University of Groningen

Intermediate to long term optimization of dead-end ultrafiltrationZondervan, Edwin

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Chapter 1

Introduction

In this chapter a brief, historic overview of the developments in water purifi-cation is given. Starting in the far past, crossing recent history and endingup with current technologies. Although water purification by means of mem-branes has become a mature industrial separation technique, there is stillscope for improvement. Particularly in the field of long term process model-ing, optimization and control, challenges exist. With water purification andmembrane technology in perspective, the framework in which this researchis conducted can be stated as follows: optimal operation of an ultra filtra-tion membrane purifying surface water. The hierarchical approach to thisproblem will be briefly introduced in this chapter.

1.1 Background

Purification of water has been an issue since biblical times, when Moses casta tree into a salt lake, and water became sweet [1]. Other sources, likeinscriptions found on the wall of the tomb of Amenhotep II from Thebessome 4000 years old (see figure 1.1), give an exact layout of ancient waterpurification tools such as an Egyptian clarifying device. The ancient Egyp-tian operators allowed impurities to settle out of the liquid, siphoned off theclarified fluid using wick siphons and, finally, stored it for later use.

In ancient Greece (around 500 B.C.) water purification was done with a”Hippocratic sleeve”, named after Hippocrates, known as the father of medi-cine, who lived from 460-354 B.C. He wrote the first treatise on public

1

2 CHAPTER 1. INTRODUCTION

Figure 1.1: Ancient Egyptian clarifying device pictured on the wall of thetomb of Amenhotep II at Thebes. The inscription was carved in 1450B.C. [2].

hygiene. In ”Air, Water and Places”, Hippocrates noted that water differedin quality, such as in taste and weight. While his main concern was withfinding the most healthy source of water, he did mention how water couldbe purified, using the ”Hippocratic sleeve,” which was a cloth bag. Heinstructed his followers to strain rain water after it had been boiled. Withoutthis treatment, Hippocrates warned that the water would have a bad smelland cause hoarseness.

Figure 1.2: Jean Antoine Nollet (1700-1770).

Europe of the 17th century, where Science was ’reborn’, allowed scientistslike Sir Francis Bacon (1627) and the Italian physician Lucas Antonius Por-tius (1685) to experiment with sea water and sand filters. However, waterpurification techniques advanced more rapidly in recent times.

The first recorded study on membranes was in the 18th century by FriarJean Antoine Nollet [3], using a pig bladder to separate alcohol from waterby osmosis. Better understanding of osmosis and diffusion came around ahundred years later with the works of Thomas Graham [4], Adolph Fick [5]and Jacobus Hendricus van ’t Hoff [6]. The first artificial membrane wasproduced by the German Scientist Moritz Traube [7], but it took until the

1.2. MEMBRANE SYSTEMS 3

beginning of the twentieth century before the first nitrocellulose ultra filterwas made by the Austrian Nobel prize laureate, Richard Zsigmondy [8]. Inthe 1950’s the first commercial reverse osmosis membranes were producedand from that moment on, developments in membrane technology took agiant leap.

1.2 Membrane systems

Nowadays membranes are produced and used extensively around the world.The estimated annual sales of membranes, world wide, are around 4 billionUS$ and more then 15 billion US$ is spent, annually, on complete membranesystems. In 2001 the average market growth was estimated at 8 to 10% [9].

Some of the largest users in the membranes world are the pharmaceutical-and (bio) medical industry, but other fields, such as the water purification in-dustry, the food industry and the chemical industry also benefit increasinglyfrom membrane technology.

The wide range of separation applications for membranes is due to the factthat membranes can be manufactured in many different ways (See figures1.3 and 1.4) with diversity in:

• Materials (polymers, ceramics, glass, metals, liquids)

• Structure (symmetric, asymmetric)

• Pore size distribution (reverse-osmosis, microfiltration, ultrafiltration,nanofiltration)

• Geometries (hollow fiber, flat plate, spiral wound)

4 CHAPTER 1. INTRODUCTION

Figure 1.3: Schematic illustrating the various materials and structures oftechnically relevant synthetic membranes,Ullmann’s Encyclopedia of Indus-trial Chemistry Published by Wiley-VCH Verlag GmbH & Co. KGaA.

1.2. MEMBRANE SYSTEMS 5

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Figure 1.4: Membrane filtration spectrum, Pore size distribution in relationto separation capacities [11].

6 CHAPTER 1. INTRODUCTION

In principle membranes can be operated in two different ways, as shownin figure 1.5. The simplest operating mode is the dead-end operation. Inthis case, all feed is forced through the membrane, resulting in a build-upof fouling, which results in a loss of performance. In dead-end membranefiltration, frequent cleaning is a necessity. For many industrial applications,the cross-flow-mode is preferred, because the membranes foul less in com-parison to the dead-end mode. In cross-flow mode the feed flows parallelto the membrane surface with the inlet stream entering the membrane at acertain position. The feed composition changes in the membrane, creatingtwo separate streams: a permeate and a retentate stream.

Feed

Permeate

Feed Retentate

Permeate

Figure 1.5: Different operating modes. Left: Dead-end mode. Right: Cross-flow mode.

The main principle of membrane separation is determined by the drivingforce that acts upon the individual components in the membrane and bythe friction which the components must overcome when moving through themembrane matrix. The driving forces acting on the components of a systemcan be expressed as mechanical, electrochemical and/or hydrostatic pressuregradients causing transport. The friction that must be overcome by the dri-ving force to achieve the transport is generally expressed by a hydrodynamicpermeability coefficient, a diffusion coefficient, or, in electrolyte solutions,also by an electrical resistance. To describe the mass transport through amembrane, the thermodynamic (i.e., driving forces) and kinetic parameters(i.e., the friction coefficients) must be mathematically related. In its mostgeneral form, mass transport through a membrane can be expressed as:

J = −PidXi

dz(1.1)

where J is the flux and Pi a general permeability coefficient. The gradientdXi/dz represents the driving force, and the subscript i refers to a com-ponent. Depending on the dominant driving force, the equation results in

1.3. SCOPE OF THE THESIS 7

the well known Hagen-Poiseuille law for mechanical force, in Fick’s law forchemical potential and in Ohm’s law for an electrical potential. A more com-plete theory on mass transfer in membrane processes can be found, amongstothers, in Ullmann’s encyclopedia of industrial chemistry [10].

Figure 1.6: Principle of UF. During filtration feed water is pressed throughthe membrane, separation takes place based on particle size, Ullmann’s En-cyclopedia of Industrial Chemistry Published by Wiley-VCH Verlag GmbH& Co. KGaA

1.3 Scope of the thesis

1.3.1 Some figures on water

Water is essential for life, yet many millions of people around the worldface water shortages and a daily struggle to secure safe water for their ba-sic needs. Millions of children continue to die every year from preventablewater-borne diseases wrote the UN secretary general Kofi Annan in March2005, in the opening booklet of the UN water program ”Water for Life, aDecade [2005 - 2015]”. In the last one hundred years, the world water con-sumption increased 6 times. In the U.S.A. the average water requirementis 400 liters/capita/day and in Europe the average consumption is around130 liters/capita/day. 20 liters/capita/day are considered to be sufficient forpeople in developing countries. However, 18% of the world population doesnot have access to safe drinking water, and the percentage of people who donot have access to water for sanitary use is even larger. The United NationsMillennium Declaration emphasizes the need for all countries to stop theunsustainable exploitation of water resources.

8 CHAPTER 1. INTRODUCTION

Surface water treatment

Ground water treatment Sand filter backwash water

recycling

Municipal & industrial waste water recycling

WWTP UF RO

UF RO

UF RO

Figure 1.7: Application of UF membrane technology in water purification.RO: Reverse osmosis, WWTP: Waste water treatment plant

1.3.2 Application area: Surface water

This research project focuses on the purification of surface water by meansof ultrafiltration. The quality of surface water is influenced by seasonalchanges, surface water contains natural organic (humic- and fulvic sub-stances), inorganic (multivalent metal ions) and microbial components (bac-teria, viruses, etc.) that can be removed by means of ultrafiltration. Ultrafil-tration membranes have a high selectivity, are easy to scale-up and becameeconomically attractive in the last 15 years as surface water purificationtechnique. Because ultrafiltration membranes are subject to fouling, fre-quent cleaning is required. Short term cleaning is achieved by means ofbackwashing, where permeate is flushed back through the membrane in or-der to restore performance. In the long term, the membrane is normallytreated by means of chemicals. Currently, installations are operated basedon pilot plant studies and rules of thumb, resulting in conservative operatingsettings. It is expected, however, that operating costs can be significantlyreduced by process optimization and improved control.

1.3. SCOPE OF THE THESIS 9

Figure 1.8: Scanning Electron Microscope footage of a fouled membranefiber.

10 CHAPTER 1. INTRODUCTION

1.3.3 Membrane type: Ultrafiltration membrane

Ultrafiltration membranes can be used for the treatment of surface water,ground water or industrial wastewater (See figure 1.7). In general, ultrafil-tration membranes are made of polymers such as PES (poly ether sulfone)or PA (Poly amide). Ultrafiltration membranes are porous; however, ingeneral the structure is more asymmetric in comparison to microfiltrationmembranes. Typical pore diameters in the dense layer of an ultrafiltrationmembrane are in the range of 10 to 100 nm. Most commonly used arethe hollow fiber membrane modules, where feed water can be filtered from’inside-to-out’ or from ’outside-to-in’.

B

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Time

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Figure 1.9: Schematic representation of the fouling state of a membrane as afunction of operating time, where F is a filtration phase, B is a backwashingphase, FC is a filtration cycle, (a sequence of one filtration and one back-wash) C is a chemical cleaning phase, CC is the chemical cleaning cycle (asequence of n filtrations and one chemical cleaning), MLTC is the membranelifetime cycle (a sequence of N chemical cleaning cycles).

1.3.4 Operating mode: Dead-end

Ultrafiltration membranes are frequently operated in dead-end mode: allfeed is forced through the membrane. In dead-end mode, fouling occurs andfrequent cleaning of the membrane is a necessity. In figure 1.8 some footageof an ultrafiltration hollow fiber with fouling deposited on the surface isshown. A filtration procedure is interrupted when the resistance reaches(for constant flux filtration) a maximum level. Subsequently clean permeate

1.3. SCOPE OF THE THESIS 11

is flushed in opposite direction through the membrane to remove the accu-mulated fouling. As a result of this backwashing, the resistance decreasesand filtration can be continued. However, often the backwashing procedureis not 100% effective, and flushing and cleaning of the membrane with che-micals is required to restore membrane resistance. Because of the switchingbetween filtration, backwashing and chemical cleaning, the ultrafiltrationprocess is sometimes referred to as a cyclic process. In figure 1.9, the cyclicnature of such a process is demonstrated, showing the fouling state as afunction of time.

1.3.5 Membrane flux and resistance

Ultrafiltration is a pressure driven process, the flux J of fluid through themembrane is proportional to the trans-membrane pressure (the pressure dropover the membrane) ∆P applied to the membrane:

J =∆P

µRt

(1.2)

Equation 1.2 is a more specific form of equation 1.1 and is often namedDarcy’s equation. Where µ is the viscosity of the fluid and Rt is the totalresistance, often the summation of all resistances in series acting on themembrane:

Rt = Rm + Rp + Ra + Rg + Rcp (1.3)

Rm is the membrane resistance, Rp the pore blocking resistance, Ra the resis-tance as a result of adsorption, Rg the resistance as result of a gel layer andRcp the resistance as an effect of concentration polarization. The resistancesare graphically represented in figure 1.10. During the filtration process, fou-ling accumulates and consequently the resistance increases. Modeling of theresistance during filtration, backwashing and chemical cleaning is imperativefor optimization purposes.

1.3.6 Outline of the thesis

From an optimization point of view, an ultrafiltration process can be sys-tematically represented in a hierarchical optimization structure, as shown infigure 1.11. In principle three hierarchical layers can be distinguished:

12 CHAPTER 1. INTRODUCTION

R a

R b

R m

R g R cp

Figure 1.10: Resistances during filtration.

• The process control level, where chemical cleaning, filtration and back-washing are optimized,

• The production control level, where the chemical cleaning cycle is op-timized,

• The strategic control level, where membrane lifetime is optimized.

Optimized control variables are passed downwards, while calculated costsare passed upwards. Optimization at each level comprises evaluation atdifferent time scales, respectively named the short term scale (hours-days),the intermediate term scale (days-months) and the long term scale (months-years).

1.3. SCOPE OF THE THESIS 13

C P

CC

MLTC

B F

FC

FIltration flow, Flocculant

concentration, Backwash flow

Chemical cleaning flow,

Chemical cleaning

concentration FIltration flow, Flocculant

concentration Backwash flow

Initial and final fouling state Overall cleaning time

Initial and final fouling state production time and volume

Minimum and maximum

allowed fouling state

Production demands, sequence

of cycles Initial and final fouling state production time and volume

production costs

production costs

cleaning costs

combined production and cleaning costs

combined production, cleaning and ageing

costs

Figure 1.11: Hierarchical optimization structure for an ultrafiltration pro-cess. F: filtration phase, B: backwashing phase, C: Chemical cleaning phase,FC: Filtration cycle, P: production phase, CC: Chemical cleaning cycle,MLTC: Membrane lifetime cycle.

14 CHAPTER 1. INTRODUCTION

This research focuses on the development of dynamic models, experimentalverification methods and optimization of the chemical cleaning phase, thechemical cleaning cycle and the membrane lifetime cycle. Modeling andoptimization of the filtration phase, the backwashing phase and the filtrationcycle receive significant attention in Blankert’s thesis [12].

It is noted that strictly spoken, chemical cleaning is a short term phe-nomenon, it is frequently addressed as an intermediate term effect. Chemicalcleaning itself takes place at the seconds to minutes time scale, while thefrequency at which chemical cleaning is performed is at the hours to daysscale.

In chapters two, three and four, intermediate term topics are discussed:chapter two proposes a chemical cleaning model. In addition the model istested for different cleaning agents and a sensitivity analysis is performed todetermine if frequent model parameter updating is required; chapter threeformulates and solves a chemical cleaning dynamic optimization problem,based on the earlier developed and validated cleaning model, chapter fourinvestigates - in a qualitative way - what type of fouling accumulates on amembrane during production, and tries to connect this knowledge to clean-ing effectiveness. In chapter five the intermediate term cleaning topics areaddressed, such as the development of a chemical cleaning sequence modelthat can be used to predict the fouling state of the membrane over multi-ple cleaning cycles and, subsequently the computation of optimal chemicalcleaning settings. Chapter six and seven concern long term topics: chaptersix evaluates the results of accelerated membrane ageing tests, determiningwhich factors are responsible for membrane ageing and in chapter seven amembrane lifetime model is proposed and validated. The membrane life-time model is used to optimize membrane lifetime. Finally in chapter eighta multi-objective control system is developed and tested to control long termirreversible fouling.

BIBLIOGRAPHY 15

Bibliography

[1] Holy Bible, King James’ version, Old Testament, Exodus 15: 22-27.

[2] Reprinted from The Quest for Pure Water. The history of the 20th Cen-tury, American Water Works Association.

[3] Nollet J.A., Recherches sur les causes du Bouillonnement des liquids,Histoir de l’Academie Royale des Sciences, Paris, 1748.

[4] Graham T., On osmotic force, Philos. Trans. R. Soc. London, 1854.114A: p. 177.

[5] Fick A., On liquid Diffusion, The London, Edinburgh and Dublin Philo-sophical magazine, 1855. p. 63.

[6] van ’t Hoff J.H., Die Rolle des osmotischen Druckes in der Analogiezwischen Losungen und Gasen, Z. Phys. Chem, 1887. 1 p. 481.

[7] Traube M., Physiologie un wissenschaftliche Medizin, Arch. Anat. Phys-iol., 1867. p. 87.

[8] Zsigmondy R. and Bachmann W., Uber neue Filter, Z. Anorg. Allg.Chem., 1918. 103: p. 119.

[9] Strathmann H., Membrane Separation Processes: Current Relevance andFuture Opportunities, AIChE J., 2001. 47 (5): p. 1077.

[10] Ullmann’s Encyclopedia of Industrial Chemistry Published by Wiley-VCH Verlag GmbH & Co. KGaA.

[11] Perry’s Chemical Engineers’ Handbook, Published by McGraw-Hill.

[12] Blankert B., (2007) Short to medium term optimization of dead-endultrafiltration Ph.D. Dissertation University of Groningen.