Hydrodynamics and Mass Transfer of Modular Catalytic Structured Packing

Hydrodynamics and Mass Transfer

of

Modular Catalytic Structured Packing

Hydrodynamics and Mass Transfer

of

Modular Catalytic Structured Packing

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. ir. J. T. Fokkema Voorzitter van het College van Promoties,

in het openbaar te verdedigen op dinsdag 24 januari 2006 om 15:30 uur

door

Marcel BEHRENS

Dit proefschrift is goedgekeurd door de promoter: Prof. dr.ir. P. J. Jansens

Toegevoegd promoter: Dr. Sc. Ž Olujić

Samenstelling promotiecommissie:

Rector Magnificus voorzitter

Prof. dr. ir. P.J. Jansens Technische Universiteit Delft, promoter

Dr. Sc. Ž Olujić Technische Universiteit Delft, toegevoegd promotor Prof. ir. J. Grievink Technische Universiteit Delft

Prof. dr. J.A. Moulijn Technische Universiteit Delft Prof. dr. ir. A. Stankiewicz Technische Universiteit Delft Prof. Dr.-Ing. A. Górak Universitaet Dortmund

Dr. Sc. Ž Olujić heeft als begeleider in belangrijke mate aan de totstandkoming van het proefschrift bijgedragen.

ISBN-10: 90-9020347-8 ISBN-13:978-90-9020347-8

© 2006 by M. Behrens

Printed by Febodruk B.V., Enschede

Summary

The Hydrodynamic and Mass Transfer Performance of Modular

Catalytic Structured Packing

Process intensification aims at the replacement of large, expensive, energy intensive processes to arrive at substantially smaller, more efficient, less costly and environmental friendly processes. The integration of two or more unit operations into single devices is the way to do this, as encountered in Catalytic Distillation where heterogeneously catalysed chemical reaction is combined with thermal separation in one shell. It presents the most significant class of multifunctional reactors/separators.

Therefore to fit the needs of catalytic distillation processes modular arranged packings are developed to allow flexibility with regard to catalyst load in the column/equipment. The modular catalytic structured packing, MCSP, is build-up of alternating catalyst containing pockets (reaction section), with corrugated sheets (distillation section). Successful implementation requires solutions regarding enormous uncertainties that exist with reliable process design and scale-up. For these purposes the knowledge of hydrodynamics (dynamic liquid holdup, pressure drop and capacity) and mass transfer (HETP) characteristics imposed by the internal configuration is vital and lacking in open literature. To overcome this, the geometry based Delft MCSP Model, a parallel channel model, is developed to predict the hydrodynamic and mass transfer performance of Modular Catalytic Structured Packings (MCSP). The model performance is validated with pilot plant hydraulic (air/water) and total reflux distillation experiments carried out without reaction, for the base case Katapak® _{SP, MCSP. Two different types were used, namely the MCSP-11 and MCSP-12,} where the difference is the number of corrugated sheets sandwiched between the catalyst filled pockets, respectively 1 and 2. Three parallel flow channels are identified in the latter, the catalyst filled pockets, closed channels directly next to the pockets and open crossing flow channels in the middle of the distillation section. In the MCSP-11 this reduces to two, the pockets and closed channels.

Liquid Hold-up and RTD

The liquid hold-up is the basic flow parameter. The total hold-up in the MCSP packed bed is determined by two contributions, the so called internal and external hold-up. The internal operating hold-up is the amount of liquid flowing in the catalyst filled pockets and is bound between the static hold-up as lower end and the hold-up at the catalytic load point, i.e. complete saturation of the catalyst beds as upper end. The basic requirement is that the catalyst is fully employed which means to approximate plug flow through the catalyst bed and this is possible at liquid loads at or above the catalytic load point and this is determined based on the experimental residence time distributions in these pockets. Above the catalytic load point the excess liquid is retained in the distillation section of the packing and adds to the dynamic liquid hold-up in this section. Loading effects are incorporated and up to flooding the model agrees well with the experimental results.

Pressure Drop

pressure drop and uniform liquid distribution. The pressure drop is predicted accurately well into the loading region.

Vapour-Liquid Mass Transfer Performance

Vapour-liquid mass transfer takes place in the space filled with corrugated sheets, including the outer surface of the pockets, since here a vapour-liquid interface is available. In the Delft MCSP model the open channels behave as the flow channels in conventional structured packing. In the closed channels both liquid and vapour are forced to follow the channel to the end. Assuming a uniform liquid distribution all three walls of the triangular channel are wetted, i.e. maximum efficiency is expected, however in practice the liquid tends to flow in the form of rivulets along the lowest point of the channel. In this way a limited interface is available for contact with the ascending vapour causing a lower efficiency. In the model at the transition between packing layers it is assumed that the flowing phases are able to fully mix and this means that the concentration in each phase over the column cross section is uniform. Of each flow channel the change in concentration is calculated and mixed according to the flow contribution. Based on the average concentration the number of equilibrium stages per layer is determined and based on the sum of the number of equilibrium stages per layer the overall HETP. Experimentally in the preloading region the MCSP generally shows a higher HETP than around loading. The difference in performance is mainly caused by maldistribution of liquid, namely bypassing of the liquid inside the pockets by the vapour and reduced lateral spreading due to these pockets which is especially seen in the MCSP-11. The MCSP’s exhibit their lowest attainable HETP around hydraulic loading of the packed bed where the model predictions agree with the experimental results.

Table of Contents

1. INTRODUCTION AND SCOPE ... 1

1.1PROCESS INTENSIFICATION... 1

1.2CATALYTIC DISTILLATION...2

1.3OUTLINE THESIS...5

1.4REFERENCES...7

2. GEOMETRY OF MODULAR CATALYTIC STRUCTURED

PACKING AND EXPERIMENTAL FACILITIES...9

2.1INTRODUCTION...9

2.2MODULAR CATALYTIC STRUCTURED PACKING... 10

2.2.1 Catalyst filled pockets ...10

2.2.2 Corrugated sheets ...12

2.2.3 The MCSP element ...13

2.3MODELLING APPROACH... 15

2.3.1 Liquid hold-up...15

2.3.2 Pressure drop...16

2.3.3 Vapour-liquid mass transfer...16

2.4EXPERIMENTAL FACILITIES... 17

2.4.1 Hydraulics’ simulator ...18

2.4.2 Total reflux distillation column...20

2.5NOMENCLATURE...22

2.6REFERENCES...23

3. LIQUID HOLD-UP IN CATALYST CONTAINING POCKETS

OF MODULAR CATALYTIC STRUCTURED PACKING... 25

3.1INTRODUCTION...25

3.2MODEL DEVELOPMENT...28

3.3DYNAMIC LIQUID HOLD-UP... 31

3.4.1 Static hold-up measurement ...33

3.4.2 Catalytic load point determination ...35

3.5RESULTS AND DISCUSSION...35

3.5.1 System Glass/Water...35

3.5.2 System Catalyst/Water ...37

3.5.3 Catalytic load point...39

3.6CONCLUSIONS... 40

3.7NOMENCLATURE... 41

3.8REFERENCES...42

4. LIQUID FLOW BEHAVIOUR IN CATALYST CONTAINING

POCKETS OF MODULAR CATALYTIC STRUCTURED PACKING

KATAPAK SP ... 45

4.1INTRODUCTION...46

4.2LIQUID RESIDENCE TIME DISTRIBUTION...48

4.2.1 Theory...48

4.2.2 Axial dispersion model ...49

4.3EXPERIMENTS... 51

4.4RESULTS AND DISCUSSION... 51

4.4.1 Flow regimes ...51

4.4.2 RTD around the catalytic load point...53

4.5CONCLUSIONS... 58

4.6NOMENCLATURE...59

4.7REFERENCES...60

5. THE DYNAMIC LIQUID HOLD-UP OF MODULAR

CATALYTIC STRUCTURED PACKING PACKED BED ... 63

5.1INTRODUCTION...63

5.2PACKING GEOMETRY...65

5.2.1 Catalyst filled pockets ...66

5.2.2 Corrugated sheets ...68

5.3.1 Distillation section...69

5.3.2 Reaction section...71

5.4EXPERIMENTS... 73

5.5RESULTS AND DISCUSSION...75

5.6CONCLUSIONS... 80

5.7NOMENCLATURE...80

5.8REFERENCES... 81

6. PRESSURE DROP AND CAPACITY OF MODULAR

CATALYTIC STRUCTURED PACKING ... 85

6.1DISTILLATION AND CATALYTIC PACKINGS...85

6.2MCSPGEOMETRY...87

6.3DELFT MCSPMODEL...89

6.3.1 Distillation section...90

6.3.2 Modular Catalytic Packing ...96

6.3.3 Pressure drop loading correction...98

6.4EXPERIMENTAL STUDIES...98

6.4.1 Hydraulics’ simulator ...99

6.4.2 Total reflux distillation column...100

6.5RESULTS AND DISCUSSION...101

6.5.1 Pressure Drop with Air/Water System ...103

6.5.2 Pressure Drop under Total Reflux Conditions ...105

6.6CONCLUSIONS... 108

6.7NOMENCLATURE... 109

6.8REFERENCES...110

7. VAPOUR – LIQUID MASS TRANSFER PERFORMANCE OF

MODULAR CATALYTIC STRUCTURED PACKING ...115

7.1INTRODUCTION...115

7.2MODEL DEVELOPMENT...118

7.2.1 MCSP Geometry...118

7.3EXPERIMENTAL STUDIES... 129

7.4RESULTS AND DISCUSSION...131

7.5CONCLUSIONS... 138

7.6NOMENCLATURE... 139

7.7REFERENCES...141

8. EPILOGUE: THE DELFT MCSP MODEL AT WORK ... 143

8.1INTRODUCTION... 143

8.2DIFFERENT MCSP TYPES:LARGER SEPARATION TO REACTION RATIOS... 143

8.3MCSP-12GEOMETRY VARIATIONS... 150

8.3.1 Reaction section geometry ...150

8.3.2 Pocket thickness ...155

8.3.3 Catalyst and bed parameters ...157

8.3.4 Corrugated sheet type ...160

8.4CONCLUDING REMARKS... 165

8.4.1 MCSP and catalytic distillation column design...166

8.5REFERENCES... 167

SAMENVATTING... 169

ACKNOWLEDGEMENTS / DANKWOORD... 173

CURRICULUM VITAE... 177

1. Introduction and Scope

Process Intensification

_

_

_

_{ Catalytic Distillation}

1.1 Process Intensification

Process intensification refers to technologies that replace large, expensive, energy intensive equipment and/or processes with ones that are substantially smaller, less costly and more efficient. The combination, integration of multiple operations into fewer or single devices is one way to achieve this goal [1]. Industrially the drive for process intensification is to develop more cost effective processes. The target is to make enormous improvements in process and plant efficiencies. Innovative developments towards novel types of equipment, novel processing techniques and process/plant development methods are necessary. The development and implementation of process intensification is for obvious reasons of great interest. However despite all possible potential advantages still several (non-)technical barriers hinder this; in short these are [2]:

- short term business targets strategy

- research and development are more focussed on new product development instead of new processes

- unawareness of novel equipment and processing methods - lack of simulation and scale-up capabilities

and as last but not least:

- conservatism in process industry

this approach is the Eastman Chemical Co.’s Reactive Distillation process, the homogeneously catalysed esterification process of methanol and acetic acid to methyl acetate [3].

1.2 Catalytic Distillation

Integration of chemical reaction with separation presents the most significant class of multifunctional reactors/separators [4]. The objective of integration of separation with equilibrium limited reactions is to benefit from the interaction between the two processes which is mainly to reach increased conversions. The best way to do this is to reduce the reaction product concentration. However care should be taken with the choice and proper design of the equipment because this is principally determined by the reaction velocity and the relative volatility of the system [5]. In case of fast reactions the residence time should be short and for slow reactions much longer residence times are necessary. This affects the choice of the column internal, i.e. gas (vapour)-liquid contacting device.

The majority of reactions are either homogeneously or heterogeneously catalysed. An increase of the reaction velocity and thus conversion in homogeneously catalysed reactions can be achieved by changing the catalyst concentration which is possible over a wide range. With heterogeneously catalysed reactions the reaction velocity can be enhanced to the limit of the concentration range that can be reached with the used contactor since the catalyst concentration is limited because in some way the catalyst particles have to be immobilised in the reaction zone [5].

Catalytic distillation refers to the integration of chemical reaction with the aid of heterogeneous catalyst and distillative separation. The most important advantage of catalytic distillation is thus an increased conversion, however besides the general benefits of process intensification in catalytic distillation additional benefits are possible [6]:

- improved product selectivity

- reduction of hot spot formation on catalyst - longer catalyst lifetime

- reduced waste production

broaden the overlap between feasible reaction and distillation conditions. Other constraints are with regard to residence time requirements and scale-up [7].

As illustrated schematically in Figure 1, the design and thus the opportunity for implementation of catalytic distillation require knowledge of reaction kinetics, equilibrium and phase behaviour and contactor characteristics. Despite the fact that considerable benefits can be obtained, implementation of such integrated operation is limited. The commercial success of methyl acetate as well as fuel ether production has led to the exploration of other potentially important applications such as acetalisation, hydrogenation, alkylation and hydratation [8] and many more reactions and separations are to be evaluated for their suitability for catalytic distillation.

(non)-Equilibrium Design model Specifications Physical Properties Fluid Dynamics Column dimensions Residence time behaviour & Flow regime Static Hold-up Dynamic Hold-up Pressure Drop Separation Efficiency Thermodynamics Reaction kinetics (non)-Equilibrium Design model Specifications Specifications Physical Properties Physical Properties Fluid Dynamics Fluid Dynamics Column dimensions Residence time behaviour & Flow regime Static Hold-up Dynamic Hold-up Pressure Drop Separation Efficiency Thermodynamics Reaction kinetics

Figure 1 Design framework for a catalytic distillation process (adopted from 8)

capable of describing the occurring complex and interrelated phenomena. The contactor selection together with the column configuration (e.g. reaction - and separation zone locations) determines the overall process performance.

The design of a catalytic distillation process can be divided into the following steps [3,11]: 1. feasibility study

2. preliminary design

3. column hardware selection and equipment sizing 4. detailed design, process dynamics and control

The feasibility analysis can be carried out using residue curve maps to depict the obtainable product compositions. In the preliminary design phase the number of equilibrium stages in the different sections is determined, feed flows and location and some optimalisation of operating parameters like reboiler load, reflux ratio, liquid hold-ups and catalyst load. Equilibrium stage models are very suitable to do this first design. The column hardware choice can have a major influence on conversion and selectivity and non-equilibrium models should take these effects properly into account. Also for an accurate description of the process dynamics non-equilibrium models are essential.

Numerous studies are presented in literature on all kinds of modelling aspects of catalytic (reactive) distillation processes. This is understandable since computer aided chemical engineering research is relatively modest in the sense that one of the few pre-requisites is calculation speed. Sophisticated non-equilibrium design models are available today. However in different forms [e.g. 4,5,6,12] one message becomes clear: “detailed information on hydrodynamics and mass transfer parameters is woefully lacking [7,11]” which is to a large extent determined by the contactor. This is the field in which this thesis wishes to contribute. The objective of this study is to provide such detailed information on the hydrodynamics and mass transfer performance imposed by the geometry of the contactor so that reliable predictive design and scale-up models become better available. Also with the results presented in this study, optimally designed, i.e. tailor made catalytic contacting devices can be arranged based on the process needs. In summary this thesis will provide information and explain the observed fluid dynamic related phenomena depicted on the right side of Figure 1.

distillation). The particle size usually employed in these types of processes is in the range of 1 – 3 mm, this to avoid intra-particle transport limitations [7]. Therefore preferably the design of the gas (vapour)-liquid-solid contactor should meet the following requirements [12]:

- uniform liquid flow in the catalytic part of the contactor without the occurrence of stagnant zones

- wide vapour and liquid loading range - limited catalyst abrasion

- variable catalyst loading

- easy catalyst exchange/renewal

The catalytic contactor considered in this study meets the majority of the above requirements.

1.3 Outline Thesis

A thorough understanding of the interaction between hydrodynamics and mass transfer performance of catalytic internals is, as said above, a prerequisite for further development and implementation of catalytic distillation processes in industry. This thesis summarises the results of a comprehensive study on geometry imposed hydrodynamic behaviour and vapour-liquid mass transfer characteristics of one class of catalytic contactors called Modular Catalytic Structured Packing (MCSP). Namely these packings have alternating reaction and distillation sections combined in a single packing element/layer. Certainly reaction could have a significant influence on especially the vapour-liquid mass transfer but due to the scale of the experimental work this is left out of this study and therefore also thermal effects due to reaction are not covered in this work.

model performance is validated with pilot scale experiments which will also be described in this chapter.

The hold-up characteristics as well as the residence time distribution in the catalytic section is presented in the following three chapters. Chapter 3 deals with the static and dynamic liquid hold-up in the reaction section, i.e. catalyst filled pockets of the MCSP. In chapter 4 the flow behaviour (RTD) through the reaction section of the MCSP is described, which is vital information for its performance in the catalytic packed bed. A full description of the dynamic liquid up of MCSP packed beds is presented in chapter 5. The dynamic hold-up contribution in the reactor section determines the utilisation of the reactor function and consequently the contribution in the distillative part determines the separation performance. The pressure drop characteristics of MCSP packed beds are presented in chapter 6. The experimental pressure drop is determined with air-water experiments as well as under total reflux conditions with an aqueous and organic system at ambient pressure. All experiments conducted with the packed beds are done on pilot scale using columns of 450 mm internal diameter.

In chapter 7 the mass transfer performance under total reflux conditions without reaction is presented. This result in the so called intrinsic geometry imposed separation efficiency of the MCSP; of course this can substantially alter when a reaction takes place [5].

The last chapter illustrates the implications predicted by the model of differences in the MCSP geometry.

1.4 References

1. Tsouris, C., Porcelli, J.V., Process intensification – Has its time finally come?, Chem. Eng. Prog., October 2003, 50-55.

2. Stankiewicz, A., Moulijn, J.A., Process intensification, Ind. Eng. Chem. Res., 2002, 41, 1920-1924.

3. Malone, M.F., Doherty, M.F., Reactive distillation, Ind. Eng. Chem. Res., 2000, 39, 3953-3957.

4. Stankiewicz, A., Reactive separations for process intensification: an industrial perspective, Chem. Eng. Proc., 42, 2003, 137-144.

5. Schoenmakers, H.G., Bessling, B., Reactive and catalytic distillation from an industrial perspective, Chem. Eng. Proc., 42, 2003, 145-155.

6. Tuchlenski, A., Beckmann, A., Reusch, D., Düssel, R., Weidlich, U., Janowsky, R., Reactive distillation – industrial applications, process design & scale-up, Chem. Eng. Sci., 56, 2001, 387-394.

7. Taylor, R., Krishna, R., Modelling reactive distillation (review), Chem. Eng. Sci., 55, 2000, 5183-5229.

8. Sundmacher, K., Kienle, A., Reactive distillation Status and future directions, chapter 1: Sharma, M.M., Mahajani, S.M., Industrial applications of reactive distillation, Wiley-VCH Verlag GmbH & Co. 2003.

9. Mortiz, P., Blagov, S., Hasse, H., Reactive distillation process design and scale-up, Conference proceeding 2001 AIChE annual meeting 2001 906-913.

10. Sundmacher, K., Kienle, A., Reactive distillation Status and future directions, chapter 9: Taylor, R., Krishna, R., Modelling of homogeneous and heterogeneous reactive distillation processes, Wiley-VCH Verlag GmbH & Co. 2003.

2. Geometry of Modular Catalytic Structured Packing

and Experimental Facilities

2.1 Introduction

The general interest in process industry to integrate separation and heterogeneous liquid phase reaction has demanded the development of a new generation of contacting devices to facilitate this operation. Namely, in situ separation and reaction require the combination of both a reactor and separation function in a single device.

The first of such a device was the Bale packing, in which the catalyst is enclosed in several pockets in a fibreglass cloth belt [1]. The pockets are generally 2.5 – 5.0 cm wide with a 0.625 – 1.25 cm stainless steel knitted web between them to provide structural strength and to allow counter-current flow. The belts are rolled up with alternating layers of an open mesh and pockets to form as such a cylindrical, spiral wound Bale. For commercial use more or less cylindrical bales are constructed with diameters of 20 – 35 cm and a height of 50 cm. Another developed structure is where the catalyst is immobilised between wire gauze corrugated sheets, the Katamax and Katapak®_{-S. The former is from Koch-Glitsch and the} latter of Sulzer Chemtech. These packings combine the features of catalyst support and the well known advantages of corrugated sheet structured packing [2,3]. The catalyst is embedded, i.e. sandwiched between two corrugated wire gauze screens resulting in a structure with open and filled cross flow channels of a defined inclination angle and hydraulic diameter. The gas flows through the open channels and the liquid trickles through the catalyst filled channels.

Since the Katapak-SP is the base case structure considered in detail in this thesis, this chapter gives a detailed description of Modular Catalytic Structured Packing (MCSP) concept. Also the Delft MCSP model, the parallel channel model framework and experimental facilities used to validate the model describing the performance of MCSP are described in greater detail here.

2.2 Modular Catalytic Structured Packing

To be able to cover a wide range of needs in catalytic distillation processes Modular Catalytic Structured Packings (MCSP) are developed in which the reaction to separation ratio is flexible. In these packings catalyst filled pockets, the reaction section of the packing, are alternated with corrugated metal sheets, the distillation section. The base case packing is the Katapak-SP where the SP stands for separation performance [3]. Two different types, the MCSP-11 and MCSP-12, are used in this study of which is a schematic depiction shown in Figure 1. The difference between the two types is the number of corrugated sheets placed between the catalyst filled pockets, respectively 1 and 2.

Figure 1 Schematic depiction of MCSP, left: MCSP-12; right MCSP-11

2.2.1 Catalyst filled pockets

As shown in Figure 1 there are two pockets per element layer height. The dense structure of the wire gauze and the catalyst beds compared to the open structure of the distillation section prevents vapour to flow through these pockets. The general geometry of the pockets is shown in Figure 2 and the corresponding geometric dimensions are summarised in Table 1. Due to a change in manufacturing, the pockets fitted in the MCSP-11 differ slightly from those in the MCSP-12. The spherical particles used as filling in the pockets have an average diameter of 1 mm, both catalyst and glass particles were used in this study.

hpe htop hpocket hd hd hmiddle hd hd hpocket htop lpocket tR lR hside

Figure 2 Schematic of catalyst filled pockets in MCSP Table 1 Geometric data of the pockets in a MCSP

Geometric parameter symbol MCSP-12 MCSP-11

height of indented part htop 12.5 mm 8.8 mm

height of indented side part hside 12.5 mm 11 mm

height of diagonal part hd 5 mm 6.85 mm

height of catalyst filled pocket hpocket 70 mm 70 mm

height of indented middle hmiddle 15 mm 15 mm

height (effective1_{) of catalyst bed hcb 150 mm 153.7 mm}

thickness of catalyst filled pocket tre 13 mm 13.2 mm

length of catalyst bed lpocket variable (lR – 2hside)

2.2.2 Corrugated sheets

The distillation section of the MCSP facilitates vapour-liquid contacting and can be fitted with corrugated sheets of certain type and size depending on system requirements. However in the MCSP’s used, the sheets are of the MellapakPlus type, namely the M752.Y. This is a well known High Capacity Packing (HCP) where in the flow channels the lower and upper ends are bended gradually to 90° with respect to the horizontal [5]. This sheet geometry ensures a smoother transition of the fluids between packing layers resulting in a lower pressure drop and a higher capacity compared to the corresponding conventional counter-part. A schematic view of the HCP sheets is shown in Figure 3. By using high capacity corrugated sheets as

a constituent of the MCSP capacity losses associated with liquid build up at the transitions between packing layers is reduced. Furthermore the corrugated sheets,

made of stainless steel, are perforated (a regular pattern of holes) and the surface is deep embossed to promote surface renewal and liquid spreading over the sheet surface. The overall corrugation angle with respect to the horizontal is determined from inlet to outlet points of the flow channel and is 45°. Since the angle at the ends of the channels bends to 90°, the corrugation angle is that in the middle of the flow channel, which is 41°.

The corrugation side length, s, is determined with:

=

2

+

2

4

b

s

h

Eq. 1

where b is the corrugation base width and h is the corrugation height. The specific geometric surface area, ap, of the HCP is calculated with:

⋅

=

⋅

4

p

s

a

b h

Eq. 2

Table 2 Geometric data of HCP; M752.Y

Geometric parameter symbol

Corrugation base length b 9.85 mm

Corrugation height h 6.50 mm

Corrugation side length s 8.16 mm

Specific geometric area ap 509.5 m2/m3

Void fraction εp 0.975

Corrugation sheet height hpe 200 mm

Overall corrugation angle α _45°

Corrugation angle αc 41°

This packing will also be used as reference packing to benchmark the performance of the different MCSP’s.

2.2.3 The MCSP element

Figure 4 shows the photograph of the MCSP-11 and MCSP-12 respectively. The catalyst filled pockets alternated with the HCP corrugated sheets make the modular catalytic structured packing elements. The MCSP-12 is the standard packing type in this study however as mentioned above also the MCSP-11 is considered.

Figure 4 Photograph of the MCSP-11 (left) and MCSP-12 (right)

=

volume occupied by the catalyst

Λ

volume occupied by MCSP element

Eq. 3

The vapour is not able to flow through the pockets of the packing due to their dense structure. This reduces the cross sectional area available for vapour and is accounted for by the cross sectional ratio, Г:

=

cross sectional area occupied by the corrugated sheets

Γ

total column cross sectional area

Eq. 4

In the MCSP-12, where two sheets are sandwiched between the pockets, in the distillation section two types of flow channels are identified. The channels directly next to the pockets are so called closed channels. The channels in the middle are open channels and these are equal to the flow channels in the HCP. To account for this the channel ratio, Χ, is defined:

=number of open channels per packing layer

Χ

total number of channels per packing layer Eq. 5

The specific installed area in each channel type is different. In the closed channels the installed area, ap,cc, is determined by the corrugation dimensions of the sheets and the outside

surface of the catalyst filled pockets. In the open channels, ap,oc, the installed area is a

function of the corrugation dimensions only. Table 3 summarises the average values of the geometrical dimensions of the MCSP types.

Table 3 Geometrical features of the tested MCSP’s

Property symbol MCSP-12 MCSP-11

height packing element hpe 200 mm 200 mm

open channel fraction Χ 0.5 0

volume fraction catalyst Λ 0.34 0.46

cross sectional ratio Γ 0.52 0.40

void fraction element εp 0.7 0.55

geometric area open channels ap,oc 132.5 m2/m3 0 m2/m3 geometric area closed channels ap,cc 208.8 m2/m3 300.2 m2/m3 outside surface area of the pockets ap,R 76.3 m2/m3 96.4 m2/m3

2.3 Modelling Approach

The modular structure of the MCSP allows the description of hydrodynamics and mass transfer performance by a parallel channel modelling approach. Three channels are identified in the base case MCSP-12: the catalyst filled pockets, open and closed channels. Each channel will contribute to the performance of the MCSP which applies to all three main design and operating parameters considered; liquid hold-up, pressure drop and mass transfer efficiency. In case of the MCSP-11 the number of parallel channels reduces to two, the pockets and closed channels. The framework of the Delft MCSP parallel channel model is shown in Figure 5. Obviously, the linking parameter is the liquid hold-up. In the MCSP model the hold-up is the basic flow parameter which in the pockets determines the reaction efficiency and in the distillation channels the pressure drop and mass transfer efficiency.

Holdup

(internal)

Holdup

(external)

static

&

dynamic

reaction

efficiency

Pressure

drop

&

HETP

exchange

flow

behaviour

dynamic

Catalyst containing pockets Open channel Closed channel Corrugated sheets

Liquid load Vapour flow

Vapour flow Liquid load

Holdup

(internal)

Holdup

(external)

static

&

dynamic

reaction

efficiency

Pressure

drop

&

HETP

exchange

flow

behaviour

dynamic

Catalyst containing pockets Open channel Closed channel Corrugated sheets

Liquid load Vapour flow

Vapour flow

Liquid load

Figure 5 Delft MCSP model, the parallel channel modelling approach

2.3.1 Liquid hold-up

external hold-up is the liquid contained in the distillation section of the MCSP. The static hold-up in these flow channels is omitted since it is relatively small and it has negligible influence on the pressure drop and mass transfer efficiency.

At the liquid loads above the catalytic load point the liquid is rejected from the pockets and this excess liquid flows through the distillation section. This gives an extra contribution to the dynamic liquid hold-up and influences the pressure drop by reducing the hydraulic diameter of the gas. The liquid is assumed to be evenly distributed over the channels in the distillation section. This because at the transition between packing layers the space is available for large scale radial mixing and this is especially promoted at higher gas loads.

2.3.2 Pressure drop

Through the pockets only liquid will flow, the dense structure of these pockets prevents inside vapour flow. This consequently means that the pressure drop will be much larger compared to the HCP of the same diameter since the pockets reduce significantly the porosity of the element.

The open channels in the distillation section exhibits the normal structure as encountered in high capacity corrugated sheet structured packing. Therefore these channels will be treated equally as in the reference HCP.

The closed channels are the channels next to the pockets. Both liquid and vapour are forced to follow the channel to the end and the liquid tends to flow in the form of rivulets along the lowest point of the channel. In this way a limited interface is available for contact with the ascending vapour. Regarding the fact that the corrugated sheets have a regular pattern of holes, it may be expected that liquid and/or vapour could partly escape to the other side of the sheet and flow in the same direction. However it is assumed that the extent of this traffic is limited and will therefore not be considered explicitly in the modelling.

2.3.3 Vapour-liquid mass transfer

The Delft MCSP model is a further development of the Delft model [6] developed for common corrugated sheet structured packings. It is a mechanistic model taking into account the packing macro geometry and system properties. All considerations and the working equations of the MCSP parallel channel model are presented in the following chapters.

2.4 Experimental Facilities

The overall performance of the MCSP was experimentally determined with two different test rigs. A packing hydraulics’ simulator was used to determine the dynamic liquid hold-up pressure drop and capacity of the packed beds. These experiments were conducted at ambient conditions with the air/water system. To determine the overall mass transfer efficiency and pressure drop total reflux distillation experiments were carried out. The separation experiments were done at atmospheric pressure with different test systems, i.e. Methanol/Water (MeOH/Wa) and Cyclohexane/n-Heptane (CH/nH) [7]. The MeOH/Wa system was used because in the first set MCSP-12 the pockets were filled with a commercial catalyst (Amberlyst) which needed an aqueous environment to survive. Thereafter the catalyst was exchanged with glass and the experiments were performed with the principal test system (for compatibility reasons) CH/nH.

2.4.1 Hydraulics’ simulator

The packing hydraulics’ simulator used in this study has an internal diameter of 450 mm and is made of Plexiglas. A schematic representation of the experimental set-up is given in Figure 6. The experiments were conducted at ambient conditions. Water is circulated within the system; it is drawn from the liquid buffer tank and pumped through the calibrated flow meters up to the distributor from which it flowed through the packed bed back into the tank. The air was taken from and subsequently blown into the surrounding after passage through the column. The air was supplied to the column from a blower and metered with a calibrated anemometer.

Figure 6 Schematic of the air-water hydraulics’ simulator

The column was always packed in the same configuration. The bottom-packing element was installed perpendicular to the gas inlet device. Every following element was rotated by 90° with respect to the previous. The liquid distributor (large turndown narrow trough distributor, drip point density of 100 per square meter (10 at the periphery and 6 in the

Column:

centre)) was placed rotated 45° with respect to the sheet orientation of the top element and approximately 50 mm above the packing. Liquid loads employed were up to 30 m3_/(m2_h). The temperature of the air leaving the column (T1) was recorded and averaged with the temperature of the surroundings in order to calculate the mean air density during the experiment. With the above the flow factor, FG, was calculated which is defined as:

=

G Gs G

F

u

ρ

Eq. 6

where uGs is the superficial gas velocity and ρG the density of the gas.

Before and between the measurements the column is operated at the desired liquid load (F2 or F3) for a period of about 30 minutes to ensure thorough wetting of the packing and to reach the desired operating conditions.

The pressure drop of the packing (P1) was measured with a water filled U-tube manometer with regard to the surrounding. The pressure drop of the narrow trough liquid distributor is negligible compared to the pressure drop over the packed bed.

2.4.2 Total reflux distillation column

The Total Reflux distillation column is part of the distillation test facilities of the Laboratory for Process Equipment (LPE).

The setup, of which a picture is shown in Figure 7, is constructed of stainless steel, four meters in height with a maximum allowable packed bed height of 2.6 m. The inner diameter is equal to that of the hydraulics’ simulator, i.e. 450 mm. The total volume of the column with bottom sump is 1.1 m3_{. The reboiler, a falling} film evaporator with a heat exchange area of 19.5 m2 _{is heated with saturated} steam with a maximum pressure of 5 bara. The condenser is a shell-tube heat exchanger with an area of 40 m2_{. The} reflux is distributed uniformly over the packing with a similar, narrow trough distributor as used in the hydraulics’ simulator. Samples of the reflux and the collected liquid flowing out of the packed bed are taken inline using a Biar Primason sampling system.

Figure 8 shows a simple process flow diagram of the setup. During operation the liquid in the bottom sump of the column was pumped to the reboiler. The reboiler partially evaporated the process liquid depending on the heat duty. The generated vapour flowed up into the column while the remaining liquid falled back into the sump. The vapour at the top was completely condensed and the liquid reflux was send back to the column. The reflux flow was measured with a coreolis type mass flow meter that measured the mass flow as well as the density and temperature. The mass flow was measured with an accuracy of 0.2 % and Figure 7 Photograph of the Total Reflux

The pressure drop over the packed bed was determined with two calibrated pressure difference meters, one ranging from 0 to 10 mbar and one over the full range, from 0 to 100 mbar. Both were connected with one side to the top of the column above the liquid distributor and the other side to the bottom just below the packed bed.

Figure 8 Process flow diagram Total reflux distillation column

Samples of the liquid streams were taken from the reflux and the liquid leaving at the bottom of the packed bed. During operation there was constant flow through the sampling system ensuring fresh samples. The composition of these samples were analysed to determine the vapour and liquid properties during the experiment and to determine the separation efficiency.

The composition of the samples with the Methanol/Water system were analysed for water content using Karl-fisher coulometric titration. The high water content (bottom) samples were diluted before analysis and the water content was determined with an accuracy of 0.5%. The composition of the samples with the CH/nH system was determined with a GC, analysed with Chrompack Cp9002 with FID detector. The column used was 25 m in length with a film of Cp-sil-5Cb (thickness 5 µm). The mass fraction analysis was within an accuracy of 0.4%.

The vapour density during the experiments was determined using the ideal gas law based on the average concentrations and with the reflux mass flow the average F-factor, in the middle of the bed, was determined. The basic performance of the total reflux distillation column with the two systems was determined with the Sulzer high capacity packing M752.Y and is presented in [9].

2.5 Nomenclature

a [m2_/m3_{] specific area} b _[

_m

_] corrugation base FG [m/s(kg/m3)0.5] F-factor h _[

_m

_] height/corrugation height s _[

_m

_] corrugation side u [m/s] velocity Greek

α [º] overall corrugation angle with respect to the horizontal αc [º] corrugation angle with respect to the horizontal

Γ [-] cross sectional ratio

ε [-] void fraction

Λ [-] volume fraction catalyst in MCSP

ρ [kg/m2_{] density}

Subscripts c corrugation cc closed channel G gas Gs gas, superficial oc open channel p packing pe packing element

2.6 References

1. Subawalla H., Gonzalez J.C., Seibert, A.F., Fair, J.R., Capacity and efficiency of reactive distillation Bale packing: Modelling and experimental validation, Ind. Eng. Chem. Res. 1997, 36, 3821-3832.

2. Moritz, P., Hasse, H., Fluid dynamics in reactive distillation packing Katapak®_{-S, Chem.} Eng. Sci., 1990, 54, 1367-1374.

3. Götze, L., Bailer, O., Moritz, P., Scala, C. von, Reactive distillation with Katapak®, Cat. Today, 69, 2001, 201-208.

4. Olujić, Ž., Kaibel, B., Jansen, H., Rietfort, T., Zich, E., Frey, G., Distillation column internals/configurations for process intensification, Chem. Biochem. Eng. Q., 17, 4, 2003, 301-309.

5. Pilling, M, Haas, K., Column Design with Structured Packings, Proceedings of the Topical Conference “Distillation Tools for the Practicing Engineer”, AIChE Spring Meeting, New Orleans, LA, USA, March 10 – 14, 2002, pp. 103-141.

6. Fair, J.R., Seibert, A.F., Behrens, M., Saraber, P.P., Olujić, Ž., Structured packing performance – Experimental evaluation of two predictive models, Ind. Eng. Chem. Res., 39, 6, 2000, 1788-1796.

7. Onken U., Arlt, W., Recommended test mixtures for distillation columns, second edition, 1990, The Institution of Chemical Engineers.

3. Liquid Hold-up in Catalyst Containing Pockets of

Modular Catalytic Structured Packing

Abstract

This chapter introduces a simple, first principles based model describing the liquid hold-up in the catalyst containing pockets of Modular Catalytic Structured Packing (MCSP) developed to enable certain degree of flexibility with respect to the variation in reaction to separation requirements in a single unit. The basic requirement for the catalyst containing pockets in this respect is to be fully filled with flowing liquid, which implies that the operating hold-up is bound between the static hold-up of the catalyst bed as the lower end and that corresponding to the upper limit the so called catalytic load point. This is the liquid load corresponding to the bed saturation point, indicating that excessive liquid will be retained, i.e. will remain in the separation part of the MCSP element and mix with the liquid leaving the catalyst filled pockets at the bottom of the element. Detailed knowledge of the liquid hold-up as well as the pattern of the trickling flow is essential because it determines the reaction performance of MCSP and consequently the hybrid unit as a whole. Both glass and resin (an industrial catalyst) particles were used in conjunction with water and a binary Water-Methanol mixture as working fluids. The model predictions for static hold-up and the catalytic load agree well with the experiments.

Keywords: Modular Catalytic Structured packing, liquid hold-up, catalytic load point, trickle bed, capillary rise.

3.1 Introduction

occurs in/on the catalyst and the reacting phase enters the distillation part for vapour/liquid contact and separation.

Anyhow, for the reaction performance information on liquid hold-up in these catalyst containing pockets is crucial and these are, due to manufacturing reasons of fixed shape and dimensions. Figure 1 shows schematically the structure of MCSP-12. The geometrical features of the catalyst containing pockets are summarised in Table 1. As shown in Figure 1, the catalyst is immobilised in two vertically separated catalyst containing pockets (short fixed beds) and the distillation (separation) part consists of two corrugated sheets. The walls of the catalyst filled pockets are made of wire gauze, with a mesh size allowing easy penetration of liquid, but impenetrable for flowing vapour and catalyst particles. This implies that reaction will take place in the liquid phase only.

Successful operation is ensured if all catalyst containing pockets are fully filled with flowing liquid. However in a hybrid process situation like catalytic distillation, it is difficult to arrange an optimal liquid load, therefore the knowledge of both the static hold-up as the lower limit and the operating, dynamic hold-up as upper limit in the catalyst filled pockets is essential information for designers of such systems.

The aim of this study was to determine experimentally the relevant static and dynamic liquid hold-up characteristics of the catalyst filled pockets in MCSP elements and as demonstrated in this chapter, it appeared to be possible to develop a simple, first principles based model capable of predicting reliably the operating hold-up for given process conditions.

Figure 1 Schematic of MCSP-12 a) catalyst containing pockets; b) corrugated structured sheets

a

Table 1 Geometric data of the catalyst containing pockets in MCSP

Geometric parameter symbol

height of reaction element hpe 200 mm

height of indented top/bottom/side part htop 12.5 mm

height of diagonal part hd 5 mm

height of reaction element pocket hpocket 70 mm

height of indented middle hmiddle 15 mm

height (effective2_{) of catalyst bed hcb 150 mm}

thickness of reaction element tre 13 mm

length of catalyst bed lpocket variable

3.2 Model Development

Considering single catalyst containing pockets as small trickle beds, the well established approach was adopted, i.e. the difference between the total and dynamic hold-up taken as static hold-up, which in turn represents an upper bound of the passive liquid fraction in the catalyst beds [12]. In the reaction engineering, liquid hold-up is often considered as liquid saturation of the bed [13], i.e. the liquid volume contained in a unit void volume of the reaction bed. In a bed of porous particles (catalyst) the total saturation is the sum of internal and external contributions. The first one is the liquid held inside the pore volume of the porous particles. The external liquid saturation which is held in the void of the catalyst filled pockets is comprised of a static and free draining or operative saturation. In fact, the saturation is equivalent to the hold-up accounted for the void fraction in the catalyst bed. Thus the total liquid hold-up in the catalyst containing pockets is defined as:

=

+

, , ,

L R L stat L dyn

h

h

h

Eq. 1

where hL,stat (m3/m3) is the total static liquid hold-up and hL,dyn (m3/m3) is the dynamic liquid

hold-up in the catalyst containing pockets.

Static liquid hold-up

The static hold-up is comprised of three individual contributions: the liquid held inside the pores of porous particles, hL,pore, the hold-up due to the capillary rise height, hL,cap, and the

so-called residual liquid, hL,res,.

=

+

+

, , , ,

L stat L pore L cap L res

h

h

h

h

Eq. 2

Pore hold-up

The pores inside porous catalyst particles are assumed to be completely filled with liquid, which holds except for systems with strong heat effects, and this is not the case here (experiments at ambient conditions). Therefore the pore hold-up is defined as:

=

,

L pore p

h

ε

Eq. 3

with εp (-) is the porosity of the particles.

Capillary rise hold-up

In general, gravity will force liquid to drain from a compact bed until a balance is reached with counteracting capillary forces. The catalyst bed is assumed to consist of a multiplicity of parallel, unconnected capillaries, with the hydraulic diameter of the voids defined as [13].

=

−

2

3 1

R h p R

ε

d

d

ε

Eq. 4

where εR (-) is the porosity of the catalyst bed and dp (m) the mean catalyst particle diameter.

For a vertical capillary, where the capillary forces are balanced with the gravitational forces and no external pressure difference exists due to vapour flow around the capillary, the capillary rise height is:

=

4

cos

cap h L

σ

h

θ

d ρ g

Eq. 5

where σ (N/m) is the liquid surface tension, ρL (kg/m3) is the density of the liquid, g (m/s2)

(

−

)

=

6 1

R

cos

cap p R L

ε σ

h

θ

d ε ρ g

Eq. 6

For the whole volume of the catalyst containing pockets, the liquid hold-up due to capillary rise becomes

=

, R R R L cap cap R

ε t l

h

h

V

Eq. 7

where tR (m) is the thickness of the catalyst containing pockets, lR (m) the total length of

catalyst pockets in a MCSP element, and VR (m3) is the total volume occupied by the catalyst

in a MCSP element.

Since the catalyst filled pockets are rather short (total catalyst bed height 0.15 m), the capillary rise height is in the same order of magnitude as the catalyst bed height.

The external pressure is not taken into account since below the hydraulic load point of the packed bed the pressure gradient around the catalyst filled pockets due to gas flow is negligible small.

Residual hold-up

The residual hold-up is defined here as the liquid retained in the catalyst filled pockets above the capillary rise height after drainage of the bed. This liquid is mainly retained by capillary forces at the contacting points (liquid pockets) of the particles. Often it is described using the empirical correlation by Saez and Carbonell [14] utilising the characteristic Eötvös number i.e. the dimensionless ratio of gravity and surface tension.

However, in this paper a simpler, physically more reliable model is adopted [15], which describes the residual hold-up as an ensemble of liquid pockets between the touching particles:

−

=

,

1

0.028

R L res R

ε

h

ε

Eq. 8

This expression is easily adapted to account for the whole catalyst filled pocket:

−





−

=

_

_





,

1

0.028

R cb cap L res R cb

h

h

ε

h

ε

h

Eq. 9

The constant on the right hand side in the above equations (i.e. 0.028) is derived based on the description of the pendular liquid volume between touching particles. This volume is determined by the number of particle-particle contact points in a packed bed, which is described with an empirical relation based on experiments, as function of εR, and the critical

wetting angle below which no more liquid can be drained.

3.3 Dynamic liquid hold-up

In a compact packed bed of small particles the upper bound of the passive liquid fraction is the static liquid hold-up. The maximum dynamic liquid hold-up is reached when the void fraction of the catalyst containing pocket is just completely filled with flowing liquid. This point is characterised as the catalytic load or catalyst bed saturation point. In [4] the catalytic load point of the Katapak-S structure is defined based on the observed flow regimes. In the model the catalytic load point is a function of the physical properties of the liquid, the particle size and the void fraction of the bed. In the catalyst filled pockets of the packing used in this study the same flow regimes were experimentally observed. This was not surprising regarding the fact that these beds are roughly of the same size and corresponding structure.

The dynamic liquid hold-up of the catalyst containing pockets is bound between the static liquid hold-up and the maximum hold-up at the liquid load where the bed is completely saturated, i.e. that corresponding to the catalytic load point. In between these points the dynamic liquid hold-up in the catalyst filled pockets is taken to be equal to the dynamic liquid hold-up in the distillation elements corrected for the volume fraction of catalyst in the packing element. The volume fraction catalyst containing pockets (catalyst) is defined as:

= volume occupied by the catalyst

Λ

volume occupied by MCSP element Eq. 10

Catalytic load point

the steady state condition, assuming that the pressure drop is negligible compared to the resistance for liquid flow through the bed and no energy is added to the system, the Bernoulli equation reduces to:

=

∆

_fr

g z e

Eq. 11

where ∆z (m) is the liquid height and efr (-) is a coefficient representing the energy dissipation

due to the drag forces associated with the liquid flow around particles. The working expression for the coefficient of energy dissipation [16] is:

=

p D _R fr m R

N F u

e

φ

ε

Eq. 12

where Np (-) is the number of particles, uR (m/s) is the superficial liquid velocity in the

catalytic bed, FD (N) is the drag force and φm (kg/s) is the mass flow rate of the liquid in the

catalyst filled pockets. The latter is described as:

=

m L R R

φ

ρ A u

Eq. 13

where AR is the cross sectional area of the catalyst filled pockets.

The drag force exerted on one particle is defined as:





=

_

_





2 2

1

4

2

R D D p L R

2

u

F

C

d

ρ

ε

Eq. 14

where CD is the drag force coefficient. The drag force coefficient is a function of the

hydraulic Reynolds number in the catalyst bed and is described with the Ergun relation.

=

2.3

+

150

Re

D h

C

_{Eq. 15} with

(

)

=

−

2

Re

3

1

R p h L R

ρu d

µ

ε

Eq. 16

The total drag force however is caused by the total number of particles in the bed and can be obtained by dividing the volume occupied by catalyst, Vcat (m3), by the volume of a single

By substituting Eq. 14, 15, 16 in 13 and rearranging, the coefficient of dissipation is expressed as:

(

−

)

=

2 3

1

3

4

_R R cb fr D R p

ε h

e

C

u

ε

d

Eq. 18

By substitution of the above in Eq 12, an implicit expression is obtained for the superficial liquid velocity:



 −

=

_

+

_





2 3

3

150 1

∆

2.3

4

Re

R cb R h R p

ε h u

g z

ε

d

Eq. 19

For ∆z = hcb this velocity corresponds to that characterising the catalytic load point. An

iterative calculation is needed to get the characteristic value.

If the overall superficial liquid velocity is larger than that corresponding to the catalyst bed load point, the excess liquid will remain in separation part of the MCSP. In this way it will contribute to the operating (dynamic) hold-up of the MCSP. This excess liquid will not participate in chemical reaction within the given MCSP element. However it will mix with liquid leaving the catalyst pockets and be redistributed at the transitions between subsequent elements/layers. On the other hand, an overall liquid load lower than that corresponding to the catalytic load point means that catalyst beds will not work under optimum conditions. Matching overall and catalyst liquid loads will be difficult to arrange in practice. This is definitely a critical point for designers of reactive distillation columns and will be addressed in a following paper discussing the hydraulic and mass transfer performance of performance of hybrid packed beds.

3.4 Experiments

3.4.1 Static hold-up measurement

however different pocket lengths, lpocket, (see figure 2) were used. The glass particles were of

the same dimensions as the catalyst particles.

Table 2 Properties of the filling in the pockets

Geometric parameter glass resin

diameter (average) dp 1 mm 1 mm

compacted bulk density ρs 1535 kg/m3bed 416dry kg/m3bed

particle porosity εp 0 0.4 bed porosity εre 0.362 0.362 contact angle θc 50° 0° hpe htop hpocket h_d h_d hmiddle h_d h_d hpocket h_top lpocket tre l_re htop

Figure 2 Schematic of the catalyst filled pockets in a MCSP

Tap water was used as liquid in the experiments. To determine the influence of surface tension and viscosity an experiment was done with pockets filled with catalyst and a methanol-water mixture (93 wt% methanol). Table 3 summarises the properties of the liquids used. The different fillings used result in a different contact angle between the particle and the liquid.

liquid draining (approximately after 15 minutes) the capillary rise height and total weight were measured.

Table 3 Liquid properties of tap water and methanol-water mixture

Property symbol tap water[17] methanol-water[18]

liquid density ρL 998 kg/m3 812 kg/m3

dynamic liquid viscosity µL 0.00115 Pa·s 0.0006 Pa·s

surface tension σL 0.073 N/m 0.024 N/m

3.4.2 Catalytic load point determination

These experiments were done with water as liquid at ambient conditions. The catalytic load point was determined indirectly, using a Perspex tube filled with glass spheres. This was done because the reaction elements could only be irrigated using drip points. With drip points the experiments showed that already below the catalytic load point liquid started to bypass the upper pocket because the momentum on the pockets of the flowing liquid was too high. In the particles filled tube above the bed, Pall rings were placed as ‘momentum breakers’. Liquid load was adjusted to the point where neither liquid accumulated above the bed nor incomplete saturation of the bed was observed.

3.5 Results and discussion

3.5.1 System Glass/Water

During the experiments with the glass/water system the capillary rise height in the lower pocket was 50 mm. Based on this capillary rise height the wetting contact angle was determined and corresponds with an angle of 50°, which is in the line with the values (between 30 and 60 degrees) reported in literature [14].

One would expect that in equal beds the capillary rise behaviour would be equal. However based on the observed effect, it is thought that this liquid is retained in the upper pocket because the intersection between both pockets forms an obstacle for free drainage. The drainage of the liquid in the upper pocket stops at a certain moment when the intersection falls dry at lower drainage velocity and capillary forces retain the remaining liquid. The drainage of the liquid in the upper pocket is larger because it is connected to the lower pocket and before the intersection falls dry the capillary forces draw more liquid downwards. It should be noted that the liquid drainage behaviour of the upper pocket in the catalyst filled pockets is determined by manufacturing. A second, different set of MCSP showed experimentally a quite higher percentage of retained liquid in the upper pocket due to a different welding method applied during the MCSP production. Therefore in the model calculations for these pockets, 15% of the lower pocket capillary rise height is added to calculate the total capillary rise hold-up in the catalyst bed.

For the residual hold-up the two predictive models gave comparable results. Owing to the fact that the model of Kramer [15] relies on a physical basis, this relation was adopted and used throughout this study. The experimental and model results of the drainage experiments are summarised in Table 4 and a graphic comparison is given in Figure 4. The model under predicts the static hold-up slightly but within tolerable accuracy (<10 %).

Table 4 Results drainage experiments, retained liquid mass; System: Glass/H2O

Reaction element length experiment model % difference

410 mm 138.25 g 129.26 g - 6.5

435 mm 148.85 g 136.31 g - 8.4

439 mm 146.85 g 140.94 g - 4.0

Figure 3 Schematic illustration of the capillary rise height in pockets in a MCSP

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 410.0 435.0 439.0

Length catalyst contaning pockets [mm]

T o ta l st a ti c l iq u id h o ld u p [ -] Experiment Water/Glass Model Water/Glass

Figure 4 Model and experimental results of the static liquid hold-up in pockets filled with glass, liquid: tap water

3.5.2 System Catalyst/Water

Here the pores inside the porous particles contribute to the static hold-up. Since there are no large heat effects the pores are completely filled. The wetting angle of porous particles is typically 0° [15]. This results in a capillary rise height of 75 mm for tap water. The lower section is completely filled with liquid which was visually observed. The upper pocket contributes again with 15 % from the lower pocket capillary rise height. Table 5 summarises the results with this system and Figure 5 shows the corresponding liquid hold-up in the tested pockets for different pocket lengths as inserted in MCSP elements, indicating that this dimension is of no influence and that the model is in excellent agreement with the experiments.

Table 5 Results drainage experiments; retained liquid mass; System: Catalyst/water

Reaction element length experiment model % difference

250 mm 99.81 g 98.60 g - 0.6

307 mm 130.23 g 126.37 g - 1.5

350 mm 142.84 g 147.84 g + 1.7

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 250.0 307.0 350.0 389.0

Length catalyst containing pockets [mm]

To ta l st a ti c l iq u id h o ld u p [-] Experiment Water/Catalyst Model Water/Catalyst Experiment MeOH/Water/Catalyst Model MeOH/Water/Catalyst

Figure 5 Model and experimental results of the static liquid hold-up in pockets filled with resin, liquid: tap water or MeOH/water mixture

Table 6 Results drainage experiments; retained liquid mass; System: Catalyst/water/Methanol

length pocket experiment model % difference

350 mm 68.22 g 68.09 g - 0.1

0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 experiment m o d e l Glass/water experiments

Catalyst resin/water experiments

Catalyst resin/water/MeOH experiment

+ 10%

- 10%

Figure 6 Parity plot of total static liquid hold-up, model vs. experiment

3.5.3 Catalytic load point

The flow rate was determined at the point where the bed is just saturated with liquid. These results are summarised in Table 7. The model agrees well with the experimentally obtained results. The model presented in [4], adapted for the reaction element structure considered in this study gives similar results.

Table 7 Results experiments catalytic load point; System: Glass/H2O

experiment experiment model % difference

3.6 Conclusions

Static hold-up and maximum liquid (saturation) load of reaction elements (metal gauze pockets filled with particles: glass/catalyst) of a commercially developed modular catalytic structured packing were determined experimentally.

The static hold-up is significant and concentrated mainly in the lower catalytic pocket. The upper pocket retained some 15 % of the liquid contained in the lower pocket, however the extent of this appears to be strongly dependent on the mechanical design of the catalyst pocket ends.

The static hold-up of the catalyst filled pockets is larger than that of glass filled elements, partly due to the contribution of pores and partly due to the difference in the contact angle. With the glass/water system the wetting contact angle was determined at 50

°

.

The static hold-up is larger in case of water than organics, mainly due to surface tension effect.

Dynamic hold-up of a catalyst filled pocket/bed is bound between the static hold-up and the hold-up at catalyst bed saturation load point. Liquid in excess will be retained, i.e. descend in the inclined flow channels.

The simple, first principles based models appeared to be capable to predict the static liquid hold-up with a satisfactory accuracy. Also the catalyst saturation/load point is predicted, which means that the upper limit for the dynamic hold-up in the catalyst filled pockets is also fixed.

Acknowledgements

3.7 Nomenclature

A [m2_{] area} hL [-] liquid holdup CD [-] drag coefficient d [m] diameter efr [-] coefficient of dissipation g [m/s2_{] gravitational acceleration} h [m] height l [m] length t [m] thickness u [m/s] velocity V [m3_{] volume} z [m] distance Greek ε [-] void fraction θ [°] contact angle

Λ [-] volume fraction catalyst in MCSP

µ [Pas] dynamic viscosity