Msc: Sustainable Engineering : Chemical and Process Engineering
Simulation
Below is the Process Flow Diagram for the thermal pre-treatment and the anaerobic digestor. For this process the NRTL model was used as it can perform highly accurate calculations with vapour liquid mixtures that this process will contain and will assume that the any vapour produced in the mixture is ideal. The compositions of the manure and DDGS were based on results found in previous studies that investigated these two feedstocks. (Budiyono, 2011), (Alagón et al., 2016)
Figure1: Thermal Hydrolysis and anaerobic digestion process simulated through ASPEN Plus simulation.
Initially the feed consisting of dairy manure, DDGS and water is led into a mixer and the to a heat exchanger at 90 degrees Celsius to create a continuous mixture that has been appropriately pre-heated before the main anaerobic digestion process. As the mixture enters the first reactor it undergoes hydrolysis, the initial stage of anaerobic digestion, where complex structures like hemicellulose, cellulose and protein are broken down to produce long chain fatty acids and sugars.
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Unlike in a typical AD process where the feed is heated between 30 and 50 degrees Celsius (Meegoda et al., 2018), for this scenario the hydrolysis stage is conducted at 160 degrees Celsius and 6 bar gauge (Kim, Lee, Kim, et al., 2023). Using a thermal hydrolysis system has two benefits. The first is to sterilise harmful microorganisms that are present in the manure such as E Coli. The second is to boost methane production by reducing the hemicellulose in the process.
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Once the hydrolysis stage is finished, the feed is led to a second heat exchanger which uses cooling water to reduce the temperature of the feed before it enters the next three stages of the AD process. Once it is cooled it enters a plug flow reactor where it undergoes acidogenesis, acetogenesis and methanogenesis at a temperature of 55 degrees Celsius and pressure of 1 atmosphere. These conditions promote the growth of microbes which assist the conversion of the hydrolysed products through the remaining three stages and finally lead to the conversion of biogas which consists of primarily methane and carbon dioxide (Rajendran et al., 2014). Once this process is finished two product streams leave the reactor, a liquid stream, and a biogas stream. The liquid stream leaves the system, either for further processing or to be spread on the farmer's field. The biogas enters the stripping section for purification.(Anaerobic Digestion & Bioresources Association, 2024)
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Figure 2: Mixing and heating of manure, DDGS and water.
Figure 3: Hydrolysis Reactor
Figure 4: Cooling of hydrolysis products and final AD reaction