Present a preliminary flow sheet (basis = 1 h) in terms of a brief description referring to a figure (see below). Show process units, connections between them, and any recycle streams. Give flow rates for potatoes, water, starch, -amylase, yeast, glucose, ethanol, amyl alcohol and CO2. (Clearly any particular stream will have only some of these components in it.) Some of the flow rates will necessarily be estimates, because you have not yet developed the design. Think about ballpark values of: (i) the glucose concentration in the feed to the fermenters; (ii) the ethanol concentration in the final fermentation broth; and (iii) the composition of the final distillate. Hand-drawn flowsheets and printouts of UniSim flowsheets are not acceptable.
1 CE 408, Spring 2020: Design Project Statement — Ethanol from Potato Processing Waste 1. Project The goal of this year’s project is to develop a preliminary design and economic analysis for a plant producing 1 million gal/yr of ethanol by enzymatic breakdown followed by fermentation of starch from potato processing waste. The purpose of this document is to define the project, provide a common basis of data upon which to base the design, and make specific statements about the scope of the project and required level of detail. Please read it carefully in order to understand what is being asked of you, so that you can direct your group’s effort appropriately. 2. Process The process in question is represented as a block diagram in Kilpimaa et al.’s (2009) Figure 1, reproduced at right. The overall scheme is to extract starch from a feedstock of potato processing waste, break it down into glucose using -amylase, and convert the glucose to ethanol and carbon dioxide by fermentation with Saccharomyces cerevisiae (baker’s yeast). Although the major alcohol product in the post-fermentation “beer” is ethanol, the yeast also produces small quantities of heavier alcohols commonly referred to as fusel oils. The raw “beer” emerging from the fermenter is purified by distillation. Meeting the purity specification for the top product (ethanol) requires special treatment because of the azeotrope in the ethanol- water phase diagram, e.g. dehydration by pressure-swing adsorption or pervaporation. Given the time constraints imposed by a one-semester project, you are being asked to limit the scope of your design and make certain simplifications according to the following points. Additional general notes regarding the process are also provided below. (a) You must determine the required amount of biomass entering the process, but you do not need to develop the actual handling steps for processing the potato waste feed. Also, you must define the conditions for enzymatic breakdown of starch, and include -amylase in your product cost analysis, but you do not need to design the reactors used. These parts of the process must be acknowledged in your flow sheet and mass balances, but do NOT have to be considered in detail. 2 (b) The fermentation process is run in a batch mode. The number and size of fermentation tanks to be used represents an important element of choice in your design (Decision Point 1). A detailed design is required for these reactors. (c) In reality, some type of filtration of the final fermentation broth is needed to remove cell debris and other solids. This operation must be acknowledged in your flow sheet and mass balances, but does NOT need to be considered in detail. (d) A holding tank will be needed downstream of the fermentation tanks to maintain a continuous feed for the distillation column. Its size should be chosen carefully. (e) CO2 must be flashed from the post-fermentation beer. You should determine the size of the requisite flash drum. (f) The distillation process typically involves two and sometimes three columns, which must be designed in detail. The configuration of these columns represents an important element of choice in your design (Decision Point 2). (g) The dehydration step by pressure swing adsorption does must be acknowledged in your flow sheet and mass balances, but does NOT have to be considered in detail. (h) Possible recycle streams should be considered carefully in developing your process flow sheet. (i) As discussed below, you must determine sizes of all lines between units, and pump power for each line. You must also specifically select the pump for the line from the hold tank to the flash drum. This element is designated formally as Decision Point 3 (although it is not actually a decision). (j) Elroy looks VERY kindly on processes exhibiting at least some degree of energy integration. Elroy frowns VERY strongly upon processes without any energy integration. 3. Data, required level of detail, and other notes Your design should ultimately produce a specification for each unit operation in terms of equipment type and size, an identified material of construction, and key operating variables. This level of detail is sufficient. As in real life, you are NOT responsible for the actual mechanical design (vessel thickness, etc.), which would be determined by a mechanical engineer from the firm producing the quotation for any given piece of equipment. Data and notes regarding requirements for the design are provided below. Feedstock. A typical potato composition is given below. Component Weight % Water 75 3 Starch 20 Extractable starch 18 Proteins, fiber, vitamins, minerals 5 Starch liquefaction. -Amylase is supplied as liquid. Dosing is typically 0.02 % (w/w) of starch. Fermenters. Dosing of yeast is typically = 10 g/L, initial glucose concentrations are typically 200 g/L, and batch times are typically 120 hours. These parameter values are only ballpark estimates; you should decide their precise values following your judgment. Assume operation at 35 °C. You may use the following rate expression for yeast growth: Here all concentrations are in units of g of (= yeast, glucose or ethanol) per liter of total fermentation broth volume. Excepting the last factor, this equation is Monod’s equation, in which — for a given — the bacterial growth rate is first order in , and in which the effective rate constant multiplying (in square brackets) depends on . The last factor accounts for inhibition of the fermentation reaction by the (ethanol) product. Thus, approaches a constant asymptotic value at long times. Each g of yeast produced consumes a given quantity of glucose. You may assume the following expression for this relationship: Each g of glucose consumed produces a definite quantity of ethanol, as described by A rigorous solution of the preceding kinetic equations must underpin your fermentation reactor design. Use amyl alcohol as a model for all fusel oils produced along with ethanol. Assume a fusel oil to ethanol mass ratio of 0.005. You must decide vessel number and size based on reactor residence time, factoring in the required time-average product flow rate, and the necessity of emptying, cleaning and charging fermenters between batches. This decision must be reported by 13 March (Decision Point 1). You must also define in detail how the fermenters will be heated or cooled to maintain the stated operating temperature. Flash drum. You must develop a detailed design of the flash drum specifying temperature, pressure and composition of feed, exiting vapor and exiting liquid streams; drum height/length and diameter; and thermal duty. Distillation. Treat the post-fermentation beer as a ternary mixture of water, ethanol and amyl alcohol, plus glucose acting as a nonvolatile component. You must develop a detailed design of the distillation column specifying reflux ratio and boilup ratio; temperature, pressure and composition of feed, distillate and bottom product streams; column height above feed and below feed, and column diameter; type of packing or trays; and thermal duty and heat transfer area for reboiler and condenser. 4 You will probably perform calculations with UniSim. However, you must also prepare one or more McCabe-Thiele diagrams that explain your design and roughly check the numbers from UniSim. The configuration of your distillation process must be reported by 3 April (Decision Point 2). Heat exchangers. Determine and state thermal duty, heat transfer area, temperature and pressure of input and output streams, and type of heat exchanger. Fluid lines. Your design must include sizes of all lines between units (must be standard sizes), and pump selection and power for each line. You must also specifically select the pump for the line from the hold tank to the flash drum. These specifications must be reported by 17 April (Decision Point 3). Flow sheet. Your completed design (and in particular, report) must contain a neat, computer- generated, detailed flow diagram. It should resemble Towler and Sinnott’s Figure 2.8 (p. 39). It may not be a UniSim flowsheet. Streams must be labeled, with information about each stream (temperature, pressure, phase, composition) tabulated below by number. Information must be given in units customary among engineers in this country (not Planet X on the galactic rim), i.e., flow rates in lb/h or kg/h, pressures in psi or kPa, temperatures in °F or °C, dimensions in ft or m, and pipe sizes in in. Although you will likely use other units in calculations, they must be converted into acceptable units in your final presentation. Also, you should retain extra significant figures in order not to let round-off error corrupt your calculations. However, you should ultimately report only a reasonable number of digits. Net: a flow rate expressed as, e.g., 1.344789002 × 108 g/week WILL NOT FLY with Elroy or any of your project supervisors. 4. Materials and services available, and other specifications (a) Materials available (f.o.b.) Potatoes Free (from another process owned by you) -Amylase $7 / kg, available as liquid Saccharomyces cerevisiae $2.30 / kg plus shipping from China at $5,000 per 18-metric-ton container (b) Services available Steam, 150 psig, saturated: $15 / 1000 kg Cooling water, 60 psig, 30 °C supply, 40 °C return: $0.20 / 1000 gal Process water (chilled), 60 psig, 15 °C: $1.50 / 1000 gal Electricity: $0.06 / kWh Wastewater treatment: $3.00 / 1000 gal Disposal of unreacted glucose from fermenters: $0.05 / lb (c) Product specification 5 Ethanol sells for $2.40 / gal. The following table gives specifications on purity. Component Specification (all w/w) ethanol ≥ 99.98% water ≤ 0.02% fusel oils (all) ≤ 0.01% other impurities ≤ 0.01% (d) On stream time: assume 8000 hours / year 5. Guidelines and requirements for economic analysis Assume 20-year project life, 6% annual effective interest rate, 40% tax rate. Results and criteria specifically to be addressed in your final report are: fixed