Resid Thermal Cracking

To give a flavor of the approach, we consider a simple lumped kinetic model for resid thermal cracking. The model needs to account for an induction time for coke formation, which is due to the phase separation of a second liquid phase formed from partially converted asphaltene cores. That is, coke forms when the solubility limit is exceeded. Wiehe developed a simple frrst-order kinetic model based on solubility classes, as follows... 

Olefins are produced primarily by thermal cracking of a hydrocarbon feedstock which takes place at low residence time in the presence of steam in the tubes of a furnace. In the United States, natural gas Hquids derived from natural gas processing, primarily ethane [74-84-0] and propane [74-98-6] have been the dominant feedstock for olefins plants, accounting for about 50 to 70% of ethylene production. Most of the remainder has been based on cracking naphtha or gas oil hydrocarbon streams which are derived from cmde oil. Naphtha is a hydrocarbon fraction boiling between 40 and 170°C, whereas the gas oil fraction bods between about 310 and 490°C. These feedstocks, which have been used primarily by producers with refinery affiliations, account for most of the remainder of olefins production. In addition a substantial amount of propylene and a small amount of ethylene ate recovered from waste gases produced in petroleum refineries. 

Thermal Cracking. Heavy petroleum fractions such as resid are thermally cracked in delayed cokers or flexicokers (44,56,57). The main products from the process are petroleum coke and off-gas which contain light olefins and butylenes. This stream also contains a considerable amount of butane. Process conditions for the flexicoker are more severe than for the delayed coker, about 550°C versus 450°C. Both are operated at low pressures, around 300—600 kPa (43—87 psi). Flexicokers produce much more linear butenes, particularly 2-butene, than delayed cokers and about half the amount of isobutylene (Table 7). This is attributed to high severity of operation for the flexicoker (43). 

It is important to separate catalyst and vapors as soon as they enter the reactor. Otherwise, the extended contact time of the vapors with the catalyst in the reactor housing will allow for non-selective catalytic recracking of some of the desirable products. The extended residence time also promotes thermal cracking of the desirable products. 

Post-riser hydrocarbon residence time leads to thermal cracking and non-selective catalytic reactions. These reactions lead to degradation of valuable products, producing dry-gas and coke at the expense of... 

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. 

State of the art riser termination devices have significantly reduced the coke deposition in the reactor disengager vessel. These modifications have significantly reduced the hydrocarbon residence time and potential thermal cracking in the disengager. 

The major industrial source of ethylene and propylene is the pyrolysis (thermal cracking) of hydrocarbons.137-139 Since there is an increase in the number of moles during cracking, low partial pressure favors alkene formation. Pyrolysis, therefore, is carried out in the presence of steam (steam cracking), which also reduces coke formation. Cracking temperature and residence time are used to control product distribution. 

Visbreaking is a relatively mild thermal (noncataly tic) cracking process that is used to reduce the viscosity of residua (Ballard et al., 1992 Dominici and Sieli, 1997). The process (Hydrocarbon Processing, 1998) uses the approach of mild thermal cracking to improve the viscosity characteristics of a residuum without attempting significant conversion to distillates. Low residence times are re-... 

The preheated feedstock enters the bottom of the fractionator, where it is mixed with the recycle oil. The mixture is pumped up to the charge heater and fed to the soaking drum (ca. atmospheric pressure, steam injection at the top and bottom), where sufficient residence time is provided to complete the thermal cracking. In the soaking drum, the feedstock and some product flows downward passing through a number of perforated plates while steam with cracked gas and distillate vapors flow through the perforated plates countercurrently. 

Although we restrict ourselves here to the coke formation reactions, the cracking reaction has to be modelled as well. This is mandatory in order to obtain proper values of concentrations of the coke precursors and the actual residence times of liquid and vapour. In addition, a proper description of the vapour-liquid equilibria (VLE) in the reactor is required. The model for the (thermal) cracking reaction involves multi-component kinetics and has been described before [8,9]. For the validation of the model on coke formation we refer to [9],... 

Visbreaking is a mild, once-through thermal cracking process. It is used to crack resid products into fuel-oil specifications. Although some light products such as naphtha and gasoline are produced, this is not the purpose of the visbreaker. 

A thermal cracking unit for waxes consists of a furnace, a primary separation column, a stabilization column and a distillation section. The feedstock is vaporized, mixed with steam to 40 per cent weight, and enters a tubular furnace in which the residence time is a few seconds (2 to 10 s) at 500 to 600°C. Once-tbrougb cbnversion is relatively low (15 to 30 per cent) to avoid side reactions. Operation is at atmospheric pressure or ghtly above. Direct quench, or quench with a heat transfer fluid, generates steam. Primary fiactionation allows the recycling of the unconverted part of the feedstock. 

Fuel Gas yield Thermal vs. Catalytic Cracking. A higher ratio of fuel gas/iso-butane was found in the case of the MR, which is to be expected with a relatively low catalyst volume fraction and longer residence time of the hydrocarbons in the reactor (see Table I). With increasing catalyst fraction (or conversion) the ratio decreases due to the formation of more iso-butane. A reduction in contact time will also result in a lower ratio, because fewer thermal cracking reactions will take place. 

Product variation was observed when the screw speed was increased from 0.7 to 11 rpm (i.e. decreasing the residence time). The results illustrated that at longer residence times higher amount of gas is produced since a greater amount of thermal cracking takes place (Figure 19.7 and Table 19.10). However, the exception to this rule is at screw speeds of... 

The conversion and the coke yield as a function of residence time with different CTOs are shown in figure 4a and 4b, respectively. The conversion in the absence of catalysts is a measure for the amount of thermal cracking. At a residence time of 4 s it is at maximum 6wt%. The catalytic conversion after 0.15 s is remarkably high, over 30 wt%. The conversion increases proportionally with residence time up to a conversion of ca. 75 wt%, then levels off and reaches a maximum value of 90-95 wt% after 3-5 s. The coke yield (figure 4b) is hardly influenced by residence time, except for a minor increase at a CTO of 6. Clearly, almost all the coke has been generated within 0.15 s. 

The conversion of a hydrowax feedstock due to thermal cracking at a residence time of 4 s was at maximum 6 wt%. The product yields obtained catalytically could all be described by the same function of conversion, regardless the reactor length or the CTO used. Up to a conversion of 95 wt% no gasoline over-cracking has been observed. 

The process (Fig. 2.3) uses the mild thermal cracking partial conversion) as a relatively low-cost and low-severity approach to improving the viscosity characteristics of the residue without attempting significant conversion to distillates. Low residence times are required to avoid polymerization and coking reactions, although additives can help to suppress coke deposits on the tubes of the furnace. 

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