Simulation of the Visbreaker

To illustrate the use of the process model previously described, the reactor volumes were assumed to obtain 2.7 and 19.1 min of residence time for coil and soaker reactors, respectively, which are typical commercial residence times of these reactors. 

The number, length, and diameter of furnace tubes as well as diameter and height of the soaker reactor vary, of course, depending on the design and type of visbreaking unit. For instance, Reza et al. (2011) reported the simulation of a commercial visbreaker unit with the following characteristics of coil and soaker reactors  

The soaker is typically a cylindrical, vertical vessel with a height/diameter ratio of about 6 and a volume that must ensure about 15-25 min residence time for the liquid effluent from the furnace (coil reactor). The need for a longer time in comparison with that practiced in coil reactor (2-3 min) can be easily understood by taking into account the difference between the corresponding reaction temperatures. 

At the entrance and exit of the coil reactor, the temperatures are 326.5°C and 439 C, and the pressure is 2.62 and 0.90MPa (1.72MPa of delta-P), respectively. A typical feed flowrate of 124,300kg/h (about 20,000 barrel per day of nominal feed flowrate) is assumed for simulation purposes. One weight percentage of water steam was also fed to the coil reactor. Delta-P and steam flowrate are typical values reported in the literature (Joshi et al., 2008). Since the coil reactor is modeled as non-isothermal reactor, the required heat transfer rate is 4.33 x lO kJ/h. For the case of the soaker reactor, it is modeled as adiabatic reactor, so that temperature will reduce due to the endothermic nature of the visbreaking reactions. 

Visbreaking being a thermal process, the yields of the various hydrocarbon products and residue conversion are commanded by the reaction temperature. That is, the temperature at which the feed is heated and fed to the furnace is a direct measure of reaction severity of the visbreaking operation. 

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Having a robust model for simulation of the visbreaking process can help predict product yields, product properties, residue conversion, and related process parameters aiming at defining the following ... 

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