Vapor Residence Time Research Articles

97/02933 Polycyclic aromatic hydrocarbons in catalytically upgraded biomass pyrolysis oils

The pyrolysis of biomass with catalyst upgrading was studied in a combined fluidised-bed pyrolysis unit with on-line fixed-bed zeolite catalytic upgrading; the biomass was in the form of mixed wood waste, and the catalyst used was zeolite HZSM-5. The influence of pyrolysis and catalyst conditions on the formation of polycyclic aromatic hydrocarbons (PAH) was investigated. There is much interest in PAH in the environment, since among the major chemical groups PAH comprise the largest group of carcinogens. Process parameters investigated included pyrolysis and catalyst temperature, the influence of catalyst ageing and regeneration, and vapour residence time over the catalyst. It was found that polycyclic aromatic hydrocarbons in biomass pyrolysis oils were in low concentration; however, after catalyst upgrading there was a marked rise in PAH, and their concentration reached high levels. The concentration of PAH increased with catalyst temperature. The freshly activated catalyst gave the highest concentrations of PAH, and as successive regenerations took place the catalyst became progressively less active, producing lower levels of PAH. Extended run-time with the catalyst also resulted in a less active catalyst and lower concentrations of PAH. Shorter vapour residence times produced lower concentrations of PAH. The pyrolysis oils after catalysis contained high concentrations of certain PAH which are of known mutagenic and/or carcinogenic activity; for example the oils contained tri- and tetra-methylphenanthrenes, chrysene, methylchrysenes and benzopyrenes. The influence of high PAH concentrations in a hydrocarbon product that may be used as a fuel are discussed. Work has shown a direct link between fuel PAH concentrations and PAH concentrations in emissions from both diesel engines and furnaces. The PAH concentration in petroleum-derived fuels are much lower than those found in zeolite catalytically upgraded biomass pyrolysis oils.

The graphite furnace and its role in atomic spectroscopy.

The diversity of applications of the graphite furnace is extraordinary, encompassing the fields of physics, thermochemistry, spectroscopy and analytical chemistry. In this respect, the graphite furnace has been used on a continuous basis as a research tool for nearly a century. Following its introduction as an atomization source for atomic absorption spectrometry by L'vov in 1959, its role in atomic spectrometry expanded considerably to encompass analytical applications in emision, fluorescence, absorption and mass spectrometry. In addition to its conspicuous use as an atomization source in these areas, it is frequently employed as a vaporizer when used in the format of combined and tandem sources with other instrumentation. The unique physico-chemical micro-environment which can be attained within the graphite furnace has also been used to advantage in a number of investigations, including the determination of gas- and solid-phase diffusion coefficients of high-temperature metal vapours, the heats of sublimation of refractory metals, fundamental optical constants and the measurement of the heats of desorption of adatoms from high-temperature surfaces. The range of such applications remains to be more fully explored. The attractive features of this source, viz., the high atomization/vaporization efficiency, comparatively long atomic vapour residence times, controllable chemical and thermal environment and its ability to handle high dissolved solids content samples (</=100%) serve to ensure its place in analytical atomic spectroscopy for years to come.

PRODUCTION OF OXYGENATED FUELS FROM BIOMASS: IMPACT OF FEEDSTOCK STORAGE

Biomass pyrolysis oils were produced from stored biomass feedstocks by rapid pyrolysis in a fluidized bed reactor. The feedstocks used for these studies were switchgrass, corn stover, and hybrid poplar. The woody and herbaceous feedstocks were stored in chip piles and bales, respectively, unprotected in an open field for 6 months. At the end of the storage period, biomass samples were taken from the interior of bales and the centers of chip piles for pyrolysis studies. The materials were ground to pass -20/+80 mesh and dried to less than 10% moisture content before pyrolyzing in the fluidized bed reactor. Pyrolysis was conducted at 500°C and with less than 0.4 s. vapor residence time. Total liquid yields were as high as 66% for the hybrid poplar and as low as 58% for the corn stover. Moisture content of the oils was between 10% and 13%. Gas and char/ash yields were 10-15% and 12-22%, respectively. The char/ash yields were feedstock dependent, but storage influence was significant for only the corn stover feedstock. Gas and liquid yields were not influenced by storage time. The oils were highly oxygenated and had higher heating values (HHV) of 23-24 MJ/kg that decreased slightly with storage time for all the feedstocks except the switchgrass. The oils, as currently produced, are high in ash and alkali metals. Ultimately, they will be upgraded and used as boiler and turbine fuels.

Distillation Efficiencies of an Industrial-Scale i-Butane/n-Butane Fractionator

Rarely published industrial-scale distillation efficiency data are presented. The Murphree tray efficiencies are determined from the i-butane/n-butane fractionator performance data. Point efficiencies, numbers of overall vapor phase transfer units, numbers of vapor and liquid phase transfer units, and liquid phase resistances of mass transfer are backcalculated from the Murphree tray efficiencies. Various efficiency prediction and scale-up methods have been tested against experimental results. A new model for the prediction of the numbers of vapor and liquid phase transfer units has been developed. The model can be applied to hydrocarbon systems at high pressure. The influence of the mass-transfer coefficients, the interfacial area, and the vapor and liquid residence times on mass transfer has been analyzed separately, and as a result the NTU correlations for vapor and liquid phases are obtained. The constants of the model can be obtained by fitting the model to experimental efficiency data from a similar system.

Fast Pyrolysis of Stored Biomass Feedstocks

Biomass pyrolysis oils were produced from stored biomass feedstocks by rapid pyrolysis in a fluidized bed reactor. The feedstocks used for these studies were switchgrass, corn stover, and hybrid poplar. The woody and herbaceous feedstocks were stored in chip piles and bales, respectively, unprotected in an open field for 6 months. At the end of the storage period, biomass samples were taken from the interior of bales and the centers of chip piles for pyrolysis studies. The materials were ground to pass -20/+80 mesh and dried to less than 10% moisture content before pyrolyzing in the fluidized bed reactor. Pyrolysis was conducted at 500 °C and with less than 0.4 s apparent vapor residence time. Total liquid yields were as high as 66% for the hybrid poplar and as low as 58% for the corn stover. Moisture content of the oils was between 10 and 13%. Gas and char/ash yields were 10-15% and 12-22%, respectively. The char/ash yields were feedstock dependent, but storage influence was significant for only the corn stover feedstock. Gas and liquid yields were not influenced by storage time. The oils were highly oxygenated and had higher heating values (HHV) of 23-24 MJ/kg that decreased slightly with storage time for all the feedstocks except the switchgrass. The oils, as currently produced, are high in ash and alkali metals. Ultimately, they may be upgraded and used as boiler and turbine fuels.

Biological vapor‐phase treatment using biofilter and biotrickling filter reactors: Practical operating regimes

AbstractThe biological treatment of volatile organic compounds (VOCs) and air toxics has received increased attention in recent years. Biotreatment of air‐borne contaminants offers an inexpensive alternative to conventional air treatment technologies such as carbon adsorption and incineration. Most biological air treatment technologies commercially available are fixed‐film systems that rely on growth of a biofilm layer on an inert organic support such as compost or peat (biofilters), or an inorganic support such as ceramic or plastic (biotrickling filters). If designed properly, these systems combine the advantages of high biomass concentration with high specific surface area for mass transfer.At economically viable vapor residence times (1 to 1.5 minutes), biofilters can be used for treating vapor streams containing up to approximately 1500 μg/L of readily biodegradable compounds. Biotrickling filters may offer greater performance than biofilters at high contaminant loadings, possibly due to higher internal biomass concentrations. Both systems are best suited for treating vapor streams containing one or two major compounds. If designed properly, biofilters are especially well suited for treating streams that vary in concentration from minute to minute.

Sample decomposition in an electrically heated cold‐trap inlet system for high speed gas chromatography

AbstractThermal decomposition of some hydrocarbon and chlorinated hydrocarbon compounds in metal capillary tubes used in an inlet system for high speed gas chromatography has been investigated. The metal tube is cooled to about −75°C by a flow of cold nitrogen gas in order to focus a vapor sample cryogenically. A capacitive discharge power supply is then used to heat the metal tube resistively in order to revaporize the sample and introduce it to the separation column as a plug 5‐10 ms wide. The effects of tube temperature, tube material, sample vapor residence time, and type of carrier gas on thermal cracking are described. Use of a copper‐nickel alloy tube resulted in less cracking than either pure platinum or pure nickel. Cracking is more significant with hydrogen as carrier gas than with helium. Cracking also increases with increasing sample residence time in the hot tube. Quantitative sample injection with minimum decomposition can be obtained for a variety of aliphatic and aromatic hydrocarbons and chlorinated hydrocarbon compounds.

Heating characteristics and atomic emission waveforms for glow discharge ion bombardment furnaces

Ion bombardment of a graphite cathode in a glow discharge plasma is used to heat the cathode to temperatures suitable for the vaporization of solution residues from the cathode surface. Two low-mass cathode configurations are considered. The use of a cylindrical post cathode results in very efficient heating with peak temperatures of about 2500°C for a 250 mA discharge in Ar at 4.0 torr (532 Pa). The use of a hollow cathode results in longer sample vapor residence time, but the greater surface area and lower plasma voltage result in less efficient heating, and peak temperatures are about 2100°C. Optical pyrometer data are presented showing the effects of cathode configuration, plasma current and argon pressure on furnace temperature. The effect of an axial magnetic field on cathode heating also is considered. The role of thermionic electron emission in limiting peak furnace temperature and in regulating the temperature is discussed.

The influence of inorganic matter on the pyrolysis of Canadian lignite in a fluidized bed

In low rank coals much of the inorganic matter is present as cations associated with organic carboxyl groups in the coal rather than as discrete mineral phases. By treating the coal with acid the inorganic content is reduced by cation exchange, as well as by acid leaching of discrete minerals. Whole and acid treated samples of pulverized lignite were pyrolyzed in pilot scale (1 to 3 kg coal/h) and bench scale (60 to 100 g coal/h) fluidized bed reactors at atmospheric pressure, 0.45 second vapour-residence time, and temperatures ranging from 500 to 730°C. Yields of char, tar, water and light gases were determined. Removal of inorganic matter from a Saskatchewan lignite resulted in increased yields of tar and total volatile matter, with little effect on the yields of light gases. Increased yields of tar are largely a result of an increased asphaltene fraction in general and specifically of an increased catechol content. Char from acid-washed coal is less reactive to carbon dioxide than is char obtained from raw lignite.

Evaluation and control of particle properties in aerosol reactors

AbstractThe production of powders by aerosol routes spans a wide range of operating temperatures depending on the type of aerosol reactors used. The dominant mechanism of particle growth and evolution depends highly on the rate at which the reactions producing the condensible species are carried out. Numerical solutions of the discrete‐sectional aerosol general dynamic equation that accounts for the interactions of the discrete clusters were obtained for conditions representative of the different types of aerosol reactors used for powder production today. Simplified reaction and coagulation equations that give fast and useful prediction of the evolution of aerosols associated with chemical reactions were derived, and a simple reaction‐coagulation model was developed. The effects of reaction rate, initial vapor concentration, residence time, seed particles, and temperature profile on the properties of tine particles produced by gas‐phase chemical reactions were evaluated using both models. Results show good agreement between the two.

Flash pyrolysis of peat in a fluidized bed

It is shown that high yields of oil can be obtained from a good quality peat by fast pyrolysis in an atmospheric pressure fluidized bed reactor. Maximum oil yields were obtained at about 500°C with an apparent vapor residence time of 0.5 seconds or less. At these conditions, on a m.a.f. basis, yields of tar were about 47–50%, gas about 13%, char 28–32% and product water about 12%. The atomic H/C ratios of both char and tar tended to decrease as temperature increased. Oxygen content of the oil decreased also with temperature and was about 24% at optimum conditions. The pyrolysis oil from peat can probably be used successfully as a substitute fuel oil, or possibly converted to higher quality fuels.

The role of temperature in the fast pyrolysis of cellulose and wood

Cellulose and maple sawdust have been pyrolyzed by different workers in two different reactors (a fluid bed and a transport reactor) in separate laboratories. The Avicel cellulose sample used by both groups was from the same batch, while the maple was different samples of the same species. Fast pyrolysis product yields were compared at a constant vapor residence time of 500 ms over a temperature range of 450-900/sup 0/C and were found to be in very good agreement. It is proposed that if particle heat-up time to 500/sup 0/C, for any reactor, is significantly less than particle residence time, or if particle weight loss is less than 10% before the particle temperature reaches 450/sup 0/C, then the temperature of the reactor will be the only variable determining the yields of char, oil, and gases for a given feed material and a given gas residence time. The implications of the results in terms of product yields and possible pyrolysis mechanisms are discussed. The oil yield as temperature increases can be described adequately by a simple kinetic model.

Pyrolysis of low rank Canadian coals

A bench scale fluidized bed reactor with a continuous feed rate of 15–30 g/h has been developed to study the flash pyrolysis of coal at atmospheric pressure. The pyrolysis behaviour of four low rank Canadian coals has been investigated at 0.44 s apparent vapour residence time over a temperature range of 500–800°C. Conditions for maximum liquid yield have been determined for each of the coals. Material balances on all products were generally satisfactory. Variation of vapour residence time from 0.2 to 1.4 s showed that the maximum liquid yield was obtained at the minimum residence time. Use of different particle sizes indicated that an optimum size probably exists for a given set of reaction conditions. Use of a hydrogenation catalyst did not increase liquid yields and the nature of the reaction atmosphere had only a minor effect on liquid yields. The H C ratio of both the pyrolysis oil and the char decreased with increasing temperature. The maximum tar yields from lignite can be approximately correlated with the atomic H C ratio of the parent coals.

On the mechanism of the rapid pyrolysis of cellulose

Experimental yields and liquid product analyses obtained from the fast pyrolysis of biomass and cellulose in a fluidized bed reactor have been used to demonstrate that the decomposition of cellulose to liquid products proceeds to a major degree by a single mechanism. At temperatures over about 450°C and at vapor residence times of one second or less, it is proposed that the monomer unit of cellulose decomposes preferentially to a two-carbon and a four-carbon fragment, with the two-carbon fragment rearranging to give a yield of hydroxyacetaldehyde (glycolaldehyde) which is 75% or more of that theoretically probable. The most likely route for decomposition or rearrangement of the four-carbon moiety is suggested. The formation of carbonyl or hydroxycarbonyl compounds with two to four carbon atoms is favored. Experimental results from the fast pyrolysis of poplar wood indicate that the cellulose decomposition in wood follows the same path as that of pure cellulose in the production of liquid products.

Light aromatics from the hydropyrolysis of coal

The hydropyrolysis of British coals was studied at hydrogen pressures of 5–15 MPa using heating rates of up to 30 K s −1. In a single-stage reactor, hydropyrolysis of a high-volatile coal gave a maximum yield of 4.5 Wt % of single-ring aromatics. The results suggested that the evolution of volatiles and their cracking to benzene, toluene and xylenes were sequential reactions. This was confirmed using a two-stage reactor in which the coal was heated in one zone and the volatiles cracked in a separate, isothermally heated zone. Yields of up to 12 wt % benzene were then obtained from a high-volatile bituminous coal in hydrogen at 15 MPa, both the cracking temperature and the vapour residence time being separately optimized. Hydropyrolysis of model compounds showed that benzene was a major product of the cracking of polynuclear aromatics under hydrogen pressure.

Product compositions in rapid hydropyrolysis of coal

The rapid hydropyrolysis of a Montana lignite and a Pittsburgh Seam bituminous coal has been examined at a high heating rate (1000 °C s −1). The results are interpreted in the light of those presented previously for pyrolysis of the two coals. A complex situation involving simultaneous chemical and mass transfer rate control is revealed, and it is apparent that the pyrolysis phase cannot be treated separately from the hydropyrolysis phase. Three key process variables, hydrogen pressure, temperature, and particle diameter, are seen to have major effects on the total yields of products. There appears to be an important tradeoff between high hydrogen pressures and high temperatures. Under the present conditions of almost zero vapour residence time at elevated temperatures, methane is the principal product of reaction between hydrogen and coal, and the yields of light aromatic liquids are small.

Direct coking of athabasca tar sand

AbstractYield patterns and product quality have been determined for the direct coking of Athabasca tar sand. Temperature and vapor residence time are the key variables affecting yield; tar sand residence time has a minor impact in yield provided that a minimum contact time is achieved. Yields are not sensitive to the composition of tar sand feed. Yields from direct coking are similar to those from commercial bitumen coking processes; previously reported variations in direct coking yields can be reconciled when liquid product endpoint and coking severity are considered.Coke deposited on the sand is readily burned, producing a flue gas high in SO2 and particulates, and a free‐flowing sand. The sand settles readily from a slurry and disposal presents no obvious environmental concerns.

The hydropyrolysis of coal to BTX

The importance of hydropyrolysis as a coal-to-liquid route is considered. Yields of BTX of up to 15% w/w have been obtained from bituminous coal and lignite by using very rapid heating (∼ 1000 Ksec −1) to 1000–1100 K in hydrogen pressures in the range 100–200 bar at optimum vapour residence times. The yields obtained by various workers and the mechanisms of the formation of BTX from coal by hydropyrolysis are discussed.

Residence time measurements in a coal hydrogenation process

Residence time of coal in the University of Utah “coiled tube” coal hydrogenation reactor has been measured. This has been done with an electronic device which detects a tracer (usually iron) as it passes specific places at the beginning and end of the reactor. The measured coal solids residence time in the reactor has varied from a few seconds to a few minutes. Calculated vapor and gas residence times are one or two seconds. Short vapor residence time terminates further hydrogenation of vapors and thus reduces gas production and hydrogen consumption. The relatively longer coal solids residence time permits further hydrogenation which increases liquid yields.

The prediction of distillation plate efficiencies from froth properties

A compound optical probe is described which is used to determine the distribution of bubble sizes and velocities when liquids are distilled on sieve trays in the froth regime. It is shown how the plate efficiencies may be thereby evaluated by thus taking into proper account the derived distribution of vapour residence times and mass transfer coefficients. The results are seen to agree with published data. Froth properties for several boiling liquids on two sieve plates are reported.