Liquid-side mass transfer characteristics of a structured packing

THE CHEMICAL ENGINEERING JOURNAL

ELSEVIER

The Chemical

Liquid-side

mass transfer

M. Laso a, M. Henriques

Engineering

Journal

58 (1995) 251-258

characteristics

of a structured

packing

de Brito b, P. Bomio ‘, U. von Stockar dy*

’ Institit fir Polymere, Swiss Federal Institute of Technology (ETH-Z), Ziirich, Switzerland

d Institute of Chemical

b Rua Paulistrinia 575171, V?Ia Madalma, Sdo Paulo, Brazil ’ Sulzer Chemtech, winterthur, Switzerland Engineering, Swiss Federal Institute of Technology (EPF-L), Lausanne,

Received 29 November

1994; accepted

10 June

Switlerland

1994

Abstract We present experimental results on the liquid-side mass transfer characteristics of a structured packing (MellapakD). We have measured the product of the liquid-side mass transfer coefficient kL and the effective specific area u, by means of the NTZJ-HTU method in a column of 0.295 m internal diameter. The column was operated in counter-current, oxygen from a saturated water stream being stripped into air. In order to elucidate the effect of the specific area, we report results on three different packing types of specific geometric areas of 125,250 and 500 m3 mm2 (Mellapakm 125.Y, 250.Y and 500.Y respectively). The dependence of k,a, on liquid load and gas velocity is qualitatively similar to that of random packings. Quantitatively, however, the values of k,a, for the structured packings investigated in this work are higher than the values reported in the literature for random packings of the same specific geometric area under the same operating conditions. Keyword-s: Mass transfer;

Packing

1. Introduction

The systematic study of mass transfer in packed columns started with the early classical work on random packings performed in the thirties and early forties [1,2]. Since then, a vast amount of data on mass transfer coefficients and specific area for random packings has been reported in the literature. However, there are but a few reports of the mass transfer characteristics of modern structured packings. Based on the development in the 1970s of highperformance structured packings for specialty applications, a new generation of structured packings has been steadily winning acceptance among chemical engineers for applications in the oil and gas, chemical and petrochemical areas. It is estimated that 25% of all refinery vacuum towers worldwide are now fitted with structured packings. It is more difficult to make a similar estimate for fine chemicals applications. The main reasons for the increase in the number of applications of structured packings are their improved capacity and efficiency, which, in general, are higher * Corresponding

author.

a 8’

A large number of experimental methods for the determination of k,_u, in packed columns can be found in the literature_ The majority of these experiments have been carried out at ambient pressure and at temperatures of between 20 and 30 “C. Except for Bereiter’s measurements [9] on metal gauze structured packing, all reported measurements were performed

252

M. Loso et al. ! The Chemical

Engineering Journal 58 (199.5) 251-258

for random packings. Most experimental work has been carried out using physical absorption [1,9-271. Water is the solvent most frequently used, although alcohols and inorganic alcaline buffered solutions have been used as well. Chemical methods rely either on Danckwert’s plot method or on slow chemical reactions [28-311. Still other systems have been used in order to study the effect of physical properties: Delaloye et al. [14] used liquids of varyingviscosity: sodium alginate, glycerol and polyethylene glycol. Although the surface tension and the wettability are difficult to change while maintaining all other parameters constant (particularly the viscosity, [28]) Onda et al. [32] used a non-foaming surfactant in water, thus reducing the surface tension of the solution to 47 X 10e3 N m-l. This reduction in surface tension resulted in smaller values for k,u,. Finally, the question of whether the mass transfer coefficients obtained under different types of operation (absorption, stripping, rectification) has been addressed by several authors. Linek et al. [18] found k,u, values for absorption and desorption experiments “identical within the margin of experimental error”. Deed et al. [33] believe that “there is no clear experimental evidence that there is any difference between HTU, or HTU, values for packed columns as found from absorption and rectification qen’rnents. Rectification tests are, however, peculiarly unsuitable for the reliable determination of these quantities”.

The thickness of the metal sheet is kept as low as mechanical stability and corrosion considerations allow, and varies typically between one tenth and half of a millimeter. The reduction in free cross-sectional area is therefore minimal ( = 2%). Besides the corrugations, the metal sheets are embossed and grooved horizontally to promote turbulence and improve spreading of the liquid. The additional area provided by the surface structure is not taken into account in the nominal packing area. The corrugations of the metal sheets are inclined at an angle 0 with respect to the vertical axis (see Fig. 2). For the Mellapak@-Y type, 0 equals 45”. The sheets are arranged vertically and parallel to each other, so that the corrugations of contiguous sheets are inclined alternately by + 0 and - 0. Therefore, in the case of MellapakO-Y type, they run perpendicular to each other. These corrugations define straight, inclined channels of triangular cross-section through which the gas flows. The three packing types, 125.Y, 250.Y and 5OO.Y, differ in the values of h,,,, which are 0.023, 0.0115 and 0.0058 m respectively, the height of the element h being the same for all of them. According to the manufacturer, these three Y types make up more than 80% of all applications. Although the flow of the liquid phase can be considered to be approximately in counter-current to the

AZ 2. Materials

column

and methods

2.1. The structured packing

The structured packing Mellapaka consists of cylindrical elements of height h made of corrugated metal, ceramic or plastic sheets, depending on the application (see Fig. 1). In this work, three different types of stainless steel MellapakB were investigated, with nominal geometric specific areas of 125, 250 and 500 m2 rnp3 (MellapakO 125.Y, 250.Y and 5OO.Y,respectively).

a = 60’

h = 210 mm h corr = 11.5 mm

b=

32.5 mm

Packing geometry for Mellapak 250.Y

Fig. 1. Element

of Mellapak@

250.Y.

Fig. 2. Geometry of a single sheet of structured packing. The numerical values correspond to type 250.Y. Types 125.Y and 5OO.Y have the same element height h and inclination angle 0, but different corrugation height, which is twice (for 125.Y) and a half (for 500.Y) the value for 250.Y.

M. Lam et al. / The Chemical

253

Engineering Journal 58 (1995) 251-258

gas, liquid hydrodynamics is actually very complex, in spite of the regular structure of the packing. Depending on the physical properties of the liquid, on its flow rate, on the mechanical finish of the surface, on its wettability, on mass transfer conditions and on gas velocity, the liquid can display a variety of flow patterns. At low loads the liquid essentially follows the regular geometry of the packing, flowing either as a film or as individual rivulets. At high loads, the liquid flows in a turbulent, chaotic fashion, detaching itself partially from the sheets of the packing to fall in showers with copious formation of droplets, somewhat similar to what happens in dumped packings. 2.2. The system air-oxygen-water An inspection of the literature on experimental determinations of k,a, shows a preference for the system air-CO,-water. This is due to the easy availability of COP, the low health hazard it represents and, above all, to the straightforward analytical techniques required to measure its concentration. Although reliable measurements can be performed with this system, it has some drawbacks. When compared to other gases such as hydrogen, nitrogen and helium, carbon dioxide possesses a relatively high solubility in water, which is undesirable. Besides, the absorption of CO, in water involves chemical reactions. Although these reactions should have a negligible effect in the mass transfer rate, if the water has not been distilled or demineralized, the presence of cations may conceivably alter the rate of transfer or lead to precipitation of insoluble salts, which adhere to the surface of the packing and, even in very low amounts, modify its wettability. A less soluble, chemically inert gas would be more convenient, provided adequate analytical techniques existed. Oxygen is some 30 times less soluble in water at room temperature than CO,, presents no health hazards and is readily available. These factors, together with the development of reliable and accurate dissolved oxygen electrodes, also known as p0, probes or sensors, make air-xygen-water well suited for the determination of k,a,. In the present investigation, we used oxygen electrodes from INGOLD AG (Urdorf, Switzerland). In these probes, oxygen diffused through a teflon membrane which separates the body of the electrode from the water whose oxygen content was being measured. The electrode contained a platinum cathode and a reference anode of Ag/AgCl immersed in an electrolyte (potassium chloride gel). The probes incorporated temperature compensation. They were polarized for at least 6 h before each experiment and care was taken to ensure that no oxygen bubbles came in contact with the probe. The general relationship between the individual (i.e. gas and liquid) mass transfer coefficients and the overall

liquid-side mass transfer known formula: 1

1

k,,u,

y k,a,

coefficient

is given by the well-

1 + $,a,H

In order to separate the liquid-side and gas-side contributions to the overall mass transfer coefficient, it would be necessary to measure accurately the concentration of the transferring component at the gas-liquid interface, which is impossible with current technology. However, the presence of Henry’s constant in the term containing k,a, implies that for a very sparingly soluble gas (i.e. for a large value of H) most of the contribution to k,, comes from the liquid-side term. Therefore, if H is very large, we can set koLa, = k,u, to good accuracy. A measurement of the overall coefficient will then be sufficient to have a reliable estimate of t’he product kLae. Since Z = HTUo,NTUo,_

(2)

where B k,,a,

HTlJ,,=

(3)

and

(4) where

the numerator

in the integral

and given that the concentrations 4 can be simplified to:

is given by:

are very small,

s

Eq.

XIX.,

NTI/,,

=

xtop

dx x* -x

(6)

On the other hand, since the solution is highly diluted and temperature and pressure are constant in the column, the equilibrium line is straight: (7)

y=mrLP

where m=_

HPL -

(8)

P The operating y

=Ybot

+

line is given by:

$ (x-Xbot)

Substituting

Eqs. (7) and (9) into Eq. (6) yields

(9)

254

M. Laso et nI. / The Chemical

NTU,,=

~

Engineering Journal 58 (1995) 251-258

1

Xln

xtop--cp--Got +L Xbot-X&:bt

mG I

(10)

Since the solubility is very small, the absorption factor LIrnG is very small and 1 -LImG is close to unity; then Eq. (10) can be simplified so that

(11) This formula was used in the evaluation of the experimental results. The error due to the assumption L/mG=O has been estimated to be smaller than 1% [34]. Throughout this work, ideal piston flow for both phases has been assumed. 2.3. The pilot plant The experiments were performed in a glass column of 0.295 m internal diameter with a packing height of 0.42 m for all packing types. The packing was held in its position by a properly designed support that did not affect gas and liquid flows. Uniform distribution of the liquid was ensured by a distributor with a high drip-point density (530 points m-‘). Demineralized water was first saturated with oxygen in a reactor of 2.5 m3 capacity and subsequently entered the packed column, where it met an air stream into which the oxygen was desorbed. The air stream had been saturated with water in a separate column placed immediately before the main column. This avoided changes in temperature in the stripping column due to evaporation. Temperature was kept constant at 295 k 3 K. Care was exercised to ensure steady-state operation of the column, accurate determination of flow-rates and 0, concentrations, etc. After the steady state had been reached, 15 measurements of oxygen concentration were performed at intervals of 1 min. The average of these points was inserted in Eq. (11) to yield a value of k,u,. Variations between individual measurements of oxygen concentration were less than 5%. The liquid and gas flowrates were determined with accuracies of 1% and 10% respectively. The relative change in oxygen concentration in the liquid phase between top and bottom (AX/ X) was measured to within 5%. These figures lead to an estimate of the experimental error of 6%. As additional verification of the method, single points were run twice, with identical results within experimental uncertainty. In order to check that the effects observed by Mangers and Ponter [19] in random packings were not affecting

the measurements, a series of runs were performed at several liquid loads, first in ascending order of B, starting from the lowest value, and then in descending order. In both cases the same results were obtained at each liquid load. Although an oxygen mass-balance check could not be performed owing to the practical impossibility of measuring the change in the oxygen concenlration with sufficient accuracy, we believe that these two experimental checks and the theoretical estimate of the error mentioned above give a reliable indication of the errors involved in the measurements. Henriques de Brito [34] provides a complete description of the pilot plant and all ancillary equipment.

3. Results

and discussion

The results obtained are presented in Figs. 3(a) (Mellapak@ 125.Y), 3(b) (250.Y) and 3(c) (500.Y) as plots of k,u, as a function of the liquid load B, with the F factor as a parameter. As expected, k,u, increases with increasing liquid load for all packing types except for 325.Y at low and average F factors. For this packing (which has the lowest geometric specific area) k,u, increases with B up to about B=O.Ol m s-l. The product k,a, is not very sensitive to the gas velocity below B=O.Ol m s-‘, but depends strongly on it above that value of B. The influence of the gas velocity is to reverse the decreasing trend in k,u, above B=O.Ol m so that it flattens and, at the S -I at low F factors, highest gas velocity, increases with increasing liquid load. This behavior contrasts notably with that of Mellapak@ 250.Y (Fig. 3(b)). For this packing type, the effect of gas velocity is very small, the main influence being due to the liquid load. The trend in the data for 250.Y (at all gas velocities) is like that for 125.Y at the highest gas velocity, i.e. increasing k,u, with increasing liquid load. The results for 500.Y display the same qualitative trend as for 25O.Y, although the effect of the gas velocity is very pronounced beyond F factor 1.4 (see the open circles in Fig. 3(c)). This comes as no surprise, since at 0.0055 m s-’ the packing is operating close to its loading point. Careful checks and repetitions of the experiments for 125.Y at F factor 1.9 indicate that this unusual behavior (decreasing k,u, with increasing liquid load) is not due to an experimental error, but is an inherent characteristic of this packing type. It probably has to do with the characteristic size of 125.Y, which has a hydraulic diameter of 31 mm. It is conceivable that the more open structure of 125.Y induces a different liquid flow regime which is responsible for its behaviour. In particular, the larger running length of the liquid film between two bends of the corrugated sheets allows flow instabilities to develop more easily than in the smaller packings, where stabilizing capillary forces are proportionally more important.

M. Laso et al. / The Chemical

a) MELLAPAK 5 -

.

4

n F-Factor

= 3.1

A F-Factor

= 2.6

0

= 1.9

F-Factor

125.Y

n I

.

4

0

Engineering Journal 58 (1995) 251-258

I

0.000

0.005

0.010

0.015

b) MELLAPAK

0.020

0.025

250.Y

0

B B Q

n

d

0.000

0.005

0.010

n F-Factor

= 3.1

A

F-Factor

= 2.6

0

F-Factor

= 1.9

0

F-Factor

= 1.4

.

F-Factor

= 0.6

0.015

c) MELLAPAK

0.020

0

k,a, = 0.574B”.62 k Lea =0 . 713B0.71

0.025

500.Y

0

0

l

3-

0

0

q un

21 . ”

0 F-Factor

= 1.9

0 F-Factor

= 1.4

0

= 0.6

F-Factor

r

0.000

0.005

0.010 B

Fig. 3. k,a, as a function of specific 125.Y, (b) 250.Y; (c) 500.Y.

0.015 [m/s]

0.020

02)

yielded a value of the exponent cZ of 0.62 for 250.Y and 0.71 for 500.Y (we did not attempt to regress the results for 125.Y in view of their unusual behavior):

0

4-

rapidly approached as the packed height is increased. It is known that columns with a high aspect ratio tend to display lower performance than those with lower aspect ratio (mainly owing to maldistribution effects). This trend is qualitatively certainly true for the packings considered in this work. For this reason, an additional set of runs (not presented in this paper) were performed with a higher packed height in order to quantify this effect [34]. The results obtained showed only a minor dependence of performance on packed height. A correlation of k,a, values for Mellapakm 25O.Y and 500.Y (using only the values far from loading, i.e. at F factors of 0.8 and 1.4) vs. B using an equation of the form k,a, = clBCZ

0

255

0.025

liquid load for MellapakB:

(a)

Another effect, already known for dumped packings and which can be observed in the present data set as well, is the stronger influence of the F factor on k,a, when the packing operates close to the capacity limit. This is traditionally attributed to the increased hydrodynamic interaction between liquid and gas as momentum is transferred from the latter to the former at the interface. There is no reason to question the plausibility of this explanation in the case of structured packings. The relatively low aspect ratio of the column (packed height to column diameter) is imposed by the high efficiency of the packing, since the equilibrium is

(250.Y) (500.Y)

(13)

These exponents are similar to those found by other investigators for random packings. If we take the values of c, available in the literature [10,14,17,22-24,26,28,35] and average them, assigning them equal weights, we obtain c2 = 0.76 + 0.17. In theoretical work, a wide range of exponents can found: between 113 [36] and 0.77 [16]. We can conclude that the behavior of structured packings, at least for the aqueous system used in this work, is qualitatively not very different from that of random However, it is packings as far as k,a, is concerned. not possible from the experimental data to ascertain whether this similarity in the value of c2 is the consequence of an underlying similarity in the hydrodynamics, or if fundamentally different momentum transfer mechanisms lead nevertheless to similar mass transfer behavior for random and structured packings. It is worth mentioning that the correlation of Bomio [4] predicts c, = 0.78 for structured gauze packings, in which capillarity effects play a major role and in which liquid hydrodynamics is conspicuously different from that in random packings. In order to facilitate the comparison between the three packing types, we have plotted in Fig. 4 the height of a transfer unit as a function of the F factor for all packing types, with the liquid load as a parameter. As expected from its specific geometric area, Mellapak@ 125.Y presents the lowest efficiency, yielding between 1.7 and 3 NTU per meter. The increase in specific geometric area as we move on to the 250-Y results in improved performance, up to 4.5 NTU per meter. The further increase in ag when we go to 500.Y is not reflected in a proportional enhancement in the efficiency. Furthermore, one has to consider that the capacity of 250.Y is clearly superior to that of 500.Y.

256

M. Laso et al. I The Chemical

Engineering Journal 58 (1995) 251-258

0.5 -

E

o.4

~ :

0.3 0.2 _ - - - Mellapak 0.1 - -Mellapak -m’1 Mellapak 0.0

125.Y 250.Y 500.Y

0

B = 0.0142 B =0.0118 H B = 0.0059 l

m/s m/s m/s

.,

0

1 1

2

F-Factor

3

4

[m/s dkg/m3]

Fig. 4. HTU, as a function of gas velocity (F factor) Mellapak@ types with various liquid loads B.

for the three

6 ,

1

5-

0.00

-

Mellapak

.l.l.l.l.

Norman

-

Mohunta

lllnlllll

Billet

0.02

0.01 B

250.Y (1961) et al (1969)

(1989)

0.03

[m/s]

Fig. 5. Comparison of k,a, for 250.Y with predictions of two correlations for random packings: 25.mm Raschig ring according to Norman [37] and Mohunta et al. [21], and 50-mm plastic Hiflow rings according to Billet [38].

Consequently, it seems that, at least for the stripping of oxygen from water, MellapakB 250.Y represents an optimum: it has high capacity (comparable to that of 2” Pall rings) and, at the same time, good efficiency (comparable to 1” Pall rings). Although the qualitative behavior of structured packings seems to be similar to that of random packings, as discussed above in connection with the exponent c,, their quantitative behavior is clearly different. The comparison with random packings can be illustrated best by plotting &a, values for 250.Y given by the first of Eqs. (12) together with the values given by the equations of Norman [37], Mohunta et al. [21] and Billet [38] (Fig. 5). The first two were applied to Raschig rings of the same specific geometric area as Mellapak@ 250.Y (25 mm nominal size), and therefore of significantly lower capacity than Mellapak@ 250.Y. The third

correlation was applied to a more modern dumped packing (plastic Hiflow rings, 50 mm) of roughly the same capacity as Mellapak@ 250.Y. The correlation of Mohunta et al. [21] was considered in the review paper by Laurent and Charpentier [39] to be the best for a large number of different packings (Pall rings, intalox saddles and Raschig rings) made of different materials (ceramic, plastic and steel), with a deviation of f 20%. Krotzsch [30] states that this correlation fits his data well. Norman’s correlation was proposed as a single equation for different types of random packings (Raschig rings and Berl saddles) with a maximum deviation of +20% and was favorably reviewed by Au-Yeung and Ponter [40]. The k,u, values for 250.Y are 40% to 70% higher than those from Mohunta’s prediction. The correlation of Norman yields k,a, values 20% higher than Mohunta’s. At any rate, for the system investigated in this work, the liquid-side volumetric mass transfer coefficient for 250.Y lies between 40% and 70% above the values for random packings of the same specific geometric area. The remaining curve corresponds to plastic Hiflow rings of 50 mm nominal size (which have roughly the same capacity as Mellapak@ 250.Y) and is based on the general correlation presented by Billet [38]. It reflects the improvement obtained during the last decades in dumped packing design: a modern dumped packing of 50 mm nominal size can deliver liquid-side mass transfer performance not much worse than traditional random packings of 25 mm nominal size, while having, as expected, significantly more capacity. In agreement with existing data and with industrial practice, structured packings seem to be able to combine both high capacity and efficiency.

4. Conclusions This work reports data on liquid-side mass transfer characteristics of three structured packings for an aqueous system. The system air-oxygen-water seems to be well suited to the determination of k,_u,. The packing with the lowest specific geometric area (125.Y) shows qualitatively different behavior from that of the finer packings (250.Y and 500.Y). The structured packings studied here show good performance characteristics.

Appendix a, a& B b

A: Nomenclature effective specific interfacial area (m’ mp3) geometric specific interfacial area (m’ m-‘) specific liquid load (m SK’) width of triangular channel (horizontally projected) (m)

M. Laso et al. / The Chemical

Engineering Journal

C D

concentration (km01 m-‘) diffusivity (m* SK’)

F factor H h h cd h con k k,

w,& (Pa”*) Henry’s constant (Pa m3 kmol-‘) height of packing element (m) column height (m) corrugation height (m) mass transfer coefficient (m s-‘) second-order reaction rate constant (m3 kmol-’ s-‘) slope of equilibrium line absorption rate (km01 s-l) mole fraction in liquid phase mole fraction in liquid phase in equilibrium with gas mole fraction in gas phase packed height (m)

I7 x x* Y z

Greek letters inclination of line of steepest descent (deg) inclination of corrugations (deg)

a

8 Subscripts b bot

bulk bottom of column carbon dioxide effective gas interface liquid overall hydroxyl ion top of column

co2

L

1,

0

OH top

References [l] [2]

[3]

[4] [5]

[6] [7]

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