Table of Contents
This document constitutes a progress report to the Client pertaining to Group J’s front end engineering design to produce 100,000 tonnes per year of butanol at 99% molar purity via acetone-butanol-ethanol fermentation using wheat as the feedstock. The plant is comprised of seven distinct sections or Areas of unit operations and processes, which are clearly demarcated and have their underlying logic explained and PFD drawn by the relevant Area Engineer. Feasibility studies and financial analysis is current underway, subject to Client approval.
2. Area 1: Wheat Milling
The wheat is delivered into three docking stations each with three silos, which can store 10 hours’ worth of wheat, subject to a maximum transportation delay of 8hrs40. The wheat is then debraned, destoned, and passed under a magnet. The wheat is milled by sequential grinders. The first grinder is designed to grind to 500μm and the second to 100μm. A sieve separates ungrounded particles for recycling to grinders. The system is air tight after the first grinder to prevent risk of explosion. The wheat is then pneumatically conveyed to Area 2 and deposited by cyclone.
3. Area 2: Starch Slurry Preparation & Gelatinisation
The objective of Area 2 is to gelatinise the starch into a soluble form so liquefaction and saccharification can be performed in Area 3. Flour from Area 1 is mixed with water forming a slurry in a tank. A pump moves the mixture towards the jet heater where steam is injected to increase the temperature to >120°C to sterilise the mixture. A screw extruder is used to cook the starch at this temperature whilst conveying it, avoiding any difficulties posed by the gelatinisation viscosity change. Cool water is added to dilute the starch solution and reduce the temperature to optimal enzymatic conditions. A tank is included as the final stage in the area to provide a buffer vessel due to irregularities downstream.
4. Area 3: Starch Hydrolysis & Saccharification
The slurry is mixed with dilute sulphuric acid to pH 5 in a mixer prior to the addition of enzymes in insulated batch reactors. 70g/l starch is used to produce about 88g/l glucose without adverse viscous effects in these reactors. Using 18 200m3 reactors with a batch cycle of 4.5hrs (including 3hrs of fermentation), a continuous outflow is produced without the use of buffer tanks to prevent the profileration of microorganisms. At the end of the liquefaction and saccharification process, the mixture is neutralized using caustic soda. In the fermentation stage the mixture is cooled to 35°C by introducing a cold water stream. Cleaning-in-place (CIP) is employed in the reactors.
5. Area 4: ABE Fermentation
5.1 Solvent Recovery Technology
Various stripping gases have been studied extensively, and the off-gas (hydrogen and CO2) recovery has been found to be 0.17g solvents per litre of broth per hour, using a gas flowrate of 3.2 litres per litre of broth per min. Solvents are subsequently recovered from the off gases by cooling to 0°C. This recovery method costs approximately £60m annually, which is four times the average cost of steam-stripping. Pervaporation and adsorption have also been extensively researched however the high expenditure with membranes makes this option less attractive. Consequently, steam is used to strip the butanol-acetone-ethanol (“solvents”) from the fermentation broth.
5.2 Number of Tanks and Batch Scheduling
The process operates in 33-hour cycles with 25 hours of fermentation. The schedule…