INVESTIGATION

OF THE ADSORPTION

OF CARBOHYDRATES AND FURANS

IN MICROPOROUS MATERIALS

by

Andrew Shah

A thesis submitted to the Faculty of the University of Delaware in partial

fulfillment of the requirements for the degree of Honors Degree in Chemical

Engineering with Distinction

Spring 2013

© 2013 Andrew Shah

All Rights Reserved

INVESTIGATION

OF THE ADSORPTION

OF CARBOHYDRATES AND FURANS

IN MICROPOROUS MATERIALS

by

Andrew Shah

Approved: __________________________________________________________

Dionisios Vlachos, Ph.D.

Professor in charge of thesis on behalf of the Advisory Committee

Approved: __________________________________________________________

Vladimiros Nikolakis, Ph.D.

Committee member from the Department of Chemical & Biomolecular

Engineering

Approved: __________________________________________________________

Susan Groh, Ph.D.

Committee member from the Board of Senior Thesis Readers

Approved: __________________________________________________________

Michael Arnold, Ph.D.

Director, University Honors Program

1

ACKNOWLEDGMENTS

There are a few people that I wish to thank for their help in the creation of this

thesis.

I would like to thank Dr. Dion Vlachos for the opportunity to research in his

lab. I have enjoyed my time studying catalysis in various forms, and feel that I made a

good decision joining his group in my sophomore year. Thanks to the Catalysis Center

for Energy Innovation, I am engaged in research outside the scope of anything I had

ever imagined.

I would especially like to thank Dr. Vladimiros Nikolakis for his guidance and

support regarding this thesis. I am so appreciative of his tireless efforts to tutor and

encourage me over the past two and a half years. From the very outset of my research

experience, Vlad made me feel comfortable asking questions and making mistakes. He

has helped me grow as a researcher, and I am truly grateful for his major contribution

to my education.

I would also like to thank Dr. Marta Leon-Garcia, Dr. Jake Kruger, and Ms.

Christina Bagia for their help around the lab and with data analysis. Marta, Jake, and

Christina were instrumental in enhancing my competence as a researcher, and for that I

am very appreciative.

Finally, I would like to thank Dr. Meredith Wesolowski and Dr. Susan Groh

for their patience and suggestions in thesis review sessions. I would especially like to

thank Dr. Groh for being a wonderful General Chemistry professor. Without their help

2

and everybody else’s, my undergraduate research could never have culminated in this

thesis and the experience gained in writing it.

3

TABLE OF CONTENTS

LIST OF TABLES ......................................................................................................... 6 LIST OF FIGURES ........................................................................................................ 7 ABSTRACT ................................................................................................................... 9

1 INTRODUCTION ............................................................................................ 10

1.1 Motivation ............................................................................................... 10 1.2 Previous Work ......................................................................................... 13

1.2.1 Adsorbents ................................................................................... 13 1.2.2 Processes ...................................................................................... 14

1.3 Aim and Outline of Thesis ...................................................................... 15

2 MATERIALS, METHODS, AND COLUMN DESIGN ................................. 16

2.1 Materials .................................................................................................. 16 2.2 Batch Isotherms ....................................................................................... 20 2.3 Column Design ........................................................................................ 21

2.3.1 Column Diameter Design ............................................................ 22 2.3.2 Volumetric Flow Rate Determination ......................................... 22 2.3.3 Column Length Determination .................................................... 23

2.4 Preliminary Column Setup ...................................................................... 23 2.5 Finalized Fixed Bed Setup ...................................................................... 24 2.6 Fixed Bed Operation ............................................................................... 25 2.7 Breakthrough Curves ............................................................................... 27 2.8 Breakthrough Curve Modeling ................................................................ 27

2.8.1 System Mass Balance .................................................................. 28 2.8.2 Modeling Input and Assumptions ............................................... 29 2.8.3 Diffusion and the Effective Mass Transfer Coefficient .............. 32

2.9 List of Symbols ....................................................................................... 34

3 BATCH ISOTHERMS ..................................................................................... 36

4

3.1 Isotherm Measurement ............................................................................ 36 3.2 Zeolite Loading Capacity Determination ................................................ 36 3.3 Equilibrium Constant Determination ...................................................... 37 3.4 13X Isotherms ......................................................................................... 37

3.4.1 Discussion .................................................................................... 40

3.5 HBEA25 Isotherms ................................................................................. 40

3.5.1 Discussion .................................................................................... 43

3.6 NaBEA25 Isotherms ................................................................................ 43

3.6.1 Discussion .................................................................................... 45

3.7 Summary of Isotherms and Langmuir Constants .................................... 46

4 BREAKTHROUGH CURVES ........................................................................ 47

4.1 Flow Experiment Setup ........................................................................... 47 4.2 Single Component Flow Testing ............................................................. 47

4.2.1 13X Flow Testing ........................................................................ 47 4.2.2 HBEA25 Flow Testing ................................................................ 51 4.2.3 NaBEA25 Flow Testing .............................................................. 53

4.3 Multicomponent Flow Testing ................................................................ 57

4.3.1 NaBEA25 Flow Testing .............................................................. 57

5 BREAKTHROUGH CURVE MODELING .................................................... 59

5.1 13X Breakthrough Curves ....................................................................... 59 5.2 H-BEA25 Breakthrough Curves ............................................................. 62 5.3 Na-BEA25 Breakthrough Curves ............................................................ 63 5.4 Multicomponent Breakthrough Curves ................................................... 65 5.5 Modeling Improvements ......................................................................... 66

5.5.1 Increase Column Diameter .......................................................... 67 5.5.2 Increase Bed Length .................................................................... 68 5.5.3 Increase Flow Rate ...................................................................... 69

6 CONCLUSIONS AND RECOMMENDATIONS ........................................... 70

7 FUTURE WORK ............................................................................................. 71

5

REFERENCES ............................................................................................................. 72

6

LIST OF TABLES

Table 1. Important Column Design Parameters ........................................................... 21

Table 2. Input Parameters for Mathematica Model ...................................................... 30

Table 3. Langmuir constants for various isotherms ..................................................... 46

Table 4. Operating conditions for 13X/Fructose flow testing ...................................... 47

Table 5. Operating conditions for 13X/HMF flow testing ........................................... 49

Table 6. Operating conditions for HBEA25/Fructose flow testing .............................. 51

Table 7. Operating conditions for NaBEA25/Fructose flow testing ............................ 54

Table 8. Operating conditions for NaBEA25/HMF flow testing ................................. 55

Table 9. Operating conditions for NaBEA25/Fructose/HMF flow testing .................. 57

7

LIST OF FIGURES

Figure 1. Reaction Network for the Dehydration of Fructose to HMF and

Byproducts ............................................................................................... 11

Figure 2. Fructose conversion to HMF, Levulinic acid, and Formic acid16

................. 12

Figure 3. Molecular framework of FAU (a) and BEA (b) zeolites18,19

........................ 17

Figure 4. Photos of the fixed bed column before and after HBEA25/HMF flow test .. 18

Figure 5. Schematic of finalized fixed bed setup ......................................................... 25

Figure 6. Types of mass transfer in a packed bed with porous media.22

1: pore

diffusion, 2: solid diffusion, 3: reaction kinetics, 4: external mass

transfer, 5: fluid mixing ........................................................................... 33

Figure 7. Isotherm for Fructose in 13X Zeolite , 25°C ................................................ 38

Figure 8. Isotherm for HMF in 13X Zeolite , 25°C...................................................... 39

Figure 9. Isotherm for Fructose in H-BEA25, 25°C .................................................... 41

Figure 10. Isotherm for HMF in H-BEA25 Zeolite, 25°C ........................................... 42

Figure 11. Isotherm for Fructose in NaBEA25 Zeolite, 25°C ...................................... 44

Figure 12. Isotherm for HMF in Na-BEA25 Zeolite, 25°C ......................................... 45

Figure 13. Breakthrough Curve for 13X/Fructose system ........................................... 48

Figure 14. Breakthrough Curve for 13X/HMF system ................................................ 50

Figure 15. Breakthrough Curve for HBEA25/Fructose system ................................... 52

Figure 16. Breakthrough Curve for NaBEA25/Fructose system .................................. 54

Figure 17. Breakthrough Curve for NaBEA25/HMF system ....................................... 56

Figure 18. Breakthrough Curve for NaBEA25/Fructose/HMF system ........................ 58

Figure 19. Model of the 13X/Fructose System ............................................................ 60

8

Figure 20. Model of the 13X/HMF System .................................................................. 61

Figure 21. Model of the H-BEA25/Fructose System ................................................... 62

Figure 22. Model of the Na-BEA25/Fructose System ................................................. 63

Figure 23. Model of the Na-BEA25/HMF System ...................................................... 64

Figure 24. Combined single component breakthrough curves for the Na-BEA25

zeolite ...................................................................................................... 65

Figure 25. Fructose/HMF/NaBEA25 multicomponent breakthrough curves .............. 66

Figure 26. Separation Effects of Increasing Column Diameter ................................... 67

Figure 27. Separation Effects of Increasing Bed Length ............................................. 68

Figure 28. Separation Effects of Increasing Volumetric Flow Rate ............................ 69

9

ABSTRACT

The feasibility of a fixed bed separation of HMF and fructose using pelletized

zeolites was investigated for applications related to HMF production via fructose

dehydration.

HMF and fructose isotherms were measured at 25°C using the batch method

for three different zeolites (13X, H-BEA25, and Na-BEA25). 13X showed a strong

affinity for both fructose and HMF. H-BEA25 demonstrated high loading capacities

for HMF and low loading capacities for fructose, while Na-BEA25 showed similar

loading capacities as the H-BEA25.

Single component breakthrough curves showed that Na-BEA25 zeolite was the

clear-cut best choice for a separations process. This zeolite showed a high loading

capacity and selectivity for HMF in both batch and breakthrough studies. While

separation in a multicomponent breakthrough process was not as ideal as expected via

inspection of single component breakthrough curves, separation was still observed. It

is hypothesized that if the length of the packed bed were increased, then the difference

in breakthrough times would be much more pronounced. As a result, the Na-BEA25

zeolite was found to be a good choice for separating fructose and HMF.

10

Chapter 1

INTRODUCTION

1.1 Motivation

As petroleum and other fossil fuels are steadily depleted, the need to

investigate sustainable methods of producing products from renewable resources

becomes of greater significance. The current push to promote sustainable sources of

energy coupled with the easy availability of biomass has made the production of

biofuels and chemicals from non-edible bio-mass a very attractive prospect. 5-

hydroxymethylfurfural (HMF), which is a promising feedstock for the production of

plastics and other chemicals1,2

, can be obtained from the dehydration of fructose

derived from cellulosic biomass. HMF can be used to produce chemicals such as

terepthalic acid, xylene, and various alkanes15

– all of which are currently synthesized

from petroleum feedstocks. These chemicals are commonly used in many commercial

applications. The possibility of creating a scalable HMF production process is a

relatively new idea, and laboratory scale testing must be conducted in order to

determine the feasibility of this process. There are currently a few challenges to

overcome in order to make this process feasible. HMF degradation to levulinic and

formic acids as well as to insoluble humins is an issue that needs to be addressed in

order to maximize its yield. HMF is an intermediate in the reaction of fructose to

levulinic and formic acids. The basic reaction network for this reaction is shown in

Figure 1.

11

Figure 1. Reaction Network for the Dehydration of Fructose to HMF and Byproducts

There have been a number of approaches proposed to remedy this problem.

Another issue is the need to separate HMF from unreacted fructose. Conversion of

fructose to products will not always necessarily reach 100%, especially without the

incorporation of a homogenous catalyst.16

HMF is an intermediate in the fructose

dehydration process, and in order to maximize HMF yield, the reaction is generally

stopped before HMF starts to rehydrate to form formic and levulinic acids. This

creates solutions with a significant amount of unreacted fructose, which must be

separated from HMF. See Figure 2 for a graphical description of HMF production

from fructose. An new method involves using solid HMF adsorbents such as zeolites

or carbons to selectively extract HMF from solution.3,4

This approach may provide a

less energy intensive method of extraction compared to the current method of

extraction, which employs a batch scale biphasic reactor. However, the lack of

experimental data about the effect of temperature and composition on adsorption and

diffusion makes the design and evaluation of such processes difficult.

12

Figure 2. Fructose conversion to HMF, Levulinic acid, and Formic acid16

This study examined the use of a fixed bed column for separating HMF and

fructose. This process first required adsorbent selection and characterization in batch

conditions before studying flow properties. This was completed by measuring the

adsorption isotherms associated with the physisorption of HMF and fructose on

different microporous solid adsorbents. Adsorption in three different types of zeolites

was investigated, specifically 13X, H-BEA-25, and Na-BEA-25. Design heuristics

were used to develop the proper specifications and operating conditions for the

construction and use of the fixed bed. This included an analysis of both equilibrium

and transport properties associated with a continuous flow system. Then a fixed bed

setup was built using pelletized zeolites and used to generate breakthrough curves,

which helped to elucidate the efficacy of the solid adsorbent on each individual

13

component. Breakthrough curves were measured in both single component and

multicomponent systems, and gave information regarding the feasibility of using a

fixed bed to separate HMF and fructose.

1.2 Previous Work

1.2.1 Adsorbents

Past work has been reviewed and used as the theoretical background for some

of this thesis.

The concept of using HMF as a platform chemical for the production of

biofuels and plastics has been well established. There has been extensive

documentation of the dehydration reaction of fructose to synthesize HMF.1 Much of

this work focuses on the synthesis of HMF in batch conditions.

Reactive extraction has been shown to effectively minimize by-product

formation. Fructose dehydration is carried out in the aqueous phase of a biphasic

reactor. HMF is selectively extracted in the organic phase from the aqueous phase, and

is recovered at a later stage by distillation of the organic solvent. This process has

been shown to be effective, but energy intensive and costly.17

Scale-up to large scale

production of this process is also an issue, due to the requirement of batch conditions,

difficulty separating HMF from the solvent, and the unwanted mixing of various

additives in the extracted HMF.

There have been some studies conducted regarding adsorption of HMF from

solution on activated carbon.3 Information regarding the setup and experimental

procedures were drawn from some of these studies. This information pertains mainly

to procedures for measuring batch isotherms of powdered catalysts. Methodology

14

regarding the measurement of batch isotherms of HMF on zeolites was derived from

previous work as well.13

1.2.2 Processes

The main process that was investigated in this thesis was the fixed bed

adsorption. There is a great deal of information in the literature regarding fixed bed

operation. This is a well-established process that is commonly-used in industry. From

a process perspective, this particular thesis focused on the work found in a Master’s

thesis by Butland.8 This thesis had thorough descriptions of a lab-scale fixed-bed

separation setup, which was helpful in the construction of the fixed bed used for flow

testing in this thesis. A patent of the Archer Daniels Midland Company described a

process for using columns and polymeric resins to separate a mixture of HMF and

fructose.20

This patent showed that this particular separation was feasible in a column-

type setup, which provided validation for experimentation. It also lent credence to the

realistic aspects of this study, as the same process was being studied for use in an

industrial setting by a large company. A fixed bed separation process for separating

fructose and HMF from DMSO was performed using carbons as a solid adsorbent,

with some success.21

It follows that this separation should also be possible in aqueous

conditions. In order to effectively determine and use transport properties such as the

mass diffusivity of HMF and fructose in water, Perry’s handbook was consulted.22

This aided in the calculation of dimensionless constants that were necessary for

determining operating parameters (such as the Peclet number) of continuous flow

systems using design heuristics. Information from Perry’s handbook was also used to

determine basic calculations for mass transfer coefficients in both the bulk solution

and within the zeolite pellets. Information on modeling breakthrough curves from a

15

fixed bed was found using texts by Ruthven5, Ingham

9, and Wankat

12. This included

guidance on mass balances for the fixed bed system and how to solve them to model

experimental data.

1.3 Aim and Outline of Thesis

Optimal design specifications for a fixed bed column for separating relevant

components of HMF production via fructose dehydration were determined through a

combination of batch experiments, flow experiments, and modeling techniques. This

design information is scalable and could be potentially used for the design of an

industrial scale process. Chapter 2 addresses the methodology used in this study.

Chapter 3 presents the batch isotherms-results and provides an analysis of the batch

isotherms for each solute/zeolite system. Chapter 4 specifically addresses each packed

bed system and operating parameters, and presents generated breakthrough curves.

Chapter 5 uses modeling to derive information from the flow data gathered from the

fixed bed.

16

Chapter 2

MATERIALS, METHODS, AND COLUMN DESIGN

2.1 Materials

Solutions of HMF and fructose in water were prepared by weighing each

solute using a balance, then mixing in volumetric flasks. D-Fructose and HMF was

purchased from Sigma Aldrich. Deionized water was used as the solvent in each

solution. Concentrations of each solution varied based on the analysis of the batch

isotherms.

Zeolites are microporous aluminosilicate materials which are commonly used

for both catalysis and adsorption. They are available in both powdered and pelletized

forms, based on the application in which they are being used. At the molecular level,

the structures of zeolites are such that they can selectively allow for the entry of

certain molecules into their “cages” via size exclusion or polarity similarities. This

property permits high loading capacities of certain types of molecules in zeolites,

while completely excluding other molecules.

The solid adsorbents used in this study were pelletized 13X (faujisite, FAU)

and H-BEA25 (beta, BEA) zeolites. These zeolites were chosen for this study for a

few different reasons. These zeolites have opposite polarities, a difference which was

hypothesized to provide different adsorption properties with HMF and fructose. 13X

and H-BEA25 zeolites are also widely used industrially. This was important for two

reasons – they were readily available commercially, and are produced in large

volumes for industrial processes. As a result, they were easy to obtain for lab

17

experiments, and should also be readily available for large-scale use. The molecular

framework of each of these zeolites is shown in Figure 3.

13X zeolites are of faujasite structure with a silica-to-alumina ratio of between

2 and 3. The pellets were spheres and had a diameter of about 1 mm. The 13X pellets

were purchased from Sigma Aldrich, the H-BEA25 pellets were purchased from Süd-

Chemie. The H-BEA25 zeolite pellets are of the beta form, with protons as the extra

framework cation. These zeolites have higher silica-to-alumina ratios, usually around

25. The pellets used in this experiment with extruded cylinders 2 mm in length.

a. b.

Figure 3. Molecular framework of FAU (a) and BEA (b) zeolites18,19

After some preliminary flow testing, it was found that the H-BEA25 zeolite acted as a

catalyst instead of an adsorbent when using an HMF-based solvent. This was realized

when the effluent flow rate of the column began to decrease, diminishing in flow until

18

it was only about half of the desired flow rate after 30 minutes. It was observed that

the majority of the HMF was held within the first 3 – 4 cm of the fixed bed after the

experiment was concluded, due to localized biological growth in the column after the

setup was left untouched for a few days. See Figure 4 below.

Figure 4. Photos of the fixed bed column before and after HBEA25/HMF flow test

In order to address this issue, it was proposed that the zeolite protons be exchanged

with Na. The Na ion proved successful with both the fructose and HMF solutions

when using the 13X zeolite, so it follows that Na will be effective in the beta zeolite as

well. Na ions are also generally less reactive than protons, and are less likely to induce

a compositional change in the solute. It was necessary to perform this ion exchange in

the fixed bed. This was completed by flowing a solution of water and NaCl through

19

the column as a primer before actual testing could begin. Approximately 6 g of NaCl

were dissolved in 60 mL of water for this solution. In order to determine that complete

ion exchange had occurred, a pH meter was used to test the pH of the effluent water in

a preliminary study. As expected, the pH initially dropped, indicating an increase in

acidity due to the new prevalence of free hydrogen ions in the solution. This signaled

that the Na ions were indeed exchanging with hydrogen ions. This pH value then

steadied, and rose back to pH values near that of pure water, indicating most of the

hydrogen ions were flushed from the column. The standard flow procedures (as stated

in section 2.6) were followed after this point. New batch isotherms were generated in

order to have a guide to the potential loading capacities of the NaBEA25 zeolite

pellets.

The key piece of equipment used for measuring breakthrough curves was the

fixed bed column. The fixed bed that was used was a glass low-pressure liquid

chromatography column, purchased from Kontes. The variety of column lengths and

diameters that were commercially available was somewhat meager, which caused

some limitations in column design. The column decided upon had a 1.5 cm inner

diameter and was 30 cm long. A flow adapter was also purchased, to allow for

adjustment of the length of the fixed bed given the amount of zeolite desired for each

experiment.

A High Performance Liquid Chromatography (HPLC) pump, made by Alltech,

was used to control the flow of the solutions into the fixed bed.

An HPLC, made by Waters, was used to accurately measure compositions of

solutions. The mobile phase used was a 0.005 M solution of H2SO4. The column used

was an Aminex HPX-87C column. The temperature of the column was 65°C, the

20

temperature of the sample was 25°C. Samples were analyzed with a Refractive Index

(RI) detector.

A refractometer, made by Bausch and Lomb, was used to measure the

composition of single component systems. The refractometer was not as sensitive as

the HPLC, but was deemed adequate when tested against the HPLC for analysis of the

same samples.

2.2 Batch Isotherms

Batch isotherms were measured for each combination of adsorbent and

solution. This process helped to determine a theoretical maximum loading capacity on

the zeolites at given solute concentrations, and helped to guide solution concentrations

for flow testing. The pelletized zeolites were first crushed into a fine powder, allowing

for the greatest possible available surface area for adsorption. This powder was then

placed on petri dishes and calcined for 1 hour at 90°C, then for 8 hours at 450°C.

Approximately 0.2g of this powder was obtained and placed in a 2mL glass gas

chromatograph vial. Solutions of each solute (HMF or fructose) in water were

prepared. Concentrations of the fructose solutions were 10, 20, 50, 100, 150, 200, 250,

300, 400, and 500 g/L. Concentrations of the HMF solutions were 2, 4, 7, 10, 15, 20,

25, 30, 40, and 50 g/L. These concentration ranges were decided based on preliminary

knowledge and testing of the loading capacities of each zeolite. Due to the highly

hygroscopic nature and reactivity of HMF, all weight measurements of HMF were

conducted in a glove box under argon gas. Approximately 1mL of each solution was

mixed with the zeolite powder in each vial. These vials were then placed in a

temperature-controlled mixing bath, using a homemade brace to suspend them in the

water of the bath. The vials were then mixed at 25°C for approximately 24 hours, so as

21

to allow for the equilibrium loading capacity to be reached. The experiment was done

in triplicate for each zeolite/solute combination. A small amount of the liquid in each

vial was obtained by using a syringe tipped with a 25μm filter, so as to obtain some of

the solution without any of the solids. The premise of this procedure was to measure

the new equilibrium concentration of the solution, and by calculating the difference

from the original concentration determine the equilibrium loading on the zeolite. The

obtained solution was diluted by 10% with DI water in preparation for analysis with

HPLC. Solution concentrations were determined in this fashion, and isotherms were

developed using Microsoft Excel.

2.3 Column Design

Before any flow experimentation could be conducted, modeling was done in

order to ensure collected data was valid. Design parameters of the column were

determined given adsorbent and solute specifications. Some of the most important

parameters are summarized below in Table 1.

Table 1. Important Column Design Parameters

Parameter Importance Addressed

Column Diameter Affects flow

properties

Theoretical

calculations based on

zeolite pellet size

Volumetric Flow Rate Affects flow

properties, diffusion

rate of solute

Calculations based on

dimensionless

constants

Column Length Affects flow

properties

Calculations based on

dimensionless

constants

22

2.3.1 Column Diameter Design

In order to determine acceptable column diameters, a literature investigation

was conducted. During the design of fixed bed columns, it was found that one of the

most important heuristics was the column diameter to pellet diameter ratio. If this ratio

is not large enough, there is a potential for channeling to occur. Channeling is a

phenomenon in which the liquid phase will take paths of least resistance in the

column. This causes non-uniform adsorption among the different the adsorbents, as

some adsorbents will saturate quickly and others will only adsorb minimal amounts of

solute. In order to prevent channeling, most literature recommended using a column

diameter to pellet diameter ratio of at least 10:1.5 In a recent paper, it was found that

column diameter to pellet size ratios as low as 7:1 gave acceptable data.6,7

With this in

mind, a column with a diameter of 1.5 cm was purchased from Kontes. This allowed

each experiment to meet this criterion (with zeolite pellet diameters of 1 mm and

approximately 1.75 mm) and still keep the column relatively narrow. This minimized

the need for excessively large solvent volumes during experimentation.

2.3.2 Volumetric Flow Rate Determination

In order to properly set the volumetric flow rate for each experiment, an

analysis of literature correlations was studied. One major pitfall regarding fixed bed

design that was frequently mentioned in the literature was the issue of axial dispersion

in the column. This occurs when the solute in the liquid phase diffuses through the

solvent faster than the solvent flows. In this fixed bed, it was assumed that the system

had plug flow throughout the column. This implies a uniform solute front when in the

column. Major axial dispersion causes this assumption to be invalid. In order to

prevent axial dispersion, it was recommended in the literature that the Peclet number

23

for the system be kept above about 40.14

The Peclet number is a dimensionless group

that measures the rate of solute diffusion versus the flow rate of solvent. It was

calculated using the following equation:

where L is the characteristic length, is the superficial velocity of the fluid, and D is

the mass diffusion coefficient of the solute in the solvent.

2.3.3 Column Length Determination

There were very few actual heuristics available in the literature for modeling

column length. This modeling was done by the researcher in a rudimentary setup,

emulating the proposed real setup. It was found that the column length directly

affected the breakthrough time for the solutes. While this information did not affect

the actual adsorption mechanism, it did help guide the amount of zeolite used in the

system based on desired breakthrough time.

2.4 Preliminary Column Setup

In order to ensure certain specific characteristics of the system were taken into

account when collecting data, preliminary experimentation was conducted. Much of

this consisted of running “dye experiments” in order to visually simulate the flow of a

solute through the fixed bed

First, the flow rate of the HPLC pump was measured to ensure it was accurate.

A volumetric flow rate was set on the pump, and water was run and collected to

guarantee the flow rates were close to the presumed value. This was found to be the

case at all pertinent flow rates.

24

The holdup time in the pump was also measured via dye experimentation. In

order to check this, the column was initially filled with only water. A solution

consisting of water colored with red food dye was then flowed through the system.

The time delay between the start of the flow and the first instant at which dye entered

the column was recorded at various flow rates. These times were used as time delays

in subsequent experiments in order to accurately predict the “start time”, defined as

the time at which the solute entered the column, for each experiment.

Finally, flow in the column was monitored by visual inspection. The column

was packed with pelletized zeolite and the dye solution was flowed through. The

predominant reason to conduct this test was to check for channeling of the dye

solution through the zeolites. It was concluded that while there may not have been the

most ideal plug flow through the column (the “wave” of dye showed some fingering

instead of a perfectly uniform flow) it was concluded that there was little to no

channeling in the system, and that the data being measured was reliable.

2.5 Finalized Fixed Bed Setup

The finalized design of the fixed bed column was comprised of an HPLC pump

and a glass column, connected by Tygon tubing. The solution to be flowed through the

fixed bed was connected with tubing to the bottom of the glass column via a flow

adapter, which allowed for different solid adsorbent bed lengths. The solution was

flowed upwards through the zeolite bed, and the effluent was collected from a tube

attached to the top of the column. The decision to flow from the bottom of the column

upwards versus the opposite was motivated by the desire to minimize channeling,

which could potentially be exacerbated by gravity. This design helps maintain a

uniform solvent front and keep the plug flow assumption valid. This basic setup was

25

used for all of the flow studies, and emulates similar studies as described in other

literature.8 See Figure 5 for a diagram of the setup.

Figure 5. Schematic of finalized fixed bed setup

2.6 Fixed Bed Operation

The full procedure of running a fixed bed adsorption study was typically a

multi-day process. First the pelletized zeolites were calcined in order to change them

their acid forms. This calcination process involved placing the pellets in an uncovered

petri dish and calcining in an oven for 1 hour at 90°C, then for 8 hours at 450°C. After

the calcination was complete, extra zeolite was stored in a desiccator to prevent

moisture absorption before massing. The zeolites used for the current study were then

weighed. Typically about 15g – 20g of pellets were used to pack the column. Before

packing the column, the zeolites were degassed. Bubble formation in the fixed bed

was a recurring issue during experimentation. Early studies found that once a liquid

was flowed through the packed bed of zeolite pellets, air would emerge from the

26

pores, displaced by water. This air then had no ability to escape from the column due

to the close packing of the zeolites. This affected not only likely the adsorption rates

of the zeolites, but it also had an impact on the quality of liquid flow in the column.

This issue was resolved by submerging the pellets in water with occasional mild

agitation for between 2-4 hours before use, in order to completely remove any air

within the pores. Once fully degassed, the zeolites were loaded into the column. Using

a funnel and a spray bottle of DI water, zeolite pellets were slowly added to the fixed

bed. Gentle tapping was employed to ensure the zeolites packed properly as they were

layered into the column. The zeolites were submerged in water as they filled the

column. After packing was complete, the flow adapter was inserted, making the entry

for the liquid at the very beginning of the actual zeolite bed (ensuring no void space).

The flow adapter was permanently attached to the HPLC pump with steel tubing. This

column was then clamped to a ring stand in preparation for experimentation. The

experiment was then initiated by selecting a volumetric flow rate and beginning a flow

of DI water through the column. The water was allowed to run for at least 5-10

minutes to begin the flow process. When ready, a solution of water and either fructose

or HMF was switched in place of the water and a timer was started. After waiting for

the pump delay associated with the chosen flow rate, the timer was restarted and the

true experiment timer began. Ideally, this is when the beginning of the solution would

be first entering the bottom of the column (defined as time=0). Sampling of the

effluent began after this point. Typically samples were collected every 3 minutes.

Collection time was approximately 30 seconds per sample. All flow testing was

conducted at room temperature (~25°C). The adsorption part of the experiment was

concluded at a specified time. Once this time was reached, desorption was tested on

27

the same system. This meant simply switching the feed to the HPLC pump from the

solution back to DI water. Sampling was conducted in the same fashion until a

specified stop time for the experiment.

2.7 Breakthrough Curves

Breakthrough curves are a measurement of the effluent concentration of a fixed

bed reactor. The effluent concentration of a specific component is measured as a

function of time. The feed concentration of said component is noted as it is fed

through the column containing the solid adsorbent. As the solid adsorbent slowly

becomes saturated by the solute, the component will begin to “break through”, or

emerge, from in the column in the effluent. The concentration profile versus time is

recorded and analyzed, giving information about adsorbent/solute interactions. In

order to adequately separate multiple components in a flow system, the breakthrough

times of each of the individual components must be different enough such that

significant adsorption of one component occurs while maintaining reasonable purity

specifications (minimizing adsorption of the other components).

2.8 Breakthrough Curve Modeling

In order to assure that experimental data was reliable, as well as determine

certain experimental characteristics, modeling software was used. The software used

was a Mathematica file designed for use in two-component liquid chromatography

systems. This program was developed by Dr. Housam Binous. The primary source for

the program was Chemical Engineering Dynamics, 3rd

Ed., by Ingham.9

28

The validity of the data generated from the code was verified by cross-referencing the

solving techniques with other literature sources.10,11,12

2.8.1 System Mass Balance

The methods that the program follows start with a basic material balance for each

component:

[ ]

Concentrations in each section are taken to be rough averages of the concentration in a

differential element of the tube where the mass transfer is occurring. Convective mass

flows are expressed as:

(

)

and

(

)

where Q is the constant volumetric flow rate of the mobile phase.

Given some of the solution is entering the nth

element of a differential length of the

tube, ΔZ, the diffusive mass flows are obtained via Fick’s Law:

(

)

(

)

and:

(

)

(

)

29

where is the void fraction in the fixed bed, A is the cross-sectional area of the

column, and D is the diffusion coefficient. The transfer rate of solute from the solvent

liquid to the porous solid is given by:

and

where keff is the effective mass transfer coefficient, ap is the specific area of the

packing medium (assumed to be

for spheres), C

*A,S is the equilibrium

concentration of solute “A” in the porous solid, and CA,S is the current concentration

of solute “A” in the solid, Dp is the particle diameter. C*

A,S is determined by batch

isotherm studies, and is also referred to as the theoretical loading capacity of the

porous solid.

These three major mass flows can be rearranged to create the overall governing

equation for mass flow in a fixed bed column:

(

) (

) (

)

2.8.2 Modeling Input and Assumptions

The Mathematica program solves this equation at small time increments to simulate

flow testing in the fixed bed. Important correlations that are used in this simulation

include:

the diffusion coefficient,

(where d is column diameter, Dp is particle diameter),

the Peclet number,

30

(

)

and the Reynolds number,

where ρ is the liquid density, ν is the superficial liquid velocity, and η is the liquid

viscosity. These correlations are used to help characterize the flow in the system.

Parameters that were input into the model are summarized in Table 2.

Table 2. Input Parameters for Mathematica Model

Parameter Value

Effective Mass Transfer Coefficient (keff) Unknown

Packing Particle Diameter (Dp) 1mm for 13X, 1.5mm for BEA25

Column Diameter (d) 1.5 cm

Void Fraction of Bed (ɛ) Assumed near ideal = 0.38

Liquid Density ( Approximated to be ~1020 kg/m3

Liquid Viscosity ( 1x10-3

Pa*s

Volumetric Flow Rate (Q) 1.5 mL/min

Equilibrium Constant (K) Dependent on Isotherm

Length of Bed (Z) Dependent on mass of zeolite

Some of the parameters in Table 2 were fixed for all cases during modeling. These

included the column diameter, the void fraction of the bed, the liquid density, the

liquid viscosity, and the volumetric flow rate were generally kept the same. The same

column was used for all testing, fixing the column diameter. Ideal packing was

assumed for all systems due to lack of information on determination of the actual

packing of the pellets. The solutions in question were relatively dilute, so the liquid

density was assumed to be slightly higher than that of pure water. There is some

literature data on viscosities of sugar solutions which was used in this model.23

The

viscosity used is nearly identical to that of pure water, so little importance was placed

31

in the actual number used. The volumetric flow rate was kept the same in most

experiments conducted, in order to better be able to compare gathered data.

There were a few other assumptions made for this model. The affinity between

zeolite and adsorbate was determined by the Langmuir constants as determined using

the Langmuir Isotherm equation with batch isotherms.24

The equation is known as:

Where q is the loading capacity of the adsorbent, is the saturation loading capacity

of the adsorbent, and b is the Langmuir equilibrium constant, and c is the equilibrium

concentration of the solute.

This method is useful for isotherms which first show a linear relationship

between concentration and loading capacity, but eventually become saturated. In this

study concentrations for flow testing were nearly always picked from the linear part of

the batch isotherm. If bc<<1, then the Langmuir equation simplifies into Henry’s Law,

which predicts linear isotherms. The equation for Henry’s Law is:

Where q is the loading capacity of the adsorbent, c is the concentration, and K is the

Henry constant. The Henry constant is equivalent to qs*b in the Langmuir equation

when bc<<1. There is a risk of oversimplification when using Henry’s Law, so the

Langmuir approach was employed.

In order to determine the Langmuir equation constants, the Langmuir equation

can be linearized into the following form:

32

Where inverse concentrations can be plotted against inverse loading capacities to give

qs and b.

Another assumption was that the column operated at a constant temperature.

This assumption is likely a good one. While it is true that nearly all adsorption is

exothermic and will give off a tangible amount of heat5, it seemed through physical

inspection that most of this heat was emitted when the pellets were first placed in

water for degassing. Degassing generally lasted a few hours, which was plenty of time

for the system to come to thermal equilibrium. It is likely true that there was a small

increase in temperature as the zeolites adsorbed solutes, but this change was not

noticed via physical inspection.

The system was assumed to have a constant flow front (plug flow) as the

solution passed through the column. The only reasons that this may have not been a

good assumption relate to the packing efficiency of the pellets. Poor packing could

lead to channeling or other wall effects which are undesirable and unpredictable from

a modeling perspective. The proper design steps were taken in order to avoid these

issues.

This model can account for two solutes in a solution. One of the implicit

assumptions made is that the two solutes are non-interacting. The simplicity of this

model does not allow for any sort of interaction parameters. This may be because there

is no efficient “catch-all” way of modeling this with any two solutes. As a result, the

model may have issues modeling multi-component testing.

2.8.3 Diffusion and the Effective Mass Transfer Coefficient

When modeling flow in and around porous media, there are multiple diffusion

considerations that must be made. Given a packed bed with a pelletized adsorbent,

33

there can be up to five different types of mass transfer that must be accounted for. This

includes pore diffusion, solid diffusion, reaction kinetics, external mass transfer, and

fluid mixing.22

See Figure 6 for a schematic of these phenomena. In zeolites, which

are microporous materials, pore diffusion is further sub-divided into macroporous,

mesoporous, and microporous diffusion. This is due to the increasingly smaller

channels that are found within the adsorbent.

Figure 6. Types of mass transfer in a packed bed with porous media.22

1: pore

diffusion, 2: solid diffusion, 3: reaction kinetics, 4: external mass

transfer, 5: fluid mixing

Many of these parameters can be modeled or determined via empirical

correlations or first principles equations. While this may be a more rigorous approach

to modeling mass transport, it is quite time-intensive without the guarantee of useful

models. It was found easier and faster to lump all of these parameters into a factor

34

called the effective mass transfer coefficient. This value accounts for all transport, and

must be experimentally determined. In the model described above, this parameter is

the only unknown in the system. This allowed for its determination by altering it to fit

model curves to experimental data. This approach gives useful information about the

specific system in question, and must be updated for each new set of input parameters.

2.9 List of Symbols

Symbol Description

L Characteristic length (m)

Superficial velocity (m/s)

D Diffusion coefficient (m2/s)

CA Concentration of component A (kg/m3)

Q Volumetric flow rate (m3/s)

ΔZ Differential Length (m)

ε Void Fraction

A Cross-sectional area of column (m2)

keff Effective mass transfer coefficient (m/s)

ap Specific area of packing medium (m2)

Dp Particle diameter (m)

CA,s Concentration of component A in solid (kg/m3)

CA*

,s

Equilibrium concentration of component A in

solid (kg/m3)

d Column diameter (m)

η Viscosity (kg/s*m)

35

Pe Peclet number

Re Reynolds number

36

Chapter 3

BATCH ISOTHERMS

Batch isotherms were measured for single component systems with each type

of zeolite. The most important data that the batch isotherms yield are the theoretical

loading capacities of each zeolite/solute system, and the equilibrium constants of each

system.

3.1 Isotherm Measurement

Batch isotherms were created by measuring the difference in concentration

before and after equilibrium was reached in each zeolite solution. This value was

divided by the mass of zeolite in each solution, giving a value in terms of mass of

adsorbate per mass of adsorbent, as shown in the following equation:

A series of these measurements were made at differing concentrations, giving

basic predictions of the equilibrium loading capacity at any given concentration.

3.2 Zeolite Loading Capacity Determination

The theoretical loading capacities primarily help to guide experimental design,

so as to ensure that breakthrough actually occurs. The theoretical loading capacity is

determined by examining the isotherm at the given solute concentration in the liquid

phase (on the x-axis), and finding the equivalent loading capacity (on the y-axis). The

37

loading capacity is called “theoretical”, as it is the theoretical maximum capacity for

the adsorbent at the given conditions. This is due to the decreased adsorption ability in

the pelletized zeolites due to transport limitations. Some isotherms will show a

“theoretical maximum” loading capacity, at which no more solute can be adsorbed,

even if the solute concentration in the liquid continues to increase.

3.3 Equilibrium Constant Determination

The equilibrium constant is determined by taking the slope of the linear portion

of the isotherm. This gives a relatively constant value that relates the expected solute

equilibrium concentration in the adsorbent at a given liquid solute concentration.

3.4 13X Isotherms

An isotherm was measured for the 13X zeolite and fructose system at 25°C.

The isotherm is shown in Figure 7.

38

Figure 7. Isotherm for Fructose in 13X Zeolite , 25°C

Based on the above isotherm, it is observed that the saturation loading capacity

for the 13X/Fructose system is approximately 80 mg fructose per gram 13X. This

value is calculated to be higher using the Langmuir equation. The Langmuir

equilibrium constant for this system was roughly 1.35e-3 L/g. The saturation loading

cannot be calculated due to the linear nature of the isotherm.

The isotherm for the 13X zeolite and HMF system at 25°C is shown below in

Figure 8.

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500

Load

ing,

q (

mg

Fru

c/gZ

)

Equilibrium Concentration (g/L)

39

Figure 8. Isotherm for HMF in 13X Zeolite , 25°C

The above isotherm shows that the theoretical loading for the 13X/HMF

system is quite low. The isotherm appears to reach the saturation loading capacity at

only about 6.5 mg HMF per gram 13X. The error bars in this system are quite

significant due to the relatively small adsorption rates. Small errors in starting HMF

concentration or zeolite mass has an amplified effect due to HMF’s low affinity to

adsorb to 13X zeolite. The Langmuir equation predicts the Langmuir equilibrium

constant to be 7.13e-2 and the saturation loading capacity to be 3.63 mg HMF/ g 13X.

0

2

4

6

8

10

12

0 10 20 30 40 50

Load

ing,

q (

mg

HM

F/gZ

)

Equilibrium Concentration (g/L)

40

3.4.1 Discussion

It is evident that fructose has a much higher affinity (determined by loading

capacity) for the 13X zeolite. This is likely due to the hydrophilicity of the 13X

zeolite. The Si/Al ratio in 13X zeolite is no greater than 3, indicating that water-loving

species are likely to have stronger temporary bonding to 13X than hydrophobic

species. This makes 13X a potential candidate for a separations system in which

fructose is extracted into the solid adsorbent, and HMF is allowed to flow through the

column.

3.5 HBEA25 Isotherms

An isotherm was measured for the HBEA25/fructose system at 25°C. This

isotherm is shown in Figure 9.

41

Figure 9. Isotherm for Fructose in H-BEA25, 25°C

The isotherm for the H-BEA25/fructose system shows a nearly negligible

loading capacity for fructose on the zeolite. Most measurements show a loading

capacity of no more than 0.4 mg fructose per gram zeolite. There is not a reliable slope

from which to draw conclusions about the equilibrium constant of the system. The

Langmuir equation predicts the Langmuir equilibrium constant to be 3.05e-2 and the

saturation loading capacity to be about 0.48 mg Fructose / g H-BEA25.

The isotherm for the H-BEA25/HMF system, measured at 25°C, is shown in

Figure 10.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 50 100 150 200 250 300 350 400 450 500

Load

ing,

q (

mg

Fru

c/gZ

)

Equilibrium Concentration (g/L)

42

Figure 10. Isotherm for HMF in H-BEA25 Zeolite, 25°C

This data in this isotherm are quite well defined. The H-BEA25 zeolite shows

a relatively high saturation loading capacity of HMF, nearly 90 mg HMF per gram

zeolite. The steep slope of the initial part of the isotherm indicates a high affinity

between the zeolite and HMF – the saturation concentration is reached quickly, even

at relatively low HMF concentrations. The Langmuir equation shows the Langmuir

equilibrium constant to be 3.88 and the saturation loading capacity to be 76 mg HMF /

g H-BEA25. The Langmuir equilibrium coefficient in this system is much higher than

those in other systems.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0 5 10 15 20 25 30 35

Load

ing,

q (

mg

HM

F/gZ

)

Equilibrium Concentration (g/L)

43

3.5.1 Discussion

The HBEA25 zeolite shows opposite effects from those demonstrated in the

13X zeolite. In this case, HMF is readily adsorbed into the microporous adsorbent,

whereas fructose showed loading capacities near zero. This is most likely due to the

high Si/Al ratio of HBEA25. The “25” designation is an indication of a Si/Al ratio for

this zeolite. Higher Si/Al ratios are typically correlated with hydrophobicity, which

explains the strong affinity between HMF and HBEA25. This makes HBEA25 a good

candidate for a separations process in which HMF is extracted into the solid adsorbent,

while fructose is allowed to flow through the column.

3.6 NaBEA25 Isotherms

An isotherm was measured for the NaBEA25 zeolite and fructose system at

25°C. The isotherm is shown in Figure 11.

44

Figure 11. Isotherm for Fructose in NaBEA25 Zeolite, 25°C

Based on the above isotherm, it is observed that the saturation loading capacity

for the Na-BEA25/Fructose system is approximately 0.2 mg fructose per gram Na-

BEA25. This value can be considered negligible, as it is observed that there is a stable

loading capacity at any given equilibrium concentration. The Langmuir equation

shows the Langmuir equilibrium constant to be 1.43e -1 and the saturation loading

capacity to be 0.23 mg Fructose / g Na-BEA25.

The isotherm for the Na-BEA25/HMF system, measured at 25°C, is shown in

Figure 12.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 50 100 150 200 250 300 350 400 450 500

Load

ing,

q (

mg

HM

F/gZ

)

Equilibrium Concentration (g/L)

45

Figure 12. Isotherm for HMF in Na-BEA25 Zeolite, 25°C

The above isotherm does not show a saturation loading capacity. The

concentrations of HMF being considered do not saturate the adsorbent. The Langmuir

equation shows the Langmuir equilibrium constant to be 9.73e-4. The saturation

loading capacity cannot be calculated due to the linear nature of the isotherm.

3.6.1 Discussion

The Na-BEA25 zeolite shows similar adsorption characteristics as the H-

BEA25 zeolite. The key difference is found in the HMF study: instead of reacting with

the free proton in the H-BEA25 zeolite, the HMF readily adsorbs to the adsorbent.

This gives the remarkably high loading capacities shown in Figure 12. This form of

the faujasite zeolite is much more likely to be useful in a flow setup.

0

50

100

150

200

250

300

0 10 20 30 40 50

Load

ing

Cap

acit

y (m

g H

MF/

gZ)

Equilibrium Concentration (g/L)

46

3.7 Summary of Isotherms and Langmuir Constants

Table 3. Langmuir constants for various isotherms

System "b" term (L/g)

Saturation Loading Capacity (mg Solute/g Z)

13X/Fruc 1.36E-03 -

13X/HMF 7.13E-02 3.63E+00

HBEA/Fruc 3.05E-02 4.87E-01

HBEA/HMF 3.94E+00 7.57E+01

NaBEA/Fruc 1.43E-01 2.26E-01

NaBEA/HMF 1.07E-03 -

47

Chapter 4

BREAKTHROUGH CURVES

4.1 Flow Experiment Setup

Flow experiments were conducted using information from batch isotherms. For

each experiment specified parameters included mass of zeolite, length of fixed bed,

feed concentration of solution, and flow rate of solution. Each of these parameters had

a significant impact on the shape of the generated breakthrough curve. They are noted

with each breakthrough curve. Concentrations (the y-axis) of the breakthrough curves

are presented in the form of C/C0. This is a ratio of the actual measured concentration

divided by the feed concentration of the solution being run through the column. This

convention is used for clarity and for the ability to easily compare curves of different

input concentrations.

4.2 Single Component Flow Testing

4.2.1 13X Flow Testing

Flow testing was conducted using 13X pelletized zeolite. The operating

conditions for the 13X/fructose system are shown in Table 4. The breakthrough curve

for the 13X/fructose system is shown below in Figure 13.

Table 4. Operating conditions for 13X/Fructose flow testing

Parameter Value

48

Mass of Zeolite (g) 19.055

Length of fixed bed (cm) 16

Flow rate (mL/min) 1.5

Feed Concentration (g/L) 50.15

Experiment Time 30 min fructose + 30 min water = 60 minutes

Figure 13. Breakthrough Curve for 13X/Fructose system

Note that both adsorption and desorption are shown in Figure 13. A

water/fructose solution was flowed for 30 minutes, and then only water was flowed for

30 minutes. Breakthrough occurs between minutes 9 and 12, and the effluent reaches a

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 500 1000 1500 2000 2500 3000 3500 4000

C/C

o

Time (sec)

49

concentration that is approximately 95% of the feed concentration. The 95%

concentration mark is considered complete breakthrough, the zeolite is considered

effectively saturated.

Flow testing was also conducted using the 13X zeolite and HMF. The

operating conditions for the 13X/HMF system are shown in Table 5. The breakthrough

curve for the 13X/HMF system is shown below in Figure 14.

Table 5. Operating conditions for 13X/HMF flow testing

Parameter Value

Mass of Zeolite (g) 19.358

Length of fixed bed (cm) 16

Flow rate (mL/min) 1.5

Feed Concentration (g/L) 20.01

Experiment Time 30 min fructose + 30 min water = 60 minutes

50

Figure 14. Breakthrough Curve for 13X/HMF system

This breakthrough curve shows a similar breakthrough time compared to the

13X/fructose system. It can be noted that the zeolite becomes completely saturated in

this particular experiment. This is likely due to the relatively low adsorption rate of

HMF on 13X. It can be noted that when comparing the two 13X breakthrough curves,

desorption of the HMF is more effective than desorption of the fructose, given very

similar operating conditions. This is also likely due to weaker bonding between 13X

and HMF.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 500 1000 1500 2000 2500 3000 3500 4000

C/C

o

Time (sec)

51

4.2.2 HBEA25 Flow Testing

Similar testing was conducted using the HBEA25 pelletized zeolite. The

operating conditions for the HBEA25/fructose system are shown in Table 6. The

breakthrough curve for the HBEA25/fructose system is shown below in Figure 15.

Table 6. Operating conditions for HBEA25/Fructose flow testing

Parameter Value

Mass of Zeolite (g) 19.794

Length of fixed bed (cm) 20

Flow rate (mL/min) 1.5

Feed Concentration (g/L) 100.64

Experiment Time 30 min fructose + 30 min water = 60 minutes

52

Figure 15. Breakthrough Curve for HBEA25/Fructose system

It can be seen that the HBEA25/fructose system breaks through between 12

and 15 minutes after flow was started. This breakthrough time is long than the

breakthrough times observed in the 13X testing due to an increased column length.

Similar masses of zeolite were used in both the 13X and HBEA25 testing, but the 13X

zeolite is both denser and has the ability to pack more efficiently due to it’s spherical

shape. This yields a shorter column. It can also be observed that the effluent

concentration of fructose does not reach the 95% saturation mark in this experiment.

This is likely due to set time constraints on the test – after 30 minutes, the flow was

switched to water only, potentially not allowing the HBEA25 to become completely

saturated. Based on the batch isotherm for this system, there should be nearly no

adsorption of the fructose on the HBEA25 pellets. It can be assumed that this curve is

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 500 1000 1500 2000 2500 3000 3500 4000

C/C

o

Time (sec)

53

a good representation of how the fructose solution flows through the column, as very

little fructose should be held up by the pores of the zeolite pellets.

Flow testing was also conducted on the HBEA25/HMF system. While this test

was being run, it became evident that there was something wrong with the flow of the

column. It was soon realized that instead of adsorbing onto the surface of HBEA25,

the HMF was reacting (bonding strongly) to the zeolite. During the experiment the

HMF did not move past the first few centimeters of the column, quickly clogging it

and causing the flow to decrease precipitously. Refer to Figure 1 for a picture of the

setup in question.

As a result, no breakthrough data was collected for this system. This did, however,

lead to the idea of ion-exchanging the hydrogen ion in the HBEA25 for a sodium ion.

The goal of this solution was to substitute for a less reactive ion, while still keeping

the same basic zeolite structure and support. This experiment motivated the

investigation of the NaBEA25 flow testing.

4.2.3 NaBEA25 Flow Testing

The HBEA25 zeolite was ion-exchanged to form NaBEA25 by flowing a NaCl

solution through the packed bed. The pH of the system was monitored periodically to

assess the degree to which ion-exchange had occurred. The pH initially dropped as the

effluent was flooded with hydrogen ions leaving the zeolite, then rose and steadied as

the bed was fully exchanged. After this was completed, similar procedures were taken

to measure breakthrough curves for the fructose and HMF single-component systems.

The operating conditions for the NaBEA25/fructose system are shown in Table 7. The

breakthrough curve for the HBEA25/fructose system is shown below in Figure 16.

54

Table 7. Operating conditions for NaBEA25/Fructose flow testing

Parameter Value

Mass of Zeolite (g) 19.467

Length of fixed bed (cm) 20

Flow rate (mL/min) 1.5

Feed Concentration (g/L) 105.28

Experiment Time 30 minutes fructose

Figure 16. Breakthrough Curve for NaBEA25/Fructose system

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 500 1000 1500 2000 2500

C/C

o

Time (sec)

55

It can be seen that the NaBEA25/Fructose breakthrough curve is similar in

shape and breakthrough time compared to the HBEA25/Fructose breakthrough curve.

This is expected, as the only factor changed in these two tests is the ion with which the

solute is interacting. The zeolite does not act as a good adsorbent for fructose, which

based on batch testing showed a very low loading capacity and poor affinity for

fructose.

Flow testing was also conducted using the NaBea25 zeolite and HMF. The

operating conditions for the 13X/HMF system are shown in Table 8. The breakthrough

curve for the 13X/HMF system is shown below in Figure 17.

Table 8. Operating conditions for NaBEA25/HMF flow testing

Parameter Value

Mass of Zeolite (g) 19.467

Length of fixed bed (cm) 20

Flow rate (mL/min) 1.5

Feed Concentration (g/L) 31.47

Experiment Time 63 minutes HMF

56

Figure 17. Breakthrough Curve for NaBEA25/HMF system

Figure 17 shows that the NaBEA25/HMF system works in a flow setup.

Instead of reacting and plugging the column, which happened when using the

HBEA25 zeolite, the introduction of sodium ions provides a means for adsorption of

HMF in the zeolite pellets. The breakthrough time of this system is at nearly 1500

seconds, which is significantly later than that of the fructose when using this system.

This result shows a potential for successful separation of the two components in a

multicomponent system.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

C/C

o

Time (sec)

57

4.3 Multicomponent Flow Testing

4.3.1 NaBEA25 Flow Testing

Based on promising single component results with the NaBEA25 zeolite,

multicomponent testing was conducted. This involved using the same process

conditions that were used in the single component tests, in order to provide an

adequate model with which to compare solute interactions. The experimental

conditions for this process are shown in Table 9. The breakthrough curve for the

NaBEA25/Fructose/HMF system is shown below in Figure 18.

Table 9. Operating conditions for NaBEA25/Fructose/HMF flow testing

Parameter Value

Mass of Zeolite (g) 19.467

Length of fixed bed (cm) 20

Flow rate (mL/min) 1.5

Feed Concentration Fructose (g/L) 99.86

Feed Concentration HMF (g/L) 29.11

Experiment Time 60 minutes Fruc/HMF + 39 min water = 99 min

58

Figure 18. Breakthrough Curve for NaBEA25/Fructose/HMF system

The above breakthrough curves show what happens in a multicomponent

system of Fructose and HMF. There are a few things to note in this graph. Perhaps the

most important feature is the loss of difference in breakthrough times of the two

components. The difference in breakthrough times in the above graph is no more than

3 minutes, whereas when comparing the two single component systems, the difference

is 6 to 9 minutes. This drastically changes the feasibility of this separations process at

the industrial level. While fructose does still breakthrough first, it may not elute fast

enough to give a reasonable separation. This indicates that there may be some sort of

intermolecular interaction between HMF and Fructose that cause HMF to elute faster

than expected. This interaction should be investigated further. It should also be noted

that the fixed bed could not be completely saturated by either component.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000 4000 5000 6000

C/C

o

Time (sec)

Fructose

HMF

59

Chapter 5

BREAKTHROUGH CURVE MODELING

In order to obtain quantifiable information from the generated breakthrough

curves, it was necessary to fit them to models. This was completed using Mathematica

code, the process of which is described in detail in Chapter 2, section 9. It was

determined that all input parameters for the model used in the Mathematica code were

either known or could be assumed, save the effective mass transfer coefficient. This

data was entered and curves were “fit” to experimental data by manipulating the

effective mass transfer coefficient in order to get the proper shape. Models were

overlaid on experimental data to show goodness-of-fit.

5.1 13X Breakthrough Curves

Single component breakthrough curves were measured for the 13X /Fructose

and 13X/HMF systems. The 13X/Fructose system is shown in Figure 19.

60

Figure 19. Model of the 13X/Fructose System

It can be seen from the graph that the model fits the experimental data quite

well. The effective mass transfer coefficient was determined to be 5.5e-7 m/s.

The model for the 13X/HMF system is shown in Figure 20.

61

Figure 20. Model of the 13X/HMF System

This model does not fit quite as well as the model in the 13X/Fructose system,

but the general shape of the curve and predicted breakthrough time fits the data

reasonably well. The effective mass coefficient for this system is approximately 6.5e-7

m/s. It appears that the two breakthrough curves (13X/Fructose and 13X/HMF) do not

show very much separation of breakthrough times. This is most likely because the

Langmuir equilibrium constant in both cases is quite small, simplifying the isotherm

equation to Henry’s Law. Calculating Henry constants show that the Henry constants

for each of these systems were quite similar. This is generally an indication of affinity

to an adsorbent, and would theoretically yield similar breakthrough times. As a result,

this zeolite would likely not be a good candidate for a separations process.

62

5.2 H-BEA25 Breakthrough Curves

Single component breakthrough curves were measured for the H-BEA25

/Fructose and H-BEA25/HMF systems. The H-BEA25/Fructose system is shown in

Figure 21.

Figure 21. Model of the H-BEA25/Fructose System

It can be seen from the graph that the model only fits the experimental data

near the bottom of the breakthrough curve. The long tail observed could be due to

better than expected mass transfer to the microporous channels in the zeolite pellets.

The effective mass transfer coefficient was determined to be 5.0e-7 m/s.

The H-BEA25/HMF system was attempted to be measured, with little success.

The column became plugged due to a suspected reaction with the zeolite. The pellets

63

acted as a catalyst as opposed to an adsorbent. This finding prompted an investigation

into the Na-BEA25 zeolite.

The H-BEA25 zeolite would most likely not make a good candidate for a

separations process due to the issue of the column plugging from reaction with HMF.

5.3 Na-BEA25 Breakthrough Curves

Single component breakthrough curves were measured for the Na-BEA25

/Fructose and Na-BEA25/HMF systems. The Na-BEA25/Fructose system is shown in

Figure 22.

Figure 22. Model of the Na-BEA25/Fructose System

It can be seen from the graph that the model fits the experimental data fairly

well. The effective mass transfer coefficient was determined to be 5.5e-7 m/s.

64

The model for the 13X/HMF system is shown in Figure 23.

Figure 23. Model of the Na-BEA25/HMF System

The model for this system fits fairly well. The effective mass transfer

coefficient for the Na-BEA25/HMF system was determined to be 6.5e-7 m/s. It

appears that the two breakthrough curves (NaBEA25/Fructose and NaBEA25/HMF)

show reasonably good separation of breakthrough times. This is observation is

corroborated by calculating the Henry constants for the two systems and seeing that

they are somewhat different (Henry constant for HMF is greater by an order or

magnitude). This is generally an indication of affinity to an adsorbent, and would

theoretically yield different breakthrough times. As a result, this zeolite has the

potential be a good candidate for a separations process.

65

5.4 Multicomponent Breakthrough Curves

Based on an assessment of data from the single component breakthrough

curves, multicomponent breakthrough curves were tested. The zeolite used was Na-

BEA25, due to its promising performance in the single component tests. See Figure 24

for the combined single component breakthrough curves using Na-BEA25.

Figure 24. Combined single component breakthrough curves for the Na-BEA25

zeolite

There seems to be a reasonable difference in breakthrough times for the two

systems. This is promising from a separations perspective.

Multicomponent testing yielded the breakthrough curves shown in Figure 25.

66

Figure 25. Fructose/HMF/NaBEA25 multicomponent breakthrough curves

It can be seen from the figure that the difference in breakthrough times that

existed when the two single component systems were overlaid has been somewhat

diminished. This may be because there are some sort of interactions between fructose

and HMF that are unaccounted for. The model assumes that the two components do

not interact in any way, which based on this data may not be a good assumption. This

decrease in difference of breakthrough times lessens the feasibility of this process.

Further investigation of this zeolite may still be worthwhile, only with different

column operating conditions.

5.5 Modeling Improvements

In order to obtain better data, a few theories were pursued using the modeling

software. These theories were aimed at getting better separation between the two

Fructose

HMF

67

components by altering operating parameters. The changes in operating parameters

that were investigated included increasing the column diameter, increasing the fixed

bed length, and increasing the liquid flow rate. In each of these scenarios, all other

operating parameters were the same as in experimental testing. Simulations assume

the Na-BEA25 pelletized zeolite is being used as the adsorbent.

5.5.1 Increase Column Diameter

The column diameter was simulated to be doubled in order see the effect on a

multicomponent separation. In order to keep flow characteristics the same, the flow

rate was increased to keep the Reynolds number and the Peclet number similar to the

experimental testing. The results are shown in Figure 26.

Figure 26. Separation Effects of Increasing Column Diameter

It can be seen that there is a noticeable increase in the difference in

breakthrough times when the column diameter is doubled. This is not too surprising,

68

as there is a greater amount of adsorbent to which HMF can adsorb, while the fructose

will likely just pass through the column in the bulk of the liquid.

5.5.2 Increase Bed Length

The fixed bed length was increased to be greater than what is was in

experimental trials. The length was increased from 20 cm to 2 m. The reasoning

behind this was to simulate the maximum length that could feasibly be achieved in a

laboratory setting. The results are shown in Figure 27.

Figure 27. Separation Effects of Increasing Bed Length

It is quite evident that the relative breakthrough times of these two components

change drastically when the bed length is increased substantially. This makes sense, as

there is a greater amount of adsorbent on which the HMF can adsorb, while the

fructose will simply pass through the column.

69

5.5.3 Increase Flow Rate

The volumetric flow rate was doubled in a simulation of breakthrough curves.

The flow rate was increased from 1.5 mL/min to 3 mL/min. The results are shown in

Figure 28.

Figure 28. Separation Effects of Increasing Volumetric Flow Rate

It can be seen that increasing the flow rate actually decreases the difference in

breakthrough times. This makes sense, as there is less time for the solute to diffuse

into the pores of the adsorbent, most of it stays in the bulk flow. This minimizes the

efficacy of the adsorbent. A longer residence time would likely be favorable to

adsorption.

70

Chapter 6

CONCLUSIONS AND RECOMMENDATIONS

After analyzing both the batch isotherm data and corresponding breakthrough

curves, a few conclusions were made. The Na-BEA25 zeolite was the clear-cut choice

for a separations process. This zeolite showed a high loading capacity and selectivity

for HMF in both batch and breakthrough studies. This was coupled with a low

selectivity for fructose. While separation in a multicomponent breakthrough process

was not as ideal as expected via inspection of single component breakthrough curves,

separation was still observed. It is theorized that if the length of the packed bed were

increased, the difference in breakthrough times would be much more pronounced. The

Na-BEA25 zeolite was found to be a good choice for separating fructose and HMF.

The H-BEA25 zeolite showed some promising results as an adsorbent until

issues arose during the breakthrough experiments. The column became plugged during

HMF testing, likely due to unexpected reactions on the surface of the zeolite, which

inhibited proper flow. As a result, this zeolite would not be a good choice for this

separations process.

The 13X zeolite did not adequately separate fructose and HMF. Breakthrough

times were too similar. This evidence is corroborated by nearly identical Henry

constants. This zeolite would not be a good choice for a separations process of

fructose and HMF.

71

Chapter 7

FUTURE WORK

There are various possible extensions to this research. From an adsorbent

perspective, different types of zeolites could be investigated to develop better trends

explaining the reasons for adsorption of the relevant components of the system. There

is not a very wide selection of commercially available pelletized zeolites, so it is

possible that homemade pellets could be effective in this type of system. Based on

literature results, it seems that using activated carbon as an adsorbent may be another

avenue of research.

From a setup perspective, it may be worth conducting the flow testing at

different temperatures to study thermal effects on adsorption and desorption. This is

generally found to change the loading capacity of most adsorbents, so it may be worth

pursuing in order to improve separation of the components under investigation.

It would also be valuable to conduct similar flow testing on other major

components of the HMF production process. Some components of interest include

levulinic acid and formic acid – two compounds formed from the rehydration of HMF

during the production process via fructose dehydration. Data on the feasibility of

separating these products from both HMF and fructose could give a more complete

picture of the entire separations process as it would likely occur at the industrial scale.

72

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