Visbreaking is a mild thermal cracking process applied to reduce the viscosity of VDR to produce fuel oil and some light products to increase the distillate yield in a refinery [1]. Depending on the feedstock properties and thermal severity in the reactor, the process will typically achieve 10–25% of conversion of the heavy ends to gas, gasoline, and distillates while producing fuel oil with the desired specifications. Carbon rejection in small quantities on the reactor surfaces during thermal cracking helps reduce the viscosity of the fuel oil product (Figure 6.3). The process decreases the demand for a cutter stock used as diluent (e.g., kerosene) that might otherwise be used to reduce the viscosity of the heavy ends to meet the fuel oil specifications. Adding a diluent may still be needed, depending on the sulfur content of the product and the fuel oil specifications. Although the principal objective of visbreaking is to reduce viscosity, some refineries may use this mild cracking process to convert fuel oil into lighter distillates.
As in all chemical reactions, conversion in visbreaking depends primarily on temperature and time. As a measure of “thermal severity” under reactions conditions, one can use a thermal severity index (TSI) as a function of temperature and time that is shown in Figure 6.4. The exponential dependence of TSI on temperature relates to the general exponential term that constitutes the chemical reaction rate constants. The chemical conversion in visbreaking reactions can be expressed as the reduction in concentration (cA) of long-chain alkane (or high-molecular weight compounds) in the feedstocks. One can see from Figure 6.4 that the conversion in the visbreaking reaction can be expressed by the integral in Figure 6.4, assuming an apparent first-order kinetics for the reaction. It can also be seen, in Figure 6.4, that the conversion that can be related to the extent of visbreaking depends on (kt); and the TSI to establish the interchangeability of T and t for a given conversion relates to (e(-Ea/RT)t), where Ea is the apparent activation energy of the reaction, R is the universal gas constant, T is the temperature, and t is time. In using the TSI for comparing thermal severity of different T and t combinations as major operating variables of visbreaking, care should be taken to use the right units for R and T. As a general convention, an apparent activation of energy of 50 kcal/mol is assumed for thermal cracking reactions involving the homolysis of C-C bonds to produce free radicals.
Higher visbreaking severity would produce a higher reduction in viscosity. Thermal severity is limited by the reactivity of the feedstock and the storage stability of the residual fuel in accordance with the desired conversion level and desired reduction in viscosity. Asphaltene content and concarbon of the feedstocks are important factors to consider when selecting an appropriate thermal severity for the process to prevent excessive coking in the visbreaking reactor.
[1] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 5, pp.111-116.
There are two types of visbreaking processes: coil or soaker visbreaking. Figure 6.5a shows a schematic diagram of the coil visbreaking process. For visbreaking, the feedstock is introduced into the coil heated in the furnace, where the thermal cracking reactions take place. At the furnace outlet, the reaction products are immediately quenched using a portion of the gas oil product from the fractionator to stop the thermal cracking reactions. The quenched products are sent to the fractionator for separation into gas, gasoline, light gas oil, and visbroken residue streams. A steam stripper can be used with the fractionator for better separation of the visbreaking products. In the soaker visbreaking process, a soak drum is placed after the furnace, Figure 6.5b. Most of the thermal cracking reactions, in this case, take place in the soaker drum.
Depending on the process objectives and feedstock characteristics, reaction temperatures range from 450°C to 485°C and pressures ranging from 3 to 10 bar. Higher temperatures and lower residence times are used in the coil visbreaking process.
Residence times can vary from 1 min (associated with high temperatures in coil visbreaking) to 10 min (for lower temperatures used in soaker visbreaking).
Similar to deasphalting and distillation, the environmental impact of visbreaking is associated with burning fuel in the furnace to provide energy for thermal cracking, and, to a lesser extent, burning off the coke deposited in the coil or soaker drum leading to emissions of CO2, oxides of nitrogen (NOx), and oxides of sulfur (SOx) in the flue gases.
A coil visbreaker operates at 500°C for 1 min. How long will it take to achieve the same thermal severity at 450°C in a soaker visbreaking process? An apparent Arrhenius activation energy for thermal cracking is given as 50 kcal/mol.