Zahida H. Khan

Identification of biomarker compounds in selected Kuwait crude oils

Eighteen crude oils from Kuwait oil fields were analyzed by capillary GC/MS spectroscopy. Thirty-two biomarker compounds, 18 pentacyclic triterpanes and 14 steranes were identified and semi-quantitatively determined by selective ion monitoring (SIM) chromatography. The data obtained were used to calculate several biomarker indices. Depending on the oil source, the variation in the pentacyclic triterpanes distribution is more significant than the variation in the sterane distribution. The C29 norhopane and C30 hopanes were the dominating biomarkers in all the crude oils analyzed. The calculated biomarker indices indicated that the analyzed oils stem from medium mature oil reservoirs that have undergone biodegradation, to a certain extent.

Composition of high boiling petroleum distillates of Kuwait crude oils

Abstract Four crude oils of diverse API gravities were distilled on an 801 Autodest, Model 800 distillation unit to obtain vacuum gas-oil (VGO) distillates in the boiling range of 360–525°C. These distillates were separated into acids and bases by anion and cation exchange resins, and neutral nitrogenous compounds were removed by ferric chloride (FeCl3) on cellulose. Saturates were separated from aromatics by silica-alumina gel chromatography. Characterization of the crude oils and VGO distillates was carried out according to IP and ASTM methods. Nuclear magnetic resonance (NMR) spectroscopy was used successfully to calculate average molecular parameters and to elucidate an average molecular structure of the separated fractions.

Improvement of the quality of heavily weathered crude oils

Abstract Ways to improve the quality of highly weathered crude oils formed as a result of uncontrolled flow and seepage from damaged Kuwaiti oil wells were developed. Seven such oils collected from various lakes in different Kuwaiti oilfields had high contents of salts (up to 15 wt%) and water (up to 50 vol.%), high kinematic viscosity (815 mm 2 s −1 at 50°C) and low API gravity (8.2° API). Washing with fresh water removed the salt and water by up to 29 and 12 vol.%, respectively. Further washing increased the oil-water emulsion owing to the flocculant precipitation of waxy materials and high-molecular-weight compounds. The application of demulsifier, fresh water and heat broke the emulsion and allowed the water to settle out. The clean, water-free crude oil was similar in quality to a typical medium-API crude residue boiling > 300°C. This demonstrates that the oil had lost most of its light hydrocarbons.

Non-destructive analysis of crude oil by gel permeation chromatography

Abstract A heavy crude and a medium crude were separated by preparative gel permeation chromatography (g.p.c.) and a good recovery (95%) was obtained. Thirteen fractions of different molecular weight were collected and characterized by g.p.c. using analytical ultrastyragel columns. Elemental analysis, including heteroatoms and metals, and number average molecular weight determination were carried out on fractions 1–10.

Influence of catalyst pore size on asphaltenes conversion andcoke-like sediments formation during catalytic hydrocracking of kuwait vacuum residues

Abstract A critical factor that limits the maximum attainable conversion of heavy residues to lighter cuts in commercial residue hydroprocessing units is coke-like sediments formation. Suppression of sediments formation is highly desirable to increase distillate yields. As part of a research program on the factors which influence sludge or sediments formation during hydroprocessing of Kuwait vacuum residue for high conversion, we have investigated the relation between catalyst pore size, asphaltene conversion and coke-like sediments formation. Five Ni-Mo/γ-Al 2 O 3 catalysts with different unimodal and bimodal pore size distribution were used in the study. A unimodal pore catalyst with maximum pore volume in medium size mesopore range (100–200dia) showed the highest activity for the overall conversion of the residual oil to distillates. However, a relatively larger percentage of sediments was also observed for this catalyst. Catalysts with a large proportion of macropores, particularly in the 800–3000range produced little or no sediments, but showed poor activity for asphaltene cracking and overall conversion of residual oil to distillates. Molecular weight of the residual asphaltenes in the product increased with decreasing catalyst pore size. The concentrations sulfur and vanadium in the residual asphaltenes were found to be higher for catalysts having only small and meso-pores without macropores. The results have been explained on the basis of the importance of the ratio of feed molecular size to catalyst pore size in determining the diffusion and reaction rates in residue hydroconversion.

SEPARATION OF SULPHUR COMPOUNDS FROM VACUUM GAS OIL DISTILLATES BY LIGAND EXCHANGE CHROMATOGRAPHY

ABSTRACT For characterizing the sulphur components present in vacuum gas oil (VGO) distillates, it is essential to develop and standardize a procedure to separate the sulphur components present in the fraction before applying any techniques for structural elucidation. Ligand exchange chromatography (LEC) has been reported for the separation of sulphur compounds (SCs) from petroleum distillates and shale oil. Therefore, in the present study, the LEC technique was applied using four metals: Cu++, Ag+, Hg++, and Pd++. These metal salts (5 wt%) were impregnated onto grade-12 silica gel in aqueous phase or in dichloromethane. All of them were tested to form complexes with different types of SC in subfractions of VGO from Kuwaiti crude oils. The efficiency of the ions in separating the SCs, was in the order of PdCl2 (aqueous) <AgNO3<HgCl2<CuSO4<PdCl2 (dichloromethane). Palladium chloride impregnated over grade 12 silica gel in aqueous phase was very effective in separating the SCs from complex mixtures of aromatics. The methodology was optimized, and it was found that a 30 : 1 ratio of the adsorbent to the sample was effective for the separation of SCs.

Hydroprocessing of Heavy Residues: Relation Between Operating Temperature, Asphaltene Conversion and Coke FORMATION

Abstract The nature of changes that take place in asphaltenic and non-asphaltenic fractions of Kuwait vacuum residue with regard to removal of heteroatom such as S, N, V and Ni as well as conversion to lighter products were investigated during hydroprocessing in the temperature range 380–450°C. The average molecular weight and the molecular weight distributions of the asphaltenes in the feed and products were also determined by Gel Permeation Chromatography (GPC). The studies revealed that operating at high temperatures enhanced depolymerization and fragmentation of asphaltenes to low molecular weight materials. Sulphur and vanadium were removed from the asphaltenes relatively easily compared with nitrogen and nickel removal. Asphaltenes conversion and the amount of toluene insoluble sediments in the product increased appreciably at high temperatures (430°C). A portion of asphaltenes was found to resist further cracking even at high temperatures. This portion contained high concentration of nitrogen and nickel. The H/C ratio of the stable asphaltene fragments was relatively low. Coke like sediments were formed in the product at high temperatures (430°c). The stable asphaltene fragments with low H/C ratio could possibly be the origin of sediment or coke formation during the process.

COMPOSITIONAL ANALYSIS OF A VACUUM RESIDUE

ABSTRACT A sample of vacuum residue (VR) was evaluated for its physical and chemical properties. The VR was separated into sixteen fractions according to molecular size distribution on preparative gel permeation chromatography (GPC) columns. Elemental analysis including heteroatoms (N, S) and metal (Ni, V) was carried out on fractions 1 through 9. Molecular weight distribution and number average molecular weight (M¯n) were obtained by analytical GPC for fractions 1 through 11. Forty percent of the VR contains high molecular weight materials (i.e., materials with molecular weights ranging from 1170 to 3920) and 64.3% total vanadium, 62.5% total nickel, 34.1% total sulphur, and 41.6% total nitrogen.