Field of the invention
[0001] The present invention relates to the production of algae and is particularly
concerned with the growth of algae in an enclosed reactor system where the sunlight
can be fed into the reactor by a mirror system. The invention provides a reactor
vessel and light-introducing means for such an algal production reactor system.
Background
[0002] Algae use light and carbon dioxide for growing and this process generates
oxygen. Normally algae are produced in open ponds, transparent piping systems,
submerged plastic bags, etc.
[0003] The disadvantages of these systems are the amount of energy necessary for
mixing, feeding the carbon source and removal of the generated oxygen. Also the
amount of water evaporation, heating of the water in daytime and the cooling in the
night is a problem. This is because the only sensible source of light for growing algae
is sunlight, which is in principle available for free, but for optimum growth, the algae
require a high influx of light. However, sites where high sunlight input is available
are almost inherently hampered by being located in arid zones of the earth, where
water, also necessary for algal production, is very scarce.
[0004] These disadvantages limit the algae concentration in the reactor systems and
increase thus the amount of reactor space necessary for a specific amount of algae
production. Thus there is need for more efficacious and up-scalable reactor vessels
for growing photosynthetic microorganisms with improved light supply.
[0005] JP-A 2000-300244 discloses a photosynthetic culturing device having lighttransmitting
plates made of acrylic, which are arranged vertically in a reactor, the
(sun)light entering on the top of the reactor. The distance of 10-70 mm between the
plates provides the reactor space containing the culture medium in an up-flow
arrangement. The top ends of the spaces between the plates are closed with covers
("first invention") or with triangular extensions of the acrylic plates for increased
irradiation surface ("second invention"). The light-transmitting plates may have lightscattering
surfaces provided by unidirectional striping, and pairs of plates may be
formed in such a way that the light- scattering surfaces are at the inner side of the
pairs, as described in JP-A H08-262232.
[0006] The reactors of the types descried in JP-A 2000-300244 and JP-A H08-
262232 do not provide for optimum irradiation efficiency and algal production rates.
Moreover, the arrangement nature of the plates used according to this prior art do not
allow the provision of robust, large scale photosynthesis reactors for use at remote
sites and requiring minimum maintenance operations.
Summary of the invention
[0007] It was found according to the invention, that improved photosynthesis can be
provided by means of a reactor of an airlift-loop type, using lateral irradiation, and by
providing unit pairs of double rectangular glass plates which are mounted in parallel
at a distance provided by glass strips arranged between the rectangular double glass
plates and along the long (vertical) sides of the double glass plates. The use of glass
plates rather than acrylic-type plates reduces fouling by microorganisms. Further
improvement is achieved by providing light- scattering layers at the inside of the
double glass plates, the light scattering layers comprising non-unidirectional layers of
non-uniformities or particles.
[0008] The invention thus provides a large-scale reactor for growing algae which has
an upward flow and a downward flow channel, which can alternate e.g. by switching
the feeds of the gas mixing flow. Gas can induce the upward and downward flow
circulation. These types of airlift loop reactors are known as such in waste water
treatment. Vertical glass plates are arranged in the upward and in the downward flow.
The front parts of these glass plates are sticking out of the reactor wall into a
(sun)light source, together with glass strips joining pairs of double glass plates along
their long (vertical) sides. These glass plates are used to transfer light into the reactor
which is necessary for algal growth inside the reactor.
Description of the invention
[0009] The invention therefore pertains to a reactor vessel for the production of
photosynthetic microorganisms. Such a reactor is also referred to by its synonyms
"photosynthesis reactor" or "photobioreactor". The reactor vessel is provided with
one or more liquid inlets and one or more liquid outlets, one or more gas inlets at the
bottom and one or more gas outlets at the top of the vessel, and vertically interspaced
sets of double glass plates which are at least partially submerged in the reactor liquid
when the reactor vessel is in operation. The double glass plates have a layer of lightscattering
particles in between and have a flat side corresponding to the thickness of
the double glass plates, which flat side is being exposed to an external light source. In
operation, the flat sides are suitably vertically. The reactor vessel is further provided
with means for vertically circulating reactor liquid. The reactor as ready for operation
also contains the further means and materials for producing algae by photosynthesis,
including a suitable inoculate of the algae to be produced, a source of carbon dioxide,
etc.
[0010] The double glass plates, which are arranged pair-wise, constitute an important
element of the reactor vessel of the invention and of the invention itself. The
refractive index of the glass is higher than the refractive index of water, which makes
the glass plates function like a glass fibre used for data transport. The layer of lightscattering
particles ensures that the light leaks out evenly over the submerged area of
the glass plate into the reactor liquid. For this purpose the glass plates consist of two
layers (referred to as "double glass plates") with a coating in between which
constitutes a matrix for small (i) inorganic (e.g. metal oxide) or (ii) organic particles
or (iii) non-uniformities to scatter the light, or for (iv) larger mirror facets to reflect
the light in such a way that the light can enter the water face.
[0011] The (single) glass plates can have a thickness varying from a few mm, e.g. 5
mm, up to about 50 mm or even more. Advantageously, the thickness of the glass
plates is between 10 and 30 mm, most preferably between 12 and 20 mm. Thus, the
double glass plates preferably have a thickness between 20 and 60 mm, most
preferably between 24 and 40 mm. Similarly, the flat sides of the double glass plates
also preferably have a breadth of 20-60, or 24-40 mm. The glass is preferably of the
so-called "ultra-clear" type, i.e. it has a high clarity and is low in iron, in particular
below 0.04 wt.% (as Fe20 3) . Such glasses are also called low-iron glass, or high
transmittance glass.
[0012] The layer of light- scattering particles can be a coating layer in which the
scattering particles are mixed in a material having the same or nearly the same
refractive index as glass, so that light can pass from glass to coating without a mirror
effect caused by a difference in refractive index. For instance it can be a ceramic
coating such as silica. In the coating layer non-uniformities can be mixed which
cause the scattering (for instance crystallites, bubbles etc.). Non-uniformities are
understood to be bodies or voids which constitute a discontinuity in the matrix, i.e.
which can be, at least electromagnetically, distinguished from the surrounding
coating, for example by their refractive index. The non-uniformities are preferably
substantially spherical.
[0013] Two types of scattering can be used: (1) Mie-scattering, with non-uniformities
or particles with about the same or slightly larger (average) size than the wavelength
of visible light, preferably in the range of 200-1200 nm, more preferably in the range
from 300 up to 1000 nm, most preferably at least 400 nm, up to e.g. 800 nm; (2)
Geometric scattering, with particles much larger than the wavelength of light,
preferably in the range from 5 micrometer up to 500 micrometer, for instance
chromium crystals.
[0014] The Mie scatterers (non-uniformities) may be metal oxides e.g. of titanium,
zinc, silicon, or aluminium, or silicates, e.g. of magnesium or aluminium.
Alternatively, the Mie scatterers may be organic particles, e.g. polymer (latex)
particles, or they may be (pores, bubbles) of the same size. Particles of metal oxides,
in particular titanium dioxide, are particularly preferred, e.g. of 0.3 to 1 mih .
[0015] The geometric scatterers are facetted (mirroring) particles, i.e. particles having
light-reflecting facets, such as in chromium crystals or mono crystalline micro
diamonds (6-20 mih) . They preferably have sizes in the range of 5-500 mih, more
preferably 10-200 mhi, in particular 20-200 m i.
[0016] The thickness of the internal coating containing the light- scattering nonuniformities
or particles depends on the type of scattering. For Mie-scattering it is
preferably in the range of 5 to 500 mih, especially between 10 and 200 mih, and for
the geometric scattering it is preferably between 100 and 1000 mih . The particles are
advantageously homogeneously distributed in the internal coating. Their density may
be e.g. between 1 and 500 mg per dm2 of the coating (matrix).
[0017] The distance between pairs of double glass plates, i.e. the effective reactor
space, may be as close as e.g. 10 mm, up to, say 20 cm or more. For an optimum light
irradiation, it is preferred that the distance is between about the thickness of the
double plates and about twice said thickness, i.e. between 20 and 120 mm, most
preferably between 24 and 80 mm. The number of double glass plates over the width
of the reactor may vary. Preferably the reactor vessel, or reactor unit, contains
between 4 and 25, more preferably between 6 and 20 double glass plates per m of
reactor width, the glass plates being essentially mounted in parallel.
[0018] According to the invention, the double glass plates are advantageously
arranged in pairs, with strips of glass joining the pairs along the long, vertical side of
the plates, as further described below, and as depicted in Figure 3 .
[0019] The invention also pertains to double glass plates (pairs of glass plates with
internal coating) as described above as well as to sets and arrays of double glass
plates which can be used in photosynthetic reactors. The double glass plates
preferentially have an essentially rectangular surface. The length (or height if
positioned vertically) of the glass plates can be e.g. between 1 and 4 m, and
preferably between 1.2 m and 2.4 m. The width of the glass plates can be e.g.
between 0.5 and 2.5 m, preferably between 0.8 m and 2 m. Smaller surfaces may be
suitable for pilot-type reactors. Larger surfaces may be feasible as well, although the
weight and the handling of the glass plates may then require special measures. The
thickness of the double glass plates can be as described above, i.e. between 20 and 60
mm, more preferably between 24 and 40 mm. The double glass plates have a layer of
light- scattering particles in between, as described above.
[0020] Advantageously, two or more, in particular two, double glass plates are
mounted together in parallel at a distance allowing an optimum reactor space between
the two plates of from 10 to 200 mm, preferably from 20 to 150 mm or more
preferably from 24 to 80 mm or even from 30 to 60 mm. The distance can be fixed by
glass strips being positioned in between and along the length of the rectangular
double glass plates. The strips thus essentially have the same length as the double
glass plates, for example between 1 and 4 m. They may also have the same width, for
example between 20 and 60 mm. Alternatively, they may have a width of e.g. 1-10
times, especially 2-4 times the thickness of the double glass plates, or alternatively
0.01 and 0.2 times, preferably 0.05-0.12 times the breadth (width) of the glass plates
(for each strip). Thus a preferred strip width is e.g. between 2 and 20 cm, preferably
between 2 and 15 cm or between 2 and 12 cm or in particular between 2.4 and 8 cm.
Preferentially, the strips create a channel for algal reactor medium of the same size as
the glass plate assembly, so the light exposure of the algal medium is the same inbetween
the glass assemblies as in the glass assembly.
[0021] The glass strips are preferably of the same glass type as the glass plates and
are fixed to the glass plates using an appropriate sealant, such as a polymethacrylate
sealant. The sets of two (or more) mounted double glass plates provide higher
strength of the glass plates, and thus more convenient handling. The reactor vessel, or
reactor unit, may thus advantageously contain between 2 and 12, more preferably
between 4 and 10 of such fixed sets of double glass plates per m of reactor width, the
sets being essentially mounted in parallel.
[0022] For facilitating the positioning and replacement of glass plates in the reactor
vessel, sets of glass plates can advantageously be combined to an array of double
glass plates or of sets of double glass plates described above. Thus, e.g. from 4 to 40,
especially from 10 to 24 equidistant double glass plates, or from 2 to 20, especially
from 5 to 12 equidistant sets of double glass plates, can be mounted together in a
steel rack covered with a suitable, preferably flexible and/or compressible material,
e.g. a thermoplastic polymer such as polypropylene or polyethylene, in such a way
that the glass does not come into contact with the steel construction. The glass plate
assemblies may be provided with suitable supports and guiding strips allowing the
arrays to be mounted in the reacor vessel and combined as horizontal and/or vertical
stacks. A useful stack of arrays of glass plates can e.g. be 3-6 racks stacked vertically
and 2-5 racks horizontally.
[0023] The reactor dimensions are preferably such that a sufficient growth efficacy
can be achieved through vertical circulation, while avoiding excessive internal
pressures. It was found that reactor liquid heights of between 3 and 12 m, preferably
between 5 and 10 m, most preferably between 7 and 9 m, provide an optimum result.
Reactor widths (diameters), i.e. in the direction of the incoming light, are limited by
the transmittance of light through normal ultra-clear float glass and typically not
more than 2.5 m, preferably not more than 2.0 meter; suitable reactor widths are e.g.
between 1 and 1.5 m. If desired, multiple reactor vessels can be stacked, so as to
further upscale and economise the production performance. It is preferred that the
reactor is made of steel, in particular coated steel or stainless steel.
[0024] In an advantageous embodiment, the reactor liquid can be provided with foam
objects, which are moving with the circulating reactor liquid in order to clean the
light emitting surfaces of the glass plates. Examples are sponge-type materials, for
instance sponges made of melamine resin, in the size from e.g. 10 mm up to 30 mm,
introduced at the bottom of the reactor in the up-flow part and collected at the top of
the reactor. In operation of the reactor, the up-flow part and the down-flow part are
regularly switched thus allowing both parts to be cleaned.
[0025] The means for vertically circulating reactor liquid are conveniently provided
by the gas inlet means being arranged vertically below part of the spaces between the
sets of glass plates. In particular, about half of the spaces between the glass plates is
positioned above a part of the bottom of the reactor vessel which is provided with gas
inlets, thus providing the upward flowing part of the reactor liquid, and the other half
is positioned above a part of the bottom of the reactor vessel which is not provided
with gas inlets, thus providing the downward flowing part of the reactor liquid.
Advantageously, the gas inlets are provided in lines below half of the spaces between
the double glass plates. The up-flow and down-flow compartments may be arranged
alternatingly, or there may be a number of up-flow spaces followed by a number of
down-flow spaces.
[0026] The light source comprises mirrors mounted on a sun tracking system
reflecting sunlight to the flat sides of the glass plates. In this way, sunlight is
reflected under a constant angle on the exposed part of the glass plates sticking in the
gas lift loop channels of the reactor during the day.
[0027] In order to reduce the heating effect of the sunlight, the light source
substantially only transmits light wavelengths between 400 and 700 nm. This means
that the light intensity (in candela) of light having wavelengths below 400 nm and
above 700 nm arriving at the glass plates is less than 50%, preferably less than 20%
of the light intensity of light having a wavelength between 400 and 700 nm. For that
purpose, the mirrors can advantageously be provided with a coating in such a way
that the light wavelengths outside the 400 and 700 nm range are absorbed by the
coating and thus not transmitted to the reactor. The skilled person will be able to
select the appropriate UV absorbers and IR absorbers from commercially available
alternatives.
[0028] The gas which is fed into the reactor vessel should comprise carbon dioxide
needed for growth and photosynthesis of the algae. The gas source contains at least
0.5% (by vol.) of C0 2, preferably at least 5%, more preferably at least 10%, most
preferably at least 30% (by vol.) of C0 2, the remainder being any gas, in particular
nitrogen. The source of carbon dioxide can be a combustion gas originating from a
fuel-combusting furnace, or a liquid or solid C0 2 source, or the like. Nutrients, such
phosphorus, nitrogen, potassium ad micronutrients, may be added to the reactor as
known in the art.
[0029] The invention also pertains to a process of producing algal products, such as
algal oils, comprising culturing algae in a reactor vessel as described above and
operated as an airlift loop reactor as known in the art per se. All photosynthetic
microorganism, including green algae, red algae, cyanobacteria, etc., can be grown in
the reactor vessel of the invention, using process parameters as known in the art. The
algae can be harvested and isolated, or the algal products, especially fats for use as
energy source "biodiesel" or as food source (long-chain polyunsaturated fatty acids),
can be collected without the algae being isolated. Other commonly known algal
product that can be produced and isolated by the process of the invention include
algal proteins and carbohydrates, carotenoids and the like.
Description of the drawings
[0030] Figure 1 schematically shows a section in the vertical plane of the reactor
showing the principal parts thereof. A reactor vessel (1) is used for growing the
algae. It has an up-flow channel (2) and a down-flow channel (3), which can alternate
by switching the feeds (4) and (5) of the gas mixing flow (6). The up-flow and downflow
are separated by a vertical plate (7), the water level in the vessel is indicated
with (8). To make this reactor suitable for algae growth, vertical glass plates (9) are
installed in the up-flow and in the down-flow channels. Foam-like objects (10) move
upward in the up-flow zone for cleaning the glass plates.
[0031] Figure 2 shows a section in the horizontal plane of the reactor with the light
mirror system. The vertical pairs of glass plates (9) in the up-flow zone (2) and
down-flow zone (3) are shown. The front parts ( 11) of these plates are protruding
from the reactor wall (12) into the light source (13). These glass plates (9) are used to
transfer light into the reactor which is necessary for growth of the algae (14) inside
the reactor (1). The glass plate consists of 2 layers (15) and (16) with a coating (17)
in between, in which small metal parts are mixed that function as mirror facets to
scatter the light in such a way that the light can enter the water.
[0032] A set of mirrors (18) is mounted on a sun-tracking system (19) to reflect the
sunlight during the day under a constant angle on the exposed part of the glass plates
sticking in the gas-lift-loop channels of the reactor. To reduce the heating effect of
the sunlight, the mirrors (18) are provided with a coating (20) in such a way that only
the wavelengths between 400 and 700 nm, necessary for algal growth, are reflected to
the reactor.
[0033] Figure 3 shows a set of double glass plates in more detail. The set of glass
plates (21) is composed of two double glass plates (9). The set of glass plates
contains 4 glass plates (15,16 twice) with layers (17) of scattering particles in
between. Two glass strips (22,23) are positioned and sealed between the inner glass
plates.
[0034] Figure 4 shows a rack containing an array of double glass plates as a
horizontal section (or top view). Sets of double glass plates (21) are mounted in a
rack having side walls (33) and front and back walls comprising spacing strips (34)
between the sets of glass plates.
[0035] Figure 5 shows the rack in perspective. The rack (30) comprises side walls
(33) and strips (34) for spacing the glass plates as front and back walls. The rack has
an upper part (3 1) and a lower part (32), for supporting the glass plates, and for
allowing the racks to be stacked. A detachable device (35) for hoisting the rack into
and out of the reactor vessel is mounted at the top of the rack.
[0036] Figure 6 shows a stacked reactor (40) containing eight racks (30) as depicted
in Figure 5 . The stacked reactor has a bottom part (41) containing gas distributors
driving the airlift loop (not shown) fed by gas pipes (42) connected to a gas supply
(43), liquid/product lines and a liquid/product exit (not shown), and a top part (46)
containing liquid inlets (not shown) and a gas exit (47). A three-way valve (44)
between gas supply (43) and gas lines (42) allows to change the gas flow from one
compartment to the other thus changing the riser part to downer part and vice versa.
Guiding strips (45) supported by carrier strips (48) allow the racks (30) to be slid out
of and into the stacked reactor assembly in case of e.g. maintenance or replacement.
[0037] The glass plates in the modules are stacked in such a way that they form
continuous vertical channels from top to bottom of the stacked reactor. The
separation baffle (7) between riser and downer parts runs over the height of the glass
modules, so that the space over the glass modules and under is open. As a result, the
reactor content (water and algae mixture) can circulate from riser to downer at the
top, and from downer to riser in the bottom to create a closed loop.
[0038] While Figure 6 shows two stacks of single racks, one serving as a riser and
one as a downer part, more than two racks are equally feasible. For example, the
reactor assembly may comprise three stacks, one as a riser, one as a downer, and the
third one which can be switched off for maintenance or the like; in this case the valve
(44) is a multi-way valve which ca be switched to either of the three or more stacks.
[0039] C0 2 can be injected in the air or gas mixture which drives the circulation loop
and ensures the stripping of oxygen produced in the reactor. There is a level control
in the reactor (not shown) and all water lost by harvesting of the algae and due to
evaporation is pumped into the reactor at any point (total mixed system). Nutrients
are fed into the reactor together with the make-up water.

CLAIMS
A reactor vessel provided with:
one or more liquid inlets and one or more liquid outlets;
one or more gas inlets at the bottom, and one or more gas outlets at the top
of the vessel;
vertically interspaced rectangular double glass plates which are at least
partially submerged in the reactor liquid, said double glass plates having a
light- scattering layer in between and having a flat vertical side being
exposed to a light source,
wherein pairs of said double glass plates are mounted in parallel at a distance of
between 10 and 200 mm, said distance being provided by glass strips having
essentially the same length as the rectangular double glass plates and being
arranged along the long sides of the double glass plates; and
wherein part of the spaces between the double glass plates is arranged vertically
above said gas inlets thus providing for vertical circulation of reactor liquid
between the double glass plates.
A reactor according to claim 1, wherein the light-scattering layer comprises nonuniformities
or particles having a size in the range of 0.2-1.2 mhi, preferably 0.4-
0.8 mih, acting as Mie scatterers.
A reactor according to claim 1, wherein the light-scattering layer comprises
facetted particles having a size in the range of 5 mih to 500 mih, acting as
geometric scatterers.
A reactor according to any one of claims 1-3, wherein the light- scattering layer
comprises a ceramic matrix, preferably silica, containing said light-scattering
particles.
A reactor according to any one of the preceding claims, wherein the glass plates
have a thickness of between 10 and 30 mm and the layer light- scattering layer
has a thickness of between 10 and 1000 mih.
A reactor according to any one of the preceding claims, wherein the pairs of
double glass plates are vertically interspaced at a distance of between 20 and 120
mm, preferably between 24 and 80 mm.
A reactor according to any one of the preceding claims, wherein the gas inlets are
connected with a source of carbon dioxide.
8 . A reactor according to any one of the preceding claims, wherein said light source
comprises mirrors mounted on a sun tracking system reflecting sunlight to said
flat sides of the glass plates, said mirrors preferably being provided with a
coating to reflect only light wavelengths between 400 and 700 nm.
9 . A reactor according to any one of the preceding claims, wherein the reactor
dimensions allow a reactor liquid height of between 5 and 10 m.
10. A reactor according to any one of the preceding claims, wherein foam objects are
moving with the circulating reactor liquid in order to clean the light emitting area
of the glass plates.
11. A set of two or more double glass plates, said double glass plates having an
essentially rectangular surface of between 1 and 4 m length and between 0.5 and
2.5 m width and a thickness of between 20 and 60 mm, having a layer of lightscattering
non-uniformities or particles in between, mounted in parallel at a
distance of between 20 and 200 mm, said distance being provided by glass strips
having essentially the same length as the rectangular double glass plates and
being arranged along the long sides of the double glass plates.
12. A set of double glass plates according to claim 11, wherein said light-scattering
non-uniformities or particles have an average size in the range of 0.2-1.2 mih .
13. A set of double glass plates according to claim 11, wherein said light-scattering
particles are facetted particles having an average size in the range of 10 mih to
500 mih, acting as geometric scatterers.
14. A set of double glass plates according to any one of claims 11-13 mounted in a
rack having means allowing the multiple arrays to be stacked in a reactor vessel.
15. A process of producing algal products, such as algal oils, comprising culturing
algae in a reactor vessel according to any one of claims 1-10, optionally
harvesting said algae and isolating said algal products.