CHAPTER 4: CONCEPTUAL PROCESS DESIGN AND SIMULATION OF MICROALGAE OIL
4.5 PROCESS DETAILS
The main details regarding the flow chart of the operations are summarized in table 12.
The table explains in details how unit processes are structured. These processes are presented in the order in which they take place, from the cultivation of microalgae to the production of jet fuel.
Table 12: details and discussion of the process design
Process
(from microalgae cultivation to jet fuel
Unit name /Code
On the flow chart
Process aim Flow of Operations and simulation conditions
Microalgae cultivation
CULTTANK Cultivation and biomass
production
Cultivation of biomass is the first step of the entire process. It is taking place between 20 and 30 OC. This is the simulated temperature range for a photobioreactor or open pond with culture growing under solar light. The biomass enters the plant via stream 1 in the cultivation tank modelled with one inlet and one outlet. Practically, nutrients such as CO2, Nitrates and phosphates will be supplied to the growing
culture via
stream 1.
Domestic wastewater or seawater can be used as a growing media. This means that a wastewater treatment plant (also suggested by Roberts et al., 2013 [37]; Max et al., 2014
[14] ) can serve as a cultivation option.
Alternatively, a farming algae facility on the sea can be also considered for the same application especially for marine species such as Nannochloropsis sp. This can be a prospective option to be studied in detail.
90 Microalgae
Harvesting
and
Physiological manipulation
HARVEST
-Collection of biomass needed ( Harvesting) for downstream processes
-To boost lipid content of algae cells
(Physiological manipulation)
This step can be achieved at ambient temperature and atmospheric pressure.
Harvesting will take place in a sedimentation tank modelled with one inlet and two outlets. The biomass generated from the cultivation step should be pumped to the sedimentation tank via stream 8. The use of flocculants or other fast and effective harvesting options can be undertaken to speed up the process. The design is flexible in this regard. Physiological modification can take place in the sedimentation tank after harvesting microalgae. After dewatering the biomass,
the volume of the wet biomass can be subjected to the stressing process to manipulate and modify the cells physiology. This is achieved through nutrient starvation as mentioned in the laboratory experiment. The simulation used biomass that is deprived with all nutrients for a period of 3 days as in the laboratory experiment. This will bring a change in cell metabolism, causing an increase in the lipid content. The cells are being stressed by keeping them in harsh conditions. An increase in lipid content of 80% can therefore be expected. This is done in highly purified water, which is nutrient free.
91 Bio-oil
extraction EXTRACT
Extraction of algae bio-oil
needed for
conversion into jet fuel.
Extraction of bio-oil is completed on wet biomass after cells physiological modification. It uses a mixture of solvent made up of chloroform and methanol in 1/1volume ratio as undertaken during the laboratory experiment. This unit, modelled with one inlet and one outlet, should have a boiling system for solvent evaporation at temperatures depending on the solvent used, in this case the temperature was between 60 and 70 °C. From the harvesting process, the wet biomass is pumped to the extraction unit via stream 12. The solvent is recycled back into the unit via stream 15.
Stream 18 is used for the removal of wet biomass sludge. Once the biomass sludge is removed via stream 18, the solvent is taken
out via stream 15.
A condenser will be needed on stream 15 to liquefy the solvent vapor. Therefore, once the wet biomass and solvent are taken out of the EXTRACT biooil can be collected into the oil tank via stream 14
Bio-Oil storage OILTANK Storage of algae bio-oil after extraction. This is done to supply continuously the plant with crude oil needed for conversion
processes.
Simulated in such a way that algae bio-oil should be stored in a tank modelled
with one outlet and one inlet operating at ambient temperature before it is pumped to conversion processes. The storage time depends on how long the conversion operations can last.
Hydrocracking CRACKER Breaking down of larger
92 hydrocarbon chains.
Algae bio-oil is pumped to the cracking unit via stream 2, the simulated cracking temperature has to vary between 200 and 400 OC. This is consistent with the specifications from the literature for effective cracking and minimizing coke formation [24,45,46]. The upgrading unit supplies hydrogen to the cracking unit via stream 19 to act as a catalyst during cracking. The CO2 generated from this step can practically be channelled to the cultivation tank to be used as nutrients via stream 1. The cracking unit has one inlet and one outlet.
Distillation (fractionation)
FRACTION Separation/fractio nation of light, middle and heavy fractions of hydrocarbon broken chains
The cracked algae bio-oil is pumped to the fractionation unit via pipe 3, simulated distillation takes place from 70 to 300OC at a pressure of 1 bar. Stream 10 will collect the light-end distillates and will send them to the reforming unit. The jet fuel made of middle-end distillates will be collected via stream 11. This will happen up to a maximum temperature of 300 OC considered as end point for middle-end distillates. Heavy-end distillate fractions will be collected at temperatures beyond 300 OC and sent to the reactor for upgrading.
The distillation equipment is modelled using resources found in the simulation package.
93 Partial
evaporation FLVS
Recovery of light- ends liquid distillates to be
sent for
reforming.
Light-end distillates and a small fraction of middle-end distillates will get out of the distillation unit in a mixed liquid-vapour phase between 50 and 70 OC. This mixture is channelled to the flash vessel via stream 21. The pressure within the flash vessel is 1 bar. From this flash vessel a liquid phase will be collected at the bottom via stream 22 and a very light vapour or liquid phase will be released via stream 23.
The liquid phase from the flash vessel will also be sent to the reforming unit. In practice this means that stream 22 will be connected to the reforming unit. The flash vessel is modelled with one inlet and two outlets.
Reforming REFORMER Partial
dehydrogenation and Converting light-ends
distillate fractions into high-end distillates
fractions which are middle and heavy-end
distillates.
From the reforming unit, all liquid phases either from the fractionation unit or flash vessel coming respectively from streams 10 and 22 will be pumped to the reactor or upgrading unit via stream 9, this stream will be made of hydrogen from the
dehydrogenation as well as high-end distillate compounds as mentioned earlier.
The equipment was modelled according to the resources available in the package with one inlet and one outlet.
94 Upgrading REACTOR Hydrocracking of
reformed hydrocarbons.
These
hydrocarbons are sent to the fractionation unit for separation.
This unit is also supplied by steam allowing the production of H2
that is sent to the cracker for hydrocracking
Upgrading is a catalytic process that will produce another cracked bio-oil to be fractionated again. This process improves the quality of a material by using chemical reactions to remove any compounds present in trace amounts that make the
fuel quality undesired. The bio-oil will be sent to the fractionation unit via stream 17. This means this stream will practically be connected to the fractionation unit.
The hydrogen from the reformer might be insufficient because depending on the volume of distillates generated by the reforming unit. The upgrading unit will therefore be supplied with more hydrogen via stream 16 to act as catalyst in order to speed up the upgrading process. This hydrogen is needed for the effective reduction of oxygen content from algae oil.
Excess of oxygen in the fuel can have a good influence on the fuel combustion but it causes nitrogen oxide (NOx) emissions due to increased combustion temperature and extra oxygen47
The remaining hydrogen in the unit will be channelled to the cracking unit as
catalyst via stream 19 as mentioned earlier after upgrading process is completely done. Two inlets and two outlets were used for this type of equipment.
95