2º artigo com transferencia de massa destilação

SEPARATION SCIENCE AND ENGINEERING Chinese Journal of Chemical Engineering, 21(7) 714723 (2013) DOI: 10.1016/S1004-9541(13)60532-7

Process Engineering Evaluation of Modern High Performance Packings for Mass Transfer Apparatus

BILLET R.1 and SCHULTES M.2,* 1 Institute for Thermo- and Fluid Dynamics, Universittsstrasse 150, 44780 Ruhr University of Bochum, Germany 2 Mundenheimerstrasse 100, 67061 Ludwigshafen, Germany

Abstract Modern packing technology is presented in terms of correlations and criteria for an extensive evaluation of up-to-date packing design used in current mass transfer operations. The corresponding basic information covers the process engineering aspects, e.g. volumetric effectiveness and optimum geometry of the packing, and the techno-economic aspects, e.g. when selecting a packing adopted in practice for a certain application task. The cor-relations required for this investigation are derived and evaluated on the basis of a comprehensive experimental re-search by testing and comparison of modern packings, such as Raschig Super-Rings and reference packings. The results thus obtained are correlated and presented in graphic presentation of diagrams, figures and tables. Keywords throughput, mass transfer, effective surface, specific cost of packing

1 INTRODUCTION

High performance packings for processes in the chemical and allied industries are distinguished by a fluid dynamically favourable basic shape with a highly specific surface related to packing volume. The de-velopment of packings in recent years has followed the goal of increasing the volumetric effectiveness by optimizing the packing geometry.

The resulting improvements in process technology properties, such as the effectiveness of mass transfer and the pressure drop of gas phase, and the maximum throughput, are accompanied from the economic point of view by further advantages. This has been deter-mined by systematic experimental studies.

In the present article, criteria and correlations for a comprehensive evaluation of packings used in mod-ern mass transfer processes are presented. Process technology aspects and economic aspects are taken into account, and among other things it is made clear where the limits of further development of packings lie.

For the quantitative evaluation of the correlations, new experimental results are used which were obtained in testing high performance packings in absorption under defined test conditions. The studies cover metal-lic Raschig Super Rings (RSR), in comparison to other metallic packings as representatives of industrially ap-plied high performance packings of recent years and also, plastic RSRs and plastic Ralu-Flow packings.

2 RELATIONSHIPS FOR EVALUATION OF THE PERFORMANCE OF DUMPED PACKED COLUMNS FOR MASS TRANSFER

In earlier studies, it was found that the RSR meets particularly well the requirements for a modern high performance packing, in terms of better fluid dynamic

properties and higher effectiveness of mass transfer, in comparison to customary packings [1, 2]. As test sys-tems, desorption and physical absorption systems were used, with various phase loads, and rectification under total reflux in the overall gas load up to the flooding limit.

In the most recent studies with the absorption system CO2-air/NaOH under normal conditions in a test apparatus with constant gas load, the liquid load was varied over a very wide range, and its influence on the efficiency and pressure loss was tested with various packing dimensions [3]. This system is representative for mass transfer with superimposed chemical reaction. The results obtained are suitable for carrying out a meaningful evaluation of the performance of these modern high performance packings.

This can be accomplished by means of the evalua-tion method described below.

In the abovementioned experiments, the volu-metric mass transfer coefficient was determined as a measure of the effectiveness of mass transfer. This is the product kOVae of the gas-side mass transfer coeffi-cient kOV and the effective phase contact surface ae. It is therefore obvious to use the HTU-NTU model as the basis for further considerations, according to which for a given volumetric gas throughput uV through a mass transfer column, the height HOV of a gas-side transfer unit NOV is expressed by

VOV

OV e OV=

u HHk a N

=

(1)

i.e. the effectiveness of the mass transfer can be ex-pressed by the number of transfer units NOV per unit of height of the packed bed H:

OVOV e

V

1N k aH u

= (2)

Received 2012-06-19, accepted 2013-02-26.

* To whom correspondence should be addressed. E-mail: [email protected]

Chin. J. Chem. Eng., Vol. 21, No. 7, July 2013 715

The volumetric effectiveness is thus determined by the specific column volume [4]

VV OV

1 Hvu N

= , (3)

which is equivalent to the reciprocal value from Eq. (2)

VVO e

1vk a

=

(4)

of the volumetric mass transfer coefficient, and from Eq. (1) the relationship

V OVV

1v Hu

= (5)

is obtained. Thus, vV is the volume of the packing column

which relates to the unit of mass transfer and the unit of volumetric gas throughput. If the geometric, vol-ume-related surface of the packing is a, then the product of a and vV

Va a v = (6)

is equal to the geometric packing surface a of the specific column volume. The smaller the value of a, the more effective is the geometric surface of the packing. The reciprocal of a thus gives information concerning the extent to which the geometric surface enables mass transfer and phase throughput, and is therefore a measure of the efficiency of the packing volume applied.

In packings which are fabricated from sheet metal or a sheet-like basic material of thickness s, with an installed specific surface a, one can achieve a rela-tive free voids volume of [5, 6]

112

a s (7)

Whereas a substantially influences the mass transfer, and is decisive for the phase throughput.

If one now applies the compact specific material volume (1 ) of the packing to the unit of mass transfer and phase throughput, one obtains for the specific volumetric input the quantity:

V(1 )v v = , (8)

and accordingly with Eqs. (7) and (8) the relationship

V1 12 2

v a s v a s = (9)

finally, with Eqs. (4) and (6), resulting in

OV e

1a ak a

=

(10)

and with Eqs. (4) and (8) accordingly

OV e

1(1 )vk a

=

(11)

and taking into account Eq. (9)

OV e

1 12

v a sk a

(12)

one obtains simple relationships between a and v and the volumetric mass transfer coefficient kOVae. These parameters are thus essential factors for the evaluation of the efficiency of a packing, even from the economic point of view, because for given costs CP per unit of geometric surface a, one obtains the investment cost KP per unit of throughput and mass transfer for a cer-tain packing from

P P POV e

1K a C a Ck a

= =

(13)

or, by taking into account Eq. (4), the following equivalent correlation:

P V PK a v C= (14) If the packing mass w per unit of packing volume

is known, the packing mass per specific column vol-ume is then given by the product:

Vw v w = (15) The reciprocal of w is thus a measure of the effec-tiveness of the packing mass deployed.

3 EVALUATION METHOD FOR ASSESSING THE MATERIAL-RELATED PERFORMANCE, BASED ON EXPERIMENTAL STUDIES

The above mentioned available results of ex-perimental investigations with the absorption system CO2-air/NaOH under normal conditions with 1% (by mol) CO2 in the air phase and 4% (by mass) NaOH in the aqueous phase consist of the measured volumetric mass transfer coefficients kOVae of dumped packings of various geometries and materials with different geometric surfaces a, with the measurements being at constant load uV [m3m2s1] in the gas phase but with different liquid loads uL = 10 to 200 m3m2h1. In these studies, further the pressure drop p in the gas per unit of packing height H was determined. It is therefore useful to determine and plot the following parameters as functions of uL and a, in order to obtain a comprehensive process engineering evaluation of the tested packings:

(1) Number NOV of mass transfer units per unit of column height H, as a measure of the height-related effectiveness of the mass transfer

( )OV OV e L constV

aN k a

f uH u =

= = (16)

and

L

OVconst( )u

Nf a

H == (17)

(2) Pressure drop p in the gas phase per unit height H


( )L constap f u

H =

= (18)

and

L const( )up f a

H =

= (19)

(3) Specific pressure drop per unit of mass transfer

( )V L constOV OV e

aup p f u

N k a H =

= =

(20)

and

L constOV

( )up f a

N =

= (21)

(4) Specific packing volume per unit of mass transfer and specific gas throughput as a measure of the volumetric performance

( )V L constav f u == (22) and

LV const( )uv f a == (23)

If it is desired to minimize the total pressure drop p, the following parameter will be decisive as to the suitability of the packing

VOV consta

pv fN

=

=

(24)

particularly in separation processes which require high numbers of separation stages.

Also significant may be the packing mass per

specific column volume, Eq. (15), according to the relationship:

L const( )uw f a = = (25)

The evaluation of the correlations of Eqs. (16) to (25) provides a comprehensive overview of the essential process technology evaluation functions, the knowl-edge of which is of central importance for overall evaluation of a high performance packing, and its se-lection for a specific mass transfer process.

Finally, the relationship

L constOV e

1 ( )ua a f ak a = = =

(26)

gives information concerning the extent to which the geometric surface is utilized for the mass transfer process, i.e. how much geometric surface is needed per unit of mass transfer and volumetric gas through-put. Correlations Eqs. (23) and (26) give information about, among other things, the technical and physical limitations of further development of packing shapes, in so far as with larger geometric surface a one can expect smaller surface-specific effectiveness, e.g. larger geometric surface a is necessary with reference to the unit of mass transfer and to the unit of volumetric gas throughput. This was previously determined in an ear-lier study with rectification systems [5].

4 RESULTS OF THE EVALUATIONS

Table 1 summarizes the characteristic geometric parameters for the packings. In particular these consist

Table 1 Geometric data of the packings tested

Packing Label d/mm a/m2m3 /m3m3 s103/m w/kgm3 Metal

0.5 20 236.2 0.96 0.3 275 0.7 25 175.9 0.97 0.4 250 1.0 30 155.5 0.98 0.4 220 1.5 38 105.8 0.98 0.4 170 2.0 50 100.6 0.98 0.4 152

Raschig Super-Ring

3.0 70 74.9 0.98 0.5 150 25 25 242.8 0.97 0.3 225 40 40 171.6 0.97 0.3 153 50 50 107.1 0.98 0.4 167

Reference packing

70 70 69.1 0.98 0.5 141

Plastic 0.6 25 206.3 0.96 0.8 62 Raschig Super-Ring

2.0 50 117.2 0.96 1.3 55

1.0 25 177.0 0.95 1.4 55 Ralu-Flow

2.0 50 98.4 0.95 2.0 54


of the geometric surface a, the relative free void frac-tion and the wall thickness s of the packing material, as well as the packing mass w per unit of filling vol-ume, listed in order of the nominal size d of the pack-ing element.

In Fig. 1, the values of , a, and s from Table 1 are plotted and supplemented with lines calculated from Eq. (7).

Figure 2 illustrates the basic versions of the metal and plastic packings tested. The significant differences in their geometric shapes are conspicuous.

Figure 2 Geometric shapes of the metal and plastic pack-ings tested

The bases for the following evaluations were provided by the experimentally determined values of the volumetric mass transfer coefficients and the pres-sure drops per unit height of the packing depending on the load for the described packings. The plots of Figs. 3

to 10 show some characteristic results. It is known that the effectiveness of a packing is

dependent to a large degree on the amount of liquid load. This is particularly clearly seen for the metal RSR packing and for the comparison packings from Figs. 3 and 4, obtained by evaluating Eq. (16), show-ing also that the specific surface of the packings has a comparably large influence on the effectiveness.

The dependence of the pressure drop per unit height of the packing Eq. (18) on liquid load uL and on the geometric surface of the packing is correspond-ingly as expected in Figs. 5 and 6.

The results shown in these plots indicate advan-tages of the metal RSR packing, i.e. better effective-ness per unit height and lower pressure drop. Accord-ingly, there are clearly lower specific pressure drops, Eq. (20), as shown in Fig. 7. The analogous Fig. 8 applies to the plastic packings (RSR and Ralu-Flow).

It may be seen from the comparative plot of Fig. 7 that the process engineering properties of metal RSR packing can be advantageously exploited in all mass transfer processes in which low specific pressure drop is a decisive criterion; e.g. in vacuum rectifications for heat sensitive mixtures, requiring a large number of separation stages, and in absorptions with regard to the power requirements of the blower.

The evaluation of the relationship of Eq. (22) leads to the diagrams of Figs. 9 and 10. Additional evaluations realize that the performance-relevant pack-ing surface in the definition according to Eq. (26) in-creases strongly with increasing geometric surface, as a consequence of the poorer surface utilisation (Fig. 11).

Based on these results, the differences in the spe-cific column volume can be determined for the metal-lic packings, operated under the conditions of the test system. These differences achieve a maximum value of approximately 10 and 15% dependent on the liquid load near a geometric surface of ca. 100 to 150 m2m3. Characteristic for all packings is the significant de-crease in surface per unit of performance with in-creasing liquid load.

Figure 1 Void fraction, geometric surface and material thickness of the packings tested


Figure 3 Number of transfer units of the metal packings as a function of liquid load with different geometric surfaces

Figure 4 Number of transfer units of the plastic packings asa function of liquid load with different geometric surfaces

Figure 5 Pressure drop of the metal packings as a function of liquid load with different geometric surfaces

Figure 6 Pressure drop of the plastic packings as a func-tion of liquid load with different geometric surfaces


Figure 7 Specific pressure drop of the metal packings as a function of liquid load with different geometric surfaces

Figure 8 Specific pressure drop of the plastic packings as a function of liquid load with different geometric surfaces

Figure 9 Specific column volume of the metal packings as a function of liquid load with different geometric surfaces

Figure 10 Specific column volume of the plastic packings asa function of liquid load with different geometric surfaces


5 LIMITS OF APPLICATION OF THE PACK-ING FROM THE ASPECT OF COST OPTIMI-ZATION

The results of the above-described evaluations clearly illustrate the process engineering characteris-tics which can be decisive in selection of packings for a certain industrial application. Since the investment cost is also an essential criterion, the cost optimization of packing taking into account the proportion of the column shell is unavoidable in any economic analysis. This is particularly true for columns for high capacity applications or columns which must operate under high pressure.

In addition to the specific packing cost KP ac-cording to Eq. (13), one must consequently determine the cost KS of the column shell with diameter dS and surface aS, which pertain to the number NOV of mass transfer units and to the throughput uV. The described surface is

SS

2OV S V4

d Ha

N d u

=

. (27)

By substitution of Eq. (3), the following relationship to the specific column volume vV is obtained:

S VS OV S e

1 44a vd k d a

= =

. (28)

If one knows the cost CS per unit of column shell sur-face, the specific cost KS is then given by

S S SK a C= . (29) The overall specific costs for the column packing

and the column shell [5, 6] are then correlated by

P S P SOV e S

1 4K K K a C Ck a d

= + = + (30)

as a function of the geometric surface a and the col-umn diameter dS. In Eq. (30) the product aCP repre-sents the packing cost per unit volume.

The numerical evaluation of K allows the deter-mination of the geometric surface with optimal cost, aopt, at which the value K achieves a minimum. This minimum comes because with packings of larger a the costs KP of the packing increase, whereas the propor-tional costs KS of the column shell decrease [5].

If one now writes Eq. (30) in the form of

SPP

OV e S P

41 KCa CK K f

k a d a C = + =

, (31)

it becomes clear that the specific overall cost K of the

Figure 11 Performance-related surface of the metal packings as a function of the geometric surface and liquid load


column

SP

P1

KK K

K = +

(32)

is greater by the factor

S

S P

41KC

fd a C

= +

(33)

than the specific packing cost KP according to Eq. (13), and the cost ratio KS/KP between the shell and the packing is correlated by Eq. (34):

S S

P S P

41KK C

fK d a C

= =

. (34)

For the special case where KS = KP, the cost factor fK assumes the numerical value by reason of Eq. (34):

S

P1 2K

K fK

= = (35)

It becomes clear from Eq. (33) that the smaller the column diameter is, the more effect the cost of the column shell takes in the overall costs of the column.

Finally the proportion of the packing cost in the overall costs of the column,

SP 1

K

K KKK K f

= = , (36)

increases the more, if the smaller the value of the cost factor fK, i.e. the greater the column diameter dS is.

The dimensionless cost factor fK represented by Eq. (33) thus imparts in a simple manner the influence of the costs of the column shell on the specific overall costs. Decisive for its amount according to Eq. (33) is the cost ratio CS/CP, comprised of the cost CS per unit of column shell surface and the cost CP per unit of geometric surface of the packing.

Evaluation of Eq. (33) leads to the plot of Fig. 12, which provides an example of the diameter-dependence on the dimensionless cost factor fK.

If one now evaluates Eq. (31) as a function of the geometric surface area a,

S const( )dK f a == (37)

and as a function of the column diameter dS, ( )S constaK f d == (38)

one obtains the plots in Figs. 13 to 15. The results lead to the conclusion that for each packing type there is a geometric surface of minimal cost, as exemplified by the cost minimums in Figs. 13 and 14. It also becomes clear, however, that the further development of pack-ings, especially with regard to increasing the geomet-ric surface a, is subject to economic limits from the financial point of view, particularly since the per-formance related specific surface a strongly increases with growing a.

Figure 12 Dimensionless factor of the specific total costs as a function of the column diameter

Figure 13 Specific total costs of columns of different di-ameter as a function of the geometric surface with different liquid loads for metal RSR


Figure 14 Specific total costs of columns of different diame-ter as a function of the geometric surface with different liquid loads for metal reference packing

Figure 15 Specific total costs as a function of column diameter with different geometric surfaces and with dif-ferent liquid loads for metal RSR

Figure 16 Dependence of the cost-optimized geometric sur-face area of the packing and of the relative cost minimum on column diameter, valid for wall thicknesses of the column shell according to Fig. 12

Figure 17 Dimensionless factor of the specific total costs as a function of column diameter with several traditional column packings on the basis of earlier investigations with rectification systems [5]


These plots were calculated on the assumption

that the cost CP per unit of geometric surface a for the metal comparison packings of a given nominal dimen-sion is generally the same as the corresponding cost of the comparable metal RSR packing, i.e. the cost

f PK a C= (39) per unit of packing volume is associated with ap-proximately the same value for RSR and the com-parison packings.

Finally, further results of the evaluations are pre-sented in Fig. 16 to convincingly demonstrate the in-fluence of the column diameter dS on the size of the cost-optimal geometric surface of the packing

minKa and is valid for wall-thicknesses s of the column shell, dealt with in Fig. 12.

A comparative evaluation of results of earlier in-vestigations of some traditional column packings, car-ried out with a rectification system [5], led to Fig. 17 and is, as expected, in accordance with Fig. 12.

6 CONCLUSIONS

The diagrams obtained by evaluating the results of experimental studies provide important information for comprehensive process engineering evaluation of the considered packings. In particular, the plots dem-onstrate the influence of the liquid load on the effec-tiveness of dumped packings with different geometric surfaces according to Fig. 1. It is made clear, among other things, that each packing shape is associated with a cost-optimal geometric surface corresponding to a minimum of overall cost, consisting of the pack-ing cost and the proportional cost of the column shell.

The particular process engineering characteristics of the Raschig ring packings (RSR metal and plastic) which were obtained in earlier studies and in practical applications were also confirmed in the present work.

Increasing energy prices make packings with low specific pressure drop extremely attractive. In this regard, Figs. 7 and 8 deserve special attention.

NOMENCLATURE

a geometric surface of a packing, m2m3 ae effective interfacial surface of a packing, m2m3 aS specific surface of a column shell per unit of mass transfer a and

per unit of volumetric gas throughput, sm1 a specific surface of a packing per unit of mass transfer and per unit

of volumetric gas throughput, sm1 CP costs per unit of the geometric surface of the packing, EURm2 CS costs per unit of the surface of the column shell, EURm2 d nominal size of the packing elements, mm dS diameter of the column, m FV factor for gas load in the free column cross section, kg1/2m1/2s1 f symbol representing a function fK dimensionless cost factor for packing plus the proportion of the

column shell H height of a packing, m HOV height of a gas-side overall transfer unit, m K sum of the specific costs of the packing and the column shell per unit

of mass transfer and of volumetric gas throughput, EURm3s1 Kf costs per unit of packing volume, EURm3 KP specific costs of a packing per unit of mass transfer and per unit

of volumetric gas throughput, EURm3s1 KS specific costs of a column shell per unit of mass transfer and per

unit of volumetric gas throughput, EURm3s1 kOV gas-side mass transfer coefficient, ms1 NOV number of gas-side overall transfer units of a packing p pressure drop of the gas in the packing, Pa s thickness of the material of a packing element, m uL liquid load referred to the free column cross section, m3m2h1 uV gas velocity in the free column cross section, ms1 vV specific column volume per unit of mass transfer and per unit of

volumetric gas throughput, m3m3s1 v specific volumetric material input per unit of mass transfer and

per unit of volumetric gas throughput, m3m3s1 w mass of the packing per unit of its volume, kgm3 w packing mass per unit of mass transfer and per unit of volumetric

gas throughput, kgm3s1 void fraction of a packing

Subscripts e effective L liquid phase O overall P packing S column shell V gas phase

REFERENCES

1 Raschig Super-Ring, Publication of Raschig GmbH, Ludwigshafen/ Rhein, Germany (1999).

2 Schultes, M., A new fourth generation packing offers new advan-tages institution of chemical engineers, Trans. IChem E, 81, Part A, 48-57 (2003).

3 Test report, Institute of Chemical Engineering, Bulgarian Academy of Sciences, Sofia (2005).

4 Billet, R., Industrielle Destillation, VCH Weinheim (1973). 5 Billet, R., Development of packings and their optimal surface ge-

ometriesstate of the art, Chem. Eng. Tech., 64, 401-416 (1992). 6 Billet, R., Packed Towers in Processing and Environmental Tech-

nology, VCH, Weinheim (1995).