Evaporation Start Temperature

The start temperature of evaporation for bitumens and vacuum residues is in the range from 180-300 °C (Tl %) or 237 to 365 °C (T5 %) respectively. Atmospheric residues start to evaporate between 97 °C and 165 °C Tl %) or 182 °C and 215 °C (T5 %). For the products from conversion processes, the evaporation start temperature depends on the degree of distillation which took place after tile conversion process. Therefore the start temperatures of the samples of this investigation show wide ranges from 98-261 °C (Tl %) and 154-324 °C (T5 %). 

Evaporation of the distillation bitumens and their colloidal components first starts at temperatures 200 °C (71 %) or 259 °C (75 %). Since thin-layer evaporation takes place in the thermobalance, and the evaporated parts of the sample are immediately removed by the gas flow, these temperatures are lower than the real start temperatures of an equilibrium evaporation (for example distillation according to Engler DIN 51 751 or ASTM D 285-62). The corresponding start temperatures for an equilibrium evaporation should be more than 400 °C. The evaporation start temperatures for bitumen, dispersion medium, and petroleum resins are lower in the case of blown bitumens, influenced by the flux oils which are added in order to facilitate the blowing process. Some of the index numbers of the thermogravimetry may be correlated with consistency data, and with analysis data (see chapters 4.3.2.1.1 and 4.3.2.1.2). Other values show only a small... 

The concentration of the dispersion medium determines the evaporation start temperature (Tl % OT T5 %) of the distillation bitumens (Fig. 4-57). For the blown bitumens no relation was found, but for both the distillation and the blown bitumens a clear correlation of the distillable fraction AG400 with the concentration of the dispersion medium was recognized (Fig. 4-58). 

Fig. 4-100 shows the plots of 75 % versus residence time and Fig. 4-101 the plot of AG400 and 7 800 versus residence time, each at two different temperatures. Depending on the reaction conditions, T5 % varies from 90 °C up to 220 °C. The evaporation start temperature is strongly influenced by the reaction temperature, as Fig. 4-100 clearly shows. Heating up to the reaction temperature influences the results a difference in the reactor temperature of 20 °C (435 °C and 455 °C) results in a decrease of the evaporation start temperature by 65 °C. The value at zero residence time is taken on quenching after the... 

Table 4-132 Index numbers from thermogravimetry of orginal samples (feed) and heavy oils. Evaporation start temperatures and distillable fraction. 

The H/C ratio only correlates with the distillable fraction AG400. Neither the evaporation start temperatures T % and T5 %, nor the coke residues 7 600 and / 800 give a satisfactory coefficient of correlation with the H/C ratio. 

Thermogravimedy in argon shows that the evaporation start temperatures of the extracts (T1 %, T5 %) increase with increasing solubility parameter 5, whereas the distillable fraction AG400 decreases and the residues J 600 and i 800 increase (Fig. 4-113). This is a consequence of the rising amounts of aromatic, coke-generating substances, whieh were extracted with solvents of increasing solubility parameter. The curves of the simulated distillation of the extracts correspond approximately to that of an atmospheric residue of a Middle East crude (for example AR Kirkuk). 

Oven aging both with and without organic contamination causes an increase of the evaporation start temperature (Tl % and T5 %) and a decrease of the quantity of the... 

The brown polymeric by-product (85 mg) is removed on a filter. The filtrate is reduced in volume on a rotary evaporator (bath temperature not above 40°) heptane is added, and then the mixture is placed in the freezer for crystallization. Filtration yields 263 mg of combined product and starting material. 

Elemental and isotopic fractionations by evaporation of silicate liquids, in particular limiting circumstances, can be simulated by equilibrium calculations, provided that an adequate thermodynamic model of the melt is available. In this approach, a particular starting temperature, pressure, and initial composition of condensed material are chosen and the gas in equilibrium with the melt is calculated from thermodynamic data. The gas is then removed from the system and equilibrium is recalculated. Repeated small steps of this sort can simulate the kinetic behavior during vacuum evaporation (i.e., the limit of fast removal of the gas relative to the rate it is generated by evaporation). This approach has been taken by Grossman et al. (2000, 2002) and Alexander (2001, 2002). 

When evaporation starts, the temperature of the surface of the body drops because of a loss of heat due to the latent heat of vaporization of the liquid. However, heat flow to the surface from the atmosphere quickly establishes thermal equilibrium. The temperature of the surface becomes steady (and is refered to as the wet-bulb temperature). The rate of evaporation during the CRP can be described by ... 

If the pressure is now reduced, analogous to method A, the phase boundary between ice and vapour state is reached. Here, the ice starts to transform directly into vapour, it sublimates. Again, heat has to be introduced, in this case not only the heat of evaporation, but also the heat of sublimation inherent to ice. This does not lead to melting of the ice, as the melting point is skipped by the detour C-D. If the heat of sublimation is not introduced, the product will cool down further and the condition of constant temperature would not be maintained. If all water is evaporated, the temperature in the product increases as a result of the positive thermal balance and the drying process has come to an end. This process, a combination of methods C and D, is called freeze drying. 

Some degree of superheating is required before nucleation sets in in the drop, depending on drop size as well as the physical and chemical purity of the fluids. Similar phenomena have also been observed in countercurrent spray column studies (S9), where the temperature of the continuous phase can be lowered to the desired value only after some evaporation starts. As is to be expected, the time required for complete evaporation of the drop is inversely proportional to the temperature driving-force. A similar relationship exists for the length of the evaporation path of single drops. This, however, may not be directly extended to populations of drops, where the onset of nucleation is not simultaneous but rather depends on the dispersed phase flow rate, holdup, and degree of turbulenee of the system. 

Heat transfer to a liquid leads to an increase of the temperature up to the boihng temperature at which evaporation starts. The vapor pressure becomes equal to the pressure of the system. In the case of the evaporation of a liquid mixture all components or only some of them or perhaps only one component can be present in the vapor. Dealing with the evaporation of an aqueous solution of an inorganic salt with a veiy low vapor pressme approximately pure steam is leaving the liquid. (Note that an entrainment of small drops can take place with the resrrlt of small salt contents in the steam.) In general, all components of the liquid rrrixtirre will be present in the vapor when there are no great differences of the vapor pressirre of the components. 

The start temperature of evaporation and of pyrolysis reaction may be defined by means of the onset temperature, which is itself defined as the temperature of the intersection of the tangents to the two branches of the curve, which have different slopes. The small drift of the curve of energy versus temperature introduces errors in the determination of the onset temperatures of evaporation events. For the experiment in methane at 10 bar pressure the onset temperatures and the peak maximum (minimum) temperatures of the evaporation are sometimes, quite close, due to the mode of determination (table 4-40) ... 

The start temperature of the cracking process was ascertained according to Adony [4-14], A mean x = 414 °C ( V=4.2 %) has been found independent of the provenance of the samples. The empirical detennination of 400 °C as the end of the evaporation range and the start of the pyrolysis (cracking) range, is thus justfied. 

Investigation of the reaction kinetics in an inert atmosphere using DSC confirms the difference between evaporation and cracking processes. However, the extrapolated onset temperatures of the evaporation exceed the T5 % index numbers of thermogravimetry considerably. On the contrary the start temperatures (onset) of the cracking reaction in DSC were found at nearly equal temperatures, to those from thermogravimetry, T ... 

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