CFD ANALYSIS OF SOAKER DRUM

Abstract

Visbreaking, a process aiming at lowering the viscosity of residues by thermal cracking, has staged a comeback recently as a convenient and inexpensive first step towards reducing fuel oil production. Thermal cracking can be attained by heating the feedstock to high temperatures in the furnace and quenching directly thereafter (furnace cracking), but it is also possible to let the cracking proceed at lower temperatures but with longer residence times in soakers (soaker cracking). The advantages of visbreakers with soakers are: 10-15 percent lower capital requirement, lower fuel consumption and longer onstream times. The relatively fast present increase in visbreaking seems to be governed by two factors: • Visbreaking is a simple and rugged technology which readily allows fuel oil production to be reduced by about 1/3 at relatively low cost. • Redundant process equipment is often available, which can easily be rearranged for visbreaking duty. In the case of visbreaking the usage of the bubble column reactor as a soaker to provide the required residence time for the thermal cracking reactions to proceed, has become an attractive alternative to furnace cracking. During this process many chain scission reactions proceed because of the thermal activation and lower molecular weight products including some gaseous products are formed. This gas generated during the process bubbles through the soaker liquid and is responsible for the liquid phase backmixing, which is responsible for the overcracking of the already cracked product. In the processes like visbreaking, where gaseous products are formed, the amount of gas generated goes on increasing along the height of the bubble column reactor due to the continuation of the thermal cracking reaction. Hence in this study the effect of varying gas velocity and liquid velocity along the height of the bubble column on liquid phase hydrodynamics has been investigated. Also the effect of internals of the bubble column on the liquid phase hydrodynamic levels has been studied to simulate the hydrodynamics of the soaker drum with vertical rods as internals in an attempt to reduce liquid phase backmixing. There is no reported published work on such a scheme of gas distribution and its effect on liquid phase backmixing. In the visbreaking process, due to continual formation of the gaseous product, the flow regime transition could take place somewhere along the height in the column. Homogeneous regime, which otherwise would have prevailed changes to heterogeneous regime effecting the liquid phase dispersion adversely (increases). Internals of the bubble column and isolating the heterogeneous section of the bubble column could substantially reduce the liquid phase backmixing. In the present work, we have computationally studied the effect of internals on dynamics of gas-liquid flow in a cylindrical bubble column, run under bubble flow and Churn turbulent conditions. The numerical simulation was done using the Computational Fluid Dynamics (CFD) based CFX-5.7.1 software (ANSYS, Canonsburg, USA). The SunFire R20 workstation computer with Solaris 8.0 (Sun Microsystems. USA) and Intel Pentium P4 with Windows XP (Microsoft Inc., USA) was used for this study. The flow in the bubble column .(both with and.. without: internals) reactor was modeled using. the Eulerian—Eulerian multiphase model incorporated, in the CFX-5 software. The simulations were .performed for an 18 in. (45.72 cm i.d.) air-water bubble column. Gas superficial velocities used were 2.0, 4.0, 6.0, 8.0, 10.0 and 12.0 cm s-1, characteristics of bubbly and churn turbulent flow regime,- respectively. Three liquid velocities of 0.0, 0.5. and 1.0 cm s'I were used which. are characteristic of homogeneous. regime. The unexpanded liquid -height -is- 175 cm. In the present simulations distributor effects have been ignored and the perforated plate had been modelled as uniform source of the gas phase. The geometry used in the simulation is same as that used by Chen et al. [4]. The internals are composed of two bundles of 1" aluminium tubes (1/16" in thickness and 170 cm in length) which are located at two different radial positions, namely, at dimensionless radius, r/R, of 0.39 and 0.61, respectively. Each bundle contains 8 equally distributed tubes. Distance from bottom of the circular domain is 0.05 cm. It is concluded that with internals flow pattern can be very well controlled. The introduction of internals brings in some regularity in the flow, results in greater mixing and higher gas hold up values. The presence of internals physically reduces the radial turbulent length scale since the internals restrict the flow in the radial direction. ►v Reasonably good agreement was obtained between the experimental data available in the literature and simulations, and the flow pattern profiles obtained for various column configurations appears to be sensible and logical. Also CFD can be used as an invaluable and efficient tool for the prediction of hydrodynamics in a bubble column. u