Soaker Visbreaking

Soaker visbreaking is characterized by low-temperature and high-residence time operation. In the soaker visbreaking process, lower conversion is achieved within the heater, and most of the reaction occurs subsequently in a reaction vessel (drum) called soaker. In the soaker, before being quenched, the heater effluent is maintained at certain temperature and time to allow for cracking reactions. Then the oil is sent to a fractionator, which separates the feed into several by-products. The low-temperature operation uses less energy and reduces the amount of residue in the process. 

The two types of visbreaking are similar and only differ in their temperatures and residence times. Product quality and yields from the coil and soaker visbreaking processes are essentially the same at a specifled severity. 

The soaker visbreaking allows the heater to operate at a lower temperature, thereby saving fuel, but it needs the heater and the soaker drum to be decoked by the use of more equipment for coke removal and handling. The low operating temperature results in low operating cost. 

Fuel consumption in soaker visbreaking is about 70% of that for the coil visbreaking. 

The coil visbreaking uses a two-zone fired heater, which provides high degree of flexibility in heat input and better control of the material being heated. 

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Shell. 2011. Shell soaker visbreaking. Technical sheet, http //www.cbi.com/images/uploads/ tech sheets/Visbreaking.pdf (retrieved on February 21, 2011). 

FIGURE 3.1 Typical soaker visbreaking unit. (Adapted from Stratiev, D. and Nikolaev, N., Petrol. Coal, 51(2), 140, 2009.)... 

Soaker visbreaking. It is a low-temperature/high-residence-time process the majority of conversion occurs in a reaction vessel or soaker drum, where the two-phase heater effluent is held at a lower temperature for a longer period of time. 

Typical process schemes of coil and soaker visbreaking technologies are illustrated in Figures 3.2 and 3.3, respectively. 

The soaker visbreaking has lower energy consumption for the same visbreaking severity. 

The operation at higher temperature of the coil visbreaking produces more recovery of the heavy visbroken gasoil, which cannot be obtained with soaker visbreaking without the addition of a vacuum flasher. 

In soaker visbreaking, due to the operation at lower temperature there is also lower deposition rate. 

Decoking is more freqnent in coil visbreaking, but for soaker visbreaking it is necessary to shut down the operation. 

Run time for coil visbreaking is 3-6 months and 6-18 months for soaker visbreaking. 

Fuel consumption is 1-1.5 wt% on feed for coil visbreaking, while for soaker visbreaking it is about 30%-35% lower. 

The heater outlet tanperature in the coil visbreaking process ( 480 C) is higher than that in the soaker visbreaking process ( 450°C). 

In soaker visbreaking, the feed is kept at relatively lower tonperature ( 450°C), which reduces due to the endothermic nature of the cracking reactions (about 10°C-20°C). Because of the operation at lower tanperature and conversion in the heater, soaker visbreaking coke buildup is much slower. 

For coil visbreaking, pressure of a few bars is sufficient to keep the feed from vaporizing. For soaker visbreaking, pressure is chosen so that the desired products are vaporized and quickly leave the reaction zone, while the heavy products soak in the liquid phase. Five to eight bars are used for vacuum residue, and 10-12 bars for atmospheric residue. 

In soaker visbreaking, the pressure at the soaker inlet is about 1.3 MPa and reduces slightly due to the hydrostatic head, cracking, and vaporization. 

The typical profiles of temperature, pressure, and conversion (expressed as weight percent of gases plus naphtha with 165°C of end boiling point) along the reaction system for coil and soaker visbreaking are depicted in Figure 3.4. 

The values of kinetic parameters (reaction rate coefficients, activation energies, pre-exponential factors) reported by the authors were obtained from experiments at different temperature, pressure, residence time, type of operation (continuous or batch reactor, coil or soaker visbreaking), and type of feed (atmospheric residue, vacuum residue, asphaltenes separated from crude oils from different sources). 

Kulkami (2005) modeled a pilot scale coil-soaker visbreaker and tried to eliminate the differences between the pilot-plant data and batch reactor data. He multiplied the batch reactor kinetic constants by appropriate scaling coefficients. The coil was assumed to be a dual PFR, while the soaker was modeled as CSTRs in series with interstage backmixing. 

As can be observed, both coil and soaker reactors have been modeled in the literature by following different approaches, and their validation has been performed with experimental data (batch reactor and pilot-plant data) and connnercial data either for coil visbreaking or for coil-soaker visbreaker. 

Reza, S., Mohaddecy, S., Sadighi, S. 2011. Simulation and kinetic modeling of vacuum residue soaker-visbreaking. Petrol. Coal 53(l) 26-34. 

Chapter 3 is devoted to the modeling of the visbreaking process. The most important and fundamental features of this process are highlighted in this chapter. The final section covers the modeling of coil and soaker visbreaking. 

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