Comparison of Flow Patterns in a Visbreaking Soaker Drum with Two

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Comparison of Flow Patterns in a Visbreaking Soaker Drum with Two Different Sieve Tray Internals Jasvinder Singh,* M. O. Garg, and S. M. Nanoti Indian Institute of Petroleum, Dehradun 248005, Uttaranchal India ABSTRACT: Sieve tray internals are a well-known method for reducing liquid phase backmixing in bubble columns. In the soaker mode of visbreaking, cracked hydrocarbon vapors increase in volume as we move up in a soaker drum because of more vapor generation due to cracking as well as reducing hydrostatic head on the gas bubbles. The free flow area in sieve trays is gradually increased as we move up in the soaker drum because of this increase in volume. A CFD analysis is presented on a soaker drum with two set of sieve tray internals with same free flow area on corresponding sieve trays, but different configuration of holes. The liquid and vapor properties have been taken identical to the hot short residue and hydrocarbon vapors, respectively. The size of flow domain for simulation has been taken equivalent to a real soaker drum, for realistic simulations. It was found that increasing of flow area in sieve trays by increasing the size of holes is more effective way for reducing liquid phase back mixing, as compared to increasing number of holes.

1. INTRODUCTION Visbreaking is a comparatively mild thermal cracking process used for upgradation of residual fraction by lowering its viscosity and pour point. The visbreaking process is operated in mainly two modes namely coil and soaker modes. During the earlier days of development of visbreaking technology, the soaker type units were once discontinued in 1933 because of excessive coking in soaker drum and short run lengths in older designs. It was basically due to poor understanding of certain important parameters of soaker type visbreaking. In 1962, M/s Shell International reintroduced soaker visbreaking technology, with a new type of soaker. The basic design difference in new soaker was that, the feed flow was from bottom to top against top to bottom in older units. Akbar and Geelen1 have tabulated comparison of old and new soaker in their research paper which clearly indicates the significant differences, which could result in the revival of soaker technology, without prejudice of negative experiences with the older soaker visbreaking units. This comparison by Akbar and Geelen1 is reproduced here in Table 1. The soaker is in principle a large cylindrical vessel that allows large residence time for the feedstock, without any external heating. Because the cracking reactions are endothermic, the feedstocks temperature drops by 1020 °C between the soaker inlet and outlet. The operating temperature of the soaker is about 420430 °C, and the operating pressure 1012 kg/cm2. The residence time of feedstock in the soaker is about 1520 min. The preheated feedstock is entered at the bottom of soaker, where it travels to the top of column, with cracking reactions taking place. The cracked products include gases and hydrocarbon vapors, which are discharged at the top and are directed to a fractionation unit. As the cracking reactions proceeds in soaker, the cracked hydrocarbon vapors bubble through the liquid phase residue feedstock. Thus, a soaker drum behaves like a bubble column reactor, in which both the phases, liquid as well as gas, move in r 2011 American Chemical Society

upward direction. The swirling motion occurring in soaking vessels, due to gas formation results in backmixing. The objective of this work was to study the effect of two different bubble column internals on the liquid phase flow pattern. These internals significantly affect the residence time distribution of the liquid phase, which in turn governs the total thermal conversion of the petroleum residue. 1.1. Development of Soaker Visbreaking Technology. A major problem encountered in the soaker is nonhomogeneous progression of the charge stock, back mixing and vortex formation, particularly in the vicinity of the walls and at the bottom of the drum.2,3 These disturbances are aggravated by the gases generated by cracking reactions and also by the fact that the residence time of the feedstock in the soaker varies markedly in the same cross-section, depending on the zone considered. As a result, there is a risk that part of the treated feedstock will be over cracked, while other fractions will be insufficiently cracked. To overcome this drawback, Akbar4 proposed to provide plural internal structures or partitions, of perforated plates. The patent suggests that 1 to 20 perforated plates, having perforation in the range of 5100 mm, are provided, in order to increase the number of mixing stages, inside the soaker. It has also been claimed in the patent that the stability of the product is improved, when the soaker with internals is used, as compared to one without internals. Sakurai and Tetsuo5 proposed a design of older type of soaker, in which the feed flows from top to bottom. Use of partition plates was suggested in order to minimize backmixing. The objective of the research was to produce a residual pitch rich in beta-resin Special Issue: Nigam Issue Received: April 3, 2011 Accepted: August 26, 2011 Revised: August 26, 2011 Published: August 26, 2011 1815

dx.doi.org/10.1021/ie200671k | Ind. Eng. Chem. Res. 2012, 51, 1815–1825

Industrial & Engineering Chemistry Research

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Table 1. Comparison of Old and New Soakers [Akbar and Geelen (1981)] item

old soaker

new soaker

flow direction

down

up

size (m3) diameter (m)

100 3

50 2

T (°C)

480

440

pressure, bar (g)

25

5 15

vapor cracking

Yes

minimum

liquid cracking

minimum

yes

backmixing

moderate (of vapor)

little (of liquid)

run length (days)

50

300

compounds. They also provided a scraper for removing coke formed on walls and perforated plates. Blauhoff and Cornelissen6 claimed that the significant reduction of back mixing in a soaking vessel might be obtained by providing the vessel with internals, which divide the interior of the vessel into a plurality of compartments. They have given four different kinds of designs, including one with horizontal soaker drum. In their designs, the feed entered from top and traveled downward. The gases were removed from the top of the column, whereas liquid product was collected at the bottom of the soaker (or from sides in the horizontal soaker). They mentioned in their patent that if the amount of gas generated in the soaking vessel is rather moderate, the provision of compartments in the vessel would normally be sufficient for generating a product having an acceptable stability. If, however, the operating conditions and/or the composition of the feedstock are such that large quantities of gas are generated, or are already present in the feed to the soaking vessel, the compartmented division of the vessel may be insufficient for producing products with optimal stability. They opined that the constant removal of produced gas improved the stability of fuel oil in such circumstances. Gouzien et al.2 suggested a novel design of internals having a large hole in the center of the partition plate (disk), with small holes surrounding it. The central circular passage represents at least 35% of the annular disk surface. The small perforations provided additional passage for the treated feedstock and thereby reduce the formation of dead zones between the continuous discs. The diameter of central hole is seven to twelve times greater than that of smaller holes. It has been reported that small holes can have diameter ranging between 0.03 to 0.1 m, and the diameter of larger (central) hole ranges between 0.72 to 1.2 m. It is stated that there is a marked increase in gasoline and VGO yield and decrease in gas and residue production, by the use of these internals. Stability of the obtained residue is also higher, and more reduction in viscosity is possible. Fersing et al.3 gave a different design of soaker, without any partition plates, but with a provision of bubbling inert gas or steam along the wall, cocurrently with feed. The inert gas bubbling was provided with the help of a perforated conduit, fitted coaxially, at the bottom of the soaker. This conduit comprises regularly spaced orifices through which compressed gas escapes (bubbles) toward the top of the soaker. It is mentioned that the bubbling of inert gas or steam helps in stripping of gases formed and with the result, the stability of cracked oil improves as compared to the product obtained under identical conditions, in a conventional visbreaking unit.

To sum-up, the patents available on soaker design invariably suggest the introduction of partition plates or compartmentalization of the soaker, for reducing liquid-phase backmixing. As mentioned above, the soaker is essentially a bubble column reactor, and therefore its design considerations are similar to those of the latter.

2. DESIGN CONSIDERATIONS FOR A BUBBLE COLUMN Bubble columns are widely used as contacting devices in chemical and process industries due to their simple construction, capability to handle multiphase flow and simple operation. Flow patterns in the bubble column and flow parameters are key factors for overall column performance. Rising gas bubbles, which entrain liquid in their wakes and carries it upward, mainly causes liquid phase dispersion in bubble column reactors. This entrained liquid then moves downward to satisfy the mass conservation. This sets up the liquid circulation pattern depending on the spatial coordinates of the gas bubbles and their rise velocities. It is known that the liquid in the central axis moves upward with most of the gas and that near the wall it moves downward. This has been a subject of number of investigations. A number of research publications are available on bubble column design.711 Design equations have been reported in the literature on the basis of various models. Schl€uter et al.9 has presented a comprehensive review on modeling and simulation of bubble column reactors. A number of studies on liquid phase backmixing1214 and other bubble column design parameters, e.g., column aspect ratio,15 particle residence time distribution in three phase bubble columns,16 interfacial area density,17 phase hold up,18,19 liquid phase backmixing,20 and axial dispersion,21,22 etc., have been reported in the literature. Sanyal et al.23 has reported numerical simulation studies on cylindrical bubble column reactor. They have verified the experimental results with the help of numerical simulations, carried out using Eulerian multiphase model available in FLUENT CFD software. Flow pattern in the bubble column and flow parameters are key factors for overall column performance. A number of research publications are available on bubble column design.11 A comparative study has been reported between different modeling approaches, in order to understand the capability of CFD to predict the flow accurately. Bertola et al.24 has discussed the application of CFD to multiphase flow in bubble columns. The experimental data of Becker et al.,10 Mudde et al.,25 Sanyal et al.,23 and Ho Yu and Kim26 have been used for comparison. 2.1. Soaker Drum with Sieve Tray Internals. The introduction of perforated sieve trays in a soaker drum was first suggested by Todt et al.27 in order to minimize liquid phase backmixing.28 The effect of stage height, superficial gas and liquid velocities, and column diameter on the overall gas holdup in a gasliquid cocurrent tray bubble column has been investigated by Kato et al.29 In another study Nishikawa et al.30 reported an increase of up to 5% in gas holdup by decreasing 40% in the tray hole diameter. Chen et al.31 study two types of plates in two different concurrent tray partitioned bubble columns; the Karr tray design with 53% of open area, and a perforated plate made of mesh screen with 64% open area. In another study Chen et al.,32 investigate the overall gas holdup for various gasliquid systems in both batch and cocurrent upward multistage units. In a study on the influence of partitioned plate on liquid-phase backmixing in different diameter columns (10 to 38 cm) and different 1816

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Figure 1. Flow Regimes in Bubble Columns. Reprinted with permission from ref 36. Copyright 2000 Wiley-VCH.

superficial gas velocities (ranging between 0.050.4 m/s), Dreher et al.14 estimated the liquid circulation velocity in bubble columns to be about 1 order of magnitude higher for columns without trays as compared to in tray partitioned bubble columns sectionalized by perforated trays of 18.6% open area. Further, they reported an increase in the axial dispersion coefficient with increase in tray open area. However, column diameter has shown negligible effect on liquid backmixing in their study. Baten and Krishna33 have reported scale up studies on partitioned bubble columns, using CFD simulation. They concluded on the basis of experimental data, that 2D simulations could predict the behavior of bubble columns qualitatively. However, 3D simulations, though computationally expensive, are required for quantitative predictions. In some recent publications, authors have reported 3D simulations on the bubble columns. Ekambra and Dhotre34 have carried out 3D CFD simulations on bubble column without internals in an effort to assess the performance of various turbulence models using experimental data published earlier.35 The models evaluated were kε model, kω model; RNG kε model, Reynolds stress model (RSM), and large eddy simulation (LES) have been tried. The key parameters used for performance evaluation of the turbulence models are axial liquid velocity, fractional gas hold up, turbulent kinetic energy, and turbulent eddy diffusion. Their studies reveal that the results obtained with RSM and LES models are comparable for a bubble column and these models give better predictions near sparger, where the flow is more anisotropic. The application of LES, though predicts the flow more accurately as compared to experimental data, but is computationally more expensive. RANS also exhibits good performance, if the objective is to understand the steady and time averaged feature of the flow. A careful scanning of literature reveals that in spite of the variety of contacting schemes introduced in bubble columns by using internals as baffles, and other geometric irregularities, most of the CFD work available in the literature is without internals. Figure 1 shows the variation of fractional gas holdup with increasing superficial gas velocity. The volume of vapor phase increases along the height in a soaker drum due to two main reasons. First, due to vapor generation by continuous cracking of the feedstock along the height and second is due to reduced hydrostatic head of the liquid phase. Thus the transition in flow regime is observed as shown in this figure. A homogeneous flow regime is observed in the bottom portion of a soaker drum due to

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absence of the radial gas hold-up profile. This homogeneous regime changes to the heterogeneous one as we go up, because of strong radial gas hold-up profile in this part. The transition points of these regimes can be manipulated by changing various operating conditions such as superficial gas velocity, type of gas sparger, the diameter of the column, liquid phase physicochemical properties etc.36 The effect of such a situation on the liquid phase dispersion in visbreaking process has neither been experimentally studied nor modeled. Researchers have also shown that the liquid phase dispersion is high in the heterogeneous regime as compared to homogeneous regime. Hence the attempt of a designer is to extend the homogeneous regime to reduce the extent of the liquid phase backmixing by reducing the liquid phase dispersion. In the visbreaking process, because of 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 affecting 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, the flow in the soaker drum was modeled using the two-phase Eularian Model available in the FLUENT software version 6.1.22, based on the finite volume method. The simulations were carried out using an HP Workstation xw6200 with two CPU quad core Xeon processor and Windows XP Professional environment. Two configurations were evaluated for the internals. In the first configuration, flow area in successive trays was varied by increasing number of holes per plate, hole diameter being constant. In the second configuration, number of holes per plate was kept same and flow area was increased by increasing diameter of each holes. The EulerianEulerian multiphase model was chosen for the simulations. This model has been described in detail as follows.

3. EULERIANEULERIAN MULTIPHASE MODEL This multiphase model in FLUENT allows for the modeling of multiple separate, yet interacting phases. The phases can be liquids, gases, or solids in nearly any combination. An Eulerian treatment is used for each phase, in contrast to the Eulerian Lagrangian treatment that is used for the discrete phase model. Applications of the Eulerian multiphase model include bubble columns, risers, particle suspension, and fluidized beds. In the EulerianEulerian two fluid approach, the different phases are treated mathematically as interpenetrating continua. Both fluids are treated as incompressible, and a single pressure field is shared by all phases. Continuity and momentum equations are solved for each phase. Momentum transfer between the phases is modeled through a drag term, which is a function of the local slip velocity between the phases. A characteristic diameter is assigned to the dispersed phase gas bubbles, and a drag formulation based on a single sphere settling in an infinite medium is used. Finally the, turbulence in either phase is modeled separately. 3.1. Basic Conservation Equations. 3.1.1. Conservation of Mass. The continuity equation for phase q is

∂ ðαq Fq Þ þ ∇ 3 ðαq Fq B v qÞ ¼ ∂t 1817

n

∑ ðm_ pq m_ qpÞ þ Sq p¼1

ð1Þ



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where vq is the velocity of phase q and m_ pq characterizes the mass transfer from the pth to qth phase and m_ qp will be the mass transfer from the qth to pth phase. By default, the source term Sq is zero. 3.1.2. Conservation of Momentum. The momentum balance for phase q yields

Table 2. Details of Mesh for the 2D and 3D Simulations soaker drum geometry mesh details

∂ ðαq Fq B v q Þ þ ∇ 3 ðαq Fq B vq B v qÞ ∂t

2D

3D

total number of elements

18 800

69 936

cell type

quadrilateral

hexhedral

total number of faces

38 524

216 153

total number of nodes

19 662

74 822

¼ αq ∇p þ ∇ 3 τ q þ αq Fq gB þ

Table 3. Details of 2D and 3D Simulations in Fluent

n

∑ ðRBpq þ m_ pq Bv pq m_ qp Bv qp Þ p¼1

Fq þ B F lift, q þ B F vm, q Þ þ αq Fq ð B Where τ q is the qth phase stressstrain tensor 2 τ q ¼ αq μq ð∇ B vq þ ∇ B v Tq Þ þ αq λq μq ∇ 3 B v qI 3

2D geometry

ð2Þ

ð3Þ

F q is an Here μq and λq are the shear and bulk viscosity of phase q, B F vm,q is a virtual mass external body force, B F lift,q is a lift force, B force, R Bpq is an interaction force between phases, and p is the pressure shared by all phases. B v pq is the interphase velocity, defined as follows. If m_ pq > 0 (i.e., phase p mass is being v p; if m_ pq > 0 (i.e., phase q mass is transferred to phase q), B v pq = B v q; Likewise, if m_ pq > 0 then being transferred to phase p), B v pq = B v q, and if m_ pq > 0 then B v qp = B v p. v qp = B B 3.1.3. Lift Forces. For multiphase flows, FLUENT can include the effect of lift forces on the secondary phase particles (or droplets or bubbles). These lift forces act on a particle mainly due to velocity gradients in the primary-phase flow field. The lift force will be more significant for larger particles, but the FLUENT model assumes that the particle diameter is much smaller than the interparticle spacing. Thus, the inclusion of lift forces is not appropriate for closely packed particles or for very small particles. The lift force acting on a secondary phase p in a primary phase q is computed from vq B v p j ð∇ B v qÞ Flift ¼ 0:5Fq αp j B

ð4Þ

By default, Flift is not included. If the lift force is significant (e.g., if the phases separate quickly), it may be appropriate to include this term. 3.1.4. Virtual Mass Force. For multiphase flows, FLUENT includes the “virtual mass effect” that occurs when a secondary phase p accelerates relative to the primary phase q. The inertia of the primary-phase mass encountered by the accelerating particles (or droplets or bubbles) exerts a “virtual mass force” on the particles. dq vq dp vp ð5Þ Fvm ¼ 0:5αp Fq dt dt The term dq/dt denotes the phase material time derivative of the form dq ðϕÞ ∂ðϕÞ þ ðvBq 3 ∇Þϕ ¼ ∂t dt

ð6Þ

By default, even Fvm is not included. The virtual mass effect is significant when the secondary phase density is much smaller

3D geometry

height (m)

14

breadth (m)

2

14 2

no. of trays liquid phase material

9 hot resid

9 hot resid

density (kg/m3)

737

737

viscosity (kg/m s)

0.0018

0.0018

liquid phase velocity (m/s)

0.0189

0.0189

vapor phase material

hydrocarbon vapor

hydrocarbon vapor

density (kg/m3)

9.2614

9.2614

viscosity (kg/(m s))

7.0 106

7.0 106

vapor phase velocity (m/s) vapor phase volume fraction

0.15 0.09

0.15 0.09

than the primary phase density (e.g., for a transient bubble column). 3.1.5. Turbulence Modeling. The standard Kε model has been used most frequently for low-speed incompressible flows in isotropic turbulence. Here, turbulent dissipation ε is transported in addition to turbulent kinetic energy k. This is due to the fact that the size of eddies depend on dissipation which eliminates the smallest eddies thus effectively increasing the average eddy size. Launder and Spalding37 proposed the Kε model in which the turbulent kinetic energy and dissipation energy transport equations can be written as follows: Dk ∂ μ ∂k ¼ μ þ t F þ Gk Dt ∂xi σ k ∂xi þ Gb Fε YM Dε ∂ ¼ F Dt ∂xi

ð7Þ

μt ∂ε μ þ σ εk ∂xi

ε ε2 þ C1ε ðGk þ C3ε Gb Þ C2ε F k k

ð8Þ

with model constants having the following values: C1ε ¼ 1:44, C2ε ¼ 1:92, Cμ ¼ 0:09, σ k ¼ 1, σε ¼ 1:3 In these equations, Gk represents the generation of turbulence kinetic energy due to the mean velocity gradients, Gb is the generation of turbulence kinetic energy due to buoyancy, YM represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, and C1ε, C2ε, and C3ε are constants. σk and σε are the turbulent Prandtl numbers for k and ε, respectively. 1818



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Figure 2. Axial velocity profiles inside the column in (a) first and (b) second configuration.

The turbulent (or eddy) viscosity μt is computed by combining k and ε as follows: μt ¼ FCμ

k2 ε

ð9Þ

Where Cμ is a constant. In a CFD model, the hydrodynamic partial differential equations (PDEs) representing the conservation of mass, momentum and energy are solved numerically. These equations are supplemented by a turbulence model, the most common of which is the kε formalism, which employs an eddyviscosity concept.

4. NUMERICAL SIMULATIONS Correct modeling of the flow domain is of crucial importance for the numerical solution procedure in CFD calculations. Particularly three-dimensional calculations were difficult to converge for the computation of local velocities and phase holdup in

multiphase flows. A fully three-dimensional soaker drum has also been modeled as a part of the present work. The numerical solution procedure implemented here makes a discretization of the flow domain into sufficiently small grid cells necessary. “Sufficiently small” in this context means that the mesh width (or grid cell size) must not be too large in order not to adversely affect convergence. On the other hand too small grid size yields numbers of cells for which the descretized equations have to be solved leading quickly to a immense computational demand with respect to processor time and memory usage. In a three-dimensional calculation, reducing grid size to one-half its original value means an 8-fold increase in the number of equations to be solved, which is proportional to computational demand. Considering all these fact, the values given in Table 2 have proved to be a reasonable compromise between accuracy and computational demand for the 2D and 3D soaker geometry under consideration here. The mesh independence study was done and test calculations with finer grids have shown that no improvement in convergence 1819



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Figure 3. Contours of hc vapors volume fraction in (a) first and (b) second configuration.

and accuracy of the results could be achieved. Increasing cell number yielded massive increases in computation time but did not significantly change modeling result or bring about a better fit to measurement results. The time-dependent, unsteady state simulations were carried out until a steady state flow could be observed in the simulated flow profile. This was achieved in approximately 309 s of actual time simulations for 2D simulations and for approximately 1300 s of actual time for 3D simulations. In the case of 2D simulations, the time steps were taken as one thousandth of a second for initial 50 s in order to capture fast gradients in flow initially. Subsequently, the time steps were gradually increased to one tenth of a second. Similarly, in the case of 3D simulations, the time steps were taken as one thousandth of a second for initial 100 s due to the same reason of initial fast gradients in flow. Subsequently, the

time steps were gradually increased to one fiftieth of a second for minimizing the simulation time. The variation in time step size in the above manner not only ensured the better convergence in optimum time, acceptable values of hydrodynamic parameters were also obtained. The geometry for the 2D simulation was taken corresponding to a soaker drum of height 14 m and diameter of 2 m. Thus the length and breadth of simulation geometry was taken as 14 and 2 m respectively. Total nine sieve trays were taken, first starting at 1.4 m from the bottom and last at 13.6 m from the bottom. The free area for flow in these trays was as follows. The first two trays were provided with 12% free hole area, followed by next four plates with 16% free hole area, and last three plates with 20% free area. Two configurations were evaluated for the internals, first with increasing number of holes per plate with increase in flow 1820



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Figure 4. Velocity Vectors of hot resid in (a) first and (b) second configuration.

area (hole diameter kept same) and second configuration with keeping number of holes per plate same, but increasing diameter with free flow area. In a soaker drum the liquid phase is hot-resid and vapor phase is hydrocarbon vapor. The physical properties for liquid and gas phase were taken from available industrial data as well as database of Fluent, and two phases were defined accordingly. Liquid phase velocity was taken as 0.00189 m/s and vapor velocity taken as 0.15 m/s n 2D simulations. Vapor volume fraction was taken as 0.09. The 2D simulations provide a quick assessment for detailed simulations in a 3D geometry, as the 3D geometry is more demanding in terms of computational time. The results obtained from these 2D simulations may not be quantitatively reliable for actual design; still these can be relied upon for finalization of the geometry for further simulations to be carried out on the threedimensional model.

For 3D simulations also, the length of the geometry was taken as 14 m and diameter 2 m. As in case of 2D simulations, total nine sieve trays were taken, first starting at 1.0 m from the bottom and last at 13 m from the bottom. The tray spacing between all the other trays was taken to be 1.5 m. The available flow area for the internal structures was as follows. In this case, three trays from the bottom were taken with 12% available area for flow, followed by three plates with 16%, followed by three plates with 20% free area for flow. The liquid velocity was taken as 0.0189 m/s and vapor velocity taken as 0.15 m/s. The vapor volume fraction was taken as 0.09. The input parameters used in the above simulations are listed in Table 3. The simulation results have been discussed below.

5. RESULTS AND DISCUSSIONS As mentioned above 2D as well as 3D simulations was carried out. The two-dimensional geometry simulations reasonably 1821



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Figure 5. Velocity contours of hot resid in (a) first and (b) second configuration.

helped in comparative evaluation of two internal sieve tray configurations. The detailed discussions on the 2D and 3D simulations have been summarized in the following paragraphs. 5.1. 2D Simulations. Figure 2 shows the axial velocity profile of the liquid phase (hot resid) inside the column after 309 s for first and second configurations. The velocity profiles have been shown at inlet and three different heights. These velocity profiles reveal that flow is much smoother in case of second configuration. Although the maximum liquid phase velocity is almost same in both the cases, but reverse velocity in the column is quantitatively less in case of second configuration. The magnitude of axial velocity of the liquid phase across the holes is maximum. The liquid velocity vectors at opaque parts of trays are in downward direction. This is due to hurdle in the path of flow. The Y velocity

plots reveal that the liquid velocity is negative in certain region near the wall of column in both the configurations. However, the region of negative velocity is more in case of first configuration. This can be attributed to the more number of holes in subsequent plates in allowing recirculation of liquid through the holes near the wall. In the case of both configurations, it is observed that the negative Y velocity decreases as we move up in the column. This shows the effectiveness of the sieve trays in arresting the liquid phase backmixing. The contours of the volume fraction of the gas phase inside the reactor are shown in Figure 3. For detailed description, the geometry has been shown in three segments. The bottom part depicts first three trays. The next four trays are depicted as middle part and rest of the column is shown as top part. It is evident from 1822



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Figure 6. Velocity vectors of axial velocity of hot resid inside the column in XZ plane.

the figure that the gas phase concentration is at the maximum at the flow passages in the column. We can also see that some of the vapor is just below the plates. This can be attributed to the restriction in flow path due to opaque region. It has been reported by many authors that the internals cause the increase in the gas holdup in the column when being compared with equipments without internals. However, in our simulations, the gas phase hold-up was slightly more in case of first internal. The mass weighted average value of vapor phase volume fraction at outlet was 0.01 for first configuration and 0.008 for second configuration. This observation is in agreement with experimental observations of Alvare et al.19 They have also observed larger gas holdup, with the trays having a greater number of holes having equal percent flow area as the trays with larger but fewer holes. This may be attributed to less number of holes allowing less gas entrainment because of recirculation currents. The back-mixing of liquid phase (hot resid) taking place inside the column can be visualized in the velocity vector plots of the liquid phase in the column. Figures 4 show the velocity vectors of the liquid phase at different regions of the column. Here also the geometry has been shown in three segments as above. Similar to the Y velocity profiles, it is evident from this figure that region of backmixing is more in case of first configuration. The velocity contours of the liquid phase within the reactor are shown in Figure 5. In this figure, we can see that the velocity of the liquid is higher at the central core region and also at the walls. This is because, though the amount of back-mixing has been reduced by introducing internals within the reactor, there is still some amount of back-mixing inside the reactor in the sections between the plates. There is some amount of liquid down flow present in the reactor, but the amount of down flow has been drastically reduced when compared to the amount of backmixing in the column without any internals. A careful analysis of the 2D simulation as above revealed that the variation of flow area by varying the size of hole is advantageous over the configuration with varying the number of holes. In view of this, 3D simulations were carried out on second

configuration, to verify the 2D results and to have more realistic simulation results in an actual cylindrical soaker drum. 5.2. 3D Simulations. For the 3D simulations also, a column of height 14 m and radius 1 m was considered. In this column, nine sieve trays are provided as internals. The first tray was placed at a distance of 1 m from the inlet and the last tray was also placed similarly at a distance of 1 m from the outlet. The distance between consecutive trays is taken to be 1.5 m. The first three trays from the bottom are considered to have a free flow area of 12%. That is, it has 31 holes, each having a diameter of 0.1244 m and a center-to-center distance of 0.3333 m. The next three trays were considered to have a free flow area of 15%. That is, it has 31 holes, each having a diameter of 0.1391 m and a center-to-center distance of 0.3333 m. The next three trays were considered to have a free flow area of 20%. That is, it has 31 holes, each having a diameter of 0.1606 m and a center-to-center distance of 0.3333 m. For the current set of simulations, we considered a gas phase velocity of 15 cm/s and a liquid phase velocity of 1.89 cm/s. The axial velocity vectors of the liquid phase (hot-resid) inside the column along XZ planes have been plotted in Figure 6. It is evident from these profiles, that backmixing has been reduced up to large extent. Most of the region of negative velocity magnitude is on the opaque portions of the sieve trays. The contours of axial velocity of hot resid along YZ and XZ planes have been shown in Figure 7. It is clearly evident that the velocity of the liquid is lower at the bottom portions of the column as compared to the middle region and is again lower at the upper regions. The axial velocity of the liquid phase is maximum at the middle portion of the column. The flow becomes more regular as the liquid moves up in the column. This kind of regularity in the flow is desirable behavior for a soaker drum. It is also clear from the velocity contours that the liquid phase velocity is minimum n the walls as compared to the central core, which is also an indication of less back-mixed flow. Figure 8 shows the vapor phase (hc-vapors) volume fractions in YZ and XZ plane. The vapor phase is present near the holes in maximum fraction. The opaque portion of trays also 1823



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effective in reducing the liquid phase backmixing. In the absence of actual experimental trial of these internals, the CFD simulations on an industrial size of flow domain provided sufficiently realistic picture of the flow inside an industrial soaker, which can be very useful in the further efforts of designing efficient soaker internals.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +91 135 2525 784.

’ REFERENCES

Figure 7. Contours of axial velocity of hot resid inside the column in the (a) YZ and (b) XZ planes.

Figure 8. Contours of vapor phase volume fraction inside the column in the (a) YZ and (b) XZ planes.

shows accumulation of hc-vapors in large quantities. But the vapor fraction near column walls is limited to the region below sieve trays. These volume fraction profiles are similar to the profiles obtained in 2D simulations shown in Figure 3. The Mass averaged value of gas volume fraction at outlet is 0.029. This value can be taken as more reliable due to more realistic simulations in 3D flow domain.

6. CONCLUSIONS An industrial size soaker drum was simulated with nine trays. Two types of sieve trays were simulated as internals. The free area for flow on respective trays was kept same in both cases. In first configuration the flow area was varied by varying the number of holes, whereas size of each hole was kept constant. In second configuration, number of holes on each tray was kept constant and hole diameter was increased. It has been established from the CFD simulations that the variation in diameter of holes is more

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Comparison of Flow Patterns in a Visbreaking Soaker Drum with Two

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