Detailed design and optimization of a sustainable micro-algal biofuel process plant

Detailed design and optimization of a sustainable micro-algal biofuel process plant Abstract A sustainable micro-algal biofuel process plant was hypothetically and detailed-designed for the production of value products from algae. The process plant cultures algae through controlled growth and processes: production of biomass, extraction of lipids and biofuels and refinement into finished products. The start-off feedstock was Nannochloropsis spp. and the four end products produced were bio-kerosene, methane, biomass slurry and CO₂. The algae were grown to maturity within the least allowable possible time under controlled conditions. From an in-depth analysis of the customer specifications, concept development and selection, the study defines an overall depiction of the end products from the concepts evaluated by using the Pugh matrix and Six sigma methodologies. DMADV approach was implemented.The process finally generated an economically viable algae biomass for conversion to biofuels and other viable products. This paper focuses on the process planning, criteria/concept selection, design optimization with necessary characterizations which were filtered into measurable characteristics for the design of an operable, sustainable functioning plant. 1 INTRODUCTION Research into harnessing natural renewable resources in a responsible and conservative manner has allowed for the discovery of the algae-biofuel concept. An alga to biofuel process plant is a technology that stems from the basic formation of the growth of algae to biomass production and to refined products. The proposed plant allows for growth, extraction and refinement. The chosen system to continue the design process is of a controlled brackish water process plant. This article presents the detailed design which is a continuation of the conceptual design study earlier published [1]. This involves the calculations, flow diagrams as well design models which were developed. This article will also detail the process of each sub-process unit/function of the plant. Emphasis was placed on the concept evaluation and the guideline/selection criteria of the selected and its working principles which were captured in our earlier study [1]. The aim of this study is hypothetically present a detailed design and optimization of a micro-alga biofuel process plant that will stand the test of time, which will be sustainable as grown in a brackish aquaculture for enhanced product ion and product recovery. 1.1 Algae strain Algae and aquatic biomass have the potential to provide a new range of ‘third generation’ biofuels, including jet fuels. Their high oil and biomass yields, widespread availability, absent (or very reduced) competition with agricultural land, high quality and versatility of the by-products, their efficient use as a mean to capture CO2 and their suitability for wastewater treatments and other industrial plants make algae and aquatic biomass one of the most promising and attractive renewable sources for a fully sustainable and low-carbon economy portfolio [1, 2]. The technologies are evolving in Africa where the potentials and capabilities of diverse strains are dominant in addition to conducive growth environments and climate. These include high solar radiation, increased environmental temperature, humidity, dense vegetation and other factors. Productivity is higher in the controlled, contained environment of a photobioreactor, but capex and opex are also both substantially higher than for open systems [1]. Significant investment in research is required before high levels of productivity can be guaranteed on a commercial scale [2]. Algae to biofuels plants may be developed on land adjacent to power stations, for converting the carbon dioxide from exhausts into fuel. 1.2 Algal bio-refineries In addition to producing oils, algae are rich sources of vitamins, protein and carbohydrates. Micro-algal species with their diverse value added products are gaining research interests in the development of strategies for integrated bio-refineries just like the conventional fossil refineries [2]. Their expeditious growth rate, in addition to their comparatively high lipids, carbohydrates, food and other nutrients production and relatively short period of production are good parameters for all-inclusive integration of different production and extraction strategies in order to maximize final products as presented by Eloka-Eboka and Inambao [2] in their recent study. The following steps have been identified for development of micro-algae bio-refineries: Development of mild and efficient cell disruption, extraction and fractionation technologies; effective technologies for separation of carbohydrates, proteins and lipids; lipid/oil refining technologies; improvement of energy consumption and environmental performance, decrease of capital costs; integrate knowledge and facilities for oil, food and fine chemical industry; and biomass provision (quantity and quality). In order to continue to develop a sustainable micro-algal biofuel process plant, an algae strain is required to grow in the concept system which in this study is in brackish water aquaculture. From Table 1 describing algae species with their corresponding lipid contents, the brackish species that are in the list exist amongst others are: Cylindrotheca sp., Nannochloropsis sp., Nitzschia sp. and Schizochytrium sp. And from the above strains from the rest of the fresh water, Nannochloropsis sp. and Schizochytrium sp. have the highest oil yield by weight, (30–68%) and (50–77%), respectively. While Schizochytrium sp. has the highest by weight oil content, the type of lipid it produces is a free fatty acid that is less efficient at producing bio-kerosene and is very specific to its natural habitat. Hence Nannochloropsis sp. is the chosen algae strain as presented in Table 2, Nannochloropsis sp. is found from the marine habitat near river mouths but is known to survive in fresh water with negligible saline conditions. It shows promise for its high lipid count and is currently used in the food industry, the cell structure of this type of algae can be modified to produce a greater amount of lipids/oil by genetically modifying its ability its structure [4, 5]. The six sigma methodology employed adopted the strategy of deconstructing the optional processes and scaling down the most optimal process. It therefore defined and evaluated each feedstock and possibility of improved yield and viability putting into consideration product and process efficiencies, feasibility, economic viability and profitability. There was futuristic room for process quality improvement and turnover on investment. These were ranked before final selection and adoption of this sustainable micro-algal biofuel plant to be designed and implemented. Out of the two main approaches of six sigma, DMAIC and DMADV; DMADV (Define, Measure, Analyse, Design and Verify) was adopted. DMADV approach is typically fitted for new processes, plants and products and so it was implemented in this study to arrive at the present design and optimization. Table 1. Algae species and their corresponding lipid content [3]. Species lipid content % Dry weight Species lipid content % Dry weight Anabaena cylindrical 4–7 Botyococcus braunii 25–80 Chlamydomonas reinhardtii 21 Chlorella emersonii 25–32 Chlorella pyremoidosa 2 Crypthecodinium cohnii 20 Chlorella vulgaris 12–22 Cylindrotheca sp. 16–37 Dunaliella bioculata 8 Dunaliella primoleta 23 Dunaliella salina 6 Dunaliella tertioleta 23 Euglena gracilis 14–20 Hormidium sp. 38 Nannochloris sp. 30–50 Scenedesmus dimorphus 16–40 Schizochytrium sp. 50–77 Scenedesmus obliquus 12–14 Spirogyra sp. 11–21 Synechoccus sp. 11 Prymnesium parvum 22–38 Nitzschia sp. 31–68 Species lipid content % Dry weight Species lipid content % Dry weight Anabaena cylindrical 4–7 Botyococcus braunii 25–80 Chlamydomonas reinhardtii 21 Chlorella emersonii 25–32 Chlorella pyremoidosa 2 Crypthecodinium cohnii 20 Chlorella vulgaris 12–22 Cylindrotheca sp. 16–37 Dunaliella bioculata 8 Dunaliella primoleta 23 Dunaliella salina 6 Dunaliella tertioleta 23 Euglena gracilis 14–20 Hormidium sp. 38 Nannochloris sp. 30–50 Scenedesmus dimorphus 16–40 Schizochytrium sp. 50–77 Scenedesmus obliquus 12–14 Spirogyra sp. 11–21 Synechoccus sp. 11 Prymnesium parvum 22–38 Nitzschia sp. 31–68 Table 1. Algae species and their corresponding lipid content [3]. Species lipid content % Dry weight Species lipid content % Dry weight Anabaena cylindrical 4–7 Botyococcus braunii 25–80 Chlamydomonas reinhardtii 21 Chlorella emersonii 25–32 Chlorella pyremoidosa 2 Crypthecodinium cohnii 20 Chlorella vulgaris 12–22 Cylindrotheca sp. 16–37 Dunaliella bioculata 8 Dunaliella primoleta 23 Dunaliella salina 6 Dunaliella tertioleta 23 Euglena gracilis 14–20 Hormidium sp. 38 Nannochloris sp. 30–50 Scenedesmus dimorphus 16–40 Schizochytrium sp. 50–77 Scenedesmus obliquus 12–14 Spirogyra sp. 11–21 Synechoccus sp. 11 Prymnesium parvum 22–38 Nitzschia sp. 31–68 Species lipid content % Dry weight Species lipid content % Dry weight Anabaena cylindrical 4–7 Botyococcus braunii 25–80 Chlamydomonas reinhardtii 21 Chlorella emersonii 25–32 Chlorella pyremoidosa 2 Crypthecodinium cohnii 20 Chlorella vulgaris 12–22 Cylindrotheca sp. 16–37 Dunaliella bioculata 8 Dunaliella primoleta 23 Dunaliella salina 6 Dunaliella tertioleta 23 Euglena gracilis 14–20 Hormidium sp. 38 Nannochloris sp. 30–50 Scenedesmus dimorphus 16–40 Schizochytrium sp. 50–77 Scenedesmus obliquus 12–14 Spirogyra sp. 11–21 Synechoccus sp. 11 Prymnesium parvum 22–38 Nitzschia sp. 31–68 Table 2. Nannochloropsis sp. specification [10]. Specification Density 920 kg/m3 Lipid percentage by weight 65% Ratio of algae to water needed for growth (per parts) 1:3 Specification Density 920 kg/m3 Lipid percentage by weight 65% Ratio of algae to water needed for growth (per parts) 1:3 Table 2. Nannochloropsis sp. specification [10]. Specification Density 920 kg/m3 Lipid percentage by weight 65% Ratio of algae to water needed for growth (per parts) 1:3 Specification Density 920 kg/m3 Lipid percentage by weight 65% Ratio of algae to water needed for growth (per parts) 1:3 2 DESIGN CALCULATIONS 2.1 General assumptions From anticipated specifications, the desired plant must produce 100 ton of algae by weight per month. The hybrid bioreactor will grow the algae to maturity in 4 weeks. 25 ton of algae will be produced per week. 5 ton will be processed per day. 8 h working per day. 2.2 General calculations [6] Volume of Algae produced per week=MassDensity (1) VolumeofAlgaeproducedperweek=MassDensity;V=25×103920;V=27.1m3≈30m3 (2) CombinedMass of substance=(ρ×V)algae+(ρ×V)water (3) Substituting, Mass=(920×30)+(1000×90)=117600kg; Densityof algae and water=MV;117600120=980(kg/m3) 2.3 Hybrid bioreactor design Assumptions: 30 m3 required hence 90 m3 water required (derived from Table 2). Dimensions of tank is 4 m × 10 m × 3 m = 120 m3. Material used is acrylic [7]. ThicknessofTank=(βqH2σ) (4) where β = coefficient from length/height; σ = maximum permissible stress (73 MPa) [8]; H = height of tank; and q = maximum permissible pressure [6, 9]: q=(ρgh) (5) And q=980×9.81×3; therefore, q=28841.4Pa (Table 3). Table 3. Ratio of length/height and its coefficients [7]. a/b 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0 ∞ β 0.2874 0.3762 0.4530 0.5172 0.5688 0.6102 0.7134 0.7410 0.7476 0.7500 α 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421 γ 0.420 0.455 0.478 0.491 0.499 0.505 0.502 0.502 0.501 0.500 a/b 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0 ∞ β 0.2874 0.3762 0.4530 0.5172 0.5688 0.6102 0.7134 0.7410 0.7476 0.7500 α 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421 γ 0.420 0.455 0.478 0.491 0.499 0.505 0.502 0.502 0.501 0.500 Table 3. Ratio of length/height and its coefficients [7]. a/b 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0 ∞ β 0.2874 0.3762 0.4530 0.5172 0.5688 0.6102 0.7134 0.7410 0.7476 0.7500 α 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421 γ 0.420 0.455 0.478 0.491 0.499 0.505 0.502 0.502 0.501 0.500 a/b 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0 ∞ β 0.2874 0.3762 0.4530 0.5172 0.5688 0.6102 0.7134 0.7410 0.7476 0.7500 α 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421 γ 0.420 0.455 0.478 0.491 0.499 0.505 0.502 0.502 0.501 0.500 Ratio of length to height, 10/3 = 3.333. Interpolation is required [11]: Y2=(X2−X1)(Y3−Y1)X3−X1+Y1Y2=(3.33−3)(0.741−0.7134)4−1+0.7134Y2=0.716 (6) Taking a safety factor of 2 which is slightly above the allowable stress, the ultimate stress, σ = 73/2 = 36.5 MPa: T=βqH2σ (7) T=0.716×28.8×103×3236.5×106 T=0.0713≈0.071m 2.4 Growth dynamics Nannochloropsis sp. will be grown over a period of a week within the structure, over this time nutrients such as thiamin (B1), cyanocobalamin (B12) and biotin are added over a set interval [12]. The growth of algae is characterized by five phases: lag or induction phase, exponential phase, declining relative growth, stationary phase and death phase [12]. Week 1—Lag phase: this is when the algae culture is introduced into the environment. Week 2—Exponential: this is the phase by which the cell density increases according to a logarithmic function which is dependent on the growth requirements. Week 3—Declining relative growth: the cell division slows down. Week 4—Stationary: the growth rate is balanced. Drained before—Death: the live cell numbers begin to decrease. These phases will be judged accordingly and tweaked for optimum growth rate once the structure is operational. An additional design feature would place the hybrid tanks on a hill to maximize the light intensity from the sun. 2.5 Design model The design model in Figure 1 shows that CO2 is to be injected into the tank. A total of 10 nozzles will be retrofitted to give an even injection every 6 h and will last for 30 s. The tank will be manufactured by Oceana Manufacturing based in Bela Bela, Limpopo, South Africa [13]. The design model depicted above is the conceptual final design of the cultivation process. The calculations from this stage can now form the flow rate of the entire plant system. There will be four tanks of this nature providing the plant 30 m3 of algae per week. Figure 1. View largeDownload slide Hybrid bioreactor model. Figure 1. View largeDownload slide Hybrid bioreactor model. 2.6 Plant flow rate calculations From the hybrid bioreactor, 120 m3 of water and algae will be sent into the plant at the beginning of the week. The working operational capacity is divided daily. capacity ofhybridtanknumberofworkingdaysinaweek=1205=24m3daily workingcapacity In order to achieve this result, the hybrid tank will need to be fed into a holding tank so that: The hybrid tank can restart its 4-week cycle. The plant can operate on a daily basis. 2.7 Holding tank [6] Volume=πr2h (8) where r = radius (assume 3 m); h = height; h = v/(πr2); h = 120/(π32); and h = 4.2 m. 2.7.1 Design model The tank’s minimum dimensions are: radius 3 m, height 4.2 m (Figure 2). Material used is stainless steel 316 as recommended by Smith and Hashemi [8] for corrosion resistance. Figure 2. View largeDownload slide Holding tank. Figure 2. View largeDownload slide Holding tank. 2.8 Extraction process The extraction process is a two part system using microwaves and a screw press. These will be sized and evaluated accordingly. 2.8.1 Microwave extraction assumptions 24 m3 of working substance is continuously fed through this system. Continuous system hence methanol will not have sufficient time to dissolve the cell structure. Operation time 8 h per day. Uses frequency of 2450 MHz as set by South African law [14]. Will use 100 L of methanol. 2.8.2 Microwave extraction calculations The extraction process is made up of four main parts: the cavity in which the substance flows through, the waveguide, microwave generator and the power source. In order to size the appropriate parts, the power required is desirable. dPdv=k×ε″×f×(Ep2+Eperp2) (9) where dP/dv = power required per cm; k = constant (5.56 × 10−13); ε″ = relative dielectric loss factor; Ep = electric field parallel to surface; and Eperp = electric field perpendicular to surface. 2.9 Screw press extraction Must be able to operate with 3 m3/h flow rate inlet. Run under constant load. With a capacity of 5 ton per day. This machine is manufactured by Blue Water Technologies the requirements were sized according to the models offered (Figure 3). Figure 3. View largeDownload slide Screw press model. Figure 3. View largeDownload slide Screw press model. 2.10 Gravity clarifier design The clarifier in Figure 4 is to aid in the progression of the working substance which from the previous process has been simplified into liquid constituents: oil, methanol and water. Figure 4. View largeDownload slide Gravity clarifier model. Figure 4. View largeDownload slide Gravity clarifier model. 2.10.1 Gravity clarifier assumptions The time required to separate the oil and water is 24 h. Material used is acrylic. Cone shape design. 2.10.2 Gravity clarifier calculations Capacity=3.9m3lipids+0.1m3methanol+18m3water: Capacity=22m3 Internal pressure=Poil+Pwater Pt=(ρgh)oil+(ρgh)water Pt=(800×9.81×1)+(1000×9.81×3) Pt=37.278kPa The dimensions will be based upon capacity hence the optimal dimensions are as follows: height = 4 m; Maximum diameter = 4 m; apex angle = 20°. These parameters are considered and incorporated in any clarifier vessel design [6]. Half the apex angle of >30° are recommended transition geometry for optimum design according to ASME codes is a mandatory compliance [15]. 2.11 Distillery design This section deals with the separation of the methanol from the algae oil. The oil is heated up to the boiling point of the methanol. The gas substance is then condensed back into liquid and returned back into the system. 2.11.1 Design model The gravity clarifier will be manufactured by Oceana Manufacturing based in Bela Bela, Limpopo, South Africa according to the specified dimensions [13]. 2.11.2 Distillery design assumptions The oil must be heated up to 64.7°C. The oil’s starting temperature is 25°C. The time required to heat the substance is 30 min. Volume of oil: 3.9 m3 and volume of methanol: 0.1 m3. Material used is stainless steel 316 (Table 4). Table 4. Specifications of oil and methanol [16]. Specification Unit Density oil 800 kg/m3 Density methanol 792 kg/m3 Cp oil 1.67 kj/kg K Cp methanol 2.51 kj/kg K Specification Unit Density oil 800 kg/m3 Density methanol 792 kg/m3 Cp oil 1.67 kj/kg K Cp methanol 2.51 kj/kg K Table 4. Specifications of oil and methanol [16]. Specification Unit Density oil 800 kg/m3 Density methanol 792 kg/m3 Cp oil 1.67 kj/kg K Cp methanol 2.51 kj/kg K Specification Unit Density oil 800 kg/m3 Density methanol 792 kg/m3 Cp oil 1.67 kj/kg K Cp methanol 2.51 kj/kg K 2.11.3 Distillery calculations q=(mCp∆T)oil+(mCp∆T)methanolt where q = heat energy rate watts [16]; m = mass (kg); Cp = heat capacity at constant pressure or specific heat (kj/kg K); ∆T = difference in temperature (°C); and t = time in seconds (s). Mass of oil: m=density×volume m=800×3.9 m=3120kg Mass of methanol: m=792×0.1 m=79.2kg q=(3120×1.67×(64.7−25))+(79.2×2.51×(64.7−25))60×30 q=119.3watts The distillery requires 119 W to heat up the mixture to 64.7°C. The distillery in Figure 5 is sized according to power consumption and the condenser is sized according to the tube and energy requirements. This unit is bought off from Taian Gaodeng Co. Ltd based in Shandong China. 2.12 Biofuel refinement design From the concept selection, this type of plant will produce bio-kerosene. To ensure there are no redesign of the jet engines, algae bio-kerosene will need to match the aviation fuel known as Jet A or Jet A-1 as well as ASTM 1655. This design will incorporate the patented design of a hydro bioreactor from Solazyme Inc known to the company as Solajet, USA. 2.12.1 Design model Figure 5. View largeDownload slide Distillery and condenser model. Figure 5. View largeDownload slide Distillery and condenser model. 2.12.2 Hydro bioreactor assumptions Capacity of 3.9 m3 of distilled oil. Runtime of process: 8 h. Adherence to specifications of Jet A-1 fuel. Hydrocarbons must match that of crude oil based kerosene. Pressure of 15 MPa. Temperature of 524°C. 2.12.3 Hydro reactor calculations Since this system is bought off according to the specifications of capacity and flow rate, stress calculation is required to ensure the vessel can withstand the pressure needed for the process. (1) σr=A−Br2 (2) σθ=A+Br2 (1) σr=A−Br2 (2) σθ=A+Br2 σr = Radial stress (15 MPa) × S.F. of 2 = 30 MPa σϴ = Hoop stress (stainless steel AISI 304 = 205 MPa) r = Internal radius A and B = Constants (1) σr=A−Br2 (2) σθ=A+Br2 (1) σr=A−Br2 (2) σθ=A+Br2 σr = Radial stress (15 MPa) × S.F. of 2 = 30 MPa σϴ = Hoop stress (stainless steel AISI 304 = 205 MPa) r = Internal radius A and B = Constants Estimated radius: Vcylinder=πr2h [6] h = 2.5 m, V = 3.9 m3 r=Vπh Inside: σr = σr r=0.7m From (1) A=B0.72−30×106 From (2) A=B0.72+205×106 Substituting (1) into (2) and solving for B B=57575000 (3) Substituting (3) into (1) A=87500000 Outside: σr = 0 0=A−Br2;r=BA;r=0.811m Estimated radius: Vcylinder=πr2h [6] h = 2.5 m, V = 3.9 m3 r=Vπh Inside: σr = σr r=0.7m From (1) A=B0.72−30×106 From (2) A=B0.72+205×106 Substituting (1) into (2) and solving for B B=57575000 (3) Substituting (3) into (1) A=87500000 Outside: σr = 0 0=A−Br2;r=BA;r=0.811m Estimated radius: Vcylinder=πr2h [6] h = 2.5 m, V = 3.9 m3 r=Vπh Inside: σr = σr r=0.7m From (1) A=B0.72−30×106 From (2) A=B0.72+205×106 Substituting (1) into (2) and solving for B B=57575000 (3) Substituting (3) into (1) A=87500000 Outside: σr = 0 0=A−Br2;r=BA;r=0.811m Estimated radius: Vcylinder=πr2h [6] h = 2.5 m, V = 3.9 m3 r=Vπh Inside: σr = σr r=0.7m From (1) A=B0.72−30×106 From (2) A=B0.72+205×106 Substituting (1) into (2) and solving for B B=57575000 (3) Substituting (3) into (1) A=87500000 Outside: σr = 0 0=A−Br2;r=BA;r=0.811m The minimum thickness of the hydro reactor must be 55.5 mm. This is related to the limits of design according to ASME codes for pressure vessels of this nature [15]. 2.12.4 Design model The model depicts an overall structure of the hydrocracking process as there is no dimensions available from Solazyme Inc. Dimensions were based on volume and stress calculations required (Figure 6). Figure 6. View largeDownload slide Model depicting hydrocracking. Figure 6. View largeDownload slide Model depicting hydrocracking. 2.13 Anaerobic digestion design The digester will allow for enzymes to break down the ruptured cells. Anaerobic means that the decomposition needs to take place without oxygen; but if oxygen is present, the sludge from the screw press will turn into compost (Figure 7). 2.13.1 Digestion assumptions Capacity of accumulated cells over 1 working week. Anaerobic (without oxygen). Separation of methane and CO2. 2.13.2 Digestion calculation Digestion volume: V=2.1×5days: V=10.5m3capacity Digester height: V=πr2h [6] Assume r = 1 m h=Vπ12 Over 1 week period: h=3.345≈3.4m Assume 10% CO2 Vc=10.5×10%=1.05m3 Assume 30% methane Vm=10.5×30%=3.15m3 Assume 70% slurry Vs=10.5×70%=7.35m3 Digestion volume: V=2.1×5days: V=10.5m3capacity Digester height: V=πr2h [6] Assume r = 1 m h=Vπ12 Over 1 week period: h=3.345≈3.4m Assume 10% CO2 Vc=10.5×10%=1.05m3 Assume 30% methane Vm=10.5×30%=3.15m3 Assume 70% slurry Vs=10.5×70%=7.35m3 Digestion volume: V=2.1×5days: V=10.5m3capacity Digester height: V=πr2h [6] Assume r = 1 m h=Vπ12 Over 1 week period: h=3.345≈3.4m Assume 10% CO2 Vc=10.5×10%=1.05m3 Assume 30% methane Vm=10.5×30%=3.15m3 Assume 70% slurry Vs=10.5×70%=7.35m3 Digestion volume: V=2.1×5days: V=10.5m3capacity Digester height: V=πr2h [6] Assume r = 1 m h=Vπ12 Over 1 week period: h=3.345≈3.4m Assume 10% CO2 Vc=10.5×10%=1.05m3 Assume 30% methane Vm=10.5×30%=3.15m3 Assume 70% slurry Vs=10.5×70%=7.35m3 The gas scrubber is required to hold 4.2 m3 of gas and will be supplied by QB Johnson Manufacturing Inc., USA. 2.14 Flow chart of final design system 2.14.1 Schematic description The schematic diagram in Figure 8 depicts the entire process of the plant system from growth to eventual refinement to biofuel. Figure 8. View largeDownload slide Flow chart for design system. Figure 8. View largeDownload slide Flow chart for design system. 2.14.2 Design model The digestion unit will be manufactured by Clarke Energy South Africa, Link Hills, KwaZulu Natal [17]. Four hybrid bioreactors 10 m higher than the plant for maximum sunlight (no shadows formed by the plant itself) and with a capacity of 120 m3, each cultivates the algae to maturity. Each week of the month, a tank is drained into a holding tank of the same capacity to be processed. From the holding tank, 24 m3 of the working substance (brackish water and algae cells) per day is sent to the microwave extraction unit. Overall, 100 L of methanol is fed into the piping system per day before the microwave extraction. The 20 kW microwave unit heats up the methanol within the cell structure rupturing the cell. The methanol does not have sufficient time to dissolve the cells since this concept was designed to be continuous. The working substance flows through a screw press separating the ruptured cells or sludge with a volume of 2.1 m3 from the liquid solution made up of 0.1 m3 methanol, 3.9 m3 lipids/oil and 18 m3 of brackish water per day. The sludge is pumped into a digester which accumulates over a week’s periods allowing for enzymes to break down the substance into 1.05 m3 CO₂, 3.15 m3 methane and 7.35 m3 slurry. The CO2 and methane flows into a gas separator for separation. The liquid solution from the screw press is pumped into one of two gravity clarifiers since the time required for separation is 24 h. The clarifier which has had time to settle separates the water at the bottom and oil and methanol on top. The water is pumped to the filtration system with a working capacity of 18 m3, from which nutrients and algae seed cells are mixed with brackish water which is returned to the hybrid tank after it has drained in order to start a new cycle. The oil and methanol is sent to the distillery which operates at 120 W heating the substance to 64.7°C. At this temperature, the methanol is converted to its gaseous phase and flows into a 2 m long condenser to be condensed back into liquid form at 20°C. The oil is then pumped to the hydrogen bioreactor operating at 15 MPa and at 524°C to produce bio-kerosene. The bio-kerosene should match to the hydrocarbon chains seen in crude oil kerosene. Figure 7. View largeDownload slide Anaerobic digestion unit. Figure 7. View largeDownload slide Anaerobic digestion unit. 2.14.3 Drawing numbering system A drawing numberings will be in place so that each part can be accountable as well as required by ISO 9001. The process is broken down into general assembly, sub assembly and parts. For general assembly—main process: a number of 1000 is given. For sub assembly—sub-process: a number of 100 is given. For parts—single units: a number of 1 is given. The subscript of the process will be in front depicting where the component is situated (Figure 9). Figure 9. View largeDownload slide Typical numbering system. Figure 9. View largeDownload slide Typical numbering system. 3 CONCLUSION The controlled environment, brackish water process was assigned to continue the process. This concept resembled most significantly the formulated characteristics. The detailed analysis followed from which a specific strain of algae was chosen on the prerequisite of the concept system that thrived in such conditions. Calculations done on the system processes, piping requirements, flow analysis and adhering to international standards allowed for the selection of definite models being articulated. The process plant consists of six main sections. Algae are grown in a hybrid bioreactor for cultivation, this method of cultivation works integrally with the five phases of the growth cycle. Extraction of the oil is done timeously due to the fact that the plant system design is continuous [18]. Microwaves heat up methanol rupturing of the cell structure of the algae [18] and by the use of a screw press, two fractions of the algae cell can be refined further. Bio-kerosene was the concepts refined biofuel; a hydrocracking bioreactor built by Solazmye Inc. will also break down the hydrocarbons and reform them into kerosene hydrocarbon chains. The ruptured cells produce three by-products in the forms of methane, CO2 and slurry. This is produced in a digester anaerobically and the gases separated by a gas separator. The plant produces little or no waste, the water is recycled, remixed to balance the salinity, nutrients and algae seed cells which are sent back to the hybrid bioreactor to repeat the entire plant process all over. Over a month of operation, the plant will cultivate 100 ton of dry algae, produce 78 m3 of bio-kerosene for research purposes and examine 12.6 and 29.4 m3 of methane and slurry for other industrial demands so that no extreme alterations to the designed plant needs to be retrofitted to suit bio-energy. This process plant has a cost projection of 13 million Rand however with the advancements in technology, this number will steadily decrease. It also shows that a viable alternative source of energy is plausible to achieve and with further research and development, will encourage less reliance on environmentally harmful and fossil-based energy. ACKNOWLEDGMENTS This study is based on the research supported by the National Research Foundation (NRF) of South Africa (Grant Number 111984). The opinions expressed is however not that of NRF but of the authors. REFERENCES 1 Onunka C, Juddhoo R, Eloka-Eboka AC. ( 2016 ). Conceptual design and optimisation of a sustainable micro-algal biofuel process plant. In: Proceedings of the 15th Sustainable Energy Technologies (SET 2016) Conference, 19–22 July, 2016 at the National University of Singapore. 2 Eloka-Eboka AC , Inambao FL . Effects of CO2 sequestration on lipid and biomass productivity in microalgal biomass production . Appl Energy 2017 ; 195 : 1100 – 11 . http://dx.doi.org/10.1016/j.apenergy.2017.03.071 . Google Scholar CrossRef Search ADS 3 Giko B . Advances in Biofuel Production , 1st edn . CRC Press , Oakville , 2014 . 4 Schlagermann P , Gottlicher G . Composition of algal oil and its potential as biofuel . J Combust 2012 ; 3 : 185 – 285 . 5 Lott R , Lee L-K ( 2010 ). Ebullated Bed Hydroprocessing System. United States of America, Patent No. US 7,815,870 B2. 6 Knight R . Physics for Scientists and Engineers , 2nd edn . Pearson Addison Wesley , San Franciso , 2008 . 7 Young W , Budynas R . Formulas for flat plates with straight boundaries and constant thickness. In Hager L . Roark’s Formulas for Stress and Strain . McGraw-Hill , New York , 2002 : 508 . 8 Smith W , Hashemi J . Foundations of Material Science and Engineering , 5th edn . McGraw-Hill , New York , 2011 . 9 Drotsky J . Strength of Materials for Technicians , 3rd edn . Heinemann Publishers (Pty) Ltd , Sandton , 2005 . 10 Crowe B , Attalah S , Agrawal S . A comparison of nannochloropsis salina growth performance in two outdoor pond designs: conventional raceways versus the ARID pond with superior temperature management . Int J Chem Eng 2012 ; 2012 : 9 . Google Scholar CrossRef Search ADS 11 Roberts J . Some practical interpolation formulas . Ann Math Stat 1935 ; 6 : 133 – 42 . Google Scholar CrossRef Search ADS 12 Lavens P , Sogeloos P . Manual on the Production and Use of Live Food for Aquaculture , 1st edn . Food and Agriculture Organisation of the United Nations , New York , 1996 . 13 Oceana Manufacturing ( 2014 ). Oceana Manufacturing. http://www.artificialcoralreef.com (10 September 2015, date last accessed). 14 RSA Government . Hazardous Substance Act 15 of 1973 as Amended 1991 . South African Government , Cape Town , 1973 . 15 Thakkar BS , Thakka SA . Design of pressure vessel using ASME Code, Section VIII, Division 1 . Int J Adv Eng Res Stud 2012 ;I: 228 – 234 . E-ISSN2249–8974. 16 Borgnakke C , Sonntag R . Fundamentals of Thermodynamics , 7th edn . John Wiley and Sons Ltd , Chichester , 2009 . 17 Clarke Energy ( 2014 ). Clarke Energy. https://www.clarke-energy.com/south-africa/ (17 September 2015, date last accessed). 18 Pozar D . Microwave Engineering , 4th edn . John Wiley and Sons, Inc , Massachusetts , 2012 . © The Author(s) 2018. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Low-Carbon Technologies Oxford University Press

Detailed design and optimization of a sustainable micro-algal biofuel process plant

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© The Author(s) 2018. Published by Oxford University Press.
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1748-1317
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Abstract

Abstract A sustainable micro-algal biofuel process plant was hypothetically and detailed-designed for the production of value products from algae. The process plant cultures algae through controlled growth and processes: production of biomass, extraction of lipids and biofuels and refinement into finished products. The start-off feedstock was Nannochloropsis spp. and the four end products produced were bio-kerosene, methane, biomass slurry and CO₂. The algae were grown to maturity within the least allowable possible time under controlled conditions. From an in-depth analysis of the customer specifications, concept development and selection, the study defines an overall depiction of the end products from the concepts evaluated by using the Pugh matrix and Six sigma methodologies. DMADV approach was implemented.The process finally generated an economically viable algae biomass for conversion to biofuels and other viable products. This paper focuses on the process planning, criteria/concept selection, design optimization with necessary characterizations which were filtered into measurable characteristics for the design of an operable, sustainable functioning plant. 1 INTRODUCTION Research into harnessing natural renewable resources in a responsible and conservative manner has allowed for the discovery of the algae-biofuel concept. An alga to biofuel process plant is a technology that stems from the basic formation of the growth of algae to biomass production and to refined products. The proposed plant allows for growth, extraction and refinement. The chosen system to continue the design process is of a controlled brackish water process plant. This article presents the detailed design which is a continuation of the conceptual design study earlier published [1]. This involves the calculations, flow diagrams as well design models which were developed. This article will also detail the process of each sub-process unit/function of the plant. Emphasis was placed on the concept evaluation and the guideline/selection criteria of the selected and its working principles which were captured in our earlier study [1]. The aim of this study is hypothetically present a detailed design and optimization of a micro-alga biofuel process plant that will stand the test of time, which will be sustainable as grown in a brackish aquaculture for enhanced product ion and product recovery. 1.1 Algae strain Algae and aquatic biomass have the potential to provide a new range of ‘third generation’ biofuels, including jet fuels. Their high oil and biomass yields, widespread availability, absent (or very reduced) competition with agricultural land, high quality and versatility of the by-products, their efficient use as a mean to capture CO2 and their suitability for wastewater treatments and other industrial plants make algae and aquatic biomass one of the most promising and attractive renewable sources for a fully sustainable and low-carbon economy portfolio [1, 2]. The technologies are evolving in Africa where the potentials and capabilities of diverse strains are dominant in addition to conducive growth environments and climate. These include high solar radiation, increased environmental temperature, humidity, dense vegetation and other factors. Productivity is higher in the controlled, contained environment of a photobioreactor, but capex and opex are also both substantially higher than for open systems [1]. Significant investment in research is required before high levels of productivity can be guaranteed on a commercial scale [2]. Algae to biofuels plants may be developed on land adjacent to power stations, for converting the carbon dioxide from exhausts into fuel. 1.2 Algal bio-refineries In addition to producing oils, algae are rich sources of vitamins, protein and carbohydrates. Micro-algal species with their diverse value added products are gaining research interests in the development of strategies for integrated bio-refineries just like the conventional fossil refineries [2]. Their expeditious growth rate, in addition to their comparatively high lipids, carbohydrates, food and other nutrients production and relatively short period of production are good parameters for all-inclusive integration of different production and extraction strategies in order to maximize final products as presented by Eloka-Eboka and Inambao [2] in their recent study. The following steps have been identified for development of micro-algae bio-refineries: Development of mild and efficient cell disruption, extraction and fractionation technologies; effective technologies for separation of carbohydrates, proteins and lipids; lipid/oil refining technologies; improvement of energy consumption and environmental performance, decrease of capital costs; integrate knowledge and facilities for oil, food and fine chemical industry; and biomass provision (quantity and quality). In order to continue to develop a sustainable micro-algal biofuel process plant, an algae strain is required to grow in the concept system which in this study is in brackish water aquaculture. From Table 1 describing algae species with their corresponding lipid contents, the brackish species that are in the list exist amongst others are: Cylindrotheca sp., Nannochloropsis sp., Nitzschia sp. and Schizochytrium sp. And from the above strains from the rest of the fresh water, Nannochloropsis sp. and Schizochytrium sp. have the highest oil yield by weight, (30–68%) and (50–77%), respectively. While Schizochytrium sp. has the highest by weight oil content, the type of lipid it produces is a free fatty acid that is less efficient at producing bio-kerosene and is very specific to its natural habitat. Hence Nannochloropsis sp. is the chosen algae strain as presented in Table 2, Nannochloropsis sp. is found from the marine habitat near river mouths but is known to survive in fresh water with negligible saline conditions. It shows promise for its high lipid count and is currently used in the food industry, the cell structure of this type of algae can be modified to produce a greater amount of lipids/oil by genetically modifying its ability its structure [4, 5]. The six sigma methodology employed adopted the strategy of deconstructing the optional processes and scaling down the most optimal process. It therefore defined and evaluated each feedstock and possibility of improved yield and viability putting into consideration product and process efficiencies, feasibility, economic viability and profitability. There was futuristic room for process quality improvement and turnover on investment. These were ranked before final selection and adoption of this sustainable micro-algal biofuel plant to be designed and implemented. Out of the two main approaches of six sigma, DMAIC and DMADV; DMADV (Define, Measure, Analyse, Design and Verify) was adopted. DMADV approach is typically fitted for new processes, plants and products and so it was implemented in this study to arrive at the present design and optimization. Table 1. Algae species and their corresponding lipid content [3]. Species lipid content % Dry weight Species lipid content % Dry weight Anabaena cylindrical 4–7 Botyococcus braunii 25–80 Chlamydomonas reinhardtii 21 Chlorella emersonii 25–32 Chlorella pyremoidosa 2 Crypthecodinium cohnii 20 Chlorella vulgaris 12–22 Cylindrotheca sp. 16–37 Dunaliella bioculata 8 Dunaliella primoleta 23 Dunaliella salina 6 Dunaliella tertioleta 23 Euglena gracilis 14–20 Hormidium sp. 38 Nannochloris sp. 30–50 Scenedesmus dimorphus 16–40 Schizochytrium sp. 50–77 Scenedesmus obliquus 12–14 Spirogyra sp. 11–21 Synechoccus sp. 11 Prymnesium parvum 22–38 Nitzschia sp. 31–68 Species lipid content % Dry weight Species lipid content % Dry weight Anabaena cylindrical 4–7 Botyococcus braunii 25–80 Chlamydomonas reinhardtii 21 Chlorella emersonii 25–32 Chlorella pyremoidosa 2 Crypthecodinium cohnii 20 Chlorella vulgaris 12–22 Cylindrotheca sp. 16–37 Dunaliella bioculata 8 Dunaliella primoleta 23 Dunaliella salina 6 Dunaliella tertioleta 23 Euglena gracilis 14–20 Hormidium sp. 38 Nannochloris sp. 30–50 Scenedesmus dimorphus 16–40 Schizochytrium sp. 50–77 Scenedesmus obliquus 12–14 Spirogyra sp. 11–21 Synechoccus sp. 11 Prymnesium parvum 22–38 Nitzschia sp. 31–68 Table 1. Algae species and their corresponding lipid content [3]. Species lipid content % Dry weight Species lipid content % Dry weight Anabaena cylindrical 4–7 Botyococcus braunii 25–80 Chlamydomonas reinhardtii 21 Chlorella emersonii 25–32 Chlorella pyremoidosa 2 Crypthecodinium cohnii 20 Chlorella vulgaris 12–22 Cylindrotheca sp. 16–37 Dunaliella bioculata 8 Dunaliella primoleta 23 Dunaliella salina 6 Dunaliella tertioleta 23 Euglena gracilis 14–20 Hormidium sp. 38 Nannochloris sp. 30–50 Scenedesmus dimorphus 16–40 Schizochytrium sp. 50–77 Scenedesmus obliquus 12–14 Spirogyra sp. 11–21 Synechoccus sp. 11 Prymnesium parvum 22–38 Nitzschia sp. 31–68 Species lipid content % Dry weight Species lipid content % Dry weight Anabaena cylindrical 4–7 Botyococcus braunii 25–80 Chlamydomonas reinhardtii 21 Chlorella emersonii 25–32 Chlorella pyremoidosa 2 Crypthecodinium cohnii 20 Chlorella vulgaris 12–22 Cylindrotheca sp. 16–37 Dunaliella bioculata 8 Dunaliella primoleta 23 Dunaliella salina 6 Dunaliella tertioleta 23 Euglena gracilis 14–20 Hormidium sp. 38 Nannochloris sp. 30–50 Scenedesmus dimorphus 16–40 Schizochytrium sp. 50–77 Scenedesmus obliquus 12–14 Spirogyra sp. 11–21 Synechoccus sp. 11 Prymnesium parvum 22–38 Nitzschia sp. 31–68 Table 2. Nannochloropsis sp. specification [10]. Specification Density 920 kg/m3 Lipid percentage by weight 65% Ratio of algae to water needed for growth (per parts) 1:3 Specification Density 920 kg/m3 Lipid percentage by weight 65% Ratio of algae to water needed for growth (per parts) 1:3 Table 2. Nannochloropsis sp. specification [10]. Specification Density 920 kg/m3 Lipid percentage by weight 65% Ratio of algae to water needed for growth (per parts) 1:3 Specification Density 920 kg/m3 Lipid percentage by weight 65% Ratio of algae to water needed for growth (per parts) 1:3 2 DESIGN CALCULATIONS 2.1 General assumptions From anticipated specifications, the desired plant must produce 100 ton of algae by weight per month. The hybrid bioreactor will grow the algae to maturity in 4 weeks. 25 ton of algae will be produced per week. 5 ton will be processed per day. 8 h working per day. 2.2 General calculations [6] Volume of Algae produced per week=MassDensity (1) VolumeofAlgaeproducedperweek=MassDensity;V=25×103920;V=27.1m3≈30m3 (2) CombinedMass of substance=(ρ×V)algae+(ρ×V)water (3) Substituting, Mass=(920×30)+(1000×90)=117600kg; Densityof algae and water=MV;117600120=980(kg/m3) 2.3 Hybrid bioreactor design Assumptions: 30 m3 required hence 90 m3 water required (derived from Table 2). Dimensions of tank is 4 m × 10 m × 3 m = 120 m3. Material used is acrylic [7]. ThicknessofTank=(βqH2σ) (4) where β = coefficient from length/height; σ = maximum permissible stress (73 MPa) [8]; H = height of tank; and q = maximum permissible pressure [6, 9]: q=(ρgh) (5) And q=980×9.81×3; therefore, q=28841.4Pa (Table 3). Table 3. Ratio of length/height and its coefficients [7]. a/b 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0 ∞ β 0.2874 0.3762 0.4530 0.5172 0.5688 0.6102 0.7134 0.7410 0.7476 0.7500 α 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421 γ 0.420 0.455 0.478 0.491 0.499 0.505 0.502 0.502 0.501 0.500 a/b 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0 ∞ β 0.2874 0.3762 0.4530 0.5172 0.5688 0.6102 0.7134 0.7410 0.7476 0.7500 α 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421 γ 0.420 0.455 0.478 0.491 0.499 0.505 0.502 0.502 0.501 0.500 Table 3. Ratio of length/height and its coefficients [7]. a/b 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0 ∞ β 0.2874 0.3762 0.4530 0.5172 0.5688 0.6102 0.7134 0.7410 0.7476 0.7500 α 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421 γ 0.420 0.455 0.478 0.491 0.499 0.505 0.502 0.502 0.501 0.500 a/b 1.0 1.2 1.4 1.6 1.8 2.0 3.0 4.0 5.0 ∞ β 0.2874 0.3762 0.4530 0.5172 0.5688 0.6102 0.7134 0.7410 0.7476 0.7500 α 0.0444 0.0616 0.0770 0.0906 0.1017 0.1110 0.1335 0.1400 0.1417 0.1421 γ 0.420 0.455 0.478 0.491 0.499 0.505 0.502 0.502 0.501 0.500 Ratio of length to height, 10/3 = 3.333. Interpolation is required [11]: Y2=(X2−X1)(Y3−Y1)X3−X1+Y1Y2=(3.33−3)(0.741−0.7134)4−1+0.7134Y2=0.716 (6) Taking a safety factor of 2 which is slightly above the allowable stress, the ultimate stress, σ = 73/2 = 36.5 MPa: T=βqH2σ (7) T=0.716×28.8×103×3236.5×106 T=0.0713≈0.071m 2.4 Growth dynamics Nannochloropsis sp. will be grown over a period of a week within the structure, over this time nutrients such as thiamin (B1), cyanocobalamin (B12) and biotin are added over a set interval [12]. The growth of algae is characterized by five phases: lag or induction phase, exponential phase, declining relative growth, stationary phase and death phase [12]. Week 1—Lag phase: this is when the algae culture is introduced into the environment. Week 2—Exponential: this is the phase by which the cell density increases according to a logarithmic function which is dependent on the growth requirements. Week 3—Declining relative growth: the cell division slows down. Week 4—Stationary: the growth rate is balanced. Drained before—Death: the live cell numbers begin to decrease. These phases will be judged accordingly and tweaked for optimum growth rate once the structure is operational. An additional design feature would place the hybrid tanks on a hill to maximize the light intensity from the sun. 2.5 Design model The design model in Figure 1 shows that CO2 is to be injected into the tank. A total of 10 nozzles will be retrofitted to give an even injection every 6 h and will last for 30 s. The tank will be manufactured by Oceana Manufacturing based in Bela Bela, Limpopo, South Africa [13]. The design model depicted above is the conceptual final design of the cultivation process. The calculations from this stage can now form the flow rate of the entire plant system. There will be four tanks of this nature providing the plant 30 m3 of algae per week. Figure 1. View largeDownload slide Hybrid bioreactor model. Figure 1. View largeDownload slide Hybrid bioreactor model. 2.6 Plant flow rate calculations From the hybrid bioreactor, 120 m3 of water and algae will be sent into the plant at the beginning of the week. The working operational capacity is divided daily. capacity ofhybridtanknumberofworkingdaysinaweek=1205=24m3daily workingcapacity In order to achieve this result, the hybrid tank will need to be fed into a holding tank so that: The hybrid tank can restart its 4-week cycle. The plant can operate on a daily basis. 2.7 Holding tank [6] Volume=πr2h (8) where r = radius (assume 3 m); h = height; h = v/(πr2); h = 120/(π32); and h = 4.2 m. 2.7.1 Design model The tank’s minimum dimensions are: radius 3 m, height 4.2 m (Figure 2). Material used is stainless steel 316 as recommended by Smith and Hashemi [8] for corrosion resistance. Figure 2. View largeDownload slide Holding tank. Figure 2. View largeDownload slide Holding tank. 2.8 Extraction process The extraction process is a two part system using microwaves and a screw press. These will be sized and evaluated accordingly. 2.8.1 Microwave extraction assumptions 24 m3 of working substance is continuously fed through this system. Continuous system hence methanol will not have sufficient time to dissolve the cell structure. Operation time 8 h per day. Uses frequency of 2450 MHz as set by South African law [14]. Will use 100 L of methanol. 2.8.2 Microwave extraction calculations The extraction process is made up of four main parts: the cavity in which the substance flows through, the waveguide, microwave generator and the power source. In order to size the appropriate parts, the power required is desirable. dPdv=k×ε″×f×(Ep2+Eperp2) (9) where dP/dv = power required per cm; k = constant (5.56 × 10−13); ε″ = relative dielectric loss factor; Ep = electric field parallel to surface; and Eperp = electric field perpendicular to surface. 2.9 Screw press extraction Must be able to operate with 3 m3/h flow rate inlet. Run under constant load. With a capacity of 5 ton per day. This machine is manufactured by Blue Water Technologies the requirements were sized according to the models offered (Figure 3). Figure 3. View largeDownload slide Screw press model. Figure 3. View largeDownload slide Screw press model. 2.10 Gravity clarifier design The clarifier in Figure 4 is to aid in the progression of the working substance which from the previous process has been simplified into liquid constituents: oil, methanol and water. Figure 4. View largeDownload slide Gravity clarifier model. Figure 4. View largeDownload slide Gravity clarifier model. 2.10.1 Gravity clarifier assumptions The time required to separate the oil and water is 24 h. Material used is acrylic. Cone shape design. 2.10.2 Gravity clarifier calculations Capacity=3.9m3lipids+0.1m3methanol+18m3water: Capacity=22m3 Internal pressure=Poil+Pwater Pt=(ρgh)oil+(ρgh)water Pt=(800×9.81×1)+(1000×9.81×3) Pt=37.278kPa The dimensions will be based upon capacity hence the optimal dimensions are as follows: height = 4 m; Maximum diameter = 4 m; apex angle = 20°. These parameters are considered and incorporated in any clarifier vessel design [6]. Half the apex angle of >30° are recommended transition geometry for optimum design according to ASME codes is a mandatory compliance [15]. 2.11 Distillery design This section deals with the separation of the methanol from the algae oil. The oil is heated up to the boiling point of the methanol. The gas substance is then condensed back into liquid and returned back into the system. 2.11.1 Design model The gravity clarifier will be manufactured by Oceana Manufacturing based in Bela Bela, Limpopo, South Africa according to the specified dimensions [13]. 2.11.2 Distillery design assumptions The oil must be heated up to 64.7°C. The oil’s starting temperature is 25°C. The time required to heat the substance is 30 min. Volume of oil: 3.9 m3 and volume of methanol: 0.1 m3. Material used is stainless steel 316 (Table 4). Table 4. Specifications of oil and methanol [16]. Specification Unit Density oil 800 kg/m3 Density methanol 792 kg/m3 Cp oil 1.67 kj/kg K Cp methanol 2.51 kj/kg K Specification Unit Density oil 800 kg/m3 Density methanol 792 kg/m3 Cp oil 1.67 kj/kg K Cp methanol 2.51 kj/kg K Table 4. Specifications of oil and methanol [16]. Specification Unit Density oil 800 kg/m3 Density methanol 792 kg/m3 Cp oil 1.67 kj/kg K Cp methanol 2.51 kj/kg K Specification Unit Density oil 800 kg/m3 Density methanol 792 kg/m3 Cp oil 1.67 kj/kg K Cp methanol 2.51 kj/kg K 2.11.3 Distillery calculations q=(mCp∆T)oil+(mCp∆T)methanolt where q = heat energy rate watts [16]; m = mass (kg); Cp = heat capacity at constant pressure or specific heat (kj/kg K); ∆T = difference in temperature (°C); and t = time in seconds (s). Mass of oil: m=density×volume m=800×3.9 m=3120kg Mass of methanol: m=792×0.1 m=79.2kg q=(3120×1.67×(64.7−25))+(79.2×2.51×(64.7−25))60×30 q=119.3watts The distillery requires 119 W to heat up the mixture to 64.7°C. The distillery in Figure 5 is sized according to power consumption and the condenser is sized according to the tube and energy requirements. This unit is bought off from Taian Gaodeng Co. Ltd based in Shandong China. 2.12 Biofuel refinement design From the concept selection, this type of plant will produce bio-kerosene. To ensure there are no redesign of the jet engines, algae bio-kerosene will need to match the aviation fuel known as Jet A or Jet A-1 as well as ASTM 1655. This design will incorporate the patented design of a hydro bioreactor from Solazyme Inc known to the company as Solajet, USA. 2.12.1 Design model Figure 5. View largeDownload slide Distillery and condenser model. Figure 5. View largeDownload slide Distillery and condenser model. 2.12.2 Hydro bioreactor assumptions Capacity of 3.9 m3 of distilled oil. Runtime of process: 8 h. Adherence to specifications of Jet A-1 fuel. Hydrocarbons must match that of crude oil based kerosene. Pressure of 15 MPa. Temperature of 524°C. 2.12.3 Hydro reactor calculations Since this system is bought off according to the specifications of capacity and flow rate, stress calculation is required to ensure the vessel can withstand the pressure needed for the process. (1) σr=A−Br2 (2) σθ=A+Br2 (1) σr=A−Br2 (2) σθ=A+Br2 σr = Radial stress (15 MPa) × S.F. of 2 = 30 MPa σϴ = Hoop stress (stainless steel AISI 304 = 205 MPa) r = Internal radius A and B = Constants (1) σr=A−Br2 (2) σθ=A+Br2 (1) σr=A−Br2 (2) σθ=A+Br2 σr = Radial stress (15 MPa) × S.F. of 2 = 30 MPa σϴ = Hoop stress (stainless steel AISI 304 = 205 MPa) r = Internal radius A and B = Constants Estimated radius: Vcylinder=πr2h [6] h = 2.5 m, V = 3.9 m3 r=Vπh Inside: σr = σr r=0.7m From (1) A=B0.72−30×106 From (2) A=B0.72+205×106 Substituting (1) into (2) and solving for B B=57575000 (3) Substituting (3) into (1) A=87500000 Outside: σr = 0 0=A−Br2;r=BA;r=0.811m Estimated radius: Vcylinder=πr2h [6] h = 2.5 m, V = 3.9 m3 r=Vπh Inside: σr = σr r=0.7m From (1) A=B0.72−30×106 From (2) A=B0.72+205×106 Substituting (1) into (2) and solving for B B=57575000 (3) Substituting (3) into (1) A=87500000 Outside: σr = 0 0=A−Br2;r=BA;r=0.811m Estimated radius: Vcylinder=πr2h [6] h = 2.5 m, V = 3.9 m3 r=Vπh Inside: σr = σr r=0.7m From (1) A=B0.72−30×106 From (2) A=B0.72+205×106 Substituting (1) into (2) and solving for B B=57575000 (3) Substituting (3) into (1) A=87500000 Outside: σr = 0 0=A−Br2;r=BA;r=0.811m Estimated radius: Vcylinder=πr2h [6] h = 2.5 m, V = 3.9 m3 r=Vπh Inside: σr = σr r=0.7m From (1) A=B0.72−30×106 From (2) A=B0.72+205×106 Substituting (1) into (2) and solving for B B=57575000 (3) Substituting (3) into (1) A=87500000 Outside: σr = 0 0=A−Br2;r=BA;r=0.811m The minimum thickness of the hydro reactor must be 55.5 mm. This is related to the limits of design according to ASME codes for pressure vessels of this nature [15]. 2.12.4 Design model The model depicts an overall structure of the hydrocracking process as there is no dimensions available from Solazyme Inc. Dimensions were based on volume and stress calculations required (Figure 6). Figure 6. View largeDownload slide Model depicting hydrocracking. Figure 6. View largeDownload slide Model depicting hydrocracking. 2.13 Anaerobic digestion design The digester will allow for enzymes to break down the ruptured cells. Anaerobic means that the decomposition needs to take place without oxygen; but if oxygen is present, the sludge from the screw press will turn into compost (Figure 7). 2.13.1 Digestion assumptions Capacity of accumulated cells over 1 working week. Anaerobic (without oxygen). Separation of methane and CO2. 2.13.2 Digestion calculation Digestion volume: V=2.1×5days: V=10.5m3capacity Digester height: V=πr2h [6] Assume r = 1 m h=Vπ12 Over 1 week period: h=3.345≈3.4m Assume 10% CO2 Vc=10.5×10%=1.05m3 Assume 30% methane Vm=10.5×30%=3.15m3 Assume 70% slurry Vs=10.5×70%=7.35m3 Digestion volume: V=2.1×5days: V=10.5m3capacity Digester height: V=πr2h [6] Assume r = 1 m h=Vπ12 Over 1 week period: h=3.345≈3.4m Assume 10% CO2 Vc=10.5×10%=1.05m3 Assume 30% methane Vm=10.5×30%=3.15m3 Assume 70% slurry Vs=10.5×70%=7.35m3 Digestion volume: V=2.1×5days: V=10.5m3capacity Digester height: V=πr2h [6] Assume r = 1 m h=Vπ12 Over 1 week period: h=3.345≈3.4m Assume 10% CO2 Vc=10.5×10%=1.05m3 Assume 30% methane Vm=10.5×30%=3.15m3 Assume 70% slurry Vs=10.5×70%=7.35m3 Digestion volume: V=2.1×5days: V=10.5m3capacity Digester height: V=πr2h [6] Assume r = 1 m h=Vπ12 Over 1 week period: h=3.345≈3.4m Assume 10% CO2 Vc=10.5×10%=1.05m3 Assume 30% methane Vm=10.5×30%=3.15m3 Assume 70% slurry Vs=10.5×70%=7.35m3 The gas scrubber is required to hold 4.2 m3 of gas and will be supplied by QB Johnson Manufacturing Inc., USA. 2.14 Flow chart of final design system 2.14.1 Schematic description The schematic diagram in Figure 8 depicts the entire process of the plant system from growth to eventual refinement to biofuel. Figure 8. View largeDownload slide Flow chart for design system. Figure 8. View largeDownload slide Flow chart for design system. 2.14.2 Design model The digestion unit will be manufactured by Clarke Energy South Africa, Link Hills, KwaZulu Natal [17]. Four hybrid bioreactors 10 m higher than the plant for maximum sunlight (no shadows formed by the plant itself) and with a capacity of 120 m3, each cultivates the algae to maturity. Each week of the month, a tank is drained into a holding tank of the same capacity to be processed. From the holding tank, 24 m3 of the working substance (brackish water and algae cells) per day is sent to the microwave extraction unit. Overall, 100 L of methanol is fed into the piping system per day before the microwave extraction. The 20 kW microwave unit heats up the methanol within the cell structure rupturing the cell. The methanol does not have sufficient time to dissolve the cells since this concept was designed to be continuous. The working substance flows through a screw press separating the ruptured cells or sludge with a volume of 2.1 m3 from the liquid solution made up of 0.1 m3 methanol, 3.9 m3 lipids/oil and 18 m3 of brackish water per day. The sludge is pumped into a digester which accumulates over a week’s periods allowing for enzymes to break down the substance into 1.05 m3 CO₂, 3.15 m3 methane and 7.35 m3 slurry. The CO2 and methane flows into a gas separator for separation. The liquid solution from the screw press is pumped into one of two gravity clarifiers since the time required for separation is 24 h. The clarifier which has had time to settle separates the water at the bottom and oil and methanol on top. The water is pumped to the filtration system with a working capacity of 18 m3, from which nutrients and algae seed cells are mixed with brackish water which is returned to the hybrid tank after it has drained in order to start a new cycle. The oil and methanol is sent to the distillery which operates at 120 W heating the substance to 64.7°C. At this temperature, the methanol is converted to its gaseous phase and flows into a 2 m long condenser to be condensed back into liquid form at 20°C. The oil is then pumped to the hydrogen bioreactor operating at 15 MPa and at 524°C to produce bio-kerosene. The bio-kerosene should match to the hydrocarbon chains seen in crude oil kerosene. Figure 7. View largeDownload slide Anaerobic digestion unit. Figure 7. View largeDownload slide Anaerobic digestion unit. 2.14.3 Drawing numbering system A drawing numberings will be in place so that each part can be accountable as well as required by ISO 9001. The process is broken down into general assembly, sub assembly and parts. For general assembly—main process: a number of 1000 is given. For sub assembly—sub-process: a number of 100 is given. For parts—single units: a number of 1 is given. The subscript of the process will be in front depicting where the component is situated (Figure 9). Figure 9. View largeDownload slide Typical numbering system. Figure 9. View largeDownload slide Typical numbering system. 3 CONCLUSION The controlled environment, brackish water process was assigned to continue the process. This concept resembled most significantly the formulated characteristics. The detailed analysis followed from which a specific strain of algae was chosen on the prerequisite of the concept system that thrived in such conditions. Calculations done on the system processes, piping requirements, flow analysis and adhering to international standards allowed for the selection of definite models being articulated. The process plant consists of six main sections. Algae are grown in a hybrid bioreactor for cultivation, this method of cultivation works integrally with the five phases of the growth cycle. Extraction of the oil is done timeously due to the fact that the plant system design is continuous [18]. Microwaves heat up methanol rupturing of the cell structure of the algae [18] and by the use of a screw press, two fractions of the algae cell can be refined further. Bio-kerosene was the concepts refined biofuel; a hydrocracking bioreactor built by Solazmye Inc. will also break down the hydrocarbons and reform them into kerosene hydrocarbon chains. The ruptured cells produce three by-products in the forms of methane, CO2 and slurry. This is produced in a digester anaerobically and the gases separated by a gas separator. The plant produces little or no waste, the water is recycled, remixed to balance the salinity, nutrients and algae seed cells which are sent back to the hybrid bioreactor to repeat the entire plant process all over. Over a month of operation, the plant will cultivate 100 ton of dry algae, produce 78 m3 of bio-kerosene for research purposes and examine 12.6 and 29.4 m3 of methane and slurry for other industrial demands so that no extreme alterations to the designed plant needs to be retrofitted to suit bio-energy. This process plant has a cost projection of 13 million Rand however with the advancements in technology, this number will steadily decrease. It also shows that a viable alternative source of energy is plausible to achieve and with further research and development, will encourage less reliance on environmentally harmful and fossil-based energy. ACKNOWLEDGMENTS This study is based on the research supported by the National Research Foundation (NRF) of South Africa (Grant Number 111984). The opinions expressed is however not that of NRF but of the authors. REFERENCES 1 Onunka C, Juddhoo R, Eloka-Eboka AC. ( 2016 ). Conceptual design and optimisation of a sustainable micro-algal biofuel process plant. In: Proceedings of the 15th Sustainable Energy Technologies (SET 2016) Conference, 19–22 July, 2016 at the National University of Singapore. 2 Eloka-Eboka AC , Inambao FL . Effects of CO2 sequestration on lipid and biomass productivity in microalgal biomass production . Appl Energy 2017 ; 195 : 1100 – 11 . http://dx.doi.org/10.1016/j.apenergy.2017.03.071 . Google Scholar CrossRef Search ADS 3 Giko B . Advances in Biofuel Production , 1st edn . CRC Press , Oakville , 2014 . 4 Schlagermann P , Gottlicher G . Composition of algal oil and its potential as biofuel . J Combust 2012 ; 3 : 185 – 285 . 5 Lott R , Lee L-K ( 2010 ). Ebullated Bed Hydroprocessing System. United States of America, Patent No. US 7,815,870 B2. 6 Knight R . Physics for Scientists and Engineers , 2nd edn . Pearson Addison Wesley , San Franciso , 2008 . 7 Young W , Budynas R . Formulas for flat plates with straight boundaries and constant thickness. In Hager L . Roark’s Formulas for Stress and Strain . McGraw-Hill , New York , 2002 : 508 . 8 Smith W , Hashemi J . Foundations of Material Science and Engineering , 5th edn . McGraw-Hill , New York , 2011 . 9 Drotsky J . Strength of Materials for Technicians , 3rd edn . Heinemann Publishers (Pty) Ltd , Sandton , 2005 . 10 Crowe B , Attalah S , Agrawal S . A comparison of nannochloropsis salina growth performance in two outdoor pond designs: conventional raceways versus the ARID pond with superior temperature management . Int J Chem Eng 2012 ; 2012 : 9 . Google Scholar CrossRef Search ADS 11 Roberts J . Some practical interpolation formulas . Ann Math Stat 1935 ; 6 : 133 – 42 . Google Scholar CrossRef Search ADS 12 Lavens P , Sogeloos P . Manual on the Production and Use of Live Food for Aquaculture , 1st edn . Food and Agriculture Organisation of the United Nations , New York , 1996 . 13 Oceana Manufacturing ( 2014 ). Oceana Manufacturing. http://www.artificialcoralreef.com (10 September 2015, date last accessed). 14 RSA Government . Hazardous Substance Act 15 of 1973 as Amended 1991 . South African Government , Cape Town , 1973 . 15 Thakkar BS , Thakka SA . Design of pressure vessel using ASME Code, Section VIII, Division 1 . Int J Adv Eng Res Stud 2012 ;I: 228 – 234 . E-ISSN2249–8974. 16 Borgnakke C , Sonntag R . Fundamentals of Thermodynamics , 7th edn . John Wiley and Sons Ltd , Chichester , 2009 . 17 Clarke Energy ( 2014 ). Clarke Energy. https://www.clarke-energy.com/south-africa/ (17 September 2015, date last accessed). 18 Pozar D . Microwave Engineering , 4th edn . John Wiley and Sons, Inc , Massachusetts , 2012 . © The Author(s) 2018. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com

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International Journal of Low-Carbon TechnologiesOxford University Press

Published: Feb 21, 2018

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