2012 cfd simulation of co distillation in structured packing

Research Article

Computational Fluid Dynamics Simulation of13CO Distillation in Structured Packing

Physical 3D models were established for corrugated packing used in the enrich-ment of the isotope 13C. Computational fluid dynamics (CFD) simulation resultsindicated that common corrugated packing was not well wetted when used forisotope distillation. It is concluded that liquid misdistribution in the packedtower results from the structure of the packing rather than from the height of thepacking beds. The existence of entrainment was also demonstrated by CFD simu-lation. It is proved that mass transfer equations based on the Nusselt theory arenot suitable for distillation calculation in such a corrugated packing system. Bycomparison, the recently developed structured packing model with a corrugationgeometry based on the right-angled triangle, known as Zigzag-pak, describesvapor-liquid distribution properties well and has significant advantages overcommon corrugated packing due to its better liquid distribution character.

Keywords: Corrugated packing, Cryogenic distillation, Isotope 13C, Transfer intensification,Zigzag-pak

Received: July 15, 2011; revised: November 01, 2011; accepted: November 02, 2011

DOI: 10.1002/ceat.201100273

1 Introduction

The stable isotope 13C is widely used as a tracer element in thefields of medicine, pharmacology, and organic chemistry. Inparticular, the rapid rise of the 13C breath test in clinical diag-nosis has gradually replaced the radioactive 14C breath test overrecent years. There are several methods for producing 13C,including thermal diffusion, chemical exchange, gas diffusion,laser, and cryogenic distillation, with the latter being the onlyindustrial technique. CH4 and CO can be used in cryogenicdistillation [13], and their separation factors are 1.00351.0054 and 1.0071.01, respectively. In order to obtain 99 %13C from the natural abundance of 1.11 % needs about 3000theoretical stages which necessitates industrial distillation col-umns with lengths of more than 100 m [4, 5]. On the otherhand, such manufacturing devices are difficult to scale upbecause of a significant shortcoming, i.e., the random packingutilized in the columns.

Structured packing has been applied to the separation ofisotopes because of its good gas-liquid distribution propertiescompared with random packing [6]. However, unfortunatelythere is little understanding of the phenomenon of two-phasedistillation in the narrow channels of the packing. In order to

improve the phase exchange on the surface of structured pack-ing, the common method is to increase the specific surfacearea of packing. This is based on the Nusselt theory whichassumes that the liquid flows as a thin film over the whole sur-face of the packing. Thus, one can conclude that the vapor-liq-uid interfaces are in positive ratio to the surfaces of the pack-ing. However, no one has confirmed by measurement that thefalling film retains a stable thickness for fully developed flow inthe channels of corrugated packing when used for 13CO distil-lation.

A corrugated gauze packing, called PACK-13C, with aspecific surface area of 1135 m2m3 was fabricated and used ina 13C-enriching pilot-scale plant through carbon monoxide(CO) cryogenic distillation [7]. The packing bed was 18 mhigh, and the column diameter was 45 mm. The packing per-formed well regarding its mass transfer properties with morethan 20 theoretical stages per meter. The cryogenic installationhas a capacity of producing about 2.1 g of 15 % 13C per day. Itis difficult to continuously increase the specific surface area ofpacking because of the difficult manufacture and operation.Therefore, it is of critical importance to study the gas-liquiddistribution in the corrugated gauze packing in order toimprove the mass transfer efficiency. However, little attentionhas been paid to the two-phase flow distribution on the surfaceof structured packing. Because of the construction of narrowchannels for corrugated gauze packing, it is extremely difficultto carry out experimental measurements on the behavior ofthis type of countercurrent flow. Previous research has simplymodeled the corrugated packing as large plate, measured the

www.cet-journal.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2012, 35, No. 2, 334340

Hu-Lin Li1,2

Yong-Lin Ju1

Liang-Jun Li2

Da-Gang Xu2

1Shanghai Jiao Tong University,Institute of Refrigeration andCryogenics, Shanghai,China.

2Shanghai Research Institute ofChemical Industry, Shanghai,China.

Correspondence: Dr. H.-L. Li ([email protected]), Shanghai Jiao TongUniversity, Institute of Refrigeration and Cryogenics, 800 DongchuanRoad, Shanghai 200240, China.

334 H.-L. Li et al.

liquid flow, and simulated its flow by computational fluiddynamics (CFD) methods [8]. One report solved the empiricalequations to obtain the parameters of mass transfer efficiency[9, 10]. Others have calculated the pressure drop throughstructured packing by CFD simulations [11]. These studiesproposed the application of CFD simulations in packedtowers. In this paper, CFD simulations were carried out tostudy the two-phase flow in the narrow channels of corrugatedpacking. Based on the CFD simulation, a new type of corru-gated packing with better liquid distribution properties,known as Zigzag-pak, has been proposed and manufactured.Parameters of the common corrugated packing and zigzagpacking are listed in Tab. 1.

2 Mathematical Models

The volume-of-fluid- (VOF) model can develop two or moreimmiscible fluids by solving a single set of momentum equa-tions and tracking the volume fraction of each of the fluidsthroughout the domain [12]. In the current simulation, theVOF model is used to calculate the properties of the CO vaporand CO liquid that flow as countercurrents in the corrugatedpacking channels. The two CO phases are treated as incom-pressible fluids in the packing channels and the RNG-basedk-e turbulence model is used for the numerical simulation.The renormalization group (RNG) model has an additionalterm in its e equation that significantly improves the accuracyfor rapidly strained flows. The effect of swirl on turbulence isincluded in the RNG model, enhancing the accuracy for swir-ling flows. The RNG theory provides an analytically deriveddifferential formula for effective viscosity that accounts for theeffects of low Reynolds numbers. These features make theRNG k-e model more accurate and reliable for two-phasevapor-liquid flows in the narrow channels of the structuredpacking than the standard k-e model.

2.1 Volume Fraction Equation

The tracking of the interfaces between the phases is accom-plished by the solution of a continuity equation for the volumefraction of the two phases. For the qth phase, the equation is:

1

qq

t aqqq

aqqqvq

np1 mpq mqp (1)

where q is the volume-averaged density. The qth fluid volumefraction in the cell is denoted as aq, mqp is the mass transfer

from phase q to phase p, and mqpis the mass transfer fromphase p to phase q.

2.2 Momentum Equation

t qv qvv p l v v

T qg F (2)where p is the static pressure, v is the component of the flowvelocity parallel to the gravitational vector, l is the viscosity, gis the gravitational acceleration, and F is the external bodyforce.

2.3 Energy Equation

t qE vqE p keffT (3)

in which keff is the effective thermal conductivity. The VOFmodel treats the energy, E, and the temperature, T, as mass-averaged variables.

2.4 Closure Equations

The RNG k-e turbulence model is derived from the instanta-neous Navier-Stokes equations, using a mathematical tech-nique called the renormalization group (RNG) method. Theanalytical derivation results in a model with constants differentfrom those in the standard k-e model, and additional termsand functions in the transport equations for k and e.

t qk

xi

qkui xj

ak leffkxj

Gk Gb qe (4)

t qe

xi

qeui

xj

aleffexj

C1e ek Gk C3eGb C2eq

e2

k Re

(5)

where Re is given by

Re Clqg31 gg0e2

1 bg3e2

k(6)

In these equations, Gk represents the genera-tion of turbulence kinetic energy due to themean velocity gradients, and Gb is the genera-tion of turbulence kinetic energy due to buoy-ancy. The quantities ak and ae are the inverseeffective Prandtl numbers for k and e, respec-tively. g = Sk/e, g0 = 4.38, and b = 0.012.

Chem. Eng. Technol. 2012, 35, No. 2, 334340 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com

Table 1. Comparison of two types of corrugated gauze packing.

Packing types Peak height[mm]

Wave length[mm]

Hydraulicdiameter[mm]

Porosity Area[m2 m3]

Common corrugatedpacking

2.5 4 2.8 0.77 1100

Zigzag-pak 2.5 2.5 1.3 0.62 1900

Transfer intensification 335

3 Geometry and Boundary Conditions

Structured packing has a complicated geometric construction,and, therefore, needs a lot of time and physical memory tosolve the above dynamic equations even for the calculation ofone packing element. In the current paper, the simulationmodel is based on one triangular channel from a piece of thepacking element. The boundary conditions are as follows: theinlet velocity of the gas is 0.743 m s1, the outlet pressure of thegas is 60 kPa, the inlet velocity of the liquid is 0.0013 m s1, andthe outlet boundary of the liquid is the outflow. The tempera-ture is 80 K, and the fluids are the two phases of CO.

Fig. 1 is a schematic of the common corrugation geometryof the packing used in the 13C distillation installation. The cor-rugation height is 2.5 mm, the corrugation width is 4 mm, andthe side length is 3.2 mm.

Fig. 2 presents schematically the new corrugation geometryZigzag-pak, where the corrugation height is 2.5 mm and thecorrugation width is 2.5 mm. The main difference in geometrybetween the Zigzag-pak and common packing is that the for-mer has a right-angled triangle corrugation whereas the latterhas an isosceles triangle corrugation.

The schematic diagrams of the current CFD modelsfor the common packing and the new Zigzag-pak areillustrated in Figs. 3 and 4, respectively. They both havethe same corrugation inclination angle of 45 and thesame length of flow channel with 50 mm. As shown inFigs. 3 and 4, there is one inlet velocity for gas, oneinlet velocity for liquid, one outlet pressure for gas,one outflow for liquid, two walls, and one symmetryboundary for each 3D model.

4 Experiments

A commonly used corrugated gauze packing,PACK-13C, with a specific surface area of 1135 m2m3

was installed in the CO cryogenic distillation column[13]. The PACK-13C was fabricated by compressing adouble-layer silk screen, made of stainless steel, intothe corrugated shape. Pieces of the screens were thenassembled with an inclination angle of 45, corruga-

tion height of 2.5 mm, porosity of 0.77, and silk diameter of0.085 mm. The packing bed was 18 m high, and the diameterof the packing was 45 mm. When the F-factor varied from 0.18to 0.90 m s1(kg m3)1/2, the numbers of theoretical stages permeter ranged between 20 and 30. In that case, the pressuredrop was less than 25 Pa for one theoretical plate, the dynamicliquid holdup was between 11 % and 21 %, and the pilot-scaleplant produced 2.1 g of 15 % 13C per day. The experimentalmeasurements demonstrated that the PACK-13C exhibited ahigher mass transfer efficiency than traditional random pack-ing when used for 13C distillation.

As demonstrated in the flow sheet of Fig. 5, the purified COgas flowed into the cryogenic distillation column at the feedingpoint, the extracted gas was removed at the top of the columnby a vacuum pump, and the concentrated 13C product was takenout at the bottom of the column. The 13CO was enriched bynumerous liquid-vapor exchanges on the surface of the packing.The column was well insulated using multilayer insulation, andthe total heat leakage of the column was less than 30 W. Rawmaterials were high-purity CO gas and commercial liquid nitro-gen. The CO vapor and liquid nitrogen exchanged heat in thecondenser located on the top of the column, and the vaporizednitrogen gas was drawn out by a pump. The installation real-ized automatic operation and ran smoothly for over sixmonths.

5 Simulation Results and Discussion

The CFD simulations were carried out using the Fluent 6.3software platform. In this solver, a first-order upwind schemeand pressure-velocity coupling algorithms were applied for thediscretization of differential equations. The two-phase fluids ofliquid CO and vapor CO flow as countercurrents in the com-mon corrugated packing installed in the cryogenic column.The operation conditions were the same as those in the experi-ments. The distribution of liquid phase in the common corru-gated gauze packing is displayed in Fig. 6.


Figure 1. Geometry of the common corrugated packing.

Figure 2. Geometry of the newly developed packing Zigzag-pak.

Figure 3. 3D model for the common corrugated packing.

336 H.-L. Li et al.

Liquid CO streams down on one side of the chan-nel even though it spreads out as a thin film on theinlet surface. Most of the packing surfaces are notmoistened. This does not agree with the commonopinion which assumes that liquids cover uniformlyall of the packing surfaces. Therefore, the CFD simu-lation proves that large areas are not wetted and theliquid is not well distributed in a distillation columnfilled with common corrugated packing. It is clearthat this structural limitation can lead to liquid mis-distribution. Previously, liquid misdistribution hasoften been considered as the result of too high pack-ing beds. It is concluded that it is not appropriate tosimplify the packing element as a large plane, as pre-vious reports have done, although these have vali-dated the flow behavior on the plates by experimentalmeasurements. The reason is that liquids are moresensitive to the structure of the flowing channels thangases. In short, fluid flow in the channels of the cor-


Figure 4. 3D model for the new Zigzag-pak.

vacuumpump

vacuumpump

purificationcolumn

wastegas

rotameter

high vacuumpump

product

coolant

CO cylindercryogenic-destillationcolumn

liquidnitrogentank

nitrogengas

structuredpacking

P

P

P

P

P

H

H

Figure 5. Flow sheet of the 13C cryogenic installation.


rugated packing cannot be studied by simplifying it as a largeplane plate.

In comparison, the CFD simulation for CO countercurrentflow in the Zigzag-pak packing is given in Fig. 7. The wavy liq-uid film flows downward and spreads to both sides of thedownside wall. The surfaces are evenly covered by liquid,which is more consistent with the assumption of the masstransfer calculations. As a result, the moistened surfaces of thispacking are much larger than those of common packing, and,therefore, the mass transfer is intensified.

The pressure drop along the y-direction on the symmetrysurface of the Zigzag-pak is plotted in Fig. 8. The result is simi-lar to the experimental result using common corrugated pack-ing.

Unlike common corrugated packing, the liquid distribution,plotted in Fig. 9, indicates that much of the liquid film islocated in the outlet of the right-angled triangle channel of theZigzag-pak. Because the liquid distribution is optimized in theZigzag-pak, there are larger vapor-liquid interfaces for massexchange for the same gross packing volume than for commonpacking. Thus, the mass transfer is certainly intensified in theZigzag-pak channels.

Fig. 10 displays the particle path lines of the vapor and liq-uid in the Zigzag-pak for 13CO cryogenic distillation.

From the above CFD simulations, the details of the vapor-liquid flow in the structured packing are clearly indicated, i.e.,liquid misdistribution in the packed towers probably resultsfrom the structural limitation of the packing, and the liquidmisdistribution is independent of the height of the packingbeds. Although the common corrugated packing used for 13Cseparation in tests proved to be superior to traditional randompacking, it has a serious geometrical limitation. The newlydeveloped Zigzag-pak with a corrugation geometry consistingof a right-angled triangle has a better liquid distribution char-acter and could be applied in distillation towers to replace thecommon corrugated packing to provide higher mass transferefficiency.

6 Conclusions

A simplified 3D physical model representing the corrugatedpacking element is established and solved by computer. TheseCFD simulations illustrate that the common corrugated pack-ing is not well wetted in precision distillation because of itsstructural limitation. It has been proved that the mass transferequations, based on the Nusselt theory, are not suitable for dis-tillation calculation in such a corrugated packing system. Liq-uid misdistribution in the packed towers results from thestructure of the packing rather than from the height of thepacking beds. The new type of corrugated packing, the Zigzag-pak, with right-angled triangle corrugation has a better liquiddistribution character and larger vapor-liquid interfaces thanthose of the common corrugated packing. Considering itsprominent advantages in vapor-liquid intensification and high


Figure 6. Distribution of liquid CO in the common corrugatedpacking.

Figure 7. Distribution of liquid CO in the Zigzag-pak.

Figure 8. Pressure drop along the y-direction on the symmetrysurface of Zigzag-pak.

338 H.-L. Li et al.

efficiency in mass transfer, the Zigzag-pak could replace com-mon corrugated packing in chemical engineering.

Acknowledgment

This research has been financially supported by the ShanghaiScience and Technology Talent Project in China (No.10QB1401700). The experiments were carried out in the labo-ratory of the Shanghai Engineering Research Center for StableIsotopes.

The authors have declared no conflict of interest.

Symbols used

m [kg s1] mass flow ratep [Pa] static pressureE [J] total energyT [K] temperaturekeff [Wm

1K1] effective thermal conductivityF [N] force vectorg [m s2] gravitational accelerationGk [] generation of turbulence kinetic energy

due to the mean velocity gradientsGb [] generation of turbulence kinetic energy

due to buoyancyS [J K1] total entropyt [s] timek [J kg1] kinetic energy per unit massu [m s1] velocity magnitudev [m s1] overall velocity vectorC1e [] constant, 1.42C2e [] constant, 1.68C3e [] constant, 0.0845

Greek symbols

q [kg m3] densityg [] effectiveness factorl [Pa s] dynamic viscousap [] p

th fluid volume fractionaq [] q

th fluid volume fractionak [] inverse effective Prandtl numbers for ka [] inverse effective Prandtl numbers for eleff [Pa s] effective viscositye [m2 s3] turbulent dissipation rate

Subscripts

q qth phasep pth phase


Figure 9. Contours of liquid volumefraction on the surface of the liquid out-let for Zigzag-pak.

Figure 10. Particle path lines for the vapor and liquid in Zigzag-pak.


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2012 cfd simulation of co distillation in structured packing

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