Hold-up of mellapak structured packings

Chemical Engineering and Processing, 31 (1992) 119- 124

119

Hold-up of Mellapak structured packings P. Sues and L. Spiegel SulzerBros. Ltd., Separation Columns, Dedicated

CH-8401

to Prof. Dr.-Zng. A. Vogelpohl

Winterthur

(Switzerland)

on the occasion

of his 6&h birthday

(Received October 23, 1991; in tinal form November 27, 1991)

Abstract The liquid hold-up of three structured packings (Mellapak 250.X, 250.Y and 500.Y) was measured with a gamma ray absorption technique using the air/water system. The results agree well with other experimental data available from the literature. A correlation was derived to calculate the hold-up as a function of liquid load, liquid viscosity and specific surface area of the packing.

Introduction

Hold-up is defined as the volume of liquid present in the form of a liquid film or rivulet on the surface of a packing, or drops in the void space of the packing. We limit ourselves to countercurrent, steady-state operation of normal two-phase flow. Knowledge of hold-up is important for understanding column dynamics, batch applications, column start-up, design of support devices for the column (because of liquid weight in operation), theoretical models of packing behaviour and calculation of residence time. Structured packings have been widely used in the process industry for more than 25 years. The Mellapak type is made from metal sheets. The performance characteristics of Mellapak have been reported elsewhere [1,21. For random packings (rings, saddles, etc.), hold-up is thought to consist of static and dynamic hold-up. Static hold-up is made up of the liquid held within the packing, principally by capillary forces, at zero gas and liquid flows. Dynamic hold-up is made up of the flowing liquid. This classical distinction cannot be made if hold-up is measured by the gamma ray absorption technique. Then the total volume of liquid is measured, irrespective of its state of motion. For the purpose of formulating theoretical models, however, the notion of the two kinds of hold-up may be helpful. McNulty and Hsieh [3] measured the static and dynamic hold-up of Flexipak lY, 2Y, 3Y, 4Y (Flexipak is the US trademark of Mellapak.) The static hold-up was determined by weighing the wet packing, the dynamic hold-up by measuring the amount of liquid leaving the packing after stopping liquid supply.

0255-2701/92/$5.00

With a similar method, Billet and Mackowiak [4] and Mackowiak [5] determined the dynamic hold-up of Mellapak 250.Y. The purpose of this paper is to present the data for liquid hold-up in three structured packings (Mellapak 250.X, 250.Y. 5OO.Y), obtained by the gamma ray absorption technique, to compare them with other published data, and to provide a correlation for the liquid hold-up in the region below the loading point.

Experimental

The hold-up measurements were made in our hydraulic simulation column SIM-1000 (Fig. 1). The SIM-1000 has an inner diameter of 1000 mm, and consists of transparent PVC, to make visual inspection of the flow conditions possible. The maximum possible packed height is 3.5 m. At the bottom of the packing the water is separated from the two-phase area by a liquid collector, from where it is led into a 4 m3 tank. Three pumps of different size pump the liquid to the top of the column with a flow rate of up to 180 m3/h. The flow rate is kept constant by means of control valves, in turn controlled by an electronic flow indicator/controller, which receives the signals of flow rate measurement turbines. Entering at the top of the column, the liquid is distributed over the packing by a distributor suitable for use over a wide range of liquid loads. The gas circulation is driven by a radial fan. The maximum possible gas flow rate is 20 000 m3/h of air at a pressure drop of 65 mbar. The gas flow rate is measured by the pressure difference, with a pitot tube, using an electronic micromanometer. The

(Q 1992 - Elsevier Sequoia. All rights reserved

TABLE 1. Characteristic and 500.Y Type

250.x 25O.Y 5OO.Y

dimensions

of Mellapak

250.X,

aI ( m’lm’)

Frns/rn’)

$eg)

250 250 500

0.98 0.975 0.975

30 45 45

25O.Y

F,, m/s (kg/m3)0.5 A 20 II

0.00

l

2.05

n

4.25

. .

n m= .

s

ri

15

I

I

.

2 9 2 B 3

. . lo-

.

t h

i

1, .

20000m3/h *j=*

::i:;::::,.%

Ag*.,

5

Fig. 1. Schematic

layout

of the test system.

flow can be adjusted by a vane controller at the suction side and a hand flap at the pressure side. The overall pressure drop is determined by a U-tube manometer, using pitot tubes before the inlet and after the outlet of the column. For small pressure drops, the micromanometer can also be used as measuring equipment. All used pitot tubes can be flushed back with compressed air to avoid corruption of the measurements by the presence of condensed liquid. The hold-up is measured with commercial radiometric density measurement equipment. The gamma rays are emitted by a 150 mCi Cs 137 source. At the opposite side of the column a detector measures the transmitted intensity of the gamma rays. It supplies a count rate dependent on radiation intensity and sends it to an evaluation unit which transforms the pulses from the counter into a linear output signal. This signal is recorded on a chart, from which the average intensity and the temporal fluctuations are determined. The hold-up of Mellapak 250.X, 250.Y and 500.Y was measured. The characteristic dimensions, surface area a,, void space E and crimp angle cp. are given in Table 1. The packings are manufactured from stainless steel.

0

5

io

i5

i0

Height above bottom layer, cm

Fig. 2. Vertical profile different gas loads.

of hold-up

with

horizontal

scanning

at

Measurement procedure At the beginning of the experimental programme the direction of the gamma ray was horizontal across the packing along a diameter. In order to get information about the accuracy of the method a preliminary measuring programme was performed. The hold-up was measured at different elevations within one packing element for different air loads at constant water loads. The result is shown in Fig. 2. The data were measured with a liquid load 1 of 100 m3/m2 h at three different air loads. It is clearly seen that at the highest air load the local hold-up varies between 7% and 20% absolute. Here, and for all data to follow, hold-up is given as percentage of liquid volume per unit packed volume. The measurements showed that flooding starts where two packing elements touch each other. Hold-up is therefore a local property of the packing above the

121 hokiup profiles

loading point

Fig. 3. Vertical hold-up profile.

0

1

2

3

4

5

6

Gas load F,, m/s (kg/m3j0.”

loading point (see Fig. 3). The loading point characterizes the condition where hold-up increases sharply due to the interaction between gas and liquid phase. Therefore to obtain reliable information on the average hold-up it was necessary to modify the experimental arrangement: instead of a horizontal an oblique traverse of the gamma ray was used. This gives a hold-up value which is an average over a packing element as a whole.

Temporal

behaviour of hold-up

The intensity of the absorbed gamma rays was recorded on a chart, from which the temporal fluctuation of the signal could be read. In Fig. 4 the mean

“‘“1

Fig. 5. Hold-up of Mellapak 250.X.

amplitude is shown as function pressed by the F-factor F,:

of the gas load ex-

F, = wcpco.5

where pG is the gas density (kg/m3) and wo the superficial gas velocity (m/s). The liquid load is the parameter. For low F-factors the amplitude is small, O.l%-0.2% absolute. Above the loading point the amplitude increases quickly to 0.3%0.5% absolute. Before flooding it decreases again, where flooding is defined as the condition where countercurrent operation is no longer possible. This behaviour indicates that above the loading point the interactions between gas and liquid are strong and that the hold-up is not a static property of the packing.

Results Liquid

load I, m/h

Gas load F,,

m/s (kq/m3)“.5

Fig. 4. Hold-up fluctuation versus gas load.

The hold-up data for Mellapak 250.X, 250.Y and 500.Y are given in Figs. 5-7. The diagrams show the hold-up as a function of the F-factor, the parameter is the liquid load, which was varied between 5 and 200 m3/m2 h. The data were obtained for a liquid temperature of 20 “C. For lower liquid loads the behaviour of the hold-up curves is as would be expected, that is, almost horizontal up to the loading point. Above the loading point the hold-up increases rapidly due to the strong interactions between gas and liquid. At high liquid loads the holdup increases with increasing gas load and reaches a plateau about 3% higher (see, in particular, Fig. 6). At yet higher gas loads the hold-up rises sharply. This effect appears above 125 m3/m2 h for Mellapak 250.X and 80 m3/m2 h for Mellapak 250.Y and 500.Y and also has an influence on the pressure drop at corresponding loads (see Fig. 8). The effect cannot yet be explained

122

I

1

/

y /

1

1

Packing

--.--

01

.

0

I 1

r

I

I

2

3

I

r

16

_. p.

6.4

-+-

96

.

!

4

I

I

5

6

.

M250.Y

l

M250.X

.

M500.Y

l

FPZY

+

FP3Y

*

M250.Y

of Mellapak

10

1

Liquid

Gas load FV. m/s (kg/m3j0.5

Fig. 6. Hold-up

II type

load I, m/h

Fig. 9. Hold-up versus liquid load for Mellapak 250.X, 250.Y and 500.Y and Flexipak IY, 2Y and 3Y. Literature data from Billet [6] and Mackowiak [5].

250.Y.

theoretically, but certainly indicates a change in flow regime. This has to be investigated further to improve holdup and pressure drop models of structured packings at very high liquid loads.

Discussion

0

1

2

9

Gas load F

Fig. 7. Hold-up

of Mellapak

V’

4

m/s (kg/m3)D5

500.Y.

5

6

In Fig. 9 the hold-up data are plotted against liquid load for zero gas flow. It can be seen that on a log-log diagram the data may be represented by two straight lines: the first with slope 0.37 in the range below 40 m3/m2 h and the second with slope 0.59 in the range above 40 m3/m2 h. This behaviour is consistent with that of Flexipak lY, 2Y and 3Y as measured by McNulty and Hsieh [3]. Billet and Mackowiak [4] measured the hold-up of Mellapak 250.Y with a volumetric method in a column of internal diameter 220 mm and packing height 1.4 m. Their data agree well with the present data within the experimental error. No effect of column diameter is visible. Because their measurements were obtained for liquid loads of up to 65 m3/m2 h only, the transition in the flow regime may be difficult to recognize from their data. Billet and Mackowiak correlated hold-up against liquid load by a 213 power law, but the data could be equally well correlated with the exponent of 0.59 suggested here for the new data. The in&em-e

Gas load F,. m/s (kg/m’)‘.5

Fig. 8. Pressure drop of Mellapak

250.X.

of packing

surface

area

In Fig. 9 it can be seen that the surface area of the packing is important. From the data, hold-up is found to increase with the surface area to the power of 0.83.

123

Again this result is supported by the data for Flexipak of McNulty and Hsieh who also found hold-up to increase with surface area to the power of 0.83. This is in contrast to the results of Billet and Mackowiak for Montz packings Bl-100 to Bl-300, for which they found the hold-up to vary with surface area to the power of 0.33. This may be explained by the fact that Billet and Mackowiak’s experiments were carried out at lower liquid loads, at which the packing may not have been completely wetted. Under these circumstances the comparison should be based on the wetted area rather than the geometric surface area.

2

4

P

B . .

-.-._._. 1 ____..___ M500

The influence of viscosity The influence of viscosity was investigated qualitatively with the liquids triethylene glycol (TEG) and aqueous solutions of monodiethanolamine (MDEA) having dynamic liquid viscosities between 6 and 30 cP. From the measurements the influence of the viscosity on the hold-up can be described by the correction factor (PLIr(LL,0)O= where ~1~is the dynamic liquid viscosity (cP) and pL,o the dynamic viscosity of water at 20 “C (cP). Correlation for hold-up The hold-up below the loading point can be calculated by the following empirical equation within 10% accuracy:

-

p&350 M700

I 100 Liquid

The infltlence of packing crimp angle The crimp angle seems to have a negligible influence on the hold-up. For Mellapak 250.X it is 30’ to the vertical, for Mellapak 250.Y it is 45”. Despite the steeper gradient of the falling liquid in the X-type packing compared to the Y-type, we do not find an important difference in hold-up. No further experimental data relating specifically to this factor appear to be available in the literature.

M500.Y M,70

.

Fig. 10. Hold-up

curves

load I, m/h

for Mellapak.

Hold-up of distributor, collectors The published data do not include the hold-up in the column internals as distributors, collectors, etc. Holdup in these elements must be calculated separately.

Conclusion The total hold-up of Mellapak 250.X, 250.Y and 500.Y has been measured by a gamma ray absorption technique in a column of 1000 mm internal diameter with the air/water system. The data agree well with the few data in the literature. An empirical correlation is given to calculate the hold-up for all Mellapak types. Our experimental technique measures the total holdup. It is not possible to distinguish between static and dynamic hold-up, as is usually done in the literature. It may be that this difference is only important for the conventional volumetric method. Considering the very local nature of hold-up in the packing it seems at least doubtful to base modelling of the liquid behaviour in the packing on data from integral, that is, non-local, measurements only.

h, = ca,0~83Z”(pL/pL, o)“~25 where p,_ is the dynamic liquid viscosity, pr,o the dynamic viscosity of water at 20 “C, 1 the liquid load (m3/m2 h), and a, the surface area (m2/m3), c = 0.0169

for

1~ 40 m3/m2 h

c = 0.0075

for

1> 40 m3/m2 h

x = 0.37

for

I < 40 m3/m2 h

x = 0.59

for

I > 40 m3/m2 h

Figure 10 shows the empirical expression given above, for the various Mellapak types, with a subset of the experimental data to indicate their accuracy.

1

Nomenclature specific area of packing, m2/m3 gas load F-factor, m/s (kg/m3)0-5 liquid hold-up, % of packing volume hold-up fluctuation, % of packing volume liquid load, m’/m* h or m/h superficial gas velocity, m/s void space of packing, m”/m’ liquid dynamic viscosity, CP dynamic viscosity of water at 20 “C, CP

124

PC

cp

gas density, kg/m3 crimp angle to vertical axis

References 1 L. Spiegel and W. Meier, Correlations of the performance characteristics of the various Mellapak types, Inst. Chem. Eng. Symp. Ser. No. 104, (1987) A203-A215. 2 Separation Columns for Distillation and Absorption, Publ. No. 22.13.06, Sulzer, Winterthur, 1991.

3 K. J. McNulty and C. Hsieh, Hydraulic performance and efficiency of Koch Flexipac structured packings, AIChE Annu. Meeting, 1982. 4 R. Billet and J. Mackowiak, Application of modern packings in thermal separation processes, Chem. Eng. Technol., 11 (1988) 213-227. 5 J. Mackowiak, Fluiddynnmik van Kolonnen mit modernen FiiIIkiirpern und Packungen ftir GaslFliissigkeitssysteme, Salle + Sauerlander, Frankfurt/Main, 1991. 6 R. Billet, Modeling of fluid dynamics in packed columns, Inst. Chem. Eng. Symp. Ser. No. 104, (1987) A171-A182.