Note: Descriptions are shown in the official language in which they were submitted.
20~41~
I
.
E'llO'rO~IOnE7~CTOn
Backqround oL Invention
~lgae have ~ee,n cul~ivated artj.icially ~or su~
diverse purposes as the productioll of food for animnls
and humans, the treatment of sewage and waste waters,
and the accumulation of radioactive wastes. More
recently, algal cultures have been used for th~ -
production oE en~ymes having industrial and researcll
applications and for producing oils and other materials
having nutritional value. Modern biotechnology o~ers
an opportunity or the gelle~ic modi~3.ca~ion o~ Dlgnr- ~o
yield cultures cnpaL]o oL produci.ng a wLde val Lel:y ~r
useful materials.
Various methods and equipment have been employed
for the artiicial culturing of algae. Perhaps the
simplest procedures have involved the ùse of shallow
open ponds exposed to sunlight. Such ponds are subject.
to contamination by dust, other microorgani~ms, insects
and environmental pollutants and provide minimal
ability to control the degree of exposure to light,
temperature, respiration and other important factors.
A more sophisticated approach has involved growing
algal cultures in plastic-covered trencl-es and ponds,
optionally having electrically powered pumps and
agitators. ~I'hese con{igurations reduce the ch.llces l-r
contamination o~ the culture and permit more accurate
control o~ temperature, respiration and other
, ::
:.- : .
....
~ . .... .....
":. '',, ' ' ' ~
I'CT/~S9~ l6,~'
2069~1~
p~rameters. Such configurations are still quiLs
ine~icient in terms or provi.cling .~dequate and ~nlirorm
amounts o~ light to ~he algal cells, p~rticulatly when
sunlight is the sole source ot light.
Unlike other microorg~nisms, the nutrient
requirements of algas are very inexpensive, carbon
dio~ide being the pri.ncipal source o carbon. On tlle
other hand, the photosylltlletic process requires thal
the algae be exposed to a reIatively constant all(l
uniform source o~ light. A primary design factor
modern photobioreactors involves providing a means
uniformly exposillg tlle cells in tlle algal culture tll
the optimum amount of visible light. Like m~ny p~an~s,
algae are qui~e sens.iti.ve ~o ~he Dnount and kincl ot
light. Excessive lighl~ sensity can damage alld kill ..
algal cells. Too little iight results in low levels o~ -
photosynthesis and consequently reduces growth.
~ number oL design factors are affected by the
means selected for supylying light to the cells. For
example, light sources, ~ncluding nat~ral sunlight,
o~ten emit substantial amounts of heat. Algal cultures -:
are sensitive to heat and many of them grow most
efficiently at temperatures of 20-35C. Thus, means
must o~ten be provided for cooling the algal culture
Dnd dissipating heat g~nt?lnl:e~l hy the light sollrce.
Two desi~n Cactors closely related Lo the
requirement fo~ a uniform and constant supply of ligllL
are the cell density and the light path length. Like
conventional fermentati.on processes, it is usually
desirable to use as high a cell density as possib~e.
Many of the same considerations apply to algal cu1l:llre.q
as to bacterial cultures. For example, in addi~i.on ~-~
the light requirements, one must take into account L~le
competition for nutrients, respiratory demands,
-
.;:- . -: ' ' - :: . ' :
'
~ 1) I'CT~(~9(1/~
20~94~4
viscosity and pumpability o~ the culture medium., an-l
the like. 11owever, an ex~remely high cell denslty
results in cells more than a lew millimeters trom ~he
light source being et~ectively shiel~e~ from t~le liyl-l
Simply increasing l.ighL intellsity will not ove~com~
this problem, because higllly intense ligllt will danl.ge
o. kill cells near ~lle ligh~ source.
The only effective way of increasing cell
densitie.s while main~inin~ a uniform amounl of li~
0 i5 to employ a rela~ively shor~ ligllt.patll lenytll. 0~
course, ~he requiremen~ ~hat Lhe photobioreacior have a
relatively short light path leng~h in~roduces a new sel.
of design problems. For industrial applications, i~ i~
usually desirable ~o employ high-volume microbial
cultures. Large cul~ure volumes are amenable ~o
continuous or large-scale batch recovery op~rations a~
generally resul~ in economies of scale. Sstisfying -:lle
requirements for large culture volumes and shor~ ligll~
path lengths has required that the photobioreac~or llave
large, txansparellt walls wlli.cll are closely spaced ~n
deine a light path and a 1uid chamber within whicll
the algal culture is contained. T~le transparellt wal~.s
are illumi~na~ed wi~h an appropriate light source to
su~tain the growth and photosynthetic reactions Or ~he
cells. ~.
Various designs of such photobioreactors have beall .:
employed. A relatively simple design which has been
successfully used in laboratory and pilot plant :-:
operations is simply a glass chamber having large,
flat, parallel side walls and a narrow bottom and ~?n~l
walls. A gas sparging tube is placed in the bottom oi
the chamber to allow caxbon dioxide o~ carbon-di.oxi li~?-
enriched air to be sparged througll a rulture medium
contained in the chamber, and banks of fluorescent
,
. ~ - ,
'.
\~ 0 '~ I'CI / ~ S~
q ~0~94~4
light tubes are arrange~ adj.cen~ ~o the si~ wall~. el
the chamber. Inocula, nn~rien~s, bn~f~rs, and the lik~
can be introduce~ into ~:h~ cl~ambe~ ongl) ~h~ l.o~-
which m~ op~.o~l]y b~ c~v~r~l wi.~l~ o lid. 'I'hi~
design has been very s~lcces~ul and useful or sma]]
scale operations.
~n alternative embodiment of a ~ioreactor
employi~g a 1uoreseent ~ube in~olves a cylindricnl
culture chamber having glas.s wall~ whicll surronlld a
single r~ uorescent ~u~e. 'I'he cul~ure chamber may al~.(.
be surrounded by a concen~ric cylindrical wa~er jacl~l
for controlli.ng the temperature oE the eulture. Sue
photobioreactor is described by ~admer, R., Behrens,
P., and Arnett, L., in a paper titled "~n Analysis of
the Productivity of a Continuous ~lgal Culture Systelll",
published in Bio~echnolo~Y and Bioenqineerina, 29
(19~), pp. 48~-492. ~rhis design has also proven very
valuable for laboratory-scale algal culturing
operations, but, for many of the reasons described
above, hafi not proven particularly useful for large-
scale operations.
Various photobioreactors designs are reviewed in
an article by Yuam-Kum Lee, ~Enclosed Bioreactors for
the Mass Cultivatlon of Photosynthetic Microorganismfi:
The Future l'rend', Tlu~rEcll~ July 1986, pgs. 186-189.
Furthermore, several problems have resulted from
photobioresctor designs which have utilized ligltt bank~c
and light compartments immersed in the liquid microllia~
culture. Firstly, it is difficult to fiafely and
effectively make the necessary electrical connectiolls
w1th the light tubes. Secondly, access to the ligh~
tubes for maintenance is made more difficult.
A significant need still exists for large-scaJe
photobioreactors which are ctpable of uslng high
,... . . .
: . , . ;: ~
.
,
:, .
': , , .
X~ ql~ ( 2~
206~41~
intensity, low-cost Jamps which provide uniform
distribution of ligh~ ove~ large surface are~s wlli]e
utilizing a sae anl les.s complicated me~l)s Eor
providing elec~i.cal poweL.
SummarY oL tlle lnvell~ioll
The present invention in accorddllce witll one
embodiment thereoC, compri~e~ a novel ptlotobioreflct.t)r
in which at least one .nd preferably ~ pluraliLy or
light transmitting ba~les are mounted side by side i
a tank containing a liquid microbial culture. Ea~h
baf1e is fo~med witll a hollow cavi~y and is mountetJ s-
~that the cavity i5 accessible Lrom outside the tank
the insertion o a ligh~ source. ~he sides ot the
- ba~fles are constructed of optically transparent
lS material to transmit the light from the light source t-
~the liquid which is ln con'act witl the outsicle
surfaces of the baffles. Each light source is made up
of a plurality of light tubes, preCerably 1uoresce
lamps, supported by braces or similar supporting
structures and mounted in the baffles. Electrical
leads are extended Lrom tlle tubes to al~ow connec~i
with an external power source.
In another embodimen~, a single higll intenslty
light source is mounted in a light compartment l~aving
walls made of in~ernally reflective prismatic sheet
material to p~ovlde unlrorm li~ht of ~ suitable
intensity to tl~e microbial liquid culture.
The invention thus provides lor sreatly simplirie-3
electrical circuitry and connections, and reduces
maintenance costs. Enabling access to the liglt
- sources from outside the tank and shielding tl~e liglm
sources from actual contac~ Wittl the mlcrobial cul-ule
makes it easier to identify and replace burned bulbs :
and it reduces the risks oC short circuits as well.
. -- , '
: ' . :
... . .
G 20~;9~
Furthermore, as the ligl)t transmitting ba~Lle~ ~r~
surrounded on its major light emit.ting surfaces by ll-~
liquid microbial culture the spacing between adjac~l~t
tubes as well D5 betweell a~ljacellt barLles is such ar. ~n
optimize absorption o ~he emitted ligh~ by ~he algne
and to assure virtually complete ~bsorpLio~l oI tlle
emitted light.
In addition tlle light ~ransmitting bafles a~o
perform a structural Iurlction in Ihat their ex1.ernnl
surfaces serve a5 walls or drDrt spaces to deIille
circulation paths through which the algae is movecl b~
means such as air liIt agitation. The baffles also
form basic building blocks or mo~ules which can be In~ed
in combination in any ~esiretl number for large sca]e
photobioreflc~or systems of any selected capaci~y.
Orief DescriDtion of the Drawin~s
Figu~e 1 is a perspective view of the
photobioreactor illustrating an embodiment of the
present invention;
Figure 2 i5 perspective view oL a bafile and
fluorescent tube arrangement forming part of the
photobioreactor of Figure l;
Figures 3-5 are top, side, and bottom views
respectively of the photobioreactor illustrating the
present invention;
.Figure 6 is a perspective view o an alternate
light source for the photobioreactor illustrating
another embodiment o~ the present inventi.on;
Figure 7 is a perspective view of the
photobioreactor illustrating the present inven~ion
utilizing the light source oI Figure 6.
- . :. . - ,
,
. - .
.. . . ~ .
'' ' ' ~' ' " :
. , .
~ Ç~ t.l 7~
7 20~9~
~etailed DescriDtioll o~ the ~oventioll
Referring now to tl~e drawings Figure 1 is .
perspective view o~ a photobiorenctor 10 embodyjllg ll
present inventiol1 to bo used in B~ ou]~uril~g ot
dispersed cells or cell ~ggregate.s Ol multice].llllal-
organisms having a liglll. reqllirelllel~ s an exnln~
this photobioreactor ma~ be used to grow unicellula
algae wllicll carry on photosynthesis. ~rhe exterior
the photobioreactor 10 i9 in the Lorm of a tank 11
capable of containing a liquid culture medium as
illustrated by numeral 12. 'I~he liqui~ culture me~j.lll-l
is sometimes referred to as an "algal" culture, but i-
will be appreciated ~ha~ the photobioreactor 10 may l)e
employed for tlle cull:ivation of any type of ~ :
photosynthetlc microorganism.
The basic unit o the photobiol-eactor is a
rectangular tank 11 as shown in Figu~e 1 with nllmerolls
internal baf1es 14 WhiCIl extend from one end o[ the
tank to the other and whose ends are sealed to the
tank's inside walls Eacl~ baffle 1~ is ormed with n
hollow cavity which is accessible through openings in
the wall of the tank. Tank end walls 13 can be cut nul.
or molded in any conventional manner to enable access
to the baffle c.~vities trom o~lLsicle tlle tank~s encl w.
2~ surface.
From the outer surface of the tank walls are
openings permitting access to the cavity of the baffle
for the lnsertion of a ligh~ source. In the embodi.n~
shown in Figure 1 a plurality of light tubes 15 are
inserted into the baffle cavity from outside the tank's
surface and housed therein. Baffles 14 serve to
protect 1.ight tubes 15 rom di.rect con~ct witl~ l.lle
liquid culture medium 12. Light is therefore emitte(l;
;'' .', ' : ` . , . ',
59/)/l~h-"
2 ~
substantially uniformally fr.om ~lhes lS and is abso-l-ed
by the a~gal culture.
~ltllougil tl~e embodimellt i.llllstrate~l in ri~re ~
shows a rectangularly sl~aped tank 1~ it is recogni~ed
that any convenient .shape may l~e l~sed.
The tank and ba[tle .strl~cl.ure Or Ll~e p~ese
invention i.s an improvemerlt over ~he pllotobiore.lctt-
disclosed in pending in applicaLion S.N. 07/163 ~41
incorporated hereill by reLerence wllich disclose~ n
compartment for protecti.ng a light bank again.st flni(l
communication with a liquid culture medil1m. rermi~ llg
insertion of a light source in~o tlle baLfle cavitie.c
through the tank s outer surfaces, as in the presenL
invention, greatly facilitates electrical connecto~s to
the light source and maintenance as well.
Located outside the tank 11 is means to control
the temperature of the contents. ~ preferred means fol-
controlling the temperature include water jackets or
internal heat transfer coils which can be connected to
refrigeration l~nits (not sllown) or lleating uni~s (IIOt
shown). ~lso the dls~olved oxygen an~ pll levels Or
the contents are continuou~ly monitored and concrol~e(l
by any conventional, well known means.
Figure 2 is a perspec~ive view of a baffle l4 alld
2S the housing of light tubes 15 t)lerein. Side panels 1?
are substantially planar walls forming th0 baffle
cavity therebetween. Planar walls 17 are major
surfaces on the opposite sides of the cavity Ior the
emission of light into said cavity. Side panels l7,
top panel l8 and bottom panel l9 are made of a
chemically inert and optically transparent material
such as glass or acry.lic. lnternal br~ces l6 ar~
mounted to Iacilitate placelllent and to support ligll~ r
tubes 15 or a bank oI ]i.ght tubes. Open baffle ends 20
~:
: :
9 2~69~1~
permit the simple insertion o~ ~ubes ~llrough either end
even wl~ile the photobjore,~c~or is in .~ll operational
state as shown in Figure 1.
In the embodiment shown in ~igure 2, light tubes
15 are fluorescent tube lamps essentially in their "otl
the shelf" condition without any modification or
customization. The advan~ages Or such lamps in ~hi~s
embodiment are that light is emitted ~rom them
substsntially uniformly along the lengtll of the tul)es
and in all directions perpendicular to the tube~.
This, along with ~he particular baffle spacing, enable.s
optimum absorption o the light by the algal culture.
Because end sec~ions 20 o2 baffle 14 are open,
electrical connections can e~sily and safely be made to
light tubes 15 at their oppo.si~e ends in Dny
conventional, well known way. As shown in Figure 2,
individuai connections cre m~de to each fluorescent
tube 15 by members 22 whereby leads are brought ou-
through wires 23 and 24 which terminste in an
electrical plug connectable to a source of electric
power (not shown). Suitable ballasts (not sllown) a~e
provided as well. These connections could also be made
by provldlng a single adapter on each end section o~
the baffle or by grouping the tubes in any number.
In an alternative embodiment of the present
invention, only one end of the baffle cavity is opened
to the tank's outside surface. Tlris structure would
require running the leaG wires connecting the
fluorescent tubes at the closed end through the barrle
cavity to the open end.
As mentioned above, the tank and bafle dimensiol-~
are selected to optimi~.e light absorption by the algae
culture durin~ the photobioreactor'~ operation.
According to one embodiment, as illustrated in Figures
:`:.- ' . - ' :
.: ~ ., ` .,
.,~ ...
20~9~
IU
3-5, baffles 14 are constr~cted to he ~ inclles widn l.o
enable the insertion of a fluorescent tube. Tlle
overall height Or the ha~fles is determi.ned by the
number of 1uorescent tubes Lo be inserLe~ hi~
embodiment, the height is approximately 2~ inches
which, as shown in Figure 2, allows for l~ rluorescellt.
tubes and three brace.s 16. I`he outer surfaces of
ad~acent baffles are separated by a distance of l illCh,
and one-half inch separates a baffle's outer surface
and the tank wall. Baffles 14 will generally extend
along the entire 48 inch tank length as .illustrated in
Figure 5. The overall height and width of the tank in
this embodiment are 31 inches and 1~ inches as shown in
Figures 4 and 3 respectively. Vsing high output, cool
white fluorescent tubes, this spacing provides a near
optimal light source for micro algae.
Furthermore, as illustrated in Flgures 2-5, planar
walls l7 form a major portion of the light emitting
surfaces of the baffle. In accordance with the baf[le
dimensions set forth in tl~e preferred embodiment
described above, approximately 90~ of the liqht emitted
by the 1uorescent tui~es 15 is transmitted through
planar walls i7. These figuses are not intended to i~e
limitations but are provided only to illustrate thaL
plannr walls are ma~or surfaces forming a major portion ~.
o the bafle l.ight emitting surfaces.
The particular tank and baff 1Q dimensions shown in
Figures 3-5 are for illustrative purpose~ only. It is
to be recognized that the baffle widths and spacing
between adjacent baf1es will depend upon the photon
flux of the light source, the optical properties o~ the
baffle walls and the cell density of the culture being .
used. Furthermore, the photobioreactor can easily t)s
constructed in any volume due ~o the tank~s in~ernal
' ' . .
o ~ n~) Pcr/~ 7'
1l 2069414
symmetry of the light-transmitting baffles. 1he
photobior.~acl:o~ in ~i gtlre 1 l~a~ rk i ng vol~lm~ Or
110-130 lil~ers. 1'hi~ vol~lm~ cao ~ily be illcr~.~s~
widening tlle ~ank increasillg ~he ll~igll~ arld increasirly
S the numbe~ o~ ~aff~es. SIICII n cllnrly~ i~l tlle
photobioreactor dssign does not a~[ec~ the operatioll or
performance ot ~he pllo~o~ioreactor in arly way.
~ s mentioned above, ~he ligllt Lransmitting barrles
also perform a structural function in that their
external surfaces serve as walls or draft spaces ~o
define circulation patlls Lor the a19A1 culture. ~s
shown in Figure 1, bafrles 1~ are mounted in tlle tank
to form passages 21 therebetween to enable the culture
circulation and to enhance the growth process. In
order to promote circulation and agitation within tlle
tank, a series of hollow tubes or cylinders 20
preferably formed of a metal or ceramic material or
inert material are placed in the preferred
embodiment, between alternate pairs of baffles as
illustrated in Figure 1. Tlle cylinders 20 can also be
positioned between any adjacent baffles at any vertical
position or between tl~e baf[le an~ the tank wall.
Furthermore cylinders 20 can be placed just below l.he
baffles such that substantially all the gas flows
there-between as illustrated in Figure 7.
The cylinders 20 contain small perforations or ~ :
apertures extending ~hrouqh the walls tllereof for~ning
gas sparging tubes througll WhiC11 a pressurized gas
~e.g. carbon dioxide) is supplied for ~he
photosynthesis requirements of the algal culture. rlle
gas may be air or nitrogen or another inert gas elther
singly or enriched with carbon dioxide. ~s gas is
bubbled into the culture, it forces the medium to rise
creating a circulation up one channel and down ano~l-er
,. . - . . ' :. .
: . . ,, . , . :
.,' : ~ , ~
- :: . . .
~l) qll~)-O~ I'Cl/~91~/1)(,,"
12 2~9~14
in the directions show1\ hy the arrows in rigurq 1. 1
is recognized, however, that the up an~l down
circulation as illustrate~ is not necessary ~o acllinve
mixing. Yositioning the sp.~Jging ~ube~ between
adjacent bafLles and betwee~ af[le and t~nk wall
will also acllieve this desired result. Other means [or
supplying nutrient source gases and ~or circulating tl-~
culture m~y also be used.
~ecause the baffles exte!ld througll the entire
length of the tank and the hollow cavities ormed
therein are accessible from the outer surface of the
tank walls on either side, supplying electrical
connections to the fluorescetlt tubes housed therein is
great~y simplified. Providing maintenance for the ~;
tubes is simplified as well. ~lectrical leads connected
to the ends of the tubes are made in th~ simple
conventional manner and are completely .shielded ~rom
the liquid culture 12. necogni~ion and replacemen~ or
burned light tubes is greatly facilitated by this nove~ ~
structure. Furthermore, by selecting the proper ligll~
path lengths as described above, virtually 100~ of the
emitted light is ~bsorbed by the culture and the lLght
absorption is relatively uni~orm and optimized
throughout the culture as well.
In accordance with another embodiment of the
inventlon, as illu~trated in Figure 6, the light source
is a single concentsated high-lntensity light 31
mounted in a light compartment 30 to provide uniform
high intensity light to the microbial liquid culture.
3~ Liqht compartment 30 contains means for substantial]~
uniformally distributinq ligllt rom source 31 wi.th
reflector 33 across ~he interior surface of transpare
walls 34 and 35. Such means maybe in the form of a
light guide constructed of internally reflective
~. -
. : .
:.:. . - : , - .
13 2~9~
prismatic sheet material (not shown)~ The internally
reflective prismatic sheet material is rormed trom
highly tran.sparent flexible shee~ mn~erial, sucll ,~
polyacrylate, on the s~lrtn~ o~ wlli~h is inscril~e~ witl.
minute 90 corrugations. ~s a result o~ ~hese
corrugations, light strj.kill~ l.he .shee~ wi~ll an c~ngJ-! or
incidence o~ about ~7 or less will be reflec~e~ Wi
nearly 100~ e~Liciency. Minor imperfe~tions in the
prismatic sheets as well ~s light iniringing tlle sh~el.s
at angles greater than about 27 result in transmiss
of light thro~lgl~ the prismatic shee~ m~terial.
~ s shown in Figure 6, a mirror 32 is located at
the bottom of light compartment 30. ~s a result o~ t:he
internal reflectDnce of the prismatic sheet material,
reflectance of mirror 32 and re~lector 33, light from
the light source 31 is, to a large extent, reflecte<l
back into the compartment. The net result of this
internal reflectance is ~hat the light from the light
source is distributed substantially uniformly across
the inner surface of wall 33 and thus provides a hiql~ly
controlled distribution of light throughout the light
paths in the walls 34 and 35. Li~ht from the light
source is there~orr? emi.tted and distril~u~ed -~
substantially uniformally from the exterior surface~ or
the baffle's planar walls.
Slnce it i.s not desirab]e to hnve light emitte~l
from end curfaces 36 and 37, a re~lective cover ~no~
shown) is placed between the compartment wall and ~lle
internally re1ective prismatic sheet so that the liqh~.
30 is redirected back into the compartment. Mirrors may
be used as end surfaces 36 and 37 as well.
Furthermore, light source 31 could be placed near end
surfaces 36 and 37 in vertical arrangement to efiect.
the uniform emittance. Constructing a light
.
: .
94~4
compartment utili~in~ a prism.~tic sl1eet material ,lnd
the light guides rormed ~herefrom is de~cribed ilt U.~.
Patent Application S.N. 07/33~,53~, Lncorpora~e~ he~Pi
by reference.
The electrical power can be supplie~ to the hit111
intensity ligll~ source 31 in any ~:onvelltional manne1.
~s illustrated in this embodimen~, leads connected ~o
source 31 are brought ou~ through wire 38 and ~ermina~e
in an electrical plug connec~able to a source of
electric power (not shown).
Figure 7 shows a perspective view of the
photobioreactor utilizi.ng ~he ligh- source 30.
Photobioreactor 40 operates in an identical way as
described with reference to photobioreactor 10.
i5 Baffles 41 are formed with hollow cavities extending
the entire tank length and are constructed to enable
the insertion of the light source from the either or
both ends of the tank, througll open ends 42.
Electrical connections to source 30 are very simply --
2~ made in any well known, conventional way as de.scribed
above.
While it is apparent that the preferred embodimelll.
shown and described provides certain advantages, many
of the advantages of the present i.nvention can
nevertheless be realized in other conflgurations, and
it will be appreciated that various modifications,
changes and adaptions can be made, all of WtliC)I are
intended to be comprellended within ~he meanlng and
range equivalen~s of the appended clnims.
r; r ' ' ' ' ~ ' '
' `
. ~, .' '. ,' ' ' . ' ~, : :' `
.
