WO*'
Lx
GAS AND LIQUID MALDISTRIBUTIONS
IN PACKED COLUMNS
PROEFSCHRIFT
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus, prof.drs. P.A. Schenck,
in het openbaar te verdedigen ten overstaan van een commissie
aangewezen door het College van Dekanen
op 27 april 1989 te 14.00 uur
door
Robertus Martinus Stikkelman
geboren te 's-Gravenhage
scheikundig ingenieur
TRdiss
1716
Dit proefschrift is goedgekeurd door de promotoren
prof.ir. J.A. Uesselingh prof.dr.ir. J. de Graauw
STELLINGEN
0. Een stelling is slechts te verdedigen binnen een geaccepteerd axiomastelsel.
1. De uitkomst van maldistributiefactormetingen in gepakte scheidingskolommen is afhankelijk van het meetsysteem.
2. Het bepalen van spreidingscoëfficiënten voor de vloeistoffase bij een lage pakkingshoogte is onnauwkeurig.
—3—Voor—de-interpretatie-van-stofoverdrachtgegevens-voor—gepakte kolommen is een nauwkeurige beschrijving van de kolom en de randapparatuur onmisbaar.
4. Het stijgen van de HETP van gestructureerde pakkkingen bij hoge gasbelasting is niet een gevolg van "flooding" maar van * het optreden van grootschalige maldistributie.
5. De inschatting van Baerends dat de tijd voor het berekenen van atomen/molekulen volgens de "discrete variational Hartree-Fock-Slater" methode in de orde ligt van het aantal electronen In het kwadraat is te optimistisch.
E.J.Baerends, D.E.Ellis and P.Ros, Chem. Phys. 2 (1973) 41 6. Alles is een perpetuum mobile.
7. Filevorming en milieuvervuiling kunnen verminderd worden door het afschaffen van:
- de reiskostenvergoedingen en
- de overdrachtsbelasting bij de huizenverkoop
8. Het belang van presentatie- en communicatievaardigheden wordt in het huidige studieprogramma voor scheikundig ingenieur onderschat.
9. Fusie leidt tot confusie.
10. De verklaring van het woord stripverhaal heeft tegenwoordig meer met de inhoud dan met de vorm te maken.
SUMMARY
Packed columns are increasingly used in distillation and absorption/stripping processes. For the application of these. colums a good understanding of the flows in the packing is desirable. This study describes the gas and liquid distributions in random as well as structured packings. The experimental column has a diameter of 0.5 m. It is equipped with a total of 1289 detectors in the top and bottom cross section. These detectors yield a detailed picture of air and water flows through elements of only 25x25 mm2.
Random packings
The observed maldistribution in the gas bulk flow is negligible compared with that of liquid bulk flow. The gas flow rate near the wall equals 1.1-1.3 times the superficial velocity for common 25 mm palEkings. No influence of the liquid flow rate has been observed. The radial spreading coëfficiënt of the gas is in the order of A mm.
The liquid flow in the bulk becomes less uniform when the superficial liquid velocity is diminished. The flow distribution is almost independent of the gas flow. The spreading of liquid shows a srnall increase up to the loading point, above which it increases more rapidly. Values of the radial spreading coëfficiënt of the liquid are approximately 3 mm. In the loading region the liquid wall flow at a fixed packing height tends to lower values at higher gas flow rates.
The measurements of the gas and liquid flow profiles can be simulated with a simple Monte Carlo cell model. It gives a good prediction of the liquid and gas spreading, of the development of wall flow, of the small scale maldistribution and of the effects of the superficial gas velocity.
Two types of tower internals have been simulated: an initial distributor and a wall wiper. Drip point densities of more than 200/m2 hardly improve the liquid distribution in a column with 25
mm packing. In a column with a diameter of 0.5 m the wall flow is only reduced over a small packing height by a wall wiper.
The effect of a severe initial liquid maldistribution in a methanol/ethanol distillation column has been investigated. This was done in a 0.45 m diameter column with a packing height of 2.65 m. Sealing half of the distributor resulted in a sharp increase of the height of a transfer unit based on over-all gas-phase resistance.
The Monte Carlo cell model, extended with a simple mass trans fer model, gives a realistic simulation of the distillation results.
The separation properties of the first meter of a typical 25 mm packing with different distributors have been simulated. With 30 drip points per m2 a packing height of 0.4 m is effectively
lost.
Structured packings
The maldistribution in the gas bulk flow is negligible. Only the observed wall flow can contribute to malperformance. The gas flows parallel to the sheet orientation, thus introducing a radial transport. Together with the change in the orientation of subsequent packing elements, this results in good gas mixing.
It was observed that the liquid wall flow decreases when the gas velocity is higher than 1.7 m/s. Up to the loading point the maldistribution of the liquid is constant. Above this point the quality of the distribution deteriorates rapidly, due to the appearance of large scale liquid segregation.
Appendix A provides a method of characterizing a flow distribution with a relatively small number of parameters. A channel mal distribution factor is defined that indicates which channel sizes contribute most to an overall maldistribution. A newly defined overall maldistribution factor is shown to give a good ranking of different distributions.
SAMENVATTING
Gepakte kolommen worden steeds vaker toegepast in de procesindustrie. Het is voor het ontwerpen van zo'n kolom wen selijk dat de stroming in de pakking nauwkeurig beschreven wordt. Dit kan helpen om tegenvallende prestaties te voorkomen. In dit proefschrift is het stromingsgedrag van water en lucht voor zowel losse als gestructureerde pakkingen bestudeerd. In een kolom met 1289 stromingsdetectoren zijn aan de top en bodem van de pakking gedetailleerde profielen gemeten. De kolom heeft een diameter van 0.5 m.
Losse pakkingen
De maldistributie van het gas in de bulk van de pakking is gering ten opzichte van die van de vloeistof. Voor gangbare 25 mm pakkingen is de gemiddelde gassnelheid langs de wand een factor 1.1-1.3 groter dan de_sup_erficiële_snelheid.—De-vloeistofstroming heeft nauwelijks invloed op het-gas. De radiale spreidingscoëffi ciënt voor het gas is ongeveer 4mm.
Bij lage vloeistofsnelheden neemt de kwaliteit van de vloei stofverdeling in de bulk van de pakking af. Deze kwaliteit gedraagt zich vrijwel onafhankelijk van de gasbelasting. De spreiding van de vloeistof neemt tot het stuwingspunt enigzins toe; daarboven is er een sterkere toename waargenomen. De waarde van de radiale spreidingscoëfficiënt bedraagt ongeveer 3 mm; In het stuwingsgebied neemt de wandstroming bij een gelijkblijvende pakkingshoogte af.
De meetresultaten van de gas- en vloeistofprofielen zijn gesimuleerd met een Monte Carlo cellenmodel. Dit model beschrijft de vloeistof- en gasspreiding, de ontwikkeling van wandstroming, gemiddelde onregelmatigheden op kleine schaal en het effect van de superficiële gassnelheid.
Voor een beginverdeler is berekend dat voor een aantal sproeipunten van meer dan 200/m2 de kwaliteit van de verdeling
nauwelijks verbetert. De simulatie van een wandschraper laat zien dat de wandstroming van de vloeistof slechts over een klein gedeelte van de pakking wordt verminderd.
In een destillatiekolom voor een methanol/ethanol mengsel is het effect van een slechte beginverdeler onderzocht. De kolom heeft een diarater van 0.45 ra en een pakkingshoogte van 2.65 m. De hoogte van een stofoverdrachtseenheid gebaseerd op de gasfase weerstand neemt sterk toe als de helft van de beginverdeler af-geblind wordt.
Het Monte Carlo cellenraodel, uitgebreid met een eenvoudig stofoverdrachtsmodel, geeft een reële beschrijving van de destillatieresultaten.
Met het model is het effect van het aantal sproeipunten op de scheidende werking van 25 mm pakkingselementen voor een kolom van 1 m hoogte gesimuleerd. Bij 30 sproeipunten per vierkante meter gaat effectief een pakkingshoogte van ongeveer 0.4 m verloren .
Gestructureerd pakkingen
De maldistributie voor de gasstroming in de bulk van de pakking is verwaarloosbaar. Alleen aan de wand treden er onregelmatigheden op. Het gas stroomt parallel aan de kanalen in de pakking. Doordat de pakkingselementen onderling verdraaid zijn treedt er een goede gasmenging op.
Bij gassnelheden groter dan 1.7 ra/s neemt de wandstroming van de vloeistof af. Onder het stuwingspunt is de maldistributie van de vloeistof constant. Daarboven neemt de kwaliteit van de verde ling sterk af, doordat er grootschalige segregatie ontstaat.
In appendix A is een methode ontwikkeld, waarmee een verdeling gekarakteriseerd kan worden met een gering aantal parameters. Met behulp van een maldistributiefactor voor verschillende kanaalgroottes kan bepaald worden welk kanaal het meest bijdraagt aan maldistributie. Een algemene maldistributiefactor geeft een goede indicatie voor de kwaliteit van verschillende verdelingen.
Aan mijn ouders
DANKWOORD
I would like to thank the Koninklijke/Shell-Laboratorium for their financial support and Norton Ltd., Raschig GmbH, Julius Montz GmbH and Gebrüder Sulzer AG for supplying us with packing.
Verder wil ik alle collega's bedanken met wie ik prettig heb samengewerkt. Speciale gevoelens gaan uit voor diegene die zich het getal driehonderdtwee-endertig herinnneren: Piet en Peter voor het 0.1 mm werk. Frits en Piet voor de electronica. Arie en Wim voor de ontwerpen. Bram en kornuiten voor de constructie. De uitvoerders van de Centrale Werkplaats.
De beide promotoren Hans Wesselingh en Jan de Graauw hebben me tijdens het onderzoek veel vrijheid gegeven. Dit vind ik belangrijk voor zowel het onderzoek als mezelf. Het zijn vier leerzame jaren geweest.
Gedurende de promotieperiode was het niet altijd even gemakkelijk. Veel ondersteuning heb ik toen gehad van de afstudeerders/stagiaires. Vooral het laatste jaar hebben zij met man (M/V) en macht gewerkt om het project tot een goed einde te brengen. Kees, Aike, Connie, Jos, Krijn, Rens, Maxim, Antonio, Manuela, Aad, Ton, Floris, Ruud, Abdel, Hessel, Jan-Jelle. Zonder jullie was het niet gelukt.
CONTENTS Summary Samenvatting Dankwoord A C F
CHAPTER I General introduction
Scope
Earlier investigations Objective of the thesis Structure of the thesis References 1 3 4 5 5
CHAPTER II The experimental setup
Introduction
Description of the equipment The raeasuring techniques used Data acquisition
Characteristics of the equipment References 9 9 12 15 15 17
CHAPTER III Measurements of the gas and liquid maldistri-bution in columns with a random packing
Introduction _ . _ ~ Literature Velocity profiles Gas profiles Liquid profiles Radial spreading Gas spreading Liquid spreading Interpretation Conclusions Symbols References -19 19 21 23 25 27 28 28 30 32 33 34
CHAPTER IV Simulation of the gas and liquid distribution
Introduction 37 Literature 38 The simulation model 39
Liquid bulk flow 40 Gas bulk flow 43 Wali flow 44 Results 45
Liquid spreading 46 Liquid profiles 48 Gas flow effect on liquid wall flow 48
Drip point density 50
Wall wiper 51 Other cell dimensions 52
Conclusions 53 Symbols 54 References 55
CHAPTER V Measurement and simulation of the influence of maldistribution on distillation in a column with a random packing
Introduction
The distillation unit HTU
OG ■value Determination of the
Distillation results Simulation of mass transfer Simulation results
Influence of the drip point density Discussion Conclusions Symbols References 57 59 60 61 63 67 69 71 71 72 73
CHAPTER VI A study of gas and liquid distributions in structured packings Introduction 75 Literature survey 76 Gas profiles 78 Gas spreading 80 Liquid profiles 80 Liquid spreading 84 Discussion 85 Conclusions 86 References 87
APPENDIX A Characterlzation of the flow distributions in a cross section of a packed column
Introduction 89 Channel and overall maldistribution 91
Sample distributions and discussion 93 A checkerboard distribution 93 A column with an irrigated outer ring 94
A point source 95 A series of checkerboard distributions 96
Conclusions 96 Symbols 97 References 97
APPENDIX B Description of the computer programs used
General information 99 The flow simulation program 99
The mass transfer simulation program 101
CHAPTER I
General introduction
Scope
For a long time gas liquid contact devices have been used in chemical engineering to separate mixtures. Destillation, absorp-tion and stripping are carried out in tray, wetted wall, spray and packed columns. Some typical applications are gas drying, crude oil refinery, monomer purification, alcohol separation and gas cleaning.
One of the problems for a process designer is to choose the economical optimum from the various devices. Although the knowledge on gas liquid contacting has reached a high technologi-cal maturity, innovations in equipment and widening of theoretitechnologi-cal backgrounds still happen. Minor improvements can result in large profits
becaus_e_of„the-enormous-quant-it-les—involved-An example to emphasize the importance of innovations is given by the petrochemical industry. During the perlod between 1950 and 1973 the world refinery capacity was rapidly extended from 13xl06
to 6Axl06 barrels a day. After the first oil shock in 1973, oil
product demand feil rather sharply, but primary distillation capacity kept rising because of the completion of plant already under construction. Refiners have reacted by closing the least efficiënt and simplest refineries. Due to the second oil shock in 1979 and shift in the oil product demand to light components the utilization rate in 1987 equaled about 75 per cent with low simple refining margins . The most important requirement for refiners became to load fully their complex conversion and upgrading capacity, like visbreaking, flexicoking and the hyconproces, to produce light oil products. Still the margins are small, so refiners have to optimize both supply and refinery operations. Especially the efficiënt use of energy is important, because refinery fuel and electricity costs increased their share of total manufacturing costs from around 20 per cent to over 40 per cent. The worldwide amount of crude oil processed in refineries during
2 the past ten years approximates 60xl06 barrels a day .
One of the possibilities to minimize manufacturing costs is to revamp plate columns with packings. The gain of flexibility and
capacity obtained in this marmer is important for the atmospheric distillation of crude oil. A second advantage is the decrease of the pressure drop per mass tranfer stage. Especially for vacuüm distillation this results in lower bottom temperatures and lower energy consumption.
Also in the chemical industry packed columns can improve the column performance. Due to the low pressure drop per mass transfer stage the decomposition of thermolabile products can be suppressed. In general the height of of mass transfer stage is lower than that of a tray tower. Revamping those towers with packing mostly results in better product specifications. For the separation of agressive chemical compounds the ceramic types of structured as well as dumped packings are very suitable.
Many applications of structured as well as dumped packings have -,_ . • -, ■ 3-17
been described in literature
More than fifty varieties of random and structured packings are commercially available on an active market. Roughly they can be divided into three types : conventional dumped packings ( Raschig rings, Pall rings, Berl saddles, Intalox saddles, etc. ) , high performance dumped packings ( Intalox Metall Tower Packing, Nutter rings, etc. ) and structured packings ( Mellapak, Gempack, Montz's BI, Ralu-pak, Rombopak, Intalox 2T(C0M), etc. ) . Table I outlines typical design data of trays and packings.
Table I Typical design data of trays, dumped packings and struc-18
tured packings according to Chen
F-factor [(kg/m/s2)0•*] HETP [m] AP/HETP [Pa] x 102 Trays 0.3- 2.4 0.6- 1.22 4 -11 Packings Dumped 0.3 -2.9 0.46-1.52 1.2 -2.4 Structured 0.12 -4.4 0.1 -0.76 0.013-1.0
In the past, the use of packings was limited to columns with a small diameter/packing height ratio, because the performance was considered to be rather unpredlctable. Today this picture has been changed, mainly for two reasons: the availability of carefully deslgned and installed packings and the improved understanding of the flow mechanisms inside the packing.
However, there still are a number of disappointing performances of large columns. A nonuniform liquid and gas distribution within the packing is thought to have an negative effect on the separa-tion efficiency. A considerable amount of literature has been produced on the so called maldistribution problem. A general overview is given in the next paragraph. A detailed survey of the studies is presented in the concerning chapters.
Earlier investipations
Many factors that can cause irregular flows in a packed column have been investigated in the literature. The most important factors are summarized below.
A principal cause of maldistribution is the packing itself. The liquid rivulets follow specific paths through the packing. Sometimes they split, sometimes they flow together thus introduc-ing irregularities on a small scale. The equillibrium—f-low-distribution in the bulk of the packing is called natural flow
19
according to Albright . This natural flow has been measured by 20
Hoek . The continuous gas flow is forced through the openings of the packing. The different orientations and dimensions of these openings result in a natural flow distribution for the gas, which
21 22 has been determined by Ali and Stikkelman
A change in the isotropy of the packing can give a departure from a uniform distribution. Practical examples are void varia-tions due to inproperly installed packing, corrosion, fouling, etc. A serious change in the isotropy is the transition between the packing and the column wall. Liquid moves more easily to the wall than vice versa causing wall flow. Liquid wall flow has been
20 23-30
studied by many authors. ' Gas wall flow received less 31,32
attention.
The quality of the initial distribution of both phases can contribute to a column malperformance. Especially for the liquid an initial maldistribution results in a decrease of the separation
33-35
efficiency . Large scale flow irregularities are diminished by radial spreading. Many spreading data are known for the liquid
20 30 36-39
without loading effects ' ' . Rough data of the gas are only 40
available for 250Y Sulzer Mellapak
The interaction between gas and liquid intensifies above the loading point. Many correlations have been proposed for the liquid hold up and the pressure drop, but only few authors studied the
32 41 effect of loading on the flow distribution '
The surface tension and viscosity of the liquid can influence the interfacial area between both phases. Even over the column length the surface tension can vary due to a change in
37 42-48
composition. Although the studies of these phenomenae ' are mostly not integrated with maldistribution, the effects on the separation efficiency can be considerably.
Obiective of the thesis
In literature little attention is given to the gas phase and its effect on the liquid phase. Therefore the two main objec-tives of this thesis are the study of:
Gas flow characteristics
The influence of gas flow on the liquid flow behaviour
Experimental data on spreading and flow profiles of both phases will be measured for structured as well as random packing in an air/water column. This information will contribute to a better understanding of the complex flow mechanism inside the packing.
A flow model will be developed to simulate the experimental results. This model can also be applied to evaluate hypothetical cases. In this way some design failures can be anticipated.
The model, extended with simple mass transfer equations, will be tested to practical distillations with severe maldistribution.
Structure of the thesis
In chapter 2 the measuring equipment will be decribed.
The next three chapters deal with random packing. They form an integrated unit starting with basic experimental flow characteris-tics and ending with a complex distillation simulation. The experiraental results will be presented in chapter 3, The flow simulation model will be explained and evaluated in chapter 4. A distillation on a pilot plant scale with severe initial liquid maldistribution is studied in chapter 5.
In chapter 6 the gas and liquid distribution results are given for structured packings.
The first appendix at the end of the thesis concerns a method of characterizing a flow distribution with a relatively small number of parameters. In the second appendix the computer programs used are outlined.
The greater part of the chapters have been submitted for publication. In this thesis their—lay-out—has—been-slrghtly
-modified to give a consistent and readable form.
References 1 De olieprijzen
Shell Brochure Series, december 1987 ISBN 90-6644-083-x 2 Energie in kort bestek
Shell Brochure Series, august 1987 ISBN 90-6644-079-1. 3 W.Meier, R.Hunkelar, W.D.Stocker
I. Chem. E. Symposium Series No.56 (1979) 3.3/1-17 4 R.F.Strigle, K.E.Porter
I. Chem. E. Symposium Series No.56 (1979) 3.3/19-33 5 R.F.Strigle, F.Rukovena
Chem. Eng. Progr., 75 (1979) 86-91 6 G.K.Chen, L.Kitterman, J.Shieh
Chem. Eng. Progr., 79 (1983) 46-49 7 N.P.Lieberman
Hydrocarbon Processing, 66 (1984) 143-145 8 R.Billet, J.Mackowiak
Chem. Ing. Tech., 57 (1985) 1-3 9 R.F.Strigle
Chem. Eng. Progr., 81 (1985) 67-71 10 R.F.Strigle
3rd World Congress of Chemical Engineering, Tokio, 1986 Paper No. 6F-354, 770-773
11 J.R.Sauter, W.E.Younts
Oil 6. Gas Journal, 84 (1986) Sept 12 P.Roy, A.C.Mercer
13 U.Bulhmann
I. Chem. E. Symposium Series No.104 (1987) A115-127 14 D.E.Nutter
I. Chem. E. Symposium Series No.104 (1987) A129-142 15 H.A.Gangriwala
I. Chem. E. Symposiun Series No.104 (1987) B89-99 16 M.Roza, R.Hunkelar, O.J.Berven, S.Ide
I. Chem. E. Symposium Series No.104 (1987) B165-178 17 L . S p i e g e l , P.Bomio
Chem. Ing. Tech., 59 (1987) 130-132 18 G.K.Chen
Chem. Eng., 91 (1984) 40-51 19 M.A.Albright
Hydrocarbon Processing, 9 (1984) 173 20 P.J.Hoek
Ph.D. Thesis, Technische Hogeschool Delft, 1983 21 Q.H.Ali
Ph.D. Thesis, University of Aston, 1984 22 R.M.Stikkelman and J.A.Wesselingh
I. Chem. E. Symposium Series No.104 (1987) B155-164 23 K.E.Porter, J.J.Templeman
Chem. Eng. Sci., 20 (1965) 1139-1140 24 K.E.Porter, J.J.Templeman
Trans. Instn. Chem. Engrs., 46 (1968) t68 25 E.Dutkai, E.Ruckenstein
Chem. Eng. Sci., 23 (1968) 1365-1373 26 V.Stanek, V.Kolar
Czech. Chem. Commun., 33 (1968) 1062-1077 27 E.A.Brignole, G.Zacharonek, J.Mangosio
Chem. Eng. Sci., 28 (1973) 1225-1229 28 H.C.Groenhof, S.Stemerding
Chemie-ing. Techn., 49 (1977) 835 29 M.M.Farid, D.J.Gunn
Chem. Eng. Sci., 33 (1978) 1221-1231 30 P.J.Hoek, J.A.Wesselingh and F.J.Zuiderweg
Chem. Eng. Res. Des., 64 (1986) 431-449 31 G.Speek
Ph.D. Thesis, Technische Hochschule Dresden, 1955 32 R.J.Kouri and J.J.Sohlo
I. Chem E. Symposium Series No.104 (1987) B193-211 33 M.Huber, R.Hiltbrunner
Chem. Eng. Sci., 21 (1966) 819-832
34 K.J.R.ter Veer, H.W.van der Klooster, A.A.H.Drinkenburg Chem. Engrs. Sci., 35 (1980) 759-761
35 J.G.Kunesh, L.L.Lahm, T.Yanigi
I. Chem. E. Symposium Series No.104 (1987) A233-244 36 K.E.Porter, V.D.Barnett and J.J.Templeman
Trans. Instn. Chem. Engrs., 46 (1968) t74-85 37 K.Onda, H.Takeuchi, Y.Maeda, N. Takeuchi
Chem. Eng. Sci., 28 (1973) 1677-1683 38 V.Stanek, M.Kolev
Chem. Eng. Sci., 33 (1978) 1049-1053 39 G.G.Bemer, F.J.Zuiderweg
Chem. Eng. Sci., 33 (1978) 1637-1643 40 W.Meier, R.Hunkler and D.Stöcker
I. Chem. E. Symposiun Series No.56 (1979) 3.3/1-17 4 1 E.Dutkai, E.Ruckenstein
Chem. Eng. Sci., 25 (1970) 483-488
42 F.J.Zuiderweg, A.Harmens
Chem. Eng. Sci., Genie Chemiqie, 9 (1958) 89-103 43 R.C.Francis, J.C.Berg
Chem. Eng. Sci., 22 (1967) 685-692 44 S.S.Paranik, A.Vogelpohl
Chem. Eng. Sci., 29 (1974) 501-507 45 A.B.Ponter, P.Trauffler, S.Vijayan
Ind. Eng. Chem. Process. Des. Dev., 15 (1976) 196-199 46 H.W.van der Klooster, A.A.H.Drinkenburg
I. Chem. E. Symposium Series no.56 (1979) 2.5/21-37 47 H.Sipma, B.J.Schram, A.A.H.Drinkenburg
I2-procestechnologie, 2 (1985) 30-33 48 T.D.Koshy, F.Rukovena
CHAPTER II
The experimental setup
Introduction
The equipment used in this study has to supply information on the gas flow profile and the liquid flow profile in the top and the bottom cross sections of a packed column. Both the gas and the liquid profiles have to be measured simultaneously. The measuring grid should be able to detect maldistribution on the scale of a reasonable sized column as well as on the scale of the packing element. These requirrements have lead to the design of a column with a diameter of 0.5 m and with a maximum packing height of 3 m. The column is placed on top of an apparatus containing 332 measur ing modules. The column is operated at atmospheric pressure, with water flowing downwards and air upwards.
___The_—measjjring_par-t_of_the-equipmenfr;—as—fuHy^described-irT~the
following paragraphs, consists of modules with a nominal diameter of 25 mm. This matches with the dimensions of 1 inch dumped packings.
The flows through the modules are collected by an automatic data acquisition system. Data reduction and interpretation are carried out on a personal computer.
Mass transfer experiments are performed on a pilot plant dis-tillation column with a diameter of 0.45 m and packing height of 2.65 m. This column is described in chapter 5.
Description of the equipment
Five different parts can be distinguished in the general flow scheme of the equipment as shown in Figure 1:
- the water circulation unit
A centrifugal water pump feeds a constant head tank. A fraction of the liquid is directed via a flow controller into the liquid distributor. The superficial velocity of the water in the column can be varied between 0 and 15 mm/s. After passing the packed
column and the bottom section the water runs into a buffer vessel. The overflow of the constant head tank is lead directly into the buffer vessel.
i constant
head tank
air o u t
Inltlal distrlbutor coollng water air cooler draln' t4 air pumpFigure 1 The general flow scheme of the equipment
the air supply unit
Hot air, supplied by a centrifugal blower, is conditioned to a fixed temperature and humidity by tap water in an air cooler filled with dumped packings. It enters the column via the bottom section, where the gas velocity profile is measured. The superfi-cial air velocity is adjustable between 0 and 3.4 m/s by means of a valve. From the top of the packed column the air is vented into the surroundings.
S u p p o r t g r L/G sepo.ro.tl G d e t e c t l o n 8< Input f l o w d e t e c t o r yfflEL O ( level detector level detector
Figure 2 The bottom section with an enlargement of one flow detec-tion module
- the bottom section
The bottom section ( Figure 2 ) combines a number of functions: The upper part ( L/G separation section ) serves as a support grid. for the packing in the column. This grid divides the cross section of the packed column into 332 modules of 25x25 mm2. Some
of the outer modules are partly covered by the column wall. Furthermore, the downcoming water flows around the gas pipes into tubes in the middle part of the bottom section and is in this way separated from the upflowing air.
In the middle part ( G detection & input section ) the gas flow from the cooler is forced through a perforated plate with a rela-tive high pressure drop to obtain a uniform initial gas distribution. The air enters the L/G separation section via 332 small pipes, each of them provided with a flow detector.
Water from the L/G separation section falls without inter-ference by air via 332 pipes located in the G detection & input section into square U-tubes in the lower part ( L detection sec tion ) . Each of these tubes contains two level sensors and a pneumatic actuated valve. After the filling time between the two levels is registered, the valve can be opened to drain the U-tube and to prepare a new measurement.
- the packed column
The column is built from perspex units with a height of 1 m and a diameter of 0.5 m. Many types of structured as well as random packings, provided by manufacturers, are available.
- the liquid distributor
The distributor, situated above the packing, consists of a hollow plate, perforated with vertical gas tubes and provided with drip points. The initial distribution can be adjusted from 149 drip points ( 760 dp/m2 )
down to one point source with all possibilities in between. A grid of 293 gas tubes, each of them con-taining a gas flow detector, enables the gas to flow out of the packing with a low resistance. A vertical cross section of the distributor is shown in Figure 3.
Figure 3 The liquid distributor Liquid
G a s
t
T T T
A
The measuring techniques used
Gas flows are measured by means of miniature NTC ( Negative Temperature Coëfficiënt ) resistors. The resistance of these
devices is dependent on their temperature in such a way, that it decreases when the temperature increases. The resistors are heated by an electrical current so that their temperature is about
180 °C in the absence of gas flow. However, if there is a gas flow, the resistor is cooled by the gas, resulting in a change of temperature and a corresponding change of resistance. The NTC resistor is part of a Wheatstone bridge ( Figure 4 ) . The relation between the measured bridge voltage and the gas velocity is given by:
U - A + B x exp(AV) (21)
For each resistor the constants A and B are calculated from calibration measuments with known gas velocities. A typical calibration curve is given in Figure 4.
150
,15 V
680 n
I
i ,, . AV . _ _ !
27 k A
2.7 k / \
TT
_i i i i—i i_4.5 5.5 6.5
Voltage (V)
Figure 4 Response of an NTC resistor in a Wheatstone circuit as a function of the gas flow through one sensor
The relative error with a reliability of 95 % is smaller than 2 X. The measuring system responds within a few seconds, thus measurements can be done almost instantaneously. As the detection of the gas flow depends on the cooling of the resistor, the gas must have exactly the same temperature as during calibration. A
convenient temperature is 25.0 °C. The consequences of raeasuring at a different temperature are negligible for differences smaller than 0.2 °C. It turned out to be easy to maintain a gas tempera ture of 25.0 °C. No deviations were found in the resistor signal under fixed conditions for a period lasting more than six weeks. However, mechanical and electrical shocks should be avoided.
9.6 V 1 k.TL
1
e m l t t e rl
4. Y -"*+ /A -f- *^^^\ -10 Vi
10 k A 7T g A V? f •
recelver
Figure 5 The Wheatstone circuit for a liquid level detector
Liquid flows are measured by means of U-formed tubes. Each tube contains a level sensor at the top and a level sensor at the bottom. A sensor consists of a pair of diodes: one LED emitting infrared light and one photodiode whose resistance varies with the amount of infrared light received. This photodiode is included in a Wheatstone bridge ( Figure 6 ) in the same way as the NTC resistor. The absorbance of infrared light in air is different from that in water. When water passes the sensor, a change in the bridge voltage is detected, and the time is registered by means of a computer. At the end of a measurement the liquid flow for each element is computed by dividing the tube volume by the time dif-ference between top and bottom sensor.
For the determination of gas spreading C02 tracer gas is
In-jected in the bottom of the packing. The C02 concentrations
leaving the packing are measured by an analyzer based on infrared light absorbtion. This analyzer works in a range from 0 to 0.3 vol-%, so the amount of tracer gas required is acceptable.
Data acouisition
There are 1289 sensors in the equipment, so an automatic system is indispensable for collecting and processing data. All signals from the sensors are directed to an analogue multiplexer, made out of ordinary CMOS-switches and some address decoding logic. An address, generated by an Olivetti M24 personal computer, selects one of the signals and connects it to an analogue-to-digital converter, which is placed on a LabMaster I/O expansion board. The resulting digitized number is, after some checking and converting, stored into memory and another sensor can be selected. In this way all sensors are measured in sequence.
The entire initial gas distribution, or final gas distribution can be detected ( each NTC is scanned five times ) , processed and saved on a floppy disk in about 5 seconds. The time needed to find the distribution of the liquid dependents on the superficial liquid velocity. Typical spans range from 10 to 60 minutes. During the measurement each photodiode is scanned 10 times per second, which—means—that—the—aceuracy—of—f ill—time-determination_is-_quite adequate.
Characteristics of the equipment
For a better understanding of the influence of the gas flow in a packed column an even initial distribution is desirable. From the initial gas distribution measurements without packing it is concluded that for the range of gas flows used the maldistribution is negligible. The Mf-factor as defined by Groenhof equals 0.005. The variance of the initial liquid distribution, based upon 149 drip points does not depend on the liquid flow rate ( Figure 6 ) . The maldistribution of the distributor is so low, that it has no negative influence on Mf-measurements of packings. An inclination of 3° has no effect on the quality of the distribution, except for very low liquid flows. However, the results are still acceptable.
A low pressure drop over the L/G separation section of the bottom section is important, otherwise the gas velocity profile caused by this part dominates the profile caused by the packing. For the same reason a low pressure drop over the liquid dis tributor is desired. The pressure drops of interest are shown with those of Sulzer 250Y and 25 mm metal Pall rings in Figure 7.
O, 0.06 O Ö tö • f—I cd 0.04 > > •i—i cd 0.02 I—I 0.00 - O - I 1 1 1 - - i 1 1 1 1 1
r-* : horizontal
O : inclined
* * * * * _ i i > i i i i i i i 1 1 1—10
15
Liquid Velocity ( m m / s )
Figure 6 The variance of the flows through 149 drip points as a function of superficial liquid velocity for a horizontal and a 3° tilted liquid distributor
1000
(ÖPu
OH O Q500
u
Vim
<D i-,CU
' 1-.
y
s^ -~~~**t^ ^*"~ï"""~^ — iPall /
1 i ~~' / '
250Y
—" " ,
Init
-■
^
■ ^^.—■—:Bot
0
1
2 3 4
Gas Velocity ( m / s )
Figure 7 Pressure drop of the equipment and over 1 m of Sulzer Mellapak 250Y and 25 rara metal Pall rings as a function of superficial gas velocity. ( Init-liquid distributor, Bot=L/G separation section ) .
These are typical examples of the structured and dumped pac kings used in this study.For 25 mm dumped packings the differences between measured gas flow profiles and ideal profiles can be atttributed to the packings. For structured packings the measured profiles will be slightly smoothed by the measuring section.
Flooding in the upper part of the bottom section does not occur for the range of gas and liquid velocities used in this study.
References
1 H.G.Groenhof
Chem. Eng. J., 14 (1977) 193
CHAPTER III
Measurements of the gas and liquid maldistribution in columns with a random packing
Introduction
Research on maldistribution in packed columns has up to now almost only been concerned with the liquid distributlon. The characteristics of the gas as well as its influence on the liquid has received little attention. However, many authors emphasize the
1 2 need of research in this field '
3
The experience at our laboratory with the liquid has therefore been extended with gas flow measurements. The equipment has been designed to study the flow of gas and liquid simultaneously on the scale of a packing element. A wide range of loadings can be ap-plied in a column with a diameter of 0.5 m and a packing height up to 3 m. The cross section at the bottom of~the packing is-divïdêd
into 332 measuring modules of 25x25 mm2. Each of these modules is provided with liquid and gas flow detectors. The liquid dis-tributor contains 149 drip points and 293 gas flow detectors. The equipment is described in detail in chapter II of this Ph.D. thesis.
The aim of the present work is to determine the maldistribution of the gas and the liquid and their influence upon each other below and above the loading point for the packings as shown in Table I. The shape of the velocity profiles of a cross section of the column and the mixing of both phases will be discussed in terms of wall flow, maldistribution and radial spreading.
Literature
Flow irregularities in the gas distribution can result in a disappointing performance of a packed column.
4
Moore and Rukovena found that the initial gas maldistribution is a function of the kinetic energy of the inlet gas, the pressure drop in the packed section, and to a lesser extent, the distance between the gas inlet and the bottom of the packed bed. According to Ali , a severe maldistribution at the gas inlet is converted to
almost uniform bulk flow within one-half a column diameter. A high pressure drop packing is better with respect to gas redistribution than a low pressure drop packing. Measurements on a small scale showed that flow deviations at the top of a deep bed provided with an elaborate gas distribution system under the bed, are negligible compared with those of the liquid. This observation was also made by Stikkelman and Wesselingh .
An excess of gas flow near the wall was observed by Speek .
Q
Krebs modelled the wall flow in columns filled with 15 mm ceramic Raschig rings by taking a bundie of channels with unequal widths, allowing complete mixing between each layer of packing. In a tower 9 of 10.2 cm diameter with 1 cm glass Raschig rings Spedding et al. absorbed ammonia into water. The gas wall flow, combined with the liquid wall flow, resulted in a radial gas concentration profile at the top of the packing. Gas wall flow is also found in beds packed with catalytic particles. Chourhary et al. used two sizes of particles ( 1/16 and 1/8 inch ) to build packed beds with a high resistance core or annulus. Measured distributions were
simulated with a vectorial form of the Ergun equation.
Kouri and Sohlo used an 500 mm diameter column fitted with special top and bottom sections for measurement of gas and liquid flow through five or six rings. The results of the gas flow dis tribution measurements showed that with good initial distributions of the liquid and gas, the gas bulk flow through plastic Pall rings may be expected to be quite uniform and independent of packing height and flow rates. The quality of the liquid distribu tion for 25 mm Pall rings was said to tend to deteriorate at high gas loads.
12
Baker et al. observed, that the gas flow hardly effects the liquid distribution, even near the loading point. Above the load-ing point, it assists in obtainload-ing a uniform liquid distribution.
13
Dutkai and Ruckenstein came to the same conclusion for Raschig rings and Intalox saddles. Their diffusion model was valid up to 703! of flooding without adjusting the radial spread factor for the liquid and the wall flow parameters. At higher gas loa-dings the radial spread factor increases while the wall flow decreases. Just the opposite occurs when using cocurrent gas flow for 6 mm Raschig rings and Berl saddles .
Stichlmair and Stemmer did not measure flow but temperature profiles. Starting with a good initial distribution, they found
that the largest deviations in temperature are located half-way up the packing.
3 From the literature on the liquid phase (discussed by Hoek and recently by Porter ) a nuraber of conclusions can be drawn:
o 1 fi _ 1 ft
The maldistribution on the scale of a packing element ' may be considered as an inherent and stabile property of the
19
packing. Albright denoted this by the terra natural distribution and concluded that an initial distribution that is better than the
20
natural one will degrade to it quickly. Zuiderweg calculated that the natural distribution appears to have only minor effects on the basic separation efficiency of the packing, due to the influence of radial mixing.
The spreading of liquid in the absence of gas has been studied 21
by many authors, dating back as far as 1893 . Their results can 22-27
be decribed as a random movement of the liquid or as rivulets 3 20 28-31
following specific paths in the packing ' ' . I n either description, a parameter, D , having the unit of length is used in combination with a diffusion like equation assuming axial ~symmëtry:
g £ ^ - D x ( °2f(Z.r) + df(z,r)
dz r dr2 rdr
The wall flow, caused by a change in the isotropy of the pack ing near the wall, is said to find its origin in a difference between the liquid flow towards the wall and the liquid flow from
9fi 97
the wall ' . Solving equation (1), using different boundary conditions for the wall and centre of the column ' ' results in relations between wall flow, initial distribution and packing height.
In summary: there are very few studies in which the mutual influence of gas and liquid have received attention. The studies
11 13 of Kouri and Sohlo and Dutkai and Ruckenstein were based on
only a few sampling areas. Furthermore gas spreading data are missing.
Velocitv profiles
The velocity profiles of the gas and liquid, using various packing heights and superficial velocities, have been measured for the packings presented in Table I. Two typical phenomena can be
recognized in such a profile as shown in Figure 1: an irregular bulk flow and wall flow.
Table I The types of dumped packings used.
Type Pall Ring Ralu Ring Ralu Ring Ralu Ring Ralu Ring Torus Saddle Torus Saddle IMTP Size [mm] 25 25 25 38 38 25 25 25 Material Stainless Steel Plastic Hydrofilated Plastic Plastic Hydrofilated Plastic Plastic Hydrofilated Plastic Stainless Steel Code PR25S R25P R25HP R38P R38HP T25P T25HP IMTP
Figure 1 A three dimensional presentation of a liquid profile showing an irregular bulk flow and pronounced wall flow.
The maldistribution factor, Mf, is used to characterize the 32
bulk flow. According to Groenhof this relative factor can be expressed as:
1 i?n ( u(i)-<u> )2
n i=l <u>2
Mf- - ^ X ^ f e r ^ - (2)
The Mf-value depends on the scale of detection of the localvelocities. In this work the scale is based upon the dimensions of the measuring module, which matches the nominal size of most packings used. Furthermore the Mf takes only the variance of the local velocities and not their spatial orientation into account.
33
An alternative method can be applied that overcomes the disad-vantages mentioned above.
The wall flow factor, Wf, is calculated from the average velocity, <u > in a ring adjacent to the wall for both the gas and
w
the liquid. The ring is chosen to have a thickness of one half of the nominal packing diameter ( •? d ) . This wall flow velocity is compared with the superficial velocity:
<u >
W f - - ^ - _ (3)
In the ideal case of plug flow the Mf-value equals 0 and the Wf-value is 1.
Gas profiles
A typical gas velocity profile ( Figure 2 ) shows a smooth bulk flow, with Mf -values smaller than 0.03, and a wall flow, with Wf -values between 1.1 and 1.3. Only for the torus saddles is
t, G
this value higher. The experimental results for the bottom and the top of the packing are summarized in Table II.
The influence of the gas velocity on the gas wall flow at the bottom of the packing has been investigated with a superficial liquid velocity of 3.4 mm/s. The column contained 1.72 m of IMTP packing. Over a gas velocity range from 1.5 to 3.9 m/s the Wf, -values varied randomly between 1.35 and 1.41. These -values approximate the value of the initial gas distribution without packing. It is assumed that the wall profile caused by the measur ing equipment determines the Wf, -value.
r 4
-0.25
0.00
Radius (m)
0.25
re 2 A typical gas velocity profile.
e II Wall flow values at the top, Wf and raaldistribution t, G
factors at the bottom, Mf, „, and at the top, Mf „, for
b,G t,u the gas phase and at the bottora, Mf, , for the liquid
phase. Code PR25S R25P R25HP R38P R38HP T25P T25HP IMTP INITIAL GAS Mf b,G 0.022 0.007 0.030 0.023 0.019 0.018 0.025 0.003 0.011 Mf t,G 0.030 0.026 0.027 0.023 0.014 0.021 0.031 0.004 Wf t,G 1.2 1.3 1.3 1.2 1.2 1.7 1.5 1.1 ... Mf b,L 0.81 0.55 0.63 0.69 0.81 1.05 1.13 0.57
24
At the top of the packing the Wf -values depend on the type of packing. No significant effects of the liquid load were found up to a superficial gas velocity of 2.5 m/s. Care should be taken to prevent irregularities in packing height, otherwise gas flow channels may occur in which packing particles are lifted.
Liquid profiles
Liquid velocity profiles are less uniform than gas profiles. Small scale maldistribution of the bulk flow is characterized by Mf, -values between 0.5 and 0.8. Again the torus saddles give
b,L
higher values of about 1.1. It was found that the gas velocity hardly influences the small scale maldistribution, except for situations close to flooding. At low liquid velocities the quality of the distribution slightly deteriorates ( Figure 3 ) . Average Mf, -values for different packings are given in Table II.
b, L
Mf
tJ.U1.5
1.0
0.5
n n A As
o
A?
— 1 A8
*
A0
<w
o
o
Liquid Velocity ( m m / s )
10
Figure 2 The maldistribution factor, Mf, of the liquid as afunc-tion of the superficial liquid velocity for 1.72 m of T25P ( ,;, ) , T25HP ( * ) , IMTP (o ) , PR25S (<? ) and R25P ( o ) packings without gas loading.
The wall flow tendency at the bottom of the packing is quite remarkable as shown for R25HP packing in Figure 4. For a gas velocity of 0 m/s the wall flow increases rapidly going downwards in the column as a function of the packing height. Starting with an Wf -value of 0.53 at the liquid distributor the Wf developes to 2 for a packing height of 1.72.
Wf
' 1-(5
o
o
<i i — i 1 —O
©
o
i , i ■©
©Initial
-0
0
0distributor
0
1
2
3 4
Gas Velocity ( m / s )
Figure 4 The development of the liquid wall flow, Wf ,as a func tion of the gas velocity for a packing height of 0.21 ( e ) , 0.43 ( (?) , 0.86 (of) and 1.72 ( © ) m of R25HP rings. The superficial liquid velocity is 3.4 mm/s.
The gas velocity stimulates or reduces the wall flow, depending on the height of packing used. At a height of 0.21 m the wall flow increases at higher gas flow rates where as at a height of 1.72 m to wall flow diminishes. Almost all packings show the same behavióur. The Wf, -values of these packings are presented in Figure 5 as a function of u for a packing height of 1.72 m.
G
Wf
3^r2H
1
-- i 1 1 r£
#
* O O ° < è *1 2 3
Gas Velocity ( m / s )
$_
Figure— 5-The^development— of— the—l-i-qu-id-wall-flow.,_Wf.as __a_func_t.ion. of the gas velocity for a packing height of 1.72 m of PR25S (*), R25P ( O ) , R25HP (•), R38P ( □ ) , R38HP ( ■ ) , T25P ( A ) , T25HP ( O AND IMTP ( $ ) packings. The super-ficial liquid velocity is 3.4 mm/s.
Radial spreading
Radial spreading coefficients have been determined for the gas as well as for the liquid under various loading conditions. Solving equation (1) for a point source with an infinite column radius results in:
f (z,r) 4TTD Z r exp(-4D z r
W
where Q is defined as the total flow rate of the point source. The flow rate passing through a circular area, Q , with radius rx
x' is obtained by integration of (4): r 2?r x r ,z x
-n
o o
f(z,r)rdrdfl - Qx(l-exp(-4D z r -)) (5)The value of D can be determined from experiments. Measured profiles are fitted to the following expression using Standard statistical methods:
- r2
Dr = 4zxln(l-Q ~^/W ( 6 )
x' Gas spreading
A point source with a high gas flow cannot be applied to measure gas spreading as with a liquid. A horizontal pressure gradiënt will cause an rapid redistribution effect within a few decimeters of packing. This is not representative for the rest of the packing. This effect can be overcome by using a homogeneous initial distribution of gas, and a point source of tracer gas. The concentration profile which leaves the top of the packing provides information for the calculation of D .-values. The flow rate in
r,G
equation (6) should then be substituted by the flow rate of tracer gas.
Carbon dioxide is introduced at the center of the bottom of the packing via a vertical pipe of 10 mm diameter. The concentration profile is measured at 49 points, located at 4 axes on top of the packing, covering the whole cross section. A packing height of 0.9 m was used; with this height the tracer gas does not reach the wall. The relative error for the D „-values is less than 10 %.
r ,G
The results of experiments with various gas velocities indicate that the radial spreading coëfficiënt is almost independent of the superficial gas velocity. Liquid loading has a small positive effect on D „ a s shown in Tabel III.
r ,G Liquid spreading
A water flow rate of 6.5 l/min is carefully fed via a jet nozzle with a diameter of 10 mm into the center of the top of the packing. A smooth countercurrent gas flow is introduced at the bottom of the packing with various superficial velocities up to 3.2 m/s. The height of packing is chosen to be 0.86 m to avoid wall effects.
Table III Gas spreading factors, D , for several dumped pac-r, o
kings with and without liquid loading
Type of p a c k i n g PR25S R25P R25HP R38P R38HP T25P T25HP IMTP u . = 0 mm/s Dr , G [ * ] 0 . 0 0 3 8 0 . 0 0 3 8 0 . 0 0 3 7 0 . 0 0 3 6 0 . 0 0 3 4 0 . 0 0 4 0 0 . 0 0 3 5 0 . 0 0 1 8 u = 3 . 4 mm/s
°r,G
[ m ] 0.0041 0.0039 0.0040 0.0037 0.0034 0.0045 0.0042 0.0027 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 6 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 0 0 0 0 0 s 10 0 0 0 0 0 0 0 0 0 0 o o o o o o o o 0 0 5 0 0 0 1 1 5 o o o o o o o o 8 5 0 10 0 12 5 7 0 0 22 19 11 0 0 14 0 0 0 0 14 17 6 7 0 0 10 11 34 11 9 9 19 52 14 28 20 1 6 27 6 15 91 0 15 45 41 5 0 13 23 49 11 36 48 6 22 10 15 0 19 14 13 0 7 5 0 0 0 0 0 0 0 0 24 29 15 28 22 0 0 13 0 0 11 11 0 11 12 15 0 0 27 18 0 15 9 0 0 0 0 0 1512 0 0 6 0 7 0 0 15 0 0 0 0 0 6 0 0 0 0 0 0 0 34 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 14 15 23 5 0 0 0 0 0 0 5 0 7 0 11 25 10 0 8 0 6 7 0 0 0 0 0 12 0 0 0 0 0 0 0 0 0 0 0 5 0 6 0 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 E 0 14 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Figure 6 An example of a liquid velocity distribution (m/sxl04)
resulting from a point source after 0.86 m of IMTP packing.
An example of a liquid flow distribution ( Figure 6 ) demonstrates the irregularity on a small scale. However, the spreadings are quite reproducible. Even after redumping of the packing the values seldom change by more than ten percent.
The dependency of the liquid spreading factor, D , of all r, L
types of packing upon the superficial gas velocity is presented in Figure 7. All the relations show approximately the same behaviour: The D -value shows a small increase up to a certain gas velocity, above which the spreading increases more rapidly.
0.010
0.005
Q
0.0001
2 3
Gas Velocity ( m / s )
Figure 7 The liquid spreading coëfficiënt, Dr, as a function of the gas velocity for PR25S ( O ) , R25P ( O ) , R25HP ( • ) , R38P ( o ) , R38HP ( ■ ) , T25P ( A ) ,. T25HP (*) AND IMTP ( « ) packing.
Interpretation
The results for the gas bulk flow confirm earlier findings ' ' , that small scale maldistribution is not important. The wall flow, however, can be serious especially for the liquid. When it is assumed that a flow equilibrium is established within a few decimeters of packing , the differences in the packing size and form are the most important factors that fix the Wf -value. As an example the IMTP packing and Torus saddles differ in size,
although they are denoted with the same nominal diameter. An IMTP element fits in a box with the dimensions of 28x23x14 mm3, whereas a Torus saddle is enclosed by the dimensions 53x28x23 mm3.
Therefore an IMTP element is better in filling the non-isotropic zone between bulk and wall and thus yields in a lower Wf -value.
The value for the gas spreading coëfficiënt of the packings tested is around 4 mm. Only the value for the IMTP packing dif-fers, which can be explained as mentioned above in combination with the high porosity of the IMTP packing.
The measured maldistribution of the liquid without gas loading 3
is consistent with the results of Hoek . The increase in mal-29
distribution for low liquid loadings as found by Bemer has also been observed for the packings used in this work. The bulk dis
tribution quality is almost independant of gas loading.
In the loading zone the flow near the wall is influenced by the gas. The equilibrium value Wf becomes lower. This is illustrated for 25 mm metal Pall rings in Figure 8. At high gas velocities more liquid is transported in a radial direction. Due to the relatively higher gas flow in the wall zone, the returning mechanism from wall to bulk is also stimulated. The higher ex-change rate between wall and bulk leads to different Wf -values.
A striking exception is given by the 25 mm plastic Torus saddle. This saddle has a form which gives only a small contact length with the wall and its material is hydrofobic. Water piek up from the wall is therefore reduced . The hydrofilated version, T25HP, has better wetting capabilities, and thus enables more back flow to the bulk.
The decline of the wall flow in the loading region might be an explanation for the minimum in the HETP-curves for several
pack-34
ings in columns with a relatively small diameter . In literature the minimum is said to be caused by a higher interaction between the gas and the liquid, but no proof is given. The absence of the
35
minimum in columns with a relatively large diameter is another support for the wall effect, because the larger the diameter the less it effects the overall performance.
3 |— 1 1 1 1 1 1 r 2
-Wf .
+ 1" $
o I 1 1 1 1 1 1 ■0.0 0.5 1.0 1.5 2.0
Packirig Depth (m)
Figure 8 The liquid wall flow, Wf, as a function of the packing depth of PR25S. The superficial gas velocity is 0 ( + ) or 2.54 ( x ) m/s and the superficial liquid velocity is 3.4 mm/s.
Conclusions.
A packed bed can be divided in a bulk and a wall zone.
In the bulk zone a poor initial distribution of the liquid has to be considered as the main cause of a possible malperformance of a column. The gas profile plays a secondary role. The gas flow distribution is smooth and much better than that of the liquid. Moreover, maldistribution at the inlet is converted to almost uniform bulk flow within a small packing height.
The maldistribution effect on the scale of a packing element is largely compensated by radial mixing of gas and liquid. However, on a large scale the radial mixing is small for the liquid ( D = 0.3 mm ) as well as for the gas ( D = 0.4 mm ) . In
r,L r,G practical applications large scale irregularities will persist in
the packing for a long distance.
Both phases show a deviation from the average velocity in the wall zone. The liquid wall flow rate develops from the initial
wall flow rate to an equilibrium value, which is higher than the average flow rate. This development is accelerated by higher gas velocities, but the equilibrium value is lower. The relative gas wall flow rate is independent of gas or liquid flow and equals 1.1-1.7 times the superficial velocity.
The increase in the spreading coëfficiënt of gas and liquid and decrease of liquid wall flow in the loading region could be an explanation for the minimum in HETP as observed in small columns.
The data as presented in this article can be used to model gas and liquid flow patterns below and above the loading point, in-cluding wall flow , maldistribution and radial spreading.
Acknowledgements
We would like to thank Norton Ltd. and Raschig GmbH for supply-ing us with packsupply-ing, the Koninklijke/Shell-Laboratorium for the financial support and all students involved for their zest of work.
Symbols
d diameter [m] f(z,r) velocity at position (z,r) [m/s]
D radial spreading coëfficiënt [m] Mf maldistribution factor [-] n number of samples [-]
Q total flow rate of the point source [m3/s]
Q the flow rate passing through a circular x'
area with radius r at a depth z [m3/s]
r radial coordinate [m] Wf wall flow factor [-] u superficial velocity [m/s] z packing height/depth [m] subscripts: b bottom of packing G gas phase L liquid phase p packing element
t top of packing w wall
=> at infinite height/depth
Greek symbols:
9 polar angle [rad]
References
1 K.E.Porter and M.C.Jones
I. Chem. E. Symposium Series No.104 (1987) A245-258 2 F.J.Zuiderweg
I. Chem. E. Symposium Series No.104 (1987) A589-596 3 P.J.Hoek
Ph.D. Thesis, Technische Hogeschool Delft, 1983 4 F.Moore and F.Rukovena
Chemical Plants & Processing, No.8 (1987) 11-15 5 Q.H.Ali
Ph.D. Thesis, Univesity of Aston, 1984 6 R.M.Stikkelman and J.A.Wesselingh
I. Chem. E. Symposium Series No.104 (1987) B155-164 7 G.Speek
Ph.D. Thesis, Technische Hochschule Dresden, 1955 9 C.Krebs
Chem. Eng. Process., 19 (1985) 129-142 9 P.L.Spedding, M.T.Jones and G.R.Lightsey
Chem. Eng. J., 32 (1986) 151-163 10 M.Choudhary, J.Szekely and S.W.Weller
AICHE journal, 22, No.6 (1979) 1021-1032 11 R.J.Kouri and J.J.Sohlo
I. Chem. E. Symposium Series No.104 (1987) B193-211 12 T.Baker, T.H.Chilton and H.C.Vernon
Trans. Am. Inst. Chem. Engrs., 31 (1935) 296 13 E.Dutkai and E.Ruckenstein
Chem. Eng. Sci., 25 (1970) 483-488 14 G.Baldi and V.Specchia
Ing. Chim. Ital., 12 (1976) 107-111 15 J.Stichlmair and A.Stemmer
I. Chem. E. Symposium Series No.104 (1987) B213-224 16 P.J.Hoek, J.A.Wesselingh and F.J.Zuiderweg
Chem. Eng. Res. Des., 64 (1986) 431-449 17 B.Lespinasse and P.le Goff
Rev. Inst. Fr. Pét., 17 (1962) 1,21,41 18 H.C.Groenhof
Ph.D. Thesis, Rijksuniversiteit Groningen, 1972 19 M.A.Albright
Hydrocarbon Processing, 63 (1984) No.9 173-177 20 F.J.Zuiderweg and P.J.Hoek
I. Chem. E. Symposium Series No.104 (1987) B247-254 21 F.Hurter
J. Soc. Chem. Ind., 12 (1893) 227 22 A.M.Scott
Trans. Ind. Che. Engng., 13 (1935) 211 23 R.S.Tour and F.Lehrman
Trans. Am. Inst. Chem. Eng., 40 (1944) 79 24 Z.Cihla and O.Schmidt
Coll. Czech. Chem. Comm., 22 (1957) 896 25 K.E.Porter and M.C.Jones
Trans. Inst. Chem. Eng., 41 (1963) 240 26 E.Dutkai and E.Ruckenstein
Chem. Eng. Sci., 23 (1968) 1365 27 V.Stanek and V.Kolar
Distribution of liquid over a random packing I to X : I Coll. Czech. Chem. Comm., 30 (1965) 1054-1059 X Coll. Czech. Chem. Comm., 42 (1977) 1129-1140 28 K.E.Porter
Trans. Inst. Chem. Eng., 46 (1968) T69 29 G.G.Bemer and F.J.Zuiderweg
Chem. Eng. Sci., 33 (1978) 1637 30 E.A.Brignole
Chem. Eng. Sci., 28 (1973) 1225 31 P.J.Hoftyzer
Trans. Instn. Chem. Engrs., 42 (1964) T09-117 32 H.G.Groenhof
Chem. Eng. J., 14 (1977) 193
33 R.M.Stikkelman, L.Feenstra, J.de Graauw and J.A.Wesselingh Submitted for publication in Chem. Eng. Res. Des.
34 K.Y.Wu and G.K.Chen
I. Chem. E. Symposium Series No.104 (1987) B225-245 35 F.J.Zuiderweg
CHAPTER IV
Simulation of the gas and liquid distribution in a column with a random packlng
Introduction
For the large scale application of random packings a good understanding of the liquid flow in the column is desirable. The effects of gas loading on the liquid distribution have been studied experimentally in a water/air column with a diameter of 0.5 m. The column has been equipped with 332 flow detectors for the liquid as well as for the gas to determine a precise image of the flow distributions. The results were evaluated in terms of a maldistribution factor Mf, a wall flow factor Wf, and a radial spreading coëfficiënt D . These global parameters give an indica-tion of the small and large scale maldistribuindica-tion and the spreading capabillties of the packing.
Flow patterns can be simulated using the diffusion equation. However, such equations cannot simulate small scale maldistribution. Also it is difficult to stipulate a good set of boundary conditions. These shortcomings can be avoided with cell models i.e. the column is built up with a three dimensional net-work of coupled cells.
In this paper a Monte Carlo cell model is presented which is capable of simulating small scale maldistribution of the liquid, wall flow and spreading of both phases. It includes loading effects. After a survey of the literature on flow modelling the basics of the cell model will be explained. The Mf, Wf and Dr
parameters will be transformed to local parameters for a unit cell. The model will be applied to the simulation of profiles as determined experimentally in the water/air column. Furthermore, the effect of the number of drip points per square meter and the effect of a wall wiper on the quality of the distribution will be discussed.
The model is not llmited to speclfic sizes of the cell or the column. A method to extend the model to any cell dimension will be presented at the end of the paper.
Literature
9 -1 ft
Several workers have investigated the liquid flow in packed columns and tried to model the distribution of liquid.
In most of the slmulation models the liquid is thought to flow as a continuüm or to flow as rivulets, following speciflc paths through the packing elements. Both mechanisms lead to a diffusion equation analogous to Fick's Law:
df^£) .
D x (*ïg£l
+m^ïl )
(1)dz r dr2 rdr
D -values have been calculated from spreading experiments using
r. n n. ., . . . . n _,. .. . 6-8,11,12,14-19
a single liquid jet as ïnitial distribution
Equation (1) has been solved for several types of initial dis-20 tribution with a radial symmetry. A survey is given by Prchlik
The boundary condition for the liquid flow near the wall is an 4 important factor of the diffusion based model. Cihla and Schmidt considered the wall of the column to be a total reflector where liquid is forced back into the packing. This allows the construc tion of a model of finite dimensions, but it does not predict wall
9
flow. Porter assumed that the wall flow is proportional to the liquid flow rate in the bulk of the packing near the wall. The wall flow was found to be over-estimated for small depths of packing. Dutkai and Ruckenstein introduced an annulus near the wall with a thichness S. They assumed that the penetration of the
liquid into the wall region cakes place via an
"adsorption-17 14 desorption" mechanism. Onda et.al and Brignole et.al presented independently a mechanism which is based on the difference between the equilibrium wall flow rate and the actual wall flow rate. Stanek and Kolar introduced a radial transfer coëfficiënt and a distribution coëfficiënt, which denotes the ratio between wall flow and bulk flow at an infinite packing depth.
Another way to simulate the liquid distribution is given by 21
Jameson . He divided the column into layers. Each layer consists of concentric rings. The thickness as well as the height of a ring is equal to the diameter of a packing element. The spread of liquid is controled by a dimensionless factor P and weighted to the circumference of a ring. Although the model predicts the same tendencies as the diffusion models, no complex mathematics is required.
Albright and Hoek et.al used a random number numerical model which is not based on diffusion theories. It accounts for the flow onto and from each individual piece of packing. Every packing tends to a "natural" flow distribution i.e. the equi-librium flow in the bulk of the packing without external effects. The models based on the diffusion equation are capable in solving radial symmetrie distribution problems for the liquid flows. However, they need complex mathematics and do not simulate small scale maldistribution. The numerical method of Jameson is more powerful, but it does not take small scale maldistribution into account and it contains some simplifications. The random number numerical method simulates small scale maldistribution, but misses links to other packing characteristics.
1 2
Gas flow data have only been published recently ' , and model
simulations of the gas have not yet been reported.
The simulation model
The aim of the present model is to predict all the observed characteristics of the liquid and gas flow patterns. This implies simulation of spreading and wall flow of both phases and of small scale maldistribution of the liquid. The simulation of small scale maldistribution of the gas will be omitted because it was found to be negllgible.
The model consists of an orthogonal network of stacked layers of small cells. Each cell has a width, w, of 25 mm and a variable height, h. The height will be chosen in such a way as to produce a proper liquid spreading. The flow model for one cell is outlined in Figure 1.
