Abstract. Membrane recycle fermentors are used successfully on laboratory scale to increase the efficiency of fermentation processes. The design of a process ...
Bioprocess Engineering 8 (1993) 2 3 5 - 2 4 6
Bioprocess Engineering © Springer-Verlag 1993
Process engineering for a membrane recycle fermentor W. J . Groot, R. G . J . M . van der Lans and K . Ch. A . M . Luyben, D e l f t , The Netherlands
Abstract. Membrane recycle fermentors are used successfully on l a b o r a t o r y scale to increase the efficiency of fermentation processes. The design of a process on larger scale however is obstructed by the lack of relevant data in literature. Compared to a stand-alone fermentor a membrane recycle fermentor presents new features which must be considered i n the design. These features include the use of high density cultures, the additional volume i n the membrane sect i o n and the circulation of the b r o t h . I n this theoretical study these aspects are analyzed w i t h the characteristic time concept, i n case of an ethanol fermentation integrated w i t h m i c r o f i l t r a t i o n . The analysis shows that depending on the reactor c o n f i g u r a t i o n used concent r a t i o n gradients can be expected. These gradients may decrease the efficiency of the fermentation, or can be advantageous, f o r example by letting the substrate conversion approach completion i n the membrane section.
in (0) M m ni max MF P rec s s s tot X
inlet membrane section medium (filtrate) mixing maximal microfiltration product recirculation flow stirrer substrate superficial total biomass
1 Introduction List of symbols A C (I D D D F H J /ci a Ks N "h P r t r T 1' V
Q
Superscript, F
area concentration substrate concentration i n feed fibre diameter stirrer diameter d i l u t i o n rate diffusion coefficient f l o w rate fermentor height membrane f l u x volumetric oxygen transfer coefficient M o n o d constant stirrer speed maintenance coefficient power input p r o d u c t i o n or consumption rate characteristic time fermentor diameter temperature liquid velocity volume yield of biomass on substrate yield of product on substrate growth rate density
kg m " ^ kg m " ^ m m h-' m^s-i m^ h ^ ' m m^ m " ^ h " ' s^i kg m " ' ' s-i kgkg-'h-' W kg m ~ ^ h ~ ' s m °C m s~' m^ kg k g " ' kg k g " ' h-' kg m " ^ subscripts
average fermentor
and
abbreviations
The last t w o decades show an increasing amount of studies on the use of recycle fermentors, notably fermentors i n conj u n c t i o n w i t h membrane modules. I n these systems the fermentor is the reaction vessel, and the fermentation h q u i d is recirculated over an external membrane module (or another unit operation). I n the membrane module h q u i d is w i t h drawn (i.e. sohd/hquid separation, cell recycle), product(s) are recovered, or substrate is added (e.g. substrate or oxygen). The principal goal of using a membrane recycle fermentor is to retain cells i n the fermentation section, which enables continuous processing, and invariably leads to an i n crease i n productivity. I n this study process engineering aspects of special interest to a membrane recycle fermentor w i l l be considered. The high cell density in such an integrated system leads to high reaction rates compared to processes w i t h o u t cell retention, which promotes the occurrence of (unwanted) concentration gradients. Compared to a standalone fermentor, a membrane recycle fermentor also features new aspects of the additional fermentation volume i n the membrane section and the recirculation of fermentation hquid. Recycling for example implies additional mixing i n the fermentor by the recirculation stream, and the m i c r o organism w i l l be exposed repeatedly to a different, and probably changing, environment i n the loop. So far hardly any studies have addressed these topics, theoreticaUy or experimentally.
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This Study attempts to summarize the different aspects of a membrane recycle fermentor for fermentation purposes, and consists of three parts; -
A n inventory of process considerations of importance to a membrane recycle fermentor. - A n analysis of characteristic times i n a membrane recycle fermentor to distinguish between more and less important mechanisms. - A n iUustration of the effect of recirculation i n a membrane recycle fermentor on the fermentation performance, by means of simulation.
3 Inventory of process engineering considerations for a membrane recycle fermentor Several aspects of a membrane recycle fermentor should be considered i n the design of a process. A summing up of these process considerations is given i n Table 1. Some important aspects w i l l n o w be explained i n more detail.
3.1
Fermentation
vohime
The fermentation volume i n a membrane recycle fermentor consists o f the w o r k i n g volume o f the fermentor and the
2 System description The model system dealt w i t h i n this paper is that of ethanol p r o d u c t i o n i n a fermentor integrated w i t h m i c r o f i l t r a t i o n for cell retention. Such a system has been investigated on a laboratory scale [1], and i n an economical evaluation some design problems have been addressed [2]. Several phenomena i n this particular system w i l l also be of importance to other integrated fermentation/membrane systems. The configuration for the ethanol production process is schematically given i n Fig. 1. Ethanol is produced by baker's yeast i n a fermentor. The fermentation liquid is mixed w i t h turbine impellers. Some air is sparged i n the b r o t h to supply a trace amount of oxygen to support g r o w t h of the yeast. A m i c r o f i l t r a t i o n module is coupled to the fermentor f o r cell retention, thus leading to a high ethanol productivity. The b r o t h is recirculated over the m i c r o f i l t r a t i o n module, and part of the hquid permeates t h r o u g h the membrane, as a result of a pressure difference over the membrane. A bleed is used to prevent an undesirable build-up of yeast cells.
CELL BLEED
SUBSTRATE
BACKWASH
FILTRATE
FER MENT A TION
MICROFIL TRA TION
Fig. 1. Schematic representation of an ethanol p r o d u c t i o n system w i t h m i c r o f i l t r a t i o n coupled to a fermentor
Table 1. Process considerations specific f o r a membrane recycle fermentor Aspects related to high cell density
Aspects related to extra fermentation volume and circulation of the b r o t h
Fermentation
H i g h reaction rates N u t r i e n t depletion (mixing problems) I n h i b i t i o n of g r o w t h [3]
N u t r i e n t depletion i n loop Shear stress H i g h CO2 pressure [4, 5] Pressure shocks Temperature shocks [6] pH-gradients
Hydrodynamics
H i g h viscosity, i.e. high solids (cell) concentration [7]
Increase of m i x i n g i n fermentor by jet of returned l i q u i d
Mass transfer
H i g h viscosity
Increase of mass transfer i n fermentor by jet of returned l i q u i d [8] F o u h n g of membranes [9]
Heat balance
Increase of heat p r o d u c t i o n
Loss of heat i n piping/membranes I n p u t of heat by pumps
Miscellaneous
Logistics: - D i s t r i b u t i o n of liquid - Spatial arrangement of modules Backwash facihties Sterilization facilities Facilities f o r the release of C O , gas
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internal volume o f tlie membrane section. The volume o f the membrane section is determined by b o t h the volume o f the membrane module and the piping, and is p r o p o r t i o n a l to the amount o f membrane area required. The volume o f the membrane module w i l l depend on the type o f module used (hollow fibre, tubular, spiral w o u n d or plate and frame). The membrane area required f o r liquid removal depends on the rate w i t h which l i q u i d must be removed f r o m the system i n combination w i t h the membrane f l u x . The internal v o l ume o f the membrane section can f o r m a substantial part o f the total volume. A n extreme case is presented by a closed loop w i t h a membrane module w i t h o u t a fermentor [10].
3.2
Configuration
of a membrane
recycle
fermentor
The integrated system consists o f a fermentor equipped w i t h one or more membrane modules (see also [11]). The configuration o f the membrane section is o f great importance to the total design. F i g . 2 schematically shows six possible configurations, which might be used i n practice. Some aspects of these membrane configurations w i l l now be commented upon.
3.2.1 Single pass or closed loop mode The performance o f a m i c r o f i l t r a t i o n module is characterized by the membrane flux. A high flux can be obtained by
favourable hydrodynamics (i.e. a high recirculation rate), a high transmembrane pressure, and efficient washing procedures to reduce fouling. I n practice the modules w i l l be coupled to a fermentor i n a single pass mode (Fig. 2 a), or the module w i l l be placed i n a closed loop (Fig. 2 b). A single pass system is relatively easy i n operation. A system w i t h a closed loop offers the advantage that the hydrodynamics in the membrane module can be controlled independently. F o r example the average transmembrane pressure i n the l o o p can be maintained relatively easy at a high level, leading to relatively high fluxes. The recirculation p u m p i n a closed loop wiU require less energy compared to a single pass system, i n w h i c h the pressure continuously falls d o w n to the atmospheric level. I n a closed loop system the ceU concentrat i o n w i l l be higher than i n the fermentor. This high cell concentration may increase problems w i t h respect to membrane f o u l i n g . However the high cell concentration may also be advantageous, since the fermentation rate can expected to be high (see section 5).
3.2.2 M u l t i p l e modules: Parallel or i n series I n a system w i t h more than one module the modules can be placed i n series (Fig. 2c) or paraUel (Fig. 2 d ) . I n practice paraUel modules are preferred, since the transmembrane pressure over the modules wUl be identical, i n contrast to a decreasing transmembrane pressure w i t h modules i n series, due to the pressure drop over the modules. The pressure
Fig. 2 a - f Configurations of the membrane section, a One module, single pass, b one module, closed loop, c multiple modules i n series, single pass, d multiple modules parallel, single pass, e multiple modules, parallel closed loops, f multiple modules, closed loops i n series
238
drop depends to a large extent on the type o f membrane. W i t h hollow fibre membranes the pressure drop over the module can be iiigh, but w i t h tubular membranes the pressure drop is less. W h e n one or more modules are required, and closed loops are used, these loops can also be placed parallel (Fig. 2e) or i n series (Fig. 2f)- Also multiple m o d ules per loop can o f course be used.
3.2.3 Logistics I t is to be expected that at large membrane areas, the logistics o f liquid distribution w i l l pose practical problems o n the system. This aspect is closely related to the configuration used: Parallel, series or separate loops. W i t h respect to logistics also other factors are i m p o r t a n t like the horizontal or verdcal arrangement o f modules, and facihties f o r backwashing, membrane replacement, sterilization, etc. I n p r i n ciple the problems can be solved, as is demonstrated by the fact that at present large plants f o r u l t r a f i l t r a t i o n or reverse osmosis w i t h several hundreds o f square meters membrane area are k n o w n to f u n c t i o n adequately.
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is expected to be well mixed i n a system w i t h closed loops (Figs. 2 b, 2e). W i t h closed loops i n series, the character o f the recirculation stream can be expected to be analogous to tanks i n series (Fig. I f ) .
3.3
Scale-up
The scale-up o f a membrane recycle fermentor w i l l consist o f the scale-up o f the fermentor and the installation o f more membrane modules. F o r the design o f a large fermentor the general approach f o r the scale-up o f a stirred, air-sparged reactor w i l l be satisfactory. One new feature i n the reactor design concerns the improvement o f mixing o f the fermentor contents by the jet o f liquid, returning f r o m the membrane section (Fig. 1). A recent article on this subject considers the increase o f oxygen transfer by enhanced m i x i n g by the jet [8]. W i t h respect to the scale-up o f the membrane section, a large membrane area w i l l be achieved by using more membrane modules, since the area per module is restricted. The use o f more than one module may lead to the use o f a specific c o n f i g u r a t i o n o f the membrane section, notably closed loop systems.
3.2.4 Consequences o f the configuration of the membrane section f o r reaction engineering 3.4 I n general the c o n f i g u r a t i o n o f the membrane section has two important and closely related consequences f o r reaction engineering. 1. The magnitude o f the recirculation stream. The flow rate f r o m the fermentor over the membrane section is usually predetermined to ensure a high flux. This stream is identical to the recirculation stream i n a simple scheme w i t h fermentation and m i c r o f i l t r a t i o n (Fig. 2 a). I n a system w i t h a closed loop the stream w i t h d r a w n f r o m the fermentor can be comparatively f a r less. I n practice the lower l i m i t o f this stream w i l l be determined by the sohds concentration of the feed i n combination w i t h the maximal solids concentration o f the liquid which can still be processed w i t h respect to the viscosity and/or membrane f o u l i n g and clogging. For dense baker's yeast suspensions the viscosity was f o u n d to increase sharply when the solids concentration exceeds about 140 kg/m^ [7]. 2. Well-mixed or plug flow character o f the recirculation stream. For reaction engineering i t is i m p o r t a n t to k n o w whether the liquid i n the membrane section has a well-mixed or plug flow character. This aspect must be considered i n the view o f reaction rates and recirculation rates, and w i l l be analyzed i n more detail i n the section o n characteristic times. Referring to the configurations i n F i g . 2, the recirculation stream in single pass systems can most hkely be considered plug flow (Figs. 2a, 2c, 2 d ) . The h q u i d i n the recirculation loop
Repealed
exposure
of the microorganism
to
changes
I n a membrane recycle fermentor the microorganism is exposed d u r i n g each cycle i n the l o o p to an environment different f r o m that i n the fermentor. W h e n concentration gradients occur i n the loop, the microorganism w i l l experience these changes each passing over the membrane module. The changes i n metabolite concentration and possibly depletion o f substrate and/or nutrients may cause changes i n the physiology o f the microorganism. The physiology o f the microorganism depends f o r example on the p H o f the medium. I n case o f ethanol production, pH-changes i n the loop w i l l be negligible since the production o f acid byproducts is l o w . However when an organic acid is the product, undesirable pH-gradients can be expected. A n o t h e r example o f a changing micro-environment f o r the cells i n a membrane recycle fermentor is the subjection of cells to shear stress i n the membrane section. Shear stress i n the recycle p u m p can even lead to rupture o f the cells. O n the other hand i t has been reported f o r Clostridium acetobutylicum that shear stress i n a m i c r o f i l t r a t i o n module i m proves the fermentation performance [12]. I t can be concluded f r o m literature that i n general little is k n o w n o f the effect o f a repeatedly changing environment on microorganisms. M o r e data are available on the steady state behaviour o f cells i n different environments, and these data w i l l give an initial impression o f the possible effects. For example literature describes yeast fermentations carried out at 60 bar o f CO 2 pressure, i n the view o f the possible in-situ extraction o f ethanol w i t h supercritical CO2 [4].
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Table 2. Basic data and assumptions f o r the characteristic time analysis Fermentation Fermentation temperature Viscosity of b r o t h [7] Feed Cell bleed Fihrate Sugar concentration i n feed Sugar concentration i n b r o t h Ethanol concentration i n b r o t h Biomass concentration i n b r o t h Biomass p r o d u c t i o n rate Substrate consumption rate Ethanol productivity Fermenter Fermenter volume W o r k i n g volume Fermenter diameter Fermenter height [H/T = 2) Stirrer diameter ( D , / r = 0.2) Aeration rate Oxygen transfer [17] Power input Stirrer speed M i x i n g [17] M i c r o f i l t r a t i o n module (hollow fibre) Total membrane area Length of module Diameter fibre Internal volume
C,
T H
N N • f„,
L d
L i q u i d velocity (recommended I - 4 m/s) Permeate flux ( ~ 0 . 0 1 0 - 0 . 0 3 0 m ^ m ^ h)
These data may prove h e l p f u l i n the analysis o f the effect o f repeated high C O j i e v e l s i n a membrane module at elevated pressures. The references o f studies relevant to repeated changes imposed on baker's yeast are included i n Table 1. W i t h respect to the effect o f a repeated change i n temperature we have performed independent measurements i n case o f ethanol p r o d u c t i o n by baker's yeast [6]. Temperature changes can be expected due to the dissipation o f heat by recirculation pumps, or i n systems w i t h evaporation o f ethanol/water mixtures f o r in-situ product recovery (e.g. pervaporation [13]). I n our study i t was f o u n d that a change in temperature o f the b r o t h i n a loop o f 5 ° C or more, at a residence time i n the loop o f 18 seconds or more, leads to significantly lower overaU fermentation rates. I t may be i m p o r t a n t to note that the repeated change i n the environment o f the cells i n a membrane recycle fermentor bears resemblance to the situation in large fermentors. I n large vessels incomplete m i x i n g occurs, which can lead to concentration gradients. I n recent years some research groups have investigated these phenomena i n more detail w i t h special experimental designs as an aid to derive rules f o r the scale-up o f fermentors. F o r example Sweere et al. have investigated the aerobic g r o w t h o f baker's yeast i n a system w i t h two interconnected fermentors w i t h different process conditions to simulate the repeated exposure o f the yeast to
30 6 • 10" 35.8 1.5 34.3 150 1.5 62 150 4.5 106 45
60 50 3.37 6.74 0.67 O.I 0.002 • (P/Lp)" ' • I 1.07 3 • N° " • {T/D,r
1,143 I 1.5 • 10-3 1.7 I 0.030
C Pa s
m7h mVh mVh kg/m^ kg/m^ kg/m^ kg/m' kg/m' h kg/m' h kg/m' h
m¬ m m m s
(1)
kW/m' (2)
m m m m^ m/s mVm^ h
aerobic and anaerobic conditions [14] and substrate excess and substrate depletion [15].
4 An analysis of characteristic times in a membrane recycle fermentor I n the design o f reactors and unit operations the concept o f characteristic times may prove a h e l p f u l t o o l to distinguish between i m p o r t a n t and less i m p o r t a n t mechanisms [16]. The characteristic time o f a process is small when the process is fast, and high when the process is slow. F o r example i n the present systems the b r o t h is pumped relatively fast t h r o u g h a module, compared w i t h the rate at which a microorganism grows. The characteristic time o f recirculation, w h i c h is the residence time o f the b r o t h i n the loop, is then l o w compared w i t h the characteristic time o f g r o w t h , which is the inverse o f the specific g r o w t h rate. This comparison learns that an increase o f the biomass concentration in the l o o p due to g r o w t h plays no role. The characteristic time concept w i l l be used to investigate which aspects o f ethanol production i n a membrane recycle fermentor are i m p o r t a n t i n the computational design o f the system (see Section 3 and Table 1). I n Table 2 the basic data and assumptions f o r the calculat i o n o f the characteristic times are given. The process par-
240
Bioprocess Engineering 8 (1993)
ameters were calculated w i t h the kinetic model that has been developed f o r ethanol production w i t h baker's yeast in a membrane recycle fermentor [1]. The model consists o f inl i i b i t i o n o f g r o w t h by ethanol w i t h linear kinetics, and the linear relation f o r substrate consumption. The model is presented i n A p p e n d i x 1.
Table 3. Characteristic times i n a membrane recycle fermenter
The process is proposed to be carried out w i t h a fermentor w i t h a w o r k i n g volume o f 50 m^. The ethanol productivity is set to 45 k g / m ' h. Substrate is fed to the system at a concentration o f 150 k g / m ' , and is converted f o r 9 9 % . The biomass concentration i n the fermentor is held constant at 150 k g / m ' w i t h the aid o f the cell bleed. I n the calculation two cases o f operation o f the membrane section have been considered: Single pass and closed loop (see also Figs. 2 a and 2 b). The reactor models are presented i n A p p e n d i x 2.
Substrate consumption
I n the analysis the internal volume o f membrane equipment has appeared to be o f great importance. O n laboratory scale the internal volume o f a module per unit membrane area is about 3 • l O ^ ' m ' / m ^ [1]. F o r commercial closed loop systems an average value o f 1.5 • 1 0 " ' m ' / m ^ was estimated. This last value was used i n the calculations, and leads to an internal volume o f the membrane section o f about 3% of the volume o f the fermentor. F o r the present calculation of characteristic times i t is assumed that the total fermentation volume is determined by the w o r k i n g volume o f the fermentor only. I n Section 5 the influence o f the internal volume o f the membrane modules is considered i n more detail.
Oxygen transfer due to impeller
The calculation o f the relevant characteristic times is presented i n Table 3. The most relevant observations provided by this table are: 1. Substrate gradients i n the fermentor The characteristic time o f substrate consumption is l o w , since the substrate consumption rate is high because o f the high cell concentration. The m i x i n g time is a factor 10 larger than the characteristic time o f substrate consumption, which means that gradients in the substrate concentration i n the fermentor can be expected. O n laboratory scale the mixing time can be f a r less ( i n the order o f seconds), and glucose gradients may not occur. However on laboratory scale care must be taken to maintain a turbulent regime, since the high viscosity and the customary smaU stirrer diameter can lead to low Reynolds-numbers. 2. Substrate gradients i n the recirculation loop I n the system w i t h a single pass o f f l u i d through the membrane section, the residence time o f b r o t h i n the reactor is very low. This is caused by the high recirculation rate, which i n t u r n is determined by the recommended f l u i d velocity i n the module. O n a large scale the high recirculation rate is unrealistic, and a closed l o o p system w i h be preferred. I n that case the recirculation stream wiU have no great effect on the mixing behaviour i n the fermentor. However i n the loop the residence time o f the b r o t h w i l l become o f the same order of magnitude as the characteristic time o f substrate con-
Formulae
Characteristic time (s)
M i c r o b i a l reaction Growth
120 • 10^
f» = 1/M 's = ( 1 - C , / C , , „ , , ) •
Oxygen consumption
19
Cs/rs
«1
CoAo
Ethanol production
5 • 10= Single pass
Closed loop
Fermenter M i x i n g by impeller
Residence time of b r o t h
200
111,1 = 3 . ^OTR
res
11
= !/(/«! a) ~
Vp/Frec
30
900
1
31
19
13
Microfiltration Residence time of b r o t h Substrate consumption
= (i-c,/c,,_) • Csh-s
Single pass: = 6 • 10^ m ^ h Closed l o o p : increase of biomass concentration f r o m 150 kg/m-' i n b r o t h to 180 k g / m ' in m i c r o f i l t r a t i o n l o o p : F„^ = 0.2 • 1 0 ' m ' / h
sumption, and a decrease i n the concentration o f substrate i n the loop is to be expected. This w i l l be considered i n more detail i n Section 5. O n laboratory scale a single pass system may be preferred f o r simplicity. The recirculation stream w i l l then be comparatively large, and its influence on mixing i n the fermentor may become notable. I t may then f o r example be i m p o r t a n t to place the pipe w i t h which the recirculat i o n stream is w i t h d r a w n f r o m the fermentor, f a r f r o m the point where the feed enters the fermentor. 3. The role o f oxygen I n an ethanol fermentation i n w h i c h yeast is g r o w n i n the absence o f g r o w t h factors i n the medium, oxygen is needed i n very small amounts. A l l oxygen that is transferred to the medium w i l l be consumed, and the concentration i n the b r o t h w i l l be practically zero. A detailed analysis o f the role of oxygen is d i f f i c u l t , since the oxygen need, and the oxygen transfer coefficient at low aeration rates are not exactly k n o w n . I t is evident that because o f the h i g h cell density in a membrane recycle fermentor the oxygen need is relatively high compared w i t h low density cultures. I n a recent article the oxygen need o f yeast is extensively discussed [18]. I t is stated that when growth factors are included i n the medium, the need f o r oxygen is not pronounced. I n the m a j o r i t y o f laboratory studies on high density cultures, including our own study [1], growth factors are supplied i n the f o r m o f yeast extract. W h e n using technical substrates, e.g. molasses, the role o f oxygen i n high density cultures should be considered i n more detail.
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241
5 An example of a reaction engineering aspect I n this section one aspect of reaction engineering of a membrane recycle fermentor w i l l be considered i n more detail. Simulations will be presented on the performance of an ethanol fermentation as influenced by the following parameters: The process configuration, the internal volume of the membrane section, the magnitude of the recirculation stream and the location of the cell bleed stream. The fermentation performance was simulated for 4 processes, the models of which are graphically represented i n Fig. 3. The rationale for these models can be f o u n d in Section 3.1, 3.2 and 4, and the features of each system relevant to reaction engineering are as follows: Figure 3 a. F u l l y mixed system The total of fermentation l i q u i d i n the fermentor and membrane section can be considered well mixed i n single pass systems w i t h one or multiple modules. Figure 3 b. Fermentor is well mixed and the membrane section is well mixed I n a system w i t h one or parallel closed loops the membrane section can be considered weU mixed. The closed loop(s) are fed w i t h the recirculation stream f r o m the fermentor The fermentor is also well mixed. Figure 3 c. Fermentor is well mixed and the membrane section consists of tanks i n series I n a system w i t h closed loops i n series each closed loop will be well mixed leading to a model w i t h tanks i n series coupled to the fermentor. Figure 3 d. Fermentor well mixed, membrane section well mixed, and the ceU bleed is placed after the membrane section I n the systems so far the cell concentration was controlled by a cell bleed directly f r o m the fermentor. I n closed loop systems however the cell concentration in the membrane sect i o n w i l l be higher than i n the fermentor. I n practice cehs w i l l then be discarded after the concentration step. A n example of the mathematical models for these systems is given i n Appendix 3, where the case of Fig. 3 b is worked out. The f o u r systems wiU be compared on the basis of the concentration of substrate i n the fermentor and filtrate. I n practice the concentration i n the filtrate should be low to avoid substrate losses, since i n ethanol production the process costs are determined mainly by the substrate costs. The low residual substrate concentration i n combination w i t h the substrate concentration in the feed of 150 k g / m ' leads to ethanol levels of about 60 k g / m ' . I n all simulations it is assumed that the total average biomass concentration is 150 k g / m ' , to enable a fair comparison of the systems. This assumption w i l l later be commented upon. The internal volume of the membrane section is set to 10% of the total volume, unless otherwise stated.
Fig. S a - d . Schematic representation of the models used to describe membrane recycle fermentors. a F u l l y mixed system, b fermentor f u l l y mixed and f u l l y mixed closed loop, c fermentor f u l l y mixed and f u l l y mixed closed loops i n series, d fermentor f u l l y mixed and closed l o o p f u l l y m i x e d ; cell bleed after m i c r o f i l t r a t i o n
First a single pass system (3 a) w i l l be compared w i t h a system w i t h one closed loop (3 b). I n this comparison also the implications of substrate l i m i t a t i o n w i h be considered, which implies the i n t r o d u c t i o n of a M o n o d - t e r m i n the equations for g r o w t h and substrate consumption (Appendix 1). The simulations are represented i n Fig. 4 (no substrate hmitation) and Fig. 5 (substrate limitation). I n b o t h figures the substrate concentrations i n the fermentor and the loop i n case of the closed loop system, are plotted against the substrate concentration i n the weU mixed single pass system. I n addition i n these figures the effect of variation of the recirculation rate is shown. The plots were constructed by calculating the respective substrate concentrations f o r a given feed rate i n each system. These feed rates can be read f r o m the upper X-axis. The figures show that i n a closed loop system, the substrate concentration i n the l o o p is lower than i n a well mixed system. The substrate concentration i n the fermentor is comparatively higher. This result means that f r o m the point of view of reaction engineering a closed l o o p system is to be preferred compared to a single pass system, since at an
Bioprocess Engineering 8 (J993)
242 0.3
Feed rate 1.2 25 I
^
1.3 ^
1.4
h-' 1.5
^
0.7
Feed rate
• •. \ ; 0.9 2 5 r^^L-i—1
^
kg m"^
0.5
1.1 1
h"'
1.3 1
1
kg
O
5
1 0 kg m - M 5
Substrate concentration in single pass system (fully mixed)
Substrate concentration in single pass system (fully mixed)
Fig. 4. C o m p a r i s o n of tlie substrate concentration i n a single pass and closed loop system ( F j , = 0.1 • V,^,); no substrate l i m i t a t i o n . Variation of the recirculation rate: D „ j = 5-Di„ {C^p = 146.5 k g / m ' , C , „ = 181.3 k g / m ' ) ; - - - ö „ , = 10 • D , , ( 0 , ^ = 1484 k g / m ' , C , „ = 164.2 k g / m ' ) ; - - - D „ , = 20-P,-,, ( C , , = 149.3 k g / m ' , C , „ = 156.8 k g / m ' )
Fig. 5. C o m p a r i s o n of the substrate concentration i n a single pass and closed loop system ( F „ = 0.1 • k;„,); substrate h m i t a t i o n . Variat i o n of the recirculation rate: ^ , . „ = 5 • ö,.„ (0^.^=146.5 k g / m ' , C , „ = 181.3 k g / m ' ) ; - - - - D„^ = 1 0 - D , , ( C , j , = 1484 k g / m ' , C,„ = 1642 k g / m ' ) ; D „ , = 20 •!»,.„ (C^f. = 149.3 k g / m ' , C ^ „ = 156.8 k g / m ' )
identical substrate concentration i n the permeate a higher feed rate can be used (higher productivity). Two factors should be mentioned to explain the low substrate concentration i n the loop. First, the biomass concentration i n the loop is higher than i n the fermentor (see the legends of Figs. 4 and 5). This leads to an increase of the substrate consumption rate. Second i t is assumed throughout this study that only the abiotic l i q u i d phase of the b r o t h contains substrate, or i n other words that the yeast contains no substrate (see ref [19], and Appendix 2 and 3). Since the b r o t h can contain more than 60% (v/v) yeast, the amount of substrate entering the m i c r o f i l t r a t i o n module is only 4 0 % of what can be expected f r o m the concentration i n the fermentor.
also show that when the recirculation rate is increased, the closed loop system approaches a well-mixed system.
I f the effect of substrate l i m i t a t i o n is ignored, i t is predicted that i n the closed l o o p system the substrate can be depleted in the loop, whereas it is still present i n the fermentor (Fig. 4). W h e n M o n o d kinetics is introduced, infinitely low substrate concentrations i n the l o o p are predicted (Fig. 5). Also the substrate consumption rate w i l l decrease, which is reflected i n the comparatively lower feed rates. B o t h figures
I n Fig. 6 the effect of the magnitude of the internal volume on the fermentation performance is shown. I t is evident that an increase i n the internal volume is profitable, since a larger volume of the system is operated at a high cell concentration. A n o p t i m u m was f o u n d when the internal volume of the membrane section is about 2 0 % of the t o t a l volume. W h e n the internal volume increases the system again approaches a well-mixed system. I t can also be concluded f r o m Figs. 3 - 6 that the ethanol productivity i n a closed loop system is higher than i n a single pass system. The figures show that at identical residual substrate concentrations, and thus identical substrate conversion and ethanol concentration, the feed rate is higher i n the closed loop system. This means that also the p r o d u c t i v i t y is higher. Depending on the different parameters, the increase in productivity i n the closed loop system can be up to 10%. I n Table 4 the effect of multiple loops i n series is considered for the present system at a given feed rate. This table
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243
Table 4. Effect of plug f l o w i n loop and position of cell bleed on the fermentation performance (D,„= 1.0 h " ' )
h-i Tank:
F
1
2
3
4
F
1
150 146.5 147.5 147.8 147.9 146.5
181.3 163.1 157.9 155.4 181.6
2
3
4
kg/m' Single pass Closed l o o p (1 tank) Closed l o o p (2 tanks) Closed loop (3 tanks) Closed loop (4 tanks) Closed l o o p (Cell bleed after M F )
0.3
0.5
4.03 6.60 6.29 6.20 6.15 6.63
0.7
1.04 2.65 3.63 4.19 1.04
0.43 1.29 2.21
0.23 0.72
0.15
1.04 1.54 1.72 1.82 1.04
Feed rate
182.5 169.4 163.6
182.8 172.8
183.0
150 181.3 171.8 170.0 168.7 181.6
61.35 62.58 62.37 62.30 62.26 62.6
0.0427 0.0440 0.0437 0.0436 0.0435 0.0356
now i n the membrane section, whereas i t is constant and high i n one closed loop. I f the cell bleed is placed after the m i c r o f i l t r a t i o n step i n a system w i t h one closed loop, the model predicts comparable substrate concentrations (Table 4). However, the ceU bleed rate is lower and contains more cells. B o t h factors lead to a decrease i n the loss of substrate. The present comparison was based on a constant average biomass concentration. I n the calculations this is experienced as a low biomass concentration i n the fermentor of the closed loop system, compared to a single pass system. I n practice one strives f o r an o p t i m a l biomass concentration i n both fermentor and membrane section. F o r a closed l o o p system this may mean that the average biomass concentrat i o n is higher than in a single pass system. This w i l l lead to a higher productivity. Furthermore the simulations show that a closed loop system w i l l be advantageous, since at an identical feed rate the substrate concentration i n the filtrate (the m a j o r product stream) is decreased, and thus the p r o d uct concentration is increased.
Substrate concentration in single pass system (fully mixed) Fig. 6. Comparison of the substrate concentration i n a single pass and closed loop system ( ö „ , = 5 • D,.„); substrate limitation. Variation of the internal volume of the membrane section: F;,„ = 0.2-F^», ( C , , = 143.2 k g / m ' , C,,„ = 177.2 k g / m ' ) ; - - - V,„[,i,= OA-V,„, ( C , , = 146.5 k g / m ' , C , „ = 181.3 k g / m ' ) ; - T/.,„,„,= 0.05 • V,,, ( C , , = 148.2 k g / m ' , C , „ = 1834 k g / m ' )
shows that the average substrate concentration i n the membrane section, and thus filtrate, increases when the number of loops i n series increases. This disadvantageous development is caused by the fact that the biomass concentration, and thus substrate consumption rate, increases progressively
I n general i t can also be concluded f r o m this treatise that in a membrane recycle fermentor the internal volume of the membrane section must be regarded as fermentation volume. This means that when an existing fermentor is equipped w i t h a membrane module, the t o t a l volume i n creases and that the capacity is higher than o n the basis of the fermentor volume alone.
6 Conclusions A n inventory was made of the design aspects of a membrane recycle fermentor, w i t h emphasis on the application i n the fermentative production of ethanol. I t is f o u n d that a membrane recycle fermentor presents several new features compared to a stand-alone fermentor. Some implications of the use of a recycle system, e.g. the possible effect of a repeated changing environment during recirculation on the physiology of the microorganism should be addressed i n future investigations.
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A n analysis of characteristic times in an ethanol product i o n process based on fermentation integrated w i t h microfiltration has shown that concentration gradients wiU become prominent, because of the high reaction rates i n high density cultures. I n case of a substrate gradient it was demonstrated i n more detail that, depending on the reactor configuration used, this gradient i n the membrane section can also profitably be used to optimize the fermentation process. The construction of an actual large scale plant w i t h a membrane recycle fermentor w i l l be preceded by a pilot plant stage. This stage must yield engineering data on the operation of the equipment, relevant to the process on a large scale. I n hterature so far practical i n f o r m a t i o n is still lacking on basic issues such as logistics, the microfiltration flux as a function of backwash and cleaning procedures, and the increase i n instrumentation for process analysis and cont r o l w i t h scale. These aspects have a direct influence on the installed membrane area and/or investments costs. A pilot plant study w i f l also yield more data on the fermentation performance i n an actual membrane recycle fermentor. This study may provide the reaction engineering framework, i n which these data should be analyzed. A total model of the integrated process must then result, which i n combination w i t h practical data on the operation of equipment w f l l enable the design of a membrane recycle fermentor on large scale.
Appendix I I : Description of an ethanol fermentation integrated with microfiltration without reaction in the membrane section I n this section the material balances used i n the description of an ethanol fermentation combined w i t h m i c r o f i l t r a t i o n w i f l be summarized (see also [1, 2]). This description is used i n the analysis of the characteristic times. The system consists of the fermentation step w i t h 3 streams: A feed (£>,•„), a cefl bleed (Z)^.) and a medium bleed of cell-free filtrate (Ö,,,). The cell bleed is w i t h d r a w n directly f r o m the fermentor. The material balances read: Biomass balance: 0 = D,C,-r,
(6)
Substrate balance: A , , •C.o = £>,„•
+
II
C,-Fr,
(7)
I t is assumed that only the abiotic phase of the b r o t h , volume fraction (1 -C,^^/C^.„,„J, contains substrate [19]. The maximal biomass concentration, C^ ^^^, is set to 240 k g / m ' . Product balance: 0 = Z)„,C, + Z ) , C ^ - ; ,
(8)
L i q u i d balance: (9)
Appendix I : Biokinetics of ethanol production by baker's yeast
The membrane area for m i c r o f i l t r a t i o n becomes: A,,,
A n extensive treatise on the biokinetics of ethanol product i o n f r o m glucose by baker's yeast, and its use i n the mathematical description of membrane recycle fermentors, can be f o u n d i n Ref [1] and [2]. I n this study the f o f l o w i n g rate equations were used. Biomass p r o d u c t i o n rate:
= D,„ • VIJ,,,
(10)
For a single pass system the recirculation flow is set by the recommended liquid velocity for the module. F o r a closed loop system the recirculation flow can be derived f r o m the biomass balance over the m i c r o f i l t r a t i o n unit: D...
Prec.MF
(11)
V
I n the calculation C,. was set to 150 k g / m ' and C^-,^i to 180 k g / m ' .
Substrate consumption rate (Pirt's equation): C„
C,. (4)
W i t h respect to the effect of substrate l i m f l a t i o n it is proposed that the M o n o d - t e r m is introduced in the equations of both the growth rate and the substrate consumption rate. Ethanol production rate ( = ethanol productivity): (5) The values of the kinetic parameters are: /(„^^^ = 0.25 h C, = 77 k g / m ' , y ™ - = 0.096 kg/kg, m, = 0.66 k g / k g h, = 0 . 4 2 k g / k g [1].
Appendix I I I : Description of an ethanol fermentation integrated with microfiltration with reaction in the membrane section I n this section an example of the set of equations is given, as used i n the simulation of the effect of the internal volume of the m i c r o f i l t r a t i o n section on the fermentation performance. The system concerns a fermentor connected to the membrane module i n a closed l o o p configuration. The liquid i n the fermentor and the membrane section is considered weflmixed (Fig. 3 b). The material balances w i f l be written i n f u l l , including the biokinetic terms given i n Appendix I .
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M a t e r i a l balances over the fermentor:
The foUowing relation f o r the recirculation rate was used,
Biomass:
w i t h a constant of 5, except otherwise stated: = constant • £),„
1 -
(20)
(12) Acknowledgement
Substrate:
This study was financed b y the l O P m (Innovative Research Programme o n membranes, M i n i s t r y of Economic Affairs, The Netherlands). (13)
1
References
^
c.,
Product: /'max-
1
•c,- y.p-ï^AC = (A.ec+ö,)-c^
(14)
M a t e r i a l balances over the membrane section: Biomass: ^
