Kinetic Studies of Low Severity Visbreaking Kamal L. Kataria, † Rohit P. Kulkarni, † Aniruddha B. Pandit,* ,† Jyeshtharaj B. Joshi, † and Manmohan Kumar ‡ Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai - 400 019, India, and Indian Institute of Petroleum, Dehradun, India Thermal cracking of vacuum residues and asphalts obtained from operating Indian refineries were studied in a batch reactor. The temperature was varied in the range 400-430 °C, and the batch time was varied from 0 to 15 min. The pressure was kept at a constant value of 1.2 MPa through out the experiment. The variation in the composition of the cracked gas fraction for each feed was studied by gas chromatography. The resulting visbroken products were further characterized in terms of its different industrially important boiling cuts. A five lump kinetic model, comprising of gas (C 1 -C 5 ), gasoline (IBP-150), LGO (150-350), VGO (350-500), and VR (500+) has been developed. The variation in the kinetic parameters with change in the feed properties has been discussed. Also, an attempt has been made to seek a relationship between the feed properties with the kinetic rate parameters and the activation energies. 1. Introduction Thermal cracking of heavy oils and vacuum residues has been advanced as an up-gradation process so as to meet the increasing demand for lighter fuels. Thermal cracking by carbon rejection methods such as visbreak- ing, delayed coking, and resid catalytic cracking and hydrogen addition methods such as hydrovisbreaking, hydrocracking, etc. are some of the important residue up-gradation processes. In these methods, visbreaking and delayed coking contribute about 32 and 30% re- spectively in terms of the total residue volume pro- cessed. These are followed by hydroprocessing and resi- due FCC with respective contributions of 19 and 15%. Deasphalting, the physical method of residue process- ing, contributes the remaining 3.5%. 1 Visbreaking, a viscosity reduction process, is a mild liquid phase py- rolysis of large molecules to smaller ones to form lighters and gaseous products and offers some additional ben- efits. The process severity is controlled by the inter- changeable operational variables (being essentially a first-order reaction) such as temperature and the resi- dence time. In the past decade, most of the visbreaking units have been modified from coil-type (high temper- ature and lower residence time route) to coil-soaker (high residence time and lower temperature route), thereby reducing the process temperature by about 10 °C. This modification has enhanced the yields of gas and gasoline. Further, the concurrent visbroken products obtained attain a relatively superior quality of the fuel oil and require zero to low quantity of cutterstock for property adjustments. For a given feed, the extent of conversion, selectivity to gas and gasoline, coking behavior, and stability of the visbroken product are directly related to the feed characteristics, such as paraffin, asphaltene content, aromaticity, heteroatom content, etc., and vary to a large extent from feed to feed. The extent of conver- sion of a particular feed within the stability range is called the crackability of the feed. 2 To investigate the role of feed properties on the crackability, several re- search groups 3-13 have studied the kinetics of visbreak- ing of heavy oils and residues from a large number of sources in a batch or continuous manner. These studies were primarily focused on thermal cracking behavior of feeds such as (i) model compounds having character- istics similar to heavy oil (average boiling point, viscos- ity, etc.) and (ii) actual heavy oils that includes atmos- pheric residue (AR), vacuum residue (VR), and their blends which act as major feeds for the visbreaking units. The residue is composed of complex organic compo- nents, and it is difficult to develop a mechanistic ap- proach for each molecule to explain the true kinetics and their thermal behavior. Thus, the pseudo-compo- nents are obtained by group composition based on their physicochemical properties such as boiling points, solu- bility, etc. The reported lumped kinetic models can be categorized into (i) parallel reaction models (Table 1) and (ii) parallel-consecutive reaction models (Table 2). A summary of the above-mentioned research can be listed as follows. 1. During thermal cracking, paraffins mainly undergo C-C splitting while asphaltenes follow dealkylation reactions. 6,10 2. The crackability of the feed was found to be a linear function of difference in the aromatic content of the asphaltenes and maltene phase. 6 3. Gas composition was found to be independent of severity. 10,11 4. The observed nonlinear trends for the distillate products were mainly due to onset of condensation reactions beyond certain severity. 4 5. The activation energy was found to vary with the feed properties (such as percentage of saturates, polar aromatics and asphaltenes present, CCR, sulfur, and metal content). Activation energies for the cracking and condensation reactions were estimated to be in the ranges 14-78 and 17-60 kcal/mol, respectively. Such a large variation in the activation energy can be attributed to the large structural variation within the lump considered and the degree of lumping. The above studies are reported for higher severity range where cracking and condensation reactions may compete with each other. The theoretical residence time and severity covered in the commercial coil-soaker * To whom correspondence should be addressed. Tel.: 91- 22-2414 5616.Fax: 91-22-2414 5614. E-mail: abp@udct.org. † University of Mumbai. ‡ Indian Institute of Petroleum. 1373 Ind. Eng. Chem. Res. 2004, 43, 1373-1387 10.1021/ie0305723 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/14/2004