Residence time, chromatography

Whereas changing catalyst volume or residence time rarely yields compHcations, changing temperature or pressure could iatroduce sintering. The properties of the catalyst should be measured both before and after deactivation and inlet and outlet streams should be analyzed by chromatography (qv) or spectrometry. 

Composition The law of mass aclion is expressed as a rate in terms of chemical compositions of the participants, so ultimately the variation of composition with time must be found. The composition is determined in terms of a property that is measured by some instrument and cahbrated in terms of composition. Among the measures that have been used are titration, pressure, refractive index, density, chromatography, spectrometry, polarimetry, conduclimetry, absorbance, and magnetic resonance. In some cases the composition may vary linearly with the observed property, but in every case a calibration is needed. Before kinetic analysis is undertaken, the data are converted to composition as a function of time (C, t), or to composition and temperature as functions of time (C, T, t). In a steady CSTR the rate is observed as a function of residence time. 

The thermal cracking of a light ffaction of mixed plastics waste was carried out in a fluidised bed reactor and the fractions obtained were analysed by elemental analysis, gas chromatography and ashing. The main components of the waste were PE and PP with a small amount of PS and the bed was fluidised by pyrolysis gas, nitrogen or preheated steam. Experiments conducted at different temperatures and residence times were compared by calculating the crack severity for each experiment. The results obtained revealed that the amounts of ethene and propene obtained by pyrolysis with steam were comparable with those obtained using a commercial steam cracker. 

Experiments of propane pyrolysis were carried out using a thin tubular CVD reactor as shown in Fig. 1 [4]. The inner diameter and heating length of the tube were 4.8 mm and 30 cm, respectively. Temperature was around 1000°C. Propane pressure was 0.1-6.7 kPa. Total pressure was 6.7 kPa. Helium was used as carrier gas. The product gas was analyzed by gas chromatography and the carbon deposition rate was calculated from the film thickness measured by electron microscopy. The effects of the residence time and the temperature... 

Compositional analysis shows a decrease in the percentage of polar compounds in the oils with increasing residence time (see Table II). The decrease in polar content is substantiated by a lower sulphur content and results in a lower viscosity (see Table II). The oil becomes more aromatic, as shown by n.m.r. spectroscopy (see Table II), with increasing time at temperature, while the molecular weights showed little change. G.l.c. analysis of the saturate hydrocarbon fractions from elution chromatography indicated little change in the saturates with residence time. 

Fig. 9. Reversed-phase separations of cytochrome c digests obtained with trypsin-modified beads (left) and trypsin-modified monolithic reactor (right) in a tandem with a chromatographic column (Reprinted with permission from [90]. Copyright 1996 Wiley-VCH). Conditions digestion (left curve) trypsin-modified beads reactor, 50 mm x 8 mm i.d., 0.2 mg of cytochrome c, digestion buffer, flow rate 0.2 ml/min, 25 °C, residence time, 15 min (right curve) trypsin immobilized onto molded monolith other conditions the same as with trypsin-modified beads. Reversed-phase chromatography column, Nova-Pak C18,150 mm x 3.9 mm i.d., mobile phase gradient 0-70% acetonitrile in 0.1% aqueous trifluoroacetic acid in 15 min, flow rate, 1 ml/min, injection volume 20 pi, UV detection at 254 nm...

Analogously to chromatography, the capacity factor kP can be defined as the ratio of the residence times of the analyte molecule in the mobile (aqueous) and stationary (micellar) phases (Nemstian distribution). The mathematical description is based on the assumption that attainment of the distribution equilibrium takes much less time than the duration of the migration of the solute. The components involved should not show any interaction with the capillary wall. As in almost all cases where UV detection is... 

The next thing we note about chromatography is that it is equivalent to tracer injection into a PFTR. Whereas in Chapter 8 we used tracer injection to determine the residence time distribution in a reactor, here we have nearly plug flow (with the pulse spread somewhat by dispersion), but adsorption from the fluid phase onto the solid reduces the flow velocity and increases the residence time to be much longer than x. ... 

Catalytic tests of n-pentane oxidation were carried out in a laboratory glass flow-reactor, operating at atmospheric pressure, and loading 3 g of catalyst diluted with inert material. Feed composition was 1 mol% n-pentane in air residence time was 2 g s/ml. The temperature of reaction was varied from 340 to 420°C. The products were collected and analyzed by means of gas chromatography. A FlP-l column (FID) was used for the separation of C5 hydrocarbons, MA and PA. A Carbosieve Sll column (TCD) was used for the separation of oxygen, carbon monoxide and carbon dioxide. 

Because of the relative facility of thermal rearrangement to phenols, melting points of arene oxides are not an entirely reliable index of purity. The checkers found variation from 119 to 135° (dec.). Purification by chromatography on activity IV alumina is also possible, but residence time on the column should be held to a minimum. 

K during the methanol conversion on zeolite HY ( si/KAi = 2.7) under flow conditions (74). In these experiments, a flow of C-enriched methanol with a modified residence time of WIF — lOOgh/mol was continuously injected into the spinning MAS NMR rotor reactor. Simultaneously, the yields of DME, Tdme, were determined by on-line gas chromatography (Fig. 32, middle). 

The essential components of a gas chromatography system are shown in Figure 3.4. The mobile phase (called the carrier gas) is inert, usually helium, nitrogen, or argon. The gas is directed past an injection port, the entry point of the sample. The sample, dissolved in a solvent, is injected with a syringe through a rubber septum into the injection port. The column, injection port, and detector are in individual ovens maintained at elevated temperatures so that the sample components remain vaporized throughout their residence time in the system. 

In NP chromatography non-polar solvents (e.g. alkanes) are used as eluents in cases of insufficient eluting strength the polarity of the eluent can be raised by addition of more polar solvent. More polar sample components have longer residence times in NP chromatography, i.e. they leave the column later because they are retained longer. 

A more modern method to determine the MMD is GPC, gel permeation chromatography, also named size-exclusion chromatography, SEC. A polymer solution is passed over a column with a porous structure. The residence time of the chains on the column depends on the diameter of the coiled chain smaller chains can migrate through more pores (they can also enter into the smaller ones), and it takes a longer time for them to pass along the column. The bigger ones cannot enter into any of the side-pores and pass in the shortest time. 

Figure 8. Size Exclusion Chromatography peak average diameters for the continuous run (17). (02 = residence time in the second reactor of the continuous train).

---