2096 Ind. Eng. Chem. Res. 1988,27, 2096-2100

Scale-up of Laboratory Data for Distillation Columns Containing Corrugated Metal-Type Structured Packing

Jeffrey R. Hufton, Jose L. Bravo, and James R. Fair* Department of Chemical Engineering, The University of Texas, Austin, Texas 78712

Laboratory-scale measurements were made of the mass-transfer efficiency of a gauze-type distillation column packing. The test mixture, chlorobenzene/ethylbenzene, was used, and several operating pressures in the range 40-750 mmHg were studied. It was found that the HETP data were correlated well by available models developed for commercial-scale structured packings. The final result of the work is a procedure that enables tests a t 25-mm column diameter size to be used for predicting performance of a large-scale system utilizing structured packing, gauze, or sheet metal.

For many years, packed columns for distillation, ab- sorption, and stripping contained solid elements that were dumped into the columns in a random fashion. The ele- ments often were metallic or ceramic pieces with ring or saddle shapes. In more recent times, the elements were also fabricated from plastic materials. These randomly packed columns operated with gas and liquid flowing countercurrently, and mass transfer between the phases occurred across interfaces formed on the packing surfaces, thought to be covered by flowing liquid films.

While the randomly packed column continues in use, a new type of packed column has emerged in recent years. This is the column containing ordered elements formed from corrugated metal gauze and sheet metal that are placed together in a careful arrangement and which are dimensioned to fit snugly into the column. In today’s parlance, these devices are known as “structured packmgs”. A photograph of a typical structured packing element, sized to fit in a relatively small diameter column, is shown in Figure 1.

Structured packing is used most often in vacuum dis- tillation service, where it exhibits a very low pressure drop in the flowing vapor while maintaining high efficiency for mass transfer. The flow of liquid and vapor through prescribed channels results in near elimination of pressure losses due to form drag. The result is a high conversion of momentum loss to effective mass transfer, a phenom- enon often sought by process engineers charged with the design of separation equipment to operate within tightly prescribed pressure limits.

Scale-up of Packed Columns Pilot-plant technology has evolved to a state such that

very small catalytic chemical reactors can be used to prepare products of commercial interest. The associated separation sequences of these reactors have not been subjected to the same degree of miniaturization, however, particularly when packed distillation columns are involved. In many instances, the size and cost of a pilot plant is determined by the smallest feasible separation device that can still produce reliable results amenable to scale-up.

The hydraulic characteristics of packed columns are such that a minimum relationship between the column and packing sizes should not be exceeded if the performance of the small column is to be adequate. A ratio of column diameter to packing diameter of 8 to 1 is frequently used. Designers have found that extrapolation of laboratory data, using specialty packings, to a commercial device is ex- tremely unreliable. As a result, the tendency has been to install commercial-type internals in pilot columns; such internals result in relatively large and expensive equip- ment.

0888-5885/88/2627-2096$01.50/0

There is a clear need for a reliable and consistent me- thod to scale from a small laboratory packing to a large commercial packed device. The use of structured packings that can vary in size significantly but maintain geometric similitude provides an opportunity to do this in a fairly rigorous manner.

Previous Work Structured packing in its present form was introduced

in the mid-l960s, largely on the basis of developmental work at Sulzer Brothers, Ltd., in Switzerland (Huber, 1966; Billet, 1969). The initial product was formed from metal gauze which had been perforated. Later versions were fabricated from less-expensive sheet metal (Huber and Meier, 1975; Meier et al., 1977, 1979), and currently there are both gauze and sheet metal versions of structured packing marketed by a number of companies throughout the world.

In 1985, Bravo et al. presented a mechanistic model which was shown to correlate quite well the mass-transfer characteristics of the commercial-size gauze-type packing (specifically the Sulzer BX geometry). Basically, a wetted wall column model was adapted to represent the flow of vapor and liquid through the channels formed by the el- ements of corrugated and perforated metal gauze. These workers concluded that, because of the enhanced liquid spreading characteristics of the gauze material, essentially the entire surface was wetted and effective for mass transfer.

Recently, Sulzer Brothers (1985) has made available a laboratory-scale version of the gauze packing (Figure 2). This packing can be placed in distillation columns of 25- 30-mm inside diameter with the advantage of reduced cost demonstration of a structured packing application. For tray-type distillation columns, the advantages of scale-up from similar size columns, using Oldershaw equipment, have been described by Fair et al. (1983). The advantages include not only reduced cost because of the smaller scale but also the capability of bypassing the expensive and sometimes ill-defined parameters of phase equilibria, theoretical stage calculations, and tray efficiency deter- mination.

The present work was initiated with the hope that a method comparable to the Oldershaw scale-up technique could be developed for structured packing.

Experimental Work A laboratory distillation system was assembled as shown

in Figure 3. The glass column was 25-mm i.d., to accom- modate gauze-type packing of 22-mm o.d., obtained from Sulzer Brothers, Ltd., Winterthur, Switzerland. The distillation setup was conventional and designed for total

0 1988 American Chemical Society

Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2097

Figure 1.

~~~

Figure 2.

Table I. Typical Values of Relative Volatility at Experimental Run Conditions for Chlorobeuzeue/Ethylbnzene System

pressure. mmHg av temp, O C re1 volatility 40 52 1.197 78 67 1.173 300 103 1.141 750 134 1.131

reflux operation. Heated parts were well-insulated to minimize heat losses. The entire setup was placed in a walk-in-type fume hood.

The test liquid mixture was the binary ethylbenzene/ chlorobenzene. This mixture has been recommended for test work by the European Federation of Chemical En- gineering (Zuiderweg, 1969) and has been used extensively for large-scale tests with structured packing (e.g., Huber and Meier (1975), Meier et al. (1977, 1979), and Billet (1986). The mixture has been shown to behave ideally in the liquid phase (Maher and Smith, 1979); values of the relative volatility for the test pressures are shown in Table I. Other properties of the mixture may be found in Zuiderweg (1969) or derived from compilations of pure

Figure 3.

Table 11. Comparison of Dimensions of Industrial-Scale and LahoratoN-Scale Sulzer Gauze-Type Structured Packin@

dimension commercial BX laboratow element diam., em typically 20+ 2.2 crimp height, mm 6.4 1.55 channel side, mm 8.9 2.02 crimp angle, deg 89 80 specific area, m2/ms 492 1300 void fraction 0.90 0.96 inclination angle, deg 60 58

'Dimensions of the laboratory packing are shown as measured (Hufton, 1987). They may differ somewhat from thme provided by the manufacturer.

component data (e.g. Yaws (1977)). Each of the mixture components had a purity of 99.8%.

The packing elements were 22-mm 0.d. by 5%" long. Ten elements were fitted together tightly, giving a total packed height of 58 cm. The reflux stream was directed to the center of the top element. It should be noted that the 22-mm-0.d. elements contain metal gauze bands of about 1-mm thickness; thus, the elements fit snugly in a 25-mm column. Communications from Sulzer Brothers indicated that a 30-mm column would represent a more convenient size for installation and removal of elemenk, however, this would provide additional free volume around the elements which would become imponderable in anal- ysis of the true mass-transfer efficiency of the packing, since the free volume would represent about 40% of the total volume for contacting. Dimensions of the laboratory packing, together with those for commercial-scale Sulzer BX packing, are given in Table 11.

Total reflux runs were made at head pressures of 40,78, 300, and 750 mmHg. Overhead and bottoms samples were analyzed by refractive index, calibrations of which con- firmed the results of Hawkins and Brent (1951). Care was taken to ensure stable operation, and separate runs were made a t replicate sampling conditions. Further details

7.0

6.0

5 5.0 d

- $ 4.0 s

7.0 1

- -

MODEL -

i l . U 0.0 0.2 0.4 0.6 0.1 1.0 1.2

Vapor Rate (F, (l/s)@g4n)o')

h 3.0 8 P

w g 2 0

1.0

7.0 1

- - PRESSURE = 40 mm Hg

6 0

5 0 -

$ 4.0 - v

2 3 0 - E 5 2.0 E

l PRESSURE = 300 mm Hg

-

' -

".U

0 0 0 .2 0.4 0.6 0.8 1.0 1.2

Vapor Rate (F,, (l/s)&g/m)O5)

PRESSURE = 750 mm Hg

Ann-

0.0 Y 0.0 0.2 0.4 0.6 0.8 1.0 1 .2

Vapor Rate F,, (l/s)Org/m)o-?

Figure 4.

may be found in the thesis of Hufton (1987).

Results Overhead and bottoms compositions for each run were

converted to height equivalent to a theoretical plate (HETP) values by means of the Fenske relationship:

where x is the mole fraction, 2 is the subscript for the more volatile component (chlorobenzene), 1 is the subscript for the less volatile component (ethylbenzene), D is the sub- script for the distillate, B is the subscript for the bottoms,

Table 111. Comparison of Model and Experimental Values of HETP for Chlorobenzene/Ethvlbenzene at Total Reflux

calcd HETP/exptl HETP mmHg F. = 0.4" F. = 0.8"

pressure,

40 1.33 1.11 78 1.44 1.15 300 1.55 1.09 750 1.32 1.00

7.0 1 6.0 1 SULZER DATA I

8 5.0

E 4.0 ' x 3.0 -

.B g 2.0 1 flu PRESSURE = 40 mm Hg

W POINTS: THIS WORK

E

1.0 - r

W 1.0 1 POINTS: THIS WORK I

SULZER DATA - yu PRESSURE: 78 mm 00

1 .o POINTS: THIS WORK

0.0 ' n ' n ' n " ' b '

0.0 0.2 0.4 0.6 0.8 1.0 1.2

~ ~ p o r ~ ~ t c (F" (l/s%um)oJ)

Figure 5.

2 is the height of the packing (58 cm), aZl is the relative volatility, and HETP is in centimeters.

The resulting HETP values are plotted in Figure 4. Also plotted are curves based on the model of Bravo et al. (1985), utilizing geometric data given in Table 11. It is clear that for most of the practical range the industrial-scale model is conservative; i.e., it predicts higher HETP values than were observed. For the particular range of F, = 0.4-1.0 (l/~)(kg/m)O.~ the deviations are given in Table 111.

Thus, if the laboratory column is operated in the vicinity of F, = 0.8-1.0 (l/~)(kg/m)O.~, the industrial-scale model gives a reasonable and slightly conservative fit. This has implications for scale-up procedures, to be discussed later.

Experimental curves (not data points) for this test mixture at 40 and 76 mmHg top pressure have been pro- vided by Sulzer Brothers (1985), and these are shown in Figure 5. At the lower pressure, the fit is excellent, whereas at the higher pressure the Sulzer HETP values are higher by about 1 to ll/z cm. Details of the Sulzer experimental work are not available, although it is known that a 30-mm column was used. The reasons for the performance differences are not known; the important conclusion is that approximately the same values of HETP were obtained in the two laboratories.

Scale-up Procedure The development of a scale-up method may be based

on the Bravo et al. (1985) model. For the system at hand,

Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2099

demonstration that the desired separation can be made, for a known packed height and reflux ratio, leads directly to the specification of required packed height for the commercial separation. This approach is similar to the one proposed for tray columns using laboratory Oldershaw tests (Fair et al., 1983). It can be a powerful tool for dealing with scale-up problems during process development.

While the method described here has been developed strictly for gauze-type structured packings, where the ef- fective area, a,, may be taken as the specific surface area, up (complete wetting of the surface), there are indications that it can be useful when sheet metal packings are to be used. The recent paper of Fair and Bravo (1987) shows that the earlier mass-transfer model (Bravo et al., 1985) can be used for such packings by applying an appropriate discount factor, p. Thus, for the laboratory scale, a, = up, then for the commercial scale, ae2 = papz. For many in- dustrial situations, /3 is in the range 0.60-1.00.

Since the value of p is inversely proportional to the HETP obtained for the large packing, an accurate pre- diction of the effective interfacial area for the conditions of the large column is important. Predictive methods for /3 are not available a t this time, but as indicated by Fair and Bravo (1987), at loadings equivalent to pressure drops of 8-10 mbar/m of packing or greater, the value of @ ap- proaches 1.0. Current research efforts are focusing on ways to predict effective interfacial areas of sheet metal struc- tured packings in distillation service.

Liquid Distribution Considerations The scale-up technique that has been outlined above can

be used effectively to obtain a value of HETP or H for a commercial packing when data for a laboratory packing are available, as long as both packings are of the corrugated metal (gauze or sheet metal) type. The value of HETP thus obtained becomes the “basic” HETP of the packing as defined by Zuiderweg et al. (1987). This basic HETP is not affected by liquid maldistribution factors and is the one achievable with near perfect liquid distribution. The HETP specified for the large-scale column should take into account the distributor design and the packing distribution characteristics. In particular, wall fit and bed depth can influence HETP in the larger size systems. Studies such as those of Hoek et al. (1986) and Zuiderweg et al. (1987) should be consulted for further information on liquid distribution in large packed columns.

The study of Zuiderweg et al. indicates that one does not need to be particularly careful about initial liquid distribution in small laboratory columns (50-mm i.d. and less) such as the one used in the experiments described above. For this reason, the column liquid feed was a simple reflux tube centered over the top packing element. Still, a reasonable effort should be made to provide the labo- ratory column with adequate initial liquid distribution.

Stepwise Scale-up Procedure On the basis of work reported here, the following step-

wise scale-up procedures are recommended: Case A. Reliable equilibrium and physical property data

are available. Al . Estimate the required height of packing for the

laboratory column. The estimate may be based on vendor packing information or on scale-down estimates from the correlation of Bravo et al. (1985).

A2. Set up the laboratory distillation system, using a column inside diameter slightly larger than the packing outside diameter. If the height of packing is greater than 1 m, more than one bed of packing should be used, with collection and redistribution of the liquid between beds.

as for most distillation systems, the majority of the mass-transfer resistance is in the vapor phase. The vapor mass-transfer coefficient for the structured packing is expressed through the Sherwood number:

This relationship was developed by Bravo et al. from an extensive collection of data on the performance of gauze- type structured packings. The data were taken for column diameters ranging from 75 to 1000 mm and for Sulzer BX packing. Eight different systems were represented and were operated at pressures ranging from 10 to 760 mmHg absolute.

The Bravo et al. model is based on defined packing geometry which determines dimensions of the flow chan- nels for vapor and liquid. This formalized approach of relating the performance of a packing to its geometry as well as to the properties and flow rates of the fluid phases constitutes the basis for using the model in the following scale-up procedure.

If large (subscript 1) and small (subscript 2) column conditions are compared at the same temperature and pressure (no change in physical properties), and if the liquid velocity is considered negligible compared with the gas velocity, then eq 2 leads to

0.8 kgldel delUge,l G G = ( G G ) (3)

The height of a vapor-phase transfer unit is defined as

(4)

and if H g - Hpg - HETP (strictly true if there is no liquid-phase resistance and the operating and equilibrium lines have the same slope), then

(5)

Since Uge = U , / ( E sin e),

where

sin O2

sin O1 (7)

For many applications, $/ - 1.0. Finally, since 2 = Nt X (HETP),

when Ntl = Ntz. If the liquid velocity is not neglected in the step from eq 2 to eq 3, then eq 8 becomes

Equation 8 is especially useful for cases where there is poor definition of vapor-liquid equilibria or even of the composition of the mixture to be separated. A laboratory

2100 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988

A3. Make experimental runs at the same pressure as planned for the commercial-scale unit. The first runs should be to determine the flooding velocity at total reflux.

A4. Run the column at an F factor of 1.0 ( l /~ ) (kg /m)”~ if lower than the flood F factor. If not, use an F factor of 75% of the flood F factor.

A5. At a design reflux ratio, sample above and below the packing to determine the separation accomplished. For cases when more than one bed is used and physical properties changed significantly throughout the column, it may be desirable to sample between beds.

A6. Compute the number of theoretical stages equiva- lent to the separation accomplished. On the basis of height of packing used, determine HETP for the laboratory packing.

A7. Compute the commercial-scale packing HETP by using eq 6 and 7, using appropriate vapor and liquid flow conditions.

Case B. Reliable equilibrium and physical property data are not available.

B1. Set up the laboratory column, and run the real system. Determine by test the necessary packed height, Z1, to perform the separation. The same guidelines for step A2 above apply here.

B2. Use eq 8 to determine the packed height require- ment, Z,, for the commercial column operating at a vapor velocity specified a t an appropriate approach to flooding for the packing specified.

Summary and Conclusions Laboratory tests with 22-mm-diameter structured

packing have shown that efficiency models developed for commercial-scale structured packings can be applied on a reasonable basis to laboratory operations. This opens the way to bench-scale testing on an economic basis that provides reliability of prediction for design. The method should be regarded as tentative, pending additional scale-up testing. It has been developed on the basis of structured packings fabricated from metal gauze, but in- dications are that with a suitable area discount factor it can be used also for scaling to commercial-sized sheet metal structured packings.

Nomenclature a, = effective interfacial area for mass transfer, m2/m3 up = specific surface area of packing, m2/m3 de = equivalent diameter of packing, m D, = gaseous diffusion coefficient, m2/s F, = superficial F factor for gas = U$s!pg)1/2, (l/~)(kg/m)O,~ H, = height of a gas-phase tranfer unit, m or cm H = height of an overall gas-phase transfer unit, m or cm H8TP = height equivalent to a theoretical plate, m or cm k , = mass-transfer coefficient for gas phase, m/s

U,, = effective gas velocity through packing, m/s U,, = superficial gas velocity through packing, m/s UL, = effective liquid velocity through packing, (3r/

x = mole fraction in liquid phase z = height of packing, m or cm Greek Letters a = relative volatility fl = discount factor for packing wetted surface, fractional r = liquid flow based on packing perimeter, kg/cm t = void fraction of packing

2 ~ ~ ) ( ~ ~ ~ g / 3 ~ ~ r ) ~ / ~ , m/s

= angle of inclination of packing channels from horizontal, deg.

I.L = vlscosity, mPa s p = density, kg/m3 $ = parameter defined by eq 7

Subscripts B = bottoms condition D = distillate condition g = gas or vapor L = liquid

Literature Cited Billet, R. Znst. Chem. Eng. Symp. Ser. 1969,32, 4/42. Billet, R. Packed Column Analysis and Design; Glitsch: Dallas,

Bravo, J. L.; Rocha, J. A.; Fair, J. R. Hydrocarbon Process. 1985,

Fair, J. R.; Bravo, J. L. Znst. Chem. Eng. Symp. Ser. 1987,104, A183. Fair, J. R.; Null, H. R.; Bolles, W. L. Znd. Eng. Chem. Process Des.

Hawkins, J. E.; Brent, A. B. Znd. Eng. Chem. 1951,43, 2611. Hoek, P. J.; Wesselingh, J. A.; Zuiderweg, F. J. Chem. Eng. Res. Des.

Huber, M. US Patent 3 285 587, 1966. Huber, M.; Meier, W. Sulzer Tech. Rev. 1975, 57(1) 1. Hufton, J. R. “Performance and Scale-up of Laboratory-Scale Dis-

tillation Columns Containing Structured Packing”. M.S. Thesis, The University of Texas a t Austin, 1987.

1986.

64(1), 91.

Deu. 1983, 22, 53.

1986, 64, 431.

Maher, P. J.; Smith, B. D. J. Chem. Eng. Data 1979,24, 363. Meier, W.; Stoecher, W. D.; Weinstein, B. Chem. Eng. Prog. 1977,

73(11) 71. Meier, W.; Hunkeler, R.; Stoecker, W. D. Znst. Chem. Eng. Symp.

Ser. 1979, 56, 3.3/1. Sulzer Brothers, Ltd. “Sulzer Laboratory-Scale Structured Packing”.

Leaflet VT2008/e, May 1985. Also, Aeschbach, S., (Sulzer), pri- vate communication to J. R. Fair, Sept 25, 1986.

Yaws, C. L. Physical Properties; McGraw-Hill: New York, 1977. Zuiderweg, F. J., Ed. Recommended Test Mixtures for Distillation

Columns; Institution of Chemical Engineers: Rugby, England, 1969.

Zuiderweg, F. J.; Hoek, P. J.; Lahm, L. Znst. Chem. Eng. Symp. Ser. 1987, 104, A217.

Received for review January 14, 1988 Revised manuscript received June 14, 1988

Accepted July 6, 1988