SOOT FORMATION IN GAS TURBINE COMBUSTORS

ABSTRACT

Soot formation in the gas turbine combustors has serious effects on liner durability, carbon deposition and smoke emission.

In this work an analytical study is made to derive quantitative relationships for calculating temperature, unburned hydrocarbon and oxygen concentration from previous detailed experimental data. The quantities of the derived parameters are used in a soot formation model to predict soot concentration along the primary zone.

The soot concentration predicted compares favourably with experiment especially at full load conditions. This supports the capability of the model with the derived quantitative relationships in predicting soot concentration for different gas turbine combustors.     

A POTENTIALLY CAPABLE KODEL FOR PREDICTING COMBUSTOR LINER TEMPERATURE IN CAS TURBINES

The continuous trend towards using higher pressure ratio engines combined with the effects of using fuels of high aromatic content and boiling range and less hydrogen content, have Increased the vulnerability of combustor wall to high temperatures.

In order to overcome this difficulty and optimize the quantity and distribution of cooling air, prediction of temperature should be as accurate as possible.

In this work an empirical model has been developed, which enables predicting liner temperature with 7% deviation for different types of combustors at different loading conditions using fuels of wide range of aromatic content, hydrogen percent and boiling range. This is in comparison with 60%-100percnt; obtained with other models.     

THE SOLVENT EXTRACTION APPROACH TO PETROLEUM DEMETALLATION

Metals are present in petroleum depending on age and conditions of diagenesis, and concentrate during refining in heavy fractions and residua. Nickel and vanadium show particularly deleterious effects on catalysts, such as poisoning, excessive gas and coke formation. Besides, removing vanadium from fuel oils reduces the environmental impact of “acid rain” due to the oxidation of sulphur dioxide in the atmosphere.

In this paper, research and industrial practice literature concerning the removal. of nickel and vanadium from petroleum and its fractions by solvent extraction and related techniques is reviewed and arranged into four groups as follows:

i) Processes employing hydrocarbon solvents, used to modify the oily matrix solubility parameter in such a way that asphalt/enes be separated. Because a fraction of total metals present are associated to asphalt/enes, deasphaltation implies partial demetallation.

ii) Processes using polar solvents, scarcely miscible with petroleum and able to selectively extract from it metallic complexes of heteroatomic ligands (e.g. metalloporphyri ns).

iii) Processes not based on molecular phase equilibria like i) and ii) but on the formation, by contact with aqueous solutions, of a separate, colloidally stabilized phase. Either surface active agents are added or the surface activity of substances naturally occurring in petroleum is promoted.

iv) Processes based on extraction with chemical reaction (oxidation, chelation) orion exchange between organometallic species in petroleum and ligandsin solution of non-miscibile media.     

EFFECT OF PREHYDROGENATION ON HYDROCONVERSION OF MAYA RESIDUUM, PART II: HYDROGEN INCORPORATION

Maya 650° F residuum (Maya AR) was prehydrogenated over a standard hydroprocessing catalyst. The 650°F residuum of this product lpar;HMaya ARrpar; and Maya AR were then separately hydroprocessed further at selected conditions. The products were examined by elemental, ¹H, and ¹³C NMR analyses to determine the how hydrogen was incorporated during processing.

For all processing steps, hydrogen was incorporated in capping fragments formed during cracking reactions, as well as in hydrogenation reactions, heteroatom removal, and hydrocarbon gas formation, but the distribution of the hydrogen was dependent upon the type and severity of the process: For the direct hydroconversion of Maya AR, 25 to 30% of the total hydrogen was incorporated for heteroatom removal and hydrocarbon gas formation. The remaining hydrogen was incorporated in hydrogenation and cracking reactions.

For the two-step hydroconversion process, 30 to 40% of the total hydrogen was incorporated for heteroatom removal and hydrocarbon gas formation. The remaining was primarily incorporated in hydrogenation reactions. Some was incorporated into cracking reactions in the moderate severity case, but almost none was seen in the low severity case.

The hydrogen incorporation during each specific processing step is discussed, along with an evaluation of the prehydrogenation step a residuum conversion process option. These results will be also compared to previously reported hydrogen incorporation measurements on other feeds and processing methods.     

IDENTIFICATION OF NITROGEN COMPOUNDS AND THEIR EVOLUTIONDURINGAGEING NABSENCE ANDPRESENCE OF ADDITIVES EFFICIENCYOFHYDROTREATMENT

As established by several previous works, nitrogen compounds play a prominent role in the evolution of middle distillates containing cracked components, particularly regarding sediment formation and color evolution.

In a first part, this paper describes and compares stability properties of fuel blends using both an accelerated ageing method at 120°C (248°F ) and long term storage methods at 43°C(110°F) -ASTM 0 4625 - and at ambient temperature. Effectiveness of stabilizing additives is also evaluated. In mixtures containing LCOs, insoluble products are formed progressively during ageings, more or less rapidly according to the chemical constitution of the mixtures.

Then, it reports the complete identification of nitrogen compounds using gas chromatography equipped with a selective nitrogen detector and mass spectrometry showing that in light cycle oils, alkyl indoles and carbazoles are the main families.

Evolution of these compounds was followed kinetically during ageings in absence and presence of additives and alkyl indoles appeared as the moat evolutionary.

It appeared that some additives avoided evolutions of alkyl indoles without preventing sediment formation and color evolution. Oxidation mechanism involving nitrogen compounds should not be the only one to explain the storage evolutions of middle distillates.

Hydrotreatment converts all the alkyl indoles of LCO and prevents coloration end deposits in the storage of the mixtures of steaight-run distillates and LCOs.     

ELECTRICAL CONDUCTIVITY OF ASPHALTENES IN ORGANIC SOLVENTS

Conductivities of asphaltenes were measured as a function of concentration in solvents of varying permittivity. The molar conductivity was calculated assuming the molecular weight of asphaltenes to be 1000 g/mol

The Fuoss conductivity model was fitted to the experimental data with two free parameters i.e. the association constant and the molar conductivity at infinite dilution. The experimental data show indication of ion-pairs and strong dipole-ion interaction. Accordingly, the measurements were consistent with the model for ion pairs as given by Fuoss and multibody interaction theory given by Petrucci

At low concentration the asphaltenes consist mainly of free ions, as the concentration increases ion pairing leads to a decrease of the molar conductivity. At higher concentration ion-dipole interaction may lead to an increase of the molar conductivity, as is seen in some of the solvents. At even higher concentrations some indications of aggregation are observed

The results indicate that part of the asphaltenic fraction is ionized and behave as ions in solvents of low dielectric constant.     

ALCOHOL-CAUSTIC STEAM PROCESS - An emerging technology for heavy oil recovery

The oil recovery potential of an alcohol-caustic steam (ACS) process has been evaluated. Experimentally, this involves propagation of one molar caustic solutions containing varying alcohol concentrations by a saturated steam at 250 kPa through a 20.7° APIg oil-bearing sand matrix. The criterion for selecting butyl-, hexyl- and octyl- alcohols used in the experiment was derived from previous studies.

At 25% alcohol concentration, oil recovery data revealed increased oil production rate accompanied respectively by overall recovery increases of 4,21 and 22% OOIP for butanol, hexanol and octanol compared with the baseline (without alcohol) case. The produced oil-water ratios are also significantly higher at higher alcohol chain length.

Increasing the octanol concentration to 50 and 80% gave further improvement in the ultimate recoveries over the 25% level, leading successively to a reduced steam injection requirement.

From these trends, ACS process clearly has a technical merit and therefore deserves consideration for future application.     

WAXING TENDENCIES AND RHEOLOGICAL EVALUATION OF CRUDE-CONDENSATE BLENDS FOR AN OFFSHORE PIPELINE TRANSPORTATION

The applicability of blending technique in an offshore transportation of a Nigerian waxy gas condensate has been studied. Experimentally, chemically treated and untreated wax-ladden gas condensate samples were blended with non-waxy crudes in the range 15 - 50% of the condensate. This was followed by measurement of the rheological properties and waxing tendencies of the resulting blends.

At all the test temperatures (60 - 100°F), increased proportion of treated condensate samples in the blends has a reducing effect on the pour point and waxing tendencies of the blends. However, for those blends with untreated condensates, significant increases in these parameters are observed.

Also, viscosities of blends showed a greater reduction at higher proportion of treated than untreated condensate samples.

All these trends appear to re-affirm the feasibility for improved pipeline transportation of treated gas condensate as blends.     

ABSORPTION OF ASPHALT INTO POROUS AGGREGATES: TEST OF A SIMPLE PHYSICOCHEMICAL MODEL

Absorption is the process in which asphalt flows into a porous aggregate, under the driving force of capillary pressure. Asphalt absorption is of interest because it represents an economic loss of effective binder, and because it may change the properties of the resulting asphalt film and lead to premature pavement distress. Incorrect air voids estimation may also result from failure to properly account for asphalt absorption in pavement design. The objective of this research was to determine the temporal dependence of absorption upon asphalt and aggregate properties and to test a rational model for this dependence.

Absorption data were obtained for several model liquids and asphalts with synthetic alumina aggregates and a natural limestone aggregate. The data were used to test a simple absorption model incorporating the effects of asphalt and aggregate properties into a single dimensionless time variable.

While quantitative agreement was not obtained in all aspects of the model, certain predictions of the model were verified. The general shape of the absorption data followed the model prediction for synthetic alumina spheres. The effect of viscosity and surface tension was correlated fairly well by the surface tension/virosity factor in the dimensionless time variable

The use of the dimensionless time variable was able to reduce the time span for absorption from ca. four order of magnitude to only ca. one order of magnitude; although, there remained a significant variation in some of the absorption data, probably caused by differences in the contact angles for the various Liquids.

The absorption behavior with a natural Iowa limestone did not follow the shape of the theoretical curve predicted by the model. This deviation is thought to be c a d by the presence of air in the pores coupled with the large and polydisperse pore size of this aggregate     

DESIGN AND OPERATION OF A FLUIDIZED BED REACTOR MINI-PILOT PLANT FOR GASOLINE SYNTHESIS

MethanoI-to-Gasoline (MTG) process is an excellent process which produces aromatics-rich gasoline from methanol over the ZSM-5 catalyst. The methanol feed in this process is usually derived from coal or natural gas based syngas.

The dehydration of methanol to dimethyl ether (DME) is a key intermediate step in converting methanol into gasoline. The substitution of syngas-to-methanol step in the MTG process by the direct one stage conversion of syngas-to-DME is thus a very attractive option. This substitution is particularly justified on the basis of the fact that DME results in virtually identical hydrocarbon product distribution as methanol.

Synthesis of gasoline via this direct DME route has several significant advantages over the MTG process, in the areas of product yield, selectivity, overall syngas conversion, exothermicity, and reactor size. The conceptual advantages of this DME-to-gasoline (DTG) process can be demonstrated in a laboratory scale fluidized bed gasoline synthesis unit.

This paper discusses the design philosophy of the fluidized bed reactor unit and its peripherals. The fabrication, assembly, and operation of the unit have also been discussed in detail.     

CHARACTERISATION OF HEAVY CRUDE OILS AND PETROLEUM RESIDUES -REVIEW OF THE RESULTS OBTAINED BY THE ASVAHL ANALYTICAL GROUP

The ASVAHL Analytical Group has worked for four years. The main objectives were :

1) To develop straight forward methods suitable for heavy products which allow an effective conversion unit follow-up and a better understanding of the phenomenon involved in. Methods which were studied include global characterizations, elementary and structural analysis.

2) To increase the knowledge of the intimate structure of heavy products. To do that, fundamental research have been done sometimes in collaboration with university laboratories and by the use of a wide range of sophisticated methods (molecular distillation, gas chromatography, mass spectrometry alone or coupled with gas chromatography, ¹H, ¹³C, ³³S nuclear magnetic resonance, FT/infra red, EXAFS/XANES, small angle X-ray and neutron scattering).

In the same way, the ASVAHL Analytical Group has collected a standard sample bank. It organized an internal symposium dedicated for the characterization of heavy products (Lyon, France, 1984).     

STABILIZATION OF COAL PARTICLE SUSPENSIONS USING COAL LIQUID DERIVED SUSPENDING MEDIA

A liquid we derived from coal, and a liquid produced from Exxon donor solvent (EDS) vacuum gas oil served to stabilize coal particle suspensions. In one case the liquids were used like commercial stabilization additives for coal-petroleum oil mixtures (COMs) with poorer but comparable results. In the other case the idea of using the coal liquid itself as the suspending medium (in place of petroleum oil) was considered, with encouraging results.

Arguments suggest interparticle repulsion enhances the stability of coal particles in coal liquid derived suspending media.     

SIMULATED DISTILLATION BY TRERMO GRAVIMETRIC MEASUREMENTS APPLIED TO THERMAL CRACKING AND THERMAL HYDROCRACKING

A suitable correlation can be made to represent the simulated distillation of heavy oils starting from thennoaravimetric measurements. This method is applicable to hydrocarbons having an initial boiling point equal or greater than 200°C.

The simulated thermogravimetric distillation fit was obtained from experiments with the standard compounds obtained from ASTM D-2887-83 (Boiling range distribution of petroleum fractions by gas chromatography). This method is simple, fast and without problems when applied to heavy feedstocks

The data were used in the determination of average boiling temperatures of products from thermal cracking and thermal hydrocracking. It was also possible to quantify coke yields

Average relative molecular masses of products from the above processes correlated well with the average boiling point temperatures. It indicates that, with respect to the hydrocarbon types, thermal cracking is not selective in comparison with thermal hydrocracking

The equation applied to find the average boiling temperature is following: T = (1/al) (Ttg-a2). T is the boiling temperature, al and a2 are the correction factors, Ttg is the thermogravimetric temperature.     

PARAMETRIC STUDY OF THE RESIDUAL GAS IN SPARK IGNITION ENGINES

The residual gas x_r affects the number of moles of unburned mixture. Consequently it affects the mixture's molecular mass, specific heats, gas constant, internal energy and enthalpy of formation and entropy. Its magnitude affects volumetric efficiency and engine torque and power directly, whereas thermal efficiency and emissions are affected by the change in thermodynamic properties of the working fluid.

A parametric study covers the variables which affect x_r namely compression ratio,equivalence ratio φ, inlet air temperature T_i, ratio of exhaust and intake pressures p_c/p_i, and fuel. A computer program was devised to calculate x_r,T_r, T_l and specific heat release, and the consequent performance namely power, η_th and bmep.

Results show the relative importance of the parameters such as r_c,φ, p_c/p_i and fuel type on residual gas particulars, and fuel on power and η_th whereas for bmep the p_c/p_i acquires additional importance.     

ASPHALTENE PRECIPITATION AND SOLVENT PROPERTIES OF CRUDE OILS

Improved prediction of the onset of asphaltene precipitation may be achieved using refractive index (RI) to characterize crude oils and their mixtures with precipitants and solvents. Experimental measurements of RI for mixtures of several crude oils with the precipitant n-heptane, are reported at ambient conditions. Theoretical developments are described that will permit extension of these observations to reservoir conditions

Measurements of RJ at the onset of precipitation have shown that the onset occurs at a characteristic RI for each oil/ precipitant combination, supporting the premise that precipitation is dominated by London dispersion interactions and thus, that RI can be used to predict the onset of precipitation. Reports in the literature showing that the onset of precipitation occurs at constant solvent-to-precipitant ratios provide additional confirmation

The theory is developed on the assumption that London dispersion forces dominate aggregation and precipitation of asphaltenes. The interaction energy of asphaltene molecules or aggregates in a medium of oil can be expressed as a function of the difference between the RI of asphaltene and oil. The RI of live crude oil during pressure depletion can be calculated from the RI of the stock tank oil, the molar refraction of the separator gas, the formation volume factor B_o and the solution gas/ oil ratio R_s     

DEASPHALTING AND CHARACTERIZATION OF A SYNCRUDE OBTAINED BY DIRECT LIQUEFACTION OF A SPANISH SUBBITUMINOUS COAL

A synciude obtained by direct liquefaction of a Spanish subbituminous coal was deasphalted as a previous step of upgrading lo get a better feedstock for further processing.

Deasphalting was carried out by ultrasonic extraction with n-hexane. Raw and deasphalted coal syncmdes have been fractionated into six fractions by an extrographic procedure that utilizes six eluents of increasing polarity. Each fraction has been analyzed by FTIR, gas chromatography. GC-MS and high resolution mass spectrometry.

Deasphalting removed 36 % of nitrogen, 44 % of sulphur and 47 % of oxygen. Nevertheless, the deasphalted liquids would require further removal of hetenatoms by hydroprocessing.

Polynuclear aromatic compounds appeared in all extrografic fractions. Nimgen-containing compounds have been found in the fractions In, IV and V, when polar solvents have been used. Some mixed heterocyclic compounds (N-0, N-S) have been observed in the most polar fractions (V and VI).     

ISOMERIZATION OF n-HEXANE AND n-PENTANE MIXTURE ON Pt-ALUMINA CATALYST

A mixture of n-hexane and n-pentane (1:1) by volume was isomerized on commercial Pt-alumina catalyst in a continuously fixed-bed reactor at atmospheric pressure. The effect of temperature, LHSV, hydrogen/hydrocarbon molar ratio and chlorine concentration were studied.

It was found that the yield of isohexanes and isopentane increases with increasing the chloride added up to 0.1 and 0.2 mole % CCl₄ respectively. The rate of isomerization became slower at higher concentrations.

Isomerization activity of the catalyst increases with increasing temperature ranging between 350-400°C. The yield of isomers decrease with increasing temperature above 400°C.

The relative conversion of n-hexane in the mixture was found to be more than the conversion of pure n-hexane at the same conditions.     

RAPID PYROLYSIS OF EASTERN CANADIAN COALS BY FREE FALL REACTORS

The effect of temperature was investigated on the rapid pyrolysis of Prince, Lingan and No. 26 coals in the temperature range of 525^° to 750^°C using a 600 cm high stainless-steel free fall reactor at atmospheric pressure.

The yields of char, condensibles and gaseous products were determined at 25^°C intervals. Gaseous products were analysed for H₂, CO, CO₂, CH₄ and C₂+.

Volatile product (condensible and gas) yields exceeded the volatile product in the A.S.T.M. methods at and above 700^°C.

Some experiments using Prince coal were repeated at reduced pressure (50 mm mercury) in both the 600 cm height stainless-steel and a 140 cm height quartz reactor in order to find out the effect of pressure and retention time.

The experimental results indicated that the condensable yield was higher at the lower pressure and at the shorter retention time. The effect of retention time was far more pronounced.     

ANALYSIS OF OIL SHALE RETORT PRODUCTION STREAM GASES FOR SULFUR SPECIES

Analysis of the sulfur species in the production gas stream of oil shale retorts is important for environmental and process development needs. Combined gas chromatography/mass spectrometry and gas chromatography with flame photometric detection have been applied to the qualitative and quantitative analysis of sulfur species in the production gas stream from six oil shale retorting experiments. Hydrogen sulfide was determined to be the most abundant species and represented approximately 95 percent (wt.) of the sulfur present. Minor sulfur species identified and quantitated include carbonyl sulfide, carbon disulfide, methylmercaptan, ethylmercaptan, propylmercaptan, and thiophene. Sulfur dioxide was not identified during any of the retorting experiments studied.

The distribution of these sulfur species from the six retorting experiments is important because it provides a baseline for the design of process control technologies. The importance of the liquid product collection train for control of sulfur emissions was also demonstrated.     

Cyclohexane Dehydrogenation Using an Economical Iron Catalyst: Influence of Alumino-Silicate Structure on the Catalytic Activity

The effect of mixing both local Egyptian hematitic ore and activated aluminosilicate material (bentonite clay) on the dehydrogenation activity of the former was studied.

Three mixtures were prepared in which bentonite percentages were 10, 20, and 40 wt%. Cyclohexane used as a model reactant for the catalytic dehydrogenation reaction carried out in catalytic flow system within reaction temperature ranged from 150 to 500°C in the presence of hydrogen stream (75 mL/min) and at constant space velocity 3.71 h^-1.

The results obtained indicated that in spite of the drop in the selectivity of the local material toward benzene formation by clay addition, a distinct increase in the benzene yield was observed. The maximum conversion attained ∼28.14% at reaction temperature 500°C using a mixture containing 20 wt% activated bentonite.