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Algae are diverse group of photosynthetic organisms. There are wide range of algae available from simplest unicellular which is cyanobacteria to the complex microalgae which is seaweeds based on multicellular and unicellular. They are most available in aquatic environments due to its fast growth rates.
There are varieties of algae species that should be chosen. Not all of them are suitable to produce green diesel. The maximum production can be obtained from algae with high lipid contents and high growth rates.
The lipid content of algae is depends on particular strain of an algae. It should be consists of triglycerides (TGA) and some other molecules such as polar lipids and fatty acids. (8) That was the factor that can lead to production of fuels. Triglyceride molecule mainly consisted of glycerol esterifies with three fatty acids.
Algae can have 85 dry weight % lipids. Algae also mainly have high production rates. Sometimes, the growth rate can be manipulated from cultivation process as well as environmental factors such as sunlight, temperature and pH. The highest production is 50 grams per square meter. (1)
Depending on particular strains, algae can be produced whether in salt water or fresh water. This is the most critical factors in order to cultivate algae in certain location.
The most suitable algae species that can be used for this purpose is Chlorella vulgaris/Chlorophyta or known as Green Algae. It is ranged from unicellular forms to large seaweeds. Their cell walls are containing cellulose and often use starch as an energy reserved. This starch property is suitable to produce ethanol as well. It also has high growth rate for the stable production of feedstock. Below is the growth rate, copper content and uptake rate for Chlorella Vulgaris/Chlorophyta:
Growth Rate (h-1)
Copper Content (ag. Cell-1)
Uptake Rate (ag. Cell-1)
ENHANCE THE PRODUCTION AND LIPID CONTENTS.
Some algae can be altered genetically to enhance the priority. Mostly the enhancement is regarding to increase the growth rates and lipids content. (2)
Nitrogen is important for algae growth. When there is less nitrogen, there will be nitrogen starvation where many algae tend to produce more TAG in order to store energy within cell. This will cause the increasing of lipid contents within algae. However, this will block the cell division since lack of nitrogen and causing the depletion of production of algae. By using nitrogen deprivation, the lipid contents might be increased but the production will be reduced.
Location is very important in order to cultivate the algae. There are many parameters that need to be concerned to optimize the growth. There are some parameters that required in order to select the location.
Constant sunlight source
In order to locate the plant, the availability is important factor that should be considered. This is to ensure the continuous source of sunlight, water and CO2. The location should be suite for both cultivation production and cost minimization.
Constant sunlight source
Sunlight is needed for algae to provide energy for production. The production will be greatly depleted during seasons when the sunlight is limited. However, too much sunlight with luminance of 195 watts/m2 or greater will harm the algae. Only 10% of these will be required to provide energy for algae. In Malaysia, this kind of problem is very rare since the climate in Malaysia is very suitable for optimal sunlight resource.
Flat land is very important for cultivation system. The inclination will affect the flow and pressure throughout the reactor tubes and disturbing the installation process. Far from urbanization region also is the best place for the plant to be built. Amount of land required depends on the amount of algae production capacity. Land permit also might be required in order to place the plant.
The cultivation system is indeed requiring a lot of water for algae to grow. Depending on particular strain, the fresh and salt water can be used. The water should be cycle able so that it will be more convenient and cost effective.
CO2 is the most important things for algae. It acts as source of carbon used by algae in order to grow. Since the environment cannot provide enough carbon, the additional CO2 should be provided. The makeup CO2 from hydrocarbon treatment is possible to provide additional carbon resource. Besides, the plant also can be built nearby with industrial region where the source of CO2 should be higher compared to other region. Some of gas released by industries contained SOX and NOX. They are actually can provide nutrients for algae. (3)
Temperature will affect the cultivation system. The surrounding temperature should be in a range that algae can live within. The temperature also might affect the lipid contents within the algae.
The cost and utilities will vary throughout the process in converting algae into fuels. Electricity might be required in order to run a process such as for cultivation, extraction and lipid processing.
There are various ways to cultivate algae. They can be open pond, closed pond and photobiorector. There are also regulating the nutrients, sunlight, CO2, temperature, pH and some other parameters.
Open air raceway ponds (Extensive Ponds)
paddles, piers & saddles
Outtake - harvest
Feed - water, nutrients
This is the simplest cultivation system. The inoculants algae being placed in a natural or artificial pond, fed the nutrients including CO2 and being allowed to grow for a period time. This method is considered as economical since the open bodies containing water can be used without having the expensive cultivation system. These open ponds are good for mass cultivation of algae since they are easy to construct and clean. (4)
However, there are some limitations to the open ponds. It is impossible for us to control the environmental conditions. Open ponds have high possibility of evaporative loses, diffusion of CO2 into atmosphere which known as pollutant and contaminants from other species of algae. The absence of stirring mechanism to agitate the algae will reducing the transfer rates of nutrients, yet reducing the production of algae.
This reactor is normally in closed system. This system involving more structured and controlled process of cultivating process. Natural or artificial sunlight can be used as energy booster for the algae. Different parameters for algae growth also can be monitored. It has large illumination areas of reactors to optimize the sunlight rays received by algae. The mass transfer rates between algae, water and nutrients more effective with help of mixing in the tanks together with low shear stress. This can lead to better productivity of algae. It is also possible to decrease photo-inhibition, which occurs when algae receive a very high concentration of solar radiation, resulting in a decrease in productivity. (4)
This cultivation system also can leads to production of hydrogen. To produce hydrogen gas from algae, it requires the use of a simple solar reactor such as transparent box with low energy requirement. (5)
This system also has limitations. Sophisticated materials and components might be required in the system. This will involves high cost in order to produce large scale of cultivation process compared to open ponds system. The rapid growth of algae at surface of reactor will inhibit the growth rate of algae at lower level due to lack of solar radiation. Due to high concentration of CO2 and oxygen in the water, the pH gradient will be formed.
OPTIMAL CONDITION FOR ALGAE CULTIVATION
The optimal condition of parameters will greatly affect the growth rates of particular strain of algae. These parameters are actually varied for different species of algae.
It is very important. If the value is too low, the growth will be slow. However, if too high temperature, the algae will be died. Optimal temperature for the algae to grow should be in range of 16-27oC. However, some species can live within 30oC. (6)
The suitable pH value is in range of 7-9 with optimum around 8.2-8.7. (6) As algae growing, the pH will be increased but can be lowered by CO2. Thus, the pH can be controlled by injection of CO2 but will be depleted due to carbon source for the algae.
Either natural or artificial can be used. Too much illumination can be harmed for the algae. Only 10% of 195watts/m2 required for algae growth.
The oxygen generated should be lower than 400% of air saturation values to prevent inhibition of photosynthesis or causing photo-oxidative damage to algae.
ALGAE OIL EXTRACTION
Basically, the algae will be lyses while it still in slurry. This process is involving injection micro bubbles of CO2 to rupture the algae cells. The broken cells then being separated into lipid, water and biomass layers in gravity clarifier. This required thermal dewatering for conventional extraction. The energy could be saved up to 90% as well as reduction in capital cost.
Open Pond System
Algae Oil Storage Tank
Quantum Fracturing Device
Breaking the Cell Wall
The raw material fed into the process is slurry from cultivation system at a certain flow rate. The first path is through the Quantum Fracturing Device. There are electromagnetic radiation and CO2 micro bubbles injection. (7) The CO2 will be injected at a high pressure in order to agitate the cell and also to alter the pH value. The injection of micro bubbles results the cell wall of algae to be ruptured. Then, the broken cell is carried in slurry into the gravity clarifier.
This will involve a number of clarifier in parallel. This is to provide an optimum residence time in order to separate the broken cells into layers of water, oil and biomass. The oil will siphoned off throughout the oil stream. The oil can be assumed as 90% oil yields. This is based on OriginOilTM reported achievable yield within range 85%-97% in bench-scale testing. (8) Lipid composition can be assumed as 80% is consists of TAG requiring further process to become fuels.
The leftover water and biomass mixture will be separated in clarifier. The dewatering process might involve series of stages. The water will be removed by centrifuge. The wet biomass will be dewatered to 20% water by weight. The water from centrifuge and clarifier will be recycled back to the algae cultivation. Most of the water from feed will eventually recycled back to the cultivation system.
The moist biomass will enter the dryer. The water will be boiled until a dry biomass less than 10% of water is left within. The dry biomass can be sold for many purposes such as for fertilizer and ethanol production.
From the beginning of the process, CO2 will be purchase in liquid formed and will be expanded for injection process. The CO2 will be get from the vendor with reasonable price. Furthermore, the CO2 from the fuel production process cannot be used since it is in gas form.
EQUIPMENT LIST AND DESCRIPTION
Quantum Fracturing Device
Technology with combination of pH modification, electromagnetic field, and CO2 micro bubbles injection so that the cell wall of the algae can be ruptured in order to separate the oil and biomass.
It enhances the separation of algae oil, water and biomass from previous device. The stream will flows into the unit and will be separated into three layers. The oil which has lower densities will be at the top of layer and will be skimming for further processing. The biomass will sinks to the bottoms and will be sent to centrifuge. The water left within will be recycled to cultivation system. This equipment operates at ambient temperature and standard atmospheric pressure. At least 3 clarifiers will be needed to provide sufficient residence time.
Series of centrifuges will be needed to dewater the biomass from clarifier. Continuous solid bowl centrifuges will be used to process large capacity of algae oil. There might be six centrifuges needed in this process.
This is to dewatering the moist biomass to reduce until less than 10% of water within the biomass before being extracted. The solid biomass can be sold to another party to generate extra income as well as to treat the waste. Number of drum dryers will be needed for this purpose. The water will be evaporated when being heated in high temperature.
HYDROTREATING OF ALGAE OIL
The purpose of this process is to produce high quality fuels. There are several ways of this process which lead to different products. First is the conversion of algae oil to FAME biodiesel through transesterification. For the second way, it is much favorable, where the algae oil is converted to green diesel or known as n-alkanes either via thermocracking or hydrotreating.
It can convert the triglycerides from algae into n- alkanes in more efficient and economical. The triglycerides will react with hydrogen at a high temperature and pressure together with catalyst. The product would be the straight alkanes (diesel), CO, CO2 water, methane and propane. After several separation methods, the product is alkanes with carbon number of C13-C20. These products are suitable to blend together with conventional fuel or further process to produce green diesel.
There are two ways to implement hydrotreating process, which are the feedstock co-fed with the conventional diesel feed and feedstock is fed into diesel hydrotreating unit. There are some drawbacks from co-fed process which are:
Large amount of hydrogen required to treat the algae oil
Large amount of water produced from algae
Large amount of CO and CO2 from algae oil
Hydraulic capacity constraint of existing plant
The feedstock might containing trace metal that could deactivate catalyst much faster
So, the standalone hydrotreating unit should be suitable to deal with algae oil and provide more efficient process and better yields.
The hydrotreating process involves the presence of catalyst. The catalyst operates with various purposes which are:
Metal function of catalyst with high hydrogen pressure will give the saturation of hydrocarbons in the triglycerides
The acid function of catalyst will give the cracking of C-O bonds
The choice of catalyst is so much important. The composition of products will be affected due to hydrodeoxygenation, decarboxylation and decarbonylation processes depends on type of catalyst selected. The typical catalysts being used in hydrotraeting process are:
Based on the study by Sotelo-Boyas, Liu and Minowa, the NiMo/Ï’-alumina is the best catalyst that can be used in this process. This is due to its hydrogenation activity and mild acidity. Besides, it is cheaper than the other catalysts. (9)
Heat Exchanger 2
Air Cooler 1
Heat Exchanger 1
Feed Surge Drum
Air Cooler 2
Air Cooler 3
The operation and background of the hydrotreating process will be discussed here. There are some components available in this process which are:
Preparation of triglycerides and hydrogen feeds
Gas scrubbing and recycle
This process has been modeled to treat the vegetable oil that being used by UOP/ Eni Green DieselTM Process. (48) This hydrotreating process is widely used in petroleum industries around the world. This process also has been licensed by vendors such as UOP, Haldor Topsoe and some others. This process also been used to improve the quality of petroleum products. But, the initiative here is to convert the crude oil into triglycerides as the feedstock. However, some of the adjustment is needed to be designed in order to account the properties of this kind of feedstock. Some of the adjustment might include:
Additional quench zone in reactor to fill the thermodynamic properties
Makeup gas stream
Recycle gas stream
Preparation of Triglyceride stream
The triglyceride from algae is collected in a large storage tank and pumped into a feed surge drum. This will ensures the steady mass flow rates and ambient temperature. The feedstream is pumped o a pressure in about 50 bars. The temperature of the outlet stream is increased when passing through two heat exchangers. A fired heater is used to heat the feed stream to target reaction temperature which is in range of 300oC- 400oC.
Makeup hydrogen from PBR and hydrogen obtained from vendors is entering the hydrotreating unit at pressure of 10-25 bars and ambient temperature. Compressors will increase the makeup hydrogen to pressure of recycle gas stream which is in range of 40-50 bars. The hydrogen stream is mixed with the recycle gas stream and is compressed to a pressure in range of 40-60 bars before entering the hydrotreating unit.
In the reactor, the feed stream will reacts with hydrogen at high temperature within range of 300 oC -400oC and high pressure within a range of 40-60 bars over a catalyst. In this unit, we would like to use NiMo catalyst due to properties and cost effectiveness. The triglycerides is hydrogenated and converted into free fatty acids and propane components. The intermediates of fatty acids will be converted into straight alkanes through a reaction. Several reactions are actually can be used which are decarboxylation, decarbonylation and hydrogenation. (47)
The carboxyl group will be split off from fatty acids which resulting the formation of n-alkane chain with loss of one carbon.
R-CH2-COOH ƒ RH-CH3 + CO2
The carbonyl group will be eliminated from fatty acids and also forming n-alkanes with loss of one carbon.
R-CH2-COOH ƒ RH-CH3 + CO + H2O
Hydrogen is added to the fatty acids which forming n-alkanes without loosing carbon
Besides, there are two side reactions that occur simultaneously in hydrotreating reactor. They are:
Water gas shift: CO2+H2 â†” CO+H2O
Methanation: CO+3H2 â†”CH4+H2O CO2 ,kkm
The extent of these two reactions can be inferred by hydrogen consumption in excess of three pathways. Studies from Haldor Topsoe found that 50% of CO2 shifted to CO (46) and the industrial consultants claimed that the extent of methanation is approximately 90%.
The triglycerides enter from the feed stream and the three reactions occur simultaneously within the reactor. Haldor Topsoe estimated that the reactions occur with distribution of 32% of fatty acids proceeds with decarboxylation, 32% proceed with decarbonylation and the remaining proceeds with hydrogenation process. (46)
In order to recover the heat contained in the reactor effluent, the effluent passing through the heat exchangers. Some heat is transferred to the feed stream before reaching High temperature Separator. The flash drum separates the liquid and n-alkanes from the gases. The liquid stream then passes through another heat exchanger then straight away to the Product Stripper.
The vapor from High Temperature Separator is cooled down using air finned cooler. Then, the vapor passes through the Low Temperature Separator. Three phases separator separates the vapor from entrained n-alkanes which returned back to the n-alkane stream. Meanwhile, water will be sent to water treatment facility. The vapor is treated by Amine Scrubber unit and recycled back to the hydrotreater unit.
The pressure in n-alkane stream should be reduced before being sent to the Product Stripper. Steam is used as the stripping fluid to remove dissolved gas from the liquid of product. The stripper removes dH2dissolved H2, H2O, CO, CO2 and light hydrocarbon gases from the main product streams.
The top product is being cooled by air cooler before passing through accumulator. The vapor or offgas is separated from water and light hydrocarbon. The bottom stream contains n-alkane with some dissolved H2O. It is further cooled by air cooler and passes through the Decanter. The residual water is separated from the n-alkane. The product is being pumped to the Product storage Tank. The product mainly consists of straight chain paraffin C13 to C20. It can be used for blending with refinery diesel or further process to produce green diesel and some other fuels.
Gas Scrubbing and Recycle
The Amine Scrubbing system removes CO2 and some other particulates. The treated gas from the scrubber is recycled back to the cultivation system to provide the carbon source for algae. This also used as waste management to prevent the emission of pollutants. The propane and methane can be recycled back to the hydrotreating unit for further reactions.
EQUIPMENT LIST AND OPERATING CONDITION
Increase the pressure of makeup H2 stream
Increase the pressure of recycle strea, to reactor pressure
Heat Exchanger 1
Heat the feed stream from ambient to higher temperature
Heat Exchanger 2
Heat the furnace stream to the furnace inlet temperature
Heat the feed to inlet temperature
Air Cooler 1
Cool the vapor stream from top product of High Temperature Separator
Air Cooler 2
Cool the bottom stream leaving the stripper
Air Cooler 3
Cool the top stream leaving the stripper
Increases the stream pressure
Converting triglycerides into alkanes
To further treatment to produce green diesel and some other types of fuel products
High Temperature Separator
Separate light gases from liquid product stream
Low Temperature Separator
Separate light gases and water from liquid product stream
removes water vapor and light gases from product stream
Remove residual water from product stream
Separate the offgas from light gases an water
Removes CO2 and other particulates before recycling to the reactor and cultivation system
Feed Surge Drum
Holds the triglycerides stream to provide steady feed to the rpocess
*****STARNDARD THERMODYNAMIC PROPERTIES OF CHEMICAL SUBSTANCES*****
Sublimes at -78OC
*m = melting point at 1 atm
*c = heat fusion at 1 atm
*b = boiling point at 1 atm