment are analyzed, on the basis of which a regression model for VI calculation is developed.
high-quality lube oils of group III grade owing to the tightening environment regulations and quality requirements by the customers, and thus the demand of group III base oil is expected to grow further.5,6 Meanwhile, the remaining base oils outside of groups I to IV are defined as group V. Naphthenic base oils are involved in group V, which have low VI but high solvency and low pour point, and they can be used for specific applications (e.g., process oil or transformer oil).2 Viscosity index (VI) is one of the most important properties to determine the quality grade of a lube base oil, which represents a measure for the degree of viscosity change with temperature. The concept was originally proposed by Standard Oil’s Dean and Davis in 1929,7 which was then used to measure the kinematic viscosity sensitivity to temperature in base oil and lubricants. In the early days, the VI of the Pennsylvanian crude which showed little change in viscosity to temperature changes was defined as 100, and one of the U.S. Gulf Coast crudes which showed a very high sensitivity to temperature was defined as 0. Then the VIs of all other sample oils were calculated based on these two reference samples. Reflecting on the several issues in measuring VI values such as the discontinuities in the VI values, particularly in the low viscosity range,8 and the identification of samples with VI values greater than 100,9,10 the ASTM 2270 method was introduced in 1964 and has been used since.11 Base oils with higher values of VI meaning lower sensitivity to temperature changes are classified as high-quality products. In particular, group III base oil has a very high viscosity index value greater than 120, which requires severe operating conditions in the lube hydroprocessing. Unlike other factors determining quality of the fuel products (e.g., sulfur fraction), VI strongly depends on the molecular structure of the constituent oil. Thus, to meet the target quality of the product, it is highly important to select an appropriate feedstock and to optimize the operating condition of the lube hydroprocessing such as the conversion rate, which determines the compositional change in the feed. For this, the effects of the feed composition and key operating conditions of the lube hydroprocessing on VI change should be quantitatively modeled, and a rigorous VI prediction model for the final products should be built based on this information. There are several previous studies on the effects of molecular structure on VI value; regression models for the VI calculation reflecting this effect have been suggested using analytical data obtained from NMR spectroscopy or IR spectroscopy. Sarpal and co-workers12 studied the relationship between VI and molecular structure such as carbon type composition and distribution of branched structure using C13 NMR spectroscopy. However, the proposed correlation between VI and hydrocarbon composition (e.g., n-paraffin + branched structure contents) was based on a qualitative analysis rather than a quantitative one. Verdier and co-workers13 analyzed C13 NMR data of 20 different oil samples and proposed a correlation model for VI based on C13 NMR spectra data in which the peaks represent ethyl branching, methyl branching, and the aromatic region. However, it was only related to a few specific branching structures (e.g., ethyl and methyl branching) and the NMR analysis is relatively complex and difficult to apply to in an actual refinery plant. Recently, Braga and co-workers14 proposed a method for estimating VIs of the finished lubricants using infrared spectroscopy for quality monitoring. Even though VI is a very important property that determines the quality grades of base oils, the studies on the compositional
2. BACKGROUND 2.1. Chemical Structure Type and Viscosity Index. The structure type of complex hydrocarbons such as petroleum fractions can broadly be classified as paraffins, naphthenes (cycloalkanes), and aromatics. The paraffin group can be further classified into n-paraffins and iso-paraffins; the naphthene group can be classified into monoring, diring, and multiring naphthenes according to its ring numbers; and the aromatic group also can be classified into substituted monoring, diring, and multiring aromatics according to its ring numbers. These hydrocarbon structure types are directly linked to the VI and they give positive or negative effects on the overall VI value. In general, more n-paraffin contents give higher values of VI, followed by iso-paraffin, mono(linear)-naphthene, multiring naphthene, and aromatics in that order.4 Table 2 shows the qualitative effect of each hydrocarbon structure type on the value of the VI and pour point.4,10,15 Among the considered hydrocarbon structures, n-paraffin has the highest VI value. The VI values of C20−C44 hydrocarbon structures are between 170 and 226.15 However, it has high pour point and therefore cannot be used for lube base oil. Branched paraffin (i-paraffin) has a relatively high VI value and good cold properties, and thus is suitable for high quality lube base oil. As the branched chain is moved to the center of the principal chain, the value of VI becomes lower.15 Most of the B
DOI: 10.1021/acs.iecr.7b00967 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
hydrotreating/cracking and hydroisomerization as major components. Hydrotreating and Hydrocracking. The main function of hydrotreating/cracking is to increase VI by changing the chemical composition and also to remove the impurities for obtaining the desired product specification. Through the catalytic reaction, unfavorable molecular types such as the aromatics and multiring naphthenes are transformed into favorable ones such as iso-paraffins and fewer-ring (1,2-ring) naphthenes, given the right catalyst and reaction condition. The main reactions that contribute to the VI increase are the hydrogenation of aromatics and the cracking reactions such as decyclization (multiring opening). Korre and co-workers18 studied the relationship between the reactivity and the molecular structure of aromatic compounds through reaction networks of hydrotreating/hydrocracking. The reactivity order is hydrogenation followed by isomerization, ring opening, and dealkylation, but this also depends on the reaction severity. The first step is the hydrogenation of aromatics, which is a thermodynamically reversible reaction and its reactivity is known to be strong in the order of polyaromatics, diaromatics, and monoaromatics. For example, a substituted naphthalene (diaromatic) is more reactive than a monoaromatic such as substituted benzene, and the reaction rate of a polyaromatic such as anthracene is two to three times faster than that of a diaromatic such as naphthalene.19,20 Under a more severe and cracking condition, the complete hydrogenation of aromatic rings, isomerization, and cracking reactions take place in order. In particular, polycyclic naphthenes are transformed into naphthenes with fewer rings, mono(linear)-naphthenes or iso-paraffins by the ring opening (decyclization) reaction. The reactivity and the degree of cracking in the hydrotreating/cracking step could be represented by reaction conversion which is defined by the fraction of fuel products (e.g., naphtha, kerosene, diesel) converted from fresh feed, expressed as shown in eq 1; high conversion means that the yield of fuel product is high whereas the amount of UCO is low, which is a feedstock for the lube base oil.
Table 2. Effect of Hydrocarbon Structure Type on the Quality of Lube Base Oil: VI and Pour Point component
effect on VI
effect on pour point
n-paraffin (nP) i-paraffin (iP) 1-ring naphthene (1rN) 2-ring naphthene (2rN) multiring naphthene (mrN) 1-ring aromatic (1rA) 2-ring aromatic (2rA) multiring aromatic (mrA)
very high VI high VI medium to high VI medium VI very low VI medium to low VI low VI very low VI
very high pour medium to low pour low pour low pour very low pour low pour low pour very low pour
naphthene and aromatic components contain one or more ring structures with n,i-paraffin as side chains bonded to the ring in petroleum fractions. Specifically, as hydrocarbons contain more ring structures, their VI values become lower.4,15 In this respect, to produce the lube base oil of high VI, it is required to select a favorable feedstock in terms of hydrocarbon structures or to transform the unfavorable components of the feed into the favorable ones. Since lube base oil is produced through up-grading of VGO and DAO, which are obtained from the distillation of the crude oil, crude oil selection is very important in determining the distribution of the molecular structure types in the feedstock and ultimately the quality of the lube base oil. There are numerous different types of crude oils around the world and all crude oils have different yield structures, impurities, and chemical compositions. Representative classification criteria for crude oils are API gravity and sulfur contents. Lane and Garton16 classified crude oil as paraffinic, intermediate, and naphthenic types by the API gravity values of the two fractions from the distillation at 250− 275 °C (482−527 °F) and 275−300 °C (736−786 °F). Figure 1 shows the distribution of the world crudes characterized based on this classification.17
conversion (%) fresh feed − unconverted oil (360 °C + ) 100 = fresh feed
(1)
Generally, the conversion level can be controlled by reaction temperature and catalyst. With the same feed, as the conversion increases in hydrotreating/cracking, the VI of the UCO portion also increases proportionally by decyclization (ring-opening) and complete hydrogenation.21 However, the degree and pattern of the VI change by the reaction conversion is different with the type of feed, and the conversion level required to satisfy a VI specification for the final product is also different.2,4,22 Paraffinic and intermediate VGOs are suitable for manufacturing group III lube base oil having a high VI, while naphthenic VGOs are not appropriate for producing such. For example, with VGOs from the California heavy crude (San Joaquin Valley, SJV), which is one of the heavy naphthenic types, even when the cracking conversion is increased to over 70%, the VI value of the dewaxed base oil from the feed is still below 60.4,22 Therefore, the naphthenic VGOs are generally used for manufacturing naphthenic base oil, which is in group V.2,23
Figure 1. Distribution of world crudes.
2.2. Lube Hydroprocessing. As explained in the previous section, the most unfavorable hydrocarbon structures from the perspective of VI are aromatic and multiring naphthenes. To have a product of high VI, their amounts must be lowered through the processes of hydrotreating and hydrocracking. Even though n-paraffins help to achieve a high VI value, they have bad cold properties such as high pour points and cloud points. Thus, n-paraffins should not be contained in the final lube base oil product in large amounts and they should be changed to i-paraffins or smaller paraffins through the hydroisomerization reaction. Thus, a typical lube hydroprocessing step to obtain a high VI base oil includes C
DOI: 10.1021/acs.iecr.7b00967 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 2. Overall flowchart of the experiment.
Figure 3. Schematic diagram of the fixed bed lube hydrotreating/cracking reactor in the pilot test.
Hydroisomerization. The catalytic hydroisomerization process is used to improve cold properties such as pour point of base oil by the reaction of hydroisomerization and selective cracking of long chain n-paraffins. Girgis and Tsao24 studied the reaction pathways and kinetics of hydroisomerization and hydrocracking of n-paraffins over bifunctional catalysts. As a result of the catalytic hydroisomerization, the VI of the final product decreases since n-paraffin structures with high VIs are eliminated to improve the cold property.
3. EXPERIMENT 3.1. Pilot Test for Lube Hydroprocessing. Figure 2 shows the overall flowchart of the experimental setup, which is composed of a hydrotreating/cracking reactor followed by a hydroisomerization reactor. The catalyst and basic operating condition such as pressure, throughput, etc. for the pilot experiment are determined based on an actual commercial lube hydrotreating and hydrocracking process in an Asia lube refinery; these factors are known to be similar from refinery to refinery.2,4 The changes in the hydrocarbon structure types and VIs of the feeds and the products were quantitatively analyzed in each lube hydroprocessing step. For each lube hydroprocessing reaction, an isothermal fixed bed down-flow reactor was used, which was composed of a reaction section, a separation section, and feed/product tanks (Figures 3 and 4). In the hydrotreating/cracking reactor, unconverted heavy oil (UCO, 360 °C+) was separated from the other products (e.g., naphtha, kerosene, and diesel), and it was fed to the subsequent hydroisomerization reactor. For each reaction, mass balances were checked for 24 h; the acceptable range for the mass balance error was chosen as ±1.0 wt %.
Figure 4. Schematic diagram of the fixed bed lube hydroisomerization reactor in the pilot test.
3.1.1. Feed VGO. For the pilot test of lube hydroprocessing, three types of feedstocks having different chemical compositions were examined: paraffinic VGO (from the South East Asia paraffinic low sulfur crude), intermediate VGO (from the Middle East intermediate high sulfur crude), and naphthenic VGO (from the Australia low sulfur naphthenic crude). The paraffinic and intermediate VGOs were obtained as intermediate products from the paraffinic and intermediate crudes, respectively, in a commercial refinery. The naphthenic VGO was obtained from a separation facility by the TBP (ASTM D2892) and Pot-Still (ASTM D5236) cut method using an actual Australia naphthenic crude sample. As mentioned previously, different products are made according to the compositional characteristics of the feed. Paraffinic and intermediate VGOs are used to produce group III base oil, D
DOI: 10.1021/acs.iecr.7b00967 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 3. Reaction Conditions of the Hydrotreating/Cracking Unit paraffinic VGO case 1 reaction catalyst WABT (HDT/HCK) LHSV, h−1 conversion, wt % a
case 2
intermediate VGO case 3
case 1
case 2
naphthenic VGO
HDTa (100%) 354
HDT (100%) 380
HDT (75%) and HCKb (25%) 380/380
HDT (55%) and HCK (45%) 385/382
HDT (55%) and HCK (45%) 385/390
HDT (100%)
0.25 15.4
0.5 19.9
0.5 23.4
0.77 49.5
0.77 63.3
0.5 12.0
360
HDT: hydrotreating. bHDK: hydrocracking.
of sulfur should be below 50 ppm and that of nitrogen below 5 ppm. Another objective for this step was to adjust the reaction conversion so as to meet the targeted VI value in the final product: The conversion of the hydrotreating/cracking reaction should be adjusted by considering the VI of the intermediated product unconverted heavy oil (UCO) which is a feedstock for the subsequent hydroisomerization process. 3.1.3. Hydroisomerization. Unconverted heavy oil (UCO), which is a bottom product of the hydrotreating/cracking reactor, was subsequently hydroisomerized (Figure 2) in order to manufacture the lube base oil with high VI and low pour point. Figure 4 shows a schematic diagram of the pilot plant for hydroisomerization composed of two serial reactors, a separator, and feed/product tanks. The reactor effluent after hydroisomerization was first separated into gas and total liquid product streams in a hot high-pressure separator. Then the total liquid product stream was accumulated in a product tank and then separated into lube base oil and other fuel products by the TBP (ASTM D2892) and Pot-Still (ASTM D5236) cut methods. For the hydroisomerization of UCO, the commercial hydroisomerization catalyst (Pt/Zeolite) and the hydrofinishing catalyst (Pt−Pd/Amorphous silica−alumina) were used and 55 cm3 of the catalyst was loaded into each reactor. And then the catalyst was dried with nitrogen (
