1433/06/28 Reactor Design 1 2 1 1433/06/28 Algae Microalgae - - PDF document

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Reactor Design

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Algae

Microalgae Macroalgae

Algae cultivation can be achieved in two ways:

Open ponds Photobioreactors (PBR)

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Contamination by

• ther

fast growing heterotrophs have restricted the commercial production of algae in open culture systems to only those organisms that can grow under extreme conditions. Their mass transfer rates are very poor resulting to low biomass productivity. Easier to construct and operate than most closed systems major limitations in open ponds include poor light utilization by the cells, evaporative losses, diffusion

• f CO2 to the atmosphere, and

requirement of large areas of land.

Open ponds

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• Optimal open pond dimensions

are:50m*5m*0.4m with 100 tons

• f water media.
• Maximum spirulina productivity

is 0.35 g/lit.

• 100

tons capacity pond is 35kg/day

A paddle wheel consists of three structural units: paddle blades, motor and gear box. A paddle blade is made of aluminium, stainless steel or fiber glass, based on the requirements.

 Fiber glass paddle wheels are preferred for algae cultivation in salt water

• ponds. The width and depth
• f the paddle wheel and the

speed of the paddles going around are customized based

• n the pond specifications.

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The Spirulina Paddle Wheel short diameter (30 cm), high rpm

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A photobioreactor is a closed equipment which provides acontrolled environment and enables high productivity of algae.. PBRs facilitate better control of culture environment such as carbon dioxide supply, water supply, optimal temperature, efficient exposure to light,, gas supply rate, mixing regime, etc.

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Cultivation of algae is in controlled circumstances, hence potential for much higher productivity Large surface-to-volume ratio. PBRs offer maximum efficiency in using light and therefore greatly improve productivity. Typically the culture density of algae produced is 10 to 20 times greater than bag culture in which algaeculture is done in bags - and can be even greater. Reduction in evaporation of growth medium.

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Space saving - Can be mounted vertically, horizontally or at an angle, indoors or outdoors. Better protection from outside contamination. More uniform temperature. Capital cost is very high.

Despite higher biomass concentration and better control of culture parameters, data accumulated in the last two decades have shown that the productivity and production cost in some enclosed photobioreactor systems are not much better than those achievable in open-pond cultures. The technical difficulty in sterilizing these photobioreactors has hindered their application for algae culture for specific end-products such as high value pharmaceutical products

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Disadvantages of Photobioreactors

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parameter Reletive Contamination risk Ponds > PBRs Space required Ponds ~ PBRs Productivity Ponds ~ PBRs Water losses Ponds > PBRs CO2 losses Ponds > PBRs O2 Inhibition Ponds < PBRs Process Control Ponds ~ PBRs Biomass Concentration Ponds < PBRs Capital/Operating Costs Ponds << PBRs

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Tubular photobioreactors Flat-plate Photobioreactors Bubble column Photobioreactors

Types of PBR

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Tubular photobioreactor is one of the most suitable types for outdoor mass cultures Most outdoor tubular photobioreactors are usually constructed with either glass or plastic tube and their cultures are re- circulated either with pump or preferably with airlift system. They can be in form of horizontal / serpentine, vertical near horizontal, conical, inclined photobioreactor

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One of the major limitations of tubular photobioreactor : some studies have shown that very high dissolved oxygen (DO) levels are easily reached in tubular photobioreactors . Max 30cm radius due to light penetration problems Motor_pump mixing Maximum spirulina productivity:0.8 g/lit 100 tons capacity tubular system=80kg/day 1200m2 area required for 100 tons of water media Tubular photobioreactor are very suitable for outdoor mass cultures

• f algae since they have large illumination surface area.

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Photoinhibition is very common in outdoor tubular photobioreactors It is difficult to control culture temperatures in most tubular

• photobioreactors. Although they can be equipped with thermostat to

maintain the desired culture temperature, this could be very expensive and difficult to implement.

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Large illumination surface area, suitable for outdoor cultures, fairly good biomass productivities, relatively cheap. Gradients of pH, dissolved oxygen and CO2 along the tubes.

Prospects Limitations

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 Flat-plate photobioreactors have received much attention for cultivation

• f photosynthetic microorganisms due to their large illumination surface
• area. The work presented by Milner (1953) paved way to the use of flat

culture vessels for cultivation of algae.

flat-plate photobioreactors

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Difficulty in controlling culture temperature Possibility of hydrodynamic stress to some algal strains Prospects Limitations Large illumination surface area Good biomass productivities Easy to clean up Low oxygen buildup.

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Bubble column reactors are cylindrical vessel with height greater than twice the diameter. Lower capital and operating cost in vertical column (without moving parts). To decrease contamination and cell damage due to shear. High surface area to volume ratio. lack of moving parts. Relatively homogenous culture environment.

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 Photobioreactor design and operation are discussed in terms of mixing, carbon utilization, and the accumulation of photosynthetically produced oxygen.

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Light energy supply Mixing Medium supply Oxygen Remoal CO2 enrichment Temperature, pH, etc

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Sun light intensity

Photoinhibition region Maximum specific growth rate Light saturation constant

When selecting a light source , both spectral quality and intensity must be considered.

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• The spectral quality of light utilized by algae is defined by

the absorption spectrum in the range of 400 to 700 nm for the chlorophylls and other photosynthetically active pigments Light must not be too strong nor too weak. In most algal- cultivation systems, light only penetrates the top 3 inches (7.6 cm) to 4 inches (10 cm) of the water. This is because as the algae grow and multiply, they become so dense that they block light from reaching deeper into the pond or tank

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Light The major factor in algal growth and productivity A schematic representation of light-response curves, i.e. the dependency of photosynthesis vs

• irradiance. The initial slope of the curve (α) is

the maximum light utilisation efficiency. When the photoinhibition occurs (for high irradiancies), also an optimal value for irradiance Iopt can be detected.

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 Availability and intensity of light are the major factors controlling productivity of photosynthetic cultures In continuous culture  Generally µ increases with increasing irradiance, reaching a maximum value µmax. Further increase in irradiance may actually inhibit growth—a phenomenon known as photoinhibition.

How light affects productivity

S.S D = µ p =DCb P=µ Cb

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Monod relation

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Equation Reference Tamiya et al. (1953) Van Oorschot (1955) Steele (1977) Bannister (1979)

Models for light–dependent specific growth rate

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Kinetic parameters (Eq. (8)) for an outdoor culture of

• P. tricornutum (Acie´n Ferna´ndez et al., 1998)

parameter value

µm .063 Ik 94.3 Ki 3426 a 3.04 b 1.209 c 514.6

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Carbon dioxide (or bicarbonate after it dissolves into the culture medium) is the source of the carbon in photoautotrophically growing cells. The upper and lower limits of CO2 are not well define, but aeration of algal cultures with 5~15% CO2 (or even pure CO2 ) is fairly routine laboratory practice. The required inorganic carbon is most commonly introduced in the form

• f bubbles of enriched CO2 gas mixture.

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 Oxygen is product of photosynthesis, but a high level of dissolved

• xygen inhibits algal growth, even at high concentrations of CO2 .

 The removal of excess O2 is a mass transfer problem comparable to that of the CO2 supply. At present, two major solution of, increasing turbulence, and O2 stripping with air, are open to the photobioreactor design and operation.

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 Nutrient availability is an important factor controlling the levels of the primary productivity of photosynthetic organisms. Media with different compositions are supplied to the cells in the form of carbon dioxide, water and mineral salts in macro or micro quantities. The macronutrients include carbon, nitrogen, phosphorus, hydrogen,

• xygen, sulfur, calcium, magnesium, sodium potassium and chlorine.

The micronutrients needed in trace quantities of micro-, nano- or even picograms per liter are iron, boron, manganese, copper, molybdenum, vanadium, cobalt, nickel, silicon and selenium.  The optimum N: P ratio for phytoplankton growth generally is about 15: 1

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To keep the cells in suspension to enhance light utilization efficiency to improve gas exchange to eliminate thermal stratification to help nutrient distribution. To prevent cells from settling

• Mechanical agitation and bubble break-up often lead to

hydrodynamic stress , resulting into restrictions to the algal growth and metabolic activity.

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 One of the most distinctive environmental factor is seasonal and diurnal in ambient temperature.  Spraying water onto the photobioreactor surface.  Heat exchangers is used to the heating of service water or for warming-up of the culture during nights.

1433/06/28 18  Each strain of algae has a narrow optimal range of pH.  The pH in algal cultures increases contiuously as a result of the depletion in carbon through photosynthesis.  Because pH is so influential, the commercial pH controllers must be used in reactors to optimize growth.

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 Batch culture is widely used for commercial cultivation of algae for its ease of operation and simple culture system. Different phases, which may occur in a batch culture  Lag phase  Exponential phase  Linear growth phase

• In the light limited linear growth phase, the relationship between the

biomass output rate and the light energy absorbed by the culture can be expressed as follows:

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μ X V = I0 A Y μ /Y = I0 A /X V

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Net increase of biomass=Growth-Biomass removal V dX = V μ X dt − F X dt Rate of biomass accumulation = Growth rate - Biomass removal rate

dX/dt = μ X − F/V X

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Net increase in energy content = Energy absorbed by biomass- Energy in outflow biomass V dE = I0 A dt − F X dt/Y dE/dt = I0 A/V − F X/(V Y )

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Model predicted values of carbon dioxide loss, the culture pH, and the dissolved oxygen concentration at tube exit for various lengths of the tubular loop (Camacho Rubio et al., 1999).

 No reliable systematic scaleup method exists for photobioreactors.

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The same value of residence time applies to a longer tube of the same diameter as the smaller one; thus, the flow velocity in the longer pipe must be greater by a factor of length ratios of the larger-to-smaller tubes. Adhering to this criterion ensures that properties of the product stream leaving the longer tube match those established at the smaller scale.

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 Several approaches have been suggested for quantifying the light– dark cycling of fluid in a bioreactor. This approach relies on the velocity of the turbulent microeddies in the fluid. The eddy velocity u is easily determined using the wellknown Kolmogoroff’s model of turbulence

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Approach one

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Approach Two

For any tube of diameter dT, the depth of the light zone dL can be calculated by applying Beer– Lambert law for known level of external incident illumination, the biomass content, the absorption coefficient, and the saturation irradiance value.