DK180369B1 - Column photobioreactor - Google Patents

Abstract

A tubular photobioreactor for production of photosynthetic microorganisms is devised. The reactor comprises a growth module, consisting of vertical, transparent tubes, mounted between two circular manifolds. Flow direction is in all transparent tubes is vertically ascending. Flow is recirculated in a closed loop to the lower manifold with a pump. Oxygen is removed in an additional loop in a central degasser column that also functions as structural support of the reactor. The degasser applies a falling-film degassing principle. Advantages of the photobioreactor is low mixing- and mass transfer energy costs; all tube and manifold connections assembled in a single factory supplied growth module, which makes the reactor economic; the vertical orientation of the tubes furthermore provides a more photosynthetically efficient use of the absorbed daily solar irradiation.

Description

| DK 180369 B1 Description Flold af invention The invention concerns a tubular photobioreactor which is a vessel for the production of photosynthetic microorganisms, including microalgae, photosynthetic bacteria and submersed plant cells. General presentation of the problems that the Invention alms to solve: Note: In the remainder of the description, the target organisms (photosynthetic microorganisms: microalgae, photosynthetic bacteria and submersed plant cells) are for clarity reasons referred to only as microalgae.

Microalgae require an environment characterized by certain ranges of several physical parameters, including light intensity, temperature, acidity, salinity, dissolved oxygen, inorganic carbon source and concentration and certain inorganic and organic nutrients. A photobioreactor is a vessel that is specialized to keep those parameters within suitable ranges.

Some microalgal species are able to grow under conditions that exclude other algal species, such as extreme salinity or alkalinity levels and can therefore be produced in open tanks, but most commercially important species need to be produced as monocultures in closed and specially designed bioreactors.

With outdoor microalgal production, light and ambient temperature vary diurnally and with changing weather conditions and photosynthesis is consequently affected. Other critical parameters are affected in turn, including oxygen concentration, inorganic carbon requirement, pH and nutrient concentrations. Photobioreactor design must therefore accommodate suitable control of those parameters.

Oxygen is a byproduct of photosynthesis and affects growth and photosynthesis negatively and must be disposed of and regulated. Oxygen produced during photosynthesis accumulates as dissolved oxygen and the algal growth medium can reach supersaturation levels up to 500 % before oxygen spontaneously bubbles off as pure O, gas. Supersaturated oxygen levels in a microalgal culture are difficult and energy demanding to reduce and represent a significant part of the operating costs.

Transferring dissolved oxygen to gas form in order to strip it off the medium is referred to as mass transfer. Mass transfer is normally done in a two-phase system by injection of gas bubbles either directly in the culture or by recirculating the culture through a specialized compartment: a degasser in which the mass transfer takes place. In both cases, the dissolved oxygen will diffuse into the gas bubbles towards an equilibrium concentration, according to Henry’s law.

The injection of gas bubbles (sparging) is associated with the problem that an extended contact period with the liquid phase is required to achieve the mass transfer. Injecting 40 atmospheric air at the bottom of a tall degassing column is the normal way of achieving sufficient mass transfer. Producing compressed air for injection at a pressure sufficient for injection at the bottom of a tall liquid filled column is energetically costly and producing compressed air for spargeing is a significant cost factor. The use of solid media such as Raschig rings in a two-phase vessel for gas transfer is not used in 45 photobioreactors because of unwanted biofilm growth on the surfaces.

The falling-film method is a mass transfer method frequently used in some fields of chemical engineering, for instance as evaporator devices, wherein a liquid film is drifting down the inner side of a column that transfers heat to the liquid while a spectrum of turbulence movements in the liquid film is driving the mass transfer. This 50 method has so-far also not found widespread use in microalgal oxygen transfer, but it is a relatively cost-efficient oxygen removal method and the column may be integrated in the scaffolding structure of the photobioreactor to further reduce the construction costs of the reactor.

, DK 1 B1 Accomplishing mass transfer in an algal culture is energy demanding; therefore, the aim in degassing is not fo reduce oxygen supersaturation completely but fo reduce the saturation to acceptable levels. It is generally considered that an oxygen saturation of up to 300 percent is acceptable.

Mixing of the culture is required fo manage (break or diminish) local nutrient and light gradients. A cerfain mixing level with respect to #Fradiafion (light administration) is required or beneficial for the algal growth. It works through shifting the algae between dark and light zones. Mixing with a high frequency (short light periods, from 0 milliseconds to hundreds of milliseconds) has been shown to increase the efficiency of light uiilization but is associated with turbulent flow at high Reynolds munbers or induced by insertion of swirl elements in the reactors or by cresting waves in a fwo- phase system. In all cases, it is energy requiring. Mixing at lower frequencies, such as tenis of seconds, is energetically optimal. H is advantsgecous to enable mixing and 13 degassing independently in two separate circuits which are the tubular growth unit and the degasser column in order that the mixing and degassing can be independently controlled and optimized. The photosynthesis unit normally has a large swriace-to-volmne ratio and surface adhesion of the algal cells fresulting in biofilm formation} can be a serious problem, requiring stopping, cleaning and restarting the reactor. Direct causes for biofilm formation are staenamt fluid zones. both because of the possibility of sedimentation/flotation and because the cessation of convective nutrient transport between cells and medium can increase the adhesive properties of the cells. Tubuicr phofobiorsactors are photobioreactors where the inadiation of the algal biomass is taking place in transparent tubes with a suitable diameter; tubular reactors are frequently chosen for the convendence of construction under craftismanship conditions and availability of transparent construction materials of suitable dimensions; (diameter 2-10 cm, but also larger diameters, 20 cm or more, are sometimes used), Tubular reactors are furthermore convenient in ferms of operation and cleaning as they exhibit a simple and homogeneous flow pattern. Funthennore, they can he designed fo allow biofilm contre} (inchsion of scouring pellets during operation or use of a "pig”) or efficient off-line cleaning. Suxiliory emergy expenditure in tubular pbotobicreactors: to reduce the avxiliary energy use with a photoldoreactor, mass transfer and mixing must be optimized separately and with respect fo mixing, the reactor should be designed to work with a low hubulence level. Flor art and reactor specific design issues, Current art includes two main fypes of tubular reactors: reactors with Sorfzondad fubes (from hereof called horizontal tubular reactors) and reactors with verficad fåes (from 4G bereofcalfed vertical tubular reactors), Horizoatal tubular reactors are known with either a single layer or stacks of horizontal fubes, sonnected as serpentine loops or with manifolds. The tube sections distance between degassing in the case of separate degassing) are generally long fup to 100 nv), The algal 48 culture is recirculated through the fubes by means of a mechanical or air HR pump and mass transfer takes place in a dedicated depasser device. Such reactors are known Fong WO2012107343A1, CNIO6754290, MX2015010837 and U8S2017037348, Horizontal tubulsr reactors are characterized by requiring rather large hydesulic flow to press air bubbles that form in ihe mbes by spontaneous degassing through the tubes and prevent 50 settling of biomass in the fubes, Horizontal mbes experience a high peak noon solar radiation which is poorly utilized by the algal culture, Vertical reactors have a fatter diunal direct sunlight absorption pattern than horizontal reactors (not peaking proncuncedly af noon). This is biologically preferable. Vertical fibular reactors are known from US200913T031AL, WO2I011035350, 55 US 4242681, US201 1037873 and U82017130181. Except for US 2009137031, these reactors are in principle coupled bubble columns with tubes alternating between being airlifts and down comers, US2009137031 describes a single-tube reactor with separate degassing taking place in sparged. recirculation tubes.

4 DK 180369 B1 An industrial plant with 8 m tall bubble columns has been constructed in the Canarias (http://www buggypower.eu/es/). A 5-meter tall, vertical air-lift reactor system based on US2017130181 is currently being installed in Austria (https://ec.europa.eu/eipp/desktop/es/projects/project-8 html).

In WO 2008/010737, a tubular photobioreactor unit with vertical, parallel, all-upstream tubes is described. This reactor is distinct from current invention in that WO 2008/010737 describes a reactor where: a. tubes are placed in a flat plane rather in a ring configuration b. tubes are fed in parallel within a unit and units are fed in series c. no degassing device is disclosed. WO 2008/010737 describes series connected units with recirculation of culture by means of acommon pump. This means that the number of tubes per unit and the number of units per reactor must either be small or the recirculated pump must be very powerful to produce the stated flow velocity. Means to ensure an equal flow in the tubes in case of a large number of tubes is also not described. No means for degassing is disclosed except that oxygen is expected to strip off spontaneously and result in overpressure to be ventilated to the ambient. This means that a working model of WO 2008/010737 could not be effectively applied to oxygenic photosynthetic processes as is the case with the — current invention. State af development of the privy art The prior art generally includes reactors that are challenged by various combinations of high mixing- and mass transfer energy requirements, complex and not easily cleanable designs and costly, craftsmanship-dominated reactor manufacturing. Improvements of prior art to he achileved: Depreciation costs of the photobioreactor, cost of maintenance and operating man-power and use of auxiliary operating energy are, more than anything else, important determinants in a commercially successful reactor design. A relatively small, septically isolated stand-alone unit is preferable both for microbial safety reasons and because landscaping interventions (ground surface levelling and anchoring, for example) can be kept at a minimum. Mass transfer and mixing mechanism should be separate. Falling- film degassing is preferable in terms of mass transfer costs whereas the film, floating over the inner surface of the degasser column is preferable for heat transfer reasons. With the present invention, ideal design basis was determined to be: a. a tubular reactor with vertically placed tubes of short length (< 5meter) b. a separate degassing stage, based on falling-film mass transfer c. amass production facilitated tube-manifold assembly d. a relatively small (< 1 m? culture volume), septically isolated unit. 40 Detaled desoription of the current invention List of figures. 45

1. Distributor manifold, showing ring-shaped conduit with one inlet port and 40 exit ports,

2. Collector manifold, showing ring-shaped conduit with one exit port and 40 inlet ports.

3. A tubular photobioreactor growth unit.

4. A tubular photobioreactor growth unit mounted on the degasser column. 50 5. Pump assembly.

DK 180369 B1 The current invention is a vertical tubular photobioreactor, wherein light absorption and the light reactions of photosynthesis take place in vertical tubes and oxygen removal in a connected degasser.

In a preferred embodiment, the transparent tubes are factory mounted between 2 circular, ring-shaped manifolds [figures 1 and 2] and together 5 constitute a tubular photobioreactor growth module [fig 3]. Another embodiment of the ring-shaped manifold is polygonal.

Circular manifolds are suitable for manufacturing by rotational molding technology, including all ports.

The two manifolds are mounted with all transparent tubes as a single module.

During assembly, the inlet to the lower manifold is placed 180 degrees opposite the outlet from the upper manifold in the horizontal plane, providing under operation opposite pressure gradients along the periphery in the circular manifolds, thereby ensuring an even flow ascending in all the vertical tubes.

In a pipe descending from the outlet port of the upper manifold to the inlet of the lower manifold, a circulation pump is inserted and provides sufficient flow to ensure the necessary upwards directed fluid velocity in the vertical transparent tubes.

The circuit through the vertical transparent tubes and down to the recirculation pump is full flowing; consequently, the required head of the circulation pump corresponds to the dynamic pressure drop in the tubes and manifold.

A preferred embodiment of the photobioreactor has 40 transparent tubes of a length of 5 meter and external diameter of 63 mm and the peripheral diameter of the manifolds is 2 meter.

In the preferred embodiment of the reactor, a commercially available axial pump can provide a velocity of 0.3 m sec" in the transparent tubes at a power consumption of less than 200 W.

At a velocity of 0.3 m sec", the hydraulic turnover time in the tubes is 20 seconds and the oxygen concentration at the maximum photosynthetic rate may increase up to 10 % saturation during the ascent, which is counteracted by the degasser column.

The centrally placed degasser column is functioning also as the support structure element of the reactor [fig.4]. The flow for to the degasser is drawn from the outlet of the upper circular manifold into a centrally placed gas-filled degasser tube.

In one embodiment of the photobioreactor, it is dropped as a free-falling film (not flowing over a surface) to the bottom of a partially gas-filled, ventilated and pressurized degasser vessel from where it is drained and injected into the lower manifold by a second pump, (the degasser pump). In another embodiment, the degasser flow flows down the inner surface of the degasser cylinder.

This allows good heat transfer to the cooling jacket, that surrounds the degasser vessel.

The static head that the degasser pump needs to provide, is equivalent with the distance from the liquid level in the degasser vessel to the top level of the ejector disc.

In a preferred embodiment of the degasser, the said distance can be selected between 1 and 3 meter by adjusting the fluid volume of the reactor, and the power for the degasser pump hence may vary between 20 and 50 W depending on oxygen transfer requirements.

The dissolved oxygen concentration in the reactor power can be used to 40 control the power of both the degasser pump and the recirculation and hence reduce the auxiliary energy consumption of the reactor to a minimum.

The mixing energy consumption is the sum of the energy consumption of the recirculation and degassing pump and is for a standard size reactor (600 liter) < 3 kWhr day™ or 5 kWhr m” day”. It is interesting to note that the auxiliary energy requirement is 45 largely proportional to solar irradiation, which means that photovoltaic panels therefore are ideal to supply the reactors.

The reactor is fitted with the following ports: a. adosing pump, dosing concentrated nutrient solution from a local reservoir 50 into the reactor (concentrated up to 1000 times in comparison with the final medium);

b. a water inlet port, fitted with a solenoid valve and a guard filter that feeds the reactor process water (fresh- or salt water) from a central water supply;

55 c. a CO, addition port on the entry side of the circulation pump;

d. a gas outlet port in the top cover of the degasser vessel, fitted with a pressostat valve;

DK 180369 B1 e. agas-bleed valve in the top-point of the outlet of the collector manifold; f. a harvest port at the bottom of the degasser; and g. adrain port at the lowest point of the reactor (the inlet to the distributor manifold). 5 The reactor thus described is a septically isolated stand-alone device, characterized by producing microalgal biomass with local or central process control requiring supply of pressurized water, mounted outdoors pressurized CO,, and pressurized air.

The septic isolation means that a potential contamination cannot pass from one reactor to another.

Placing reactors in a cluster, there is wide freedom in the choice of distance between reactors and topographical elevation due to the only liquid connections between individual reactors being pressurized medium supply lines.

Mounting a reactor on-site implies mounting and fixing the degasser column and pump assembly [figure 5], placing and fixing the photobioreactor growth unit around the degasser column and mounting a small number of process control items.

Claims (6)

DK 180369 B1 Requirements: A tubular, photobioreactor photosynthesis unit, comprising a plurality of translucent tubes connecting an annular manifold manifold with an annular manifold, wherein the main conductor of the manifold may be: a. Circular or polygonal in the plane described by the ring; b. circular or polygonal in cross section; and c. either open throughout the course or closed by a septum. An entubular photobioreactor growth unit, comprising: a. A photosynthesis unit, according to claim 1; and b. a recirculation conductor connecting the output ports of the above manifold manifold to the input ports of the above manifold manifold and containing a recirculation pump. An entubular photobioreactor unit, comprising: a. A tubular photobioreactor culture unit, according to claim 2; and b. a degasser through which a portion of the stream in the recirculation conductor is pumped. An entubular photobioreactor according to claim 3, wherein the degasser allows said above portion of the stream in the recirculation conductor to be distributed over the inner surface of the upper portion of the degasser chamber and by gravity flows down to the bottom of the degasser chamber from which it can be reintroduced into the manifold manifold. . A tubular photobioreactor according to claim 3, wherein the degasser allows the above-mentioned portion of the stream in the recirculation unit as a free-falling liquid film to flow down to the bottom of the degasser chamber, from where it can be reintroduced into the manifold manifold. The entubular photobioreactor, according to claim 3, further comprising: a. A single liquid inlet port, preferably located on top of the degasser chamber b. A liquid drain port, preferably located on the bottom of the degasser chamber.