| Compared with traditional anaerobic digestion methanogenesis technology,synthetic gas biological fermentation directed acid production process has the advantages of higher utilization rate of biomass,lower reaction process cost,higher separation value of products,etc.,which is more in line with our energy strategic goals.The existing mechanism models of acid production(such as ADM1 model,ASM model,etc.)cannot be used to simulate the acid production process of syngas.(1)There is a lack of descriptions of variables and equations related to the acid production process of syngas,such as descriptions of CO and the biochemical reaction and physicochemical reaction in which CO participates,descriptions of valeric acid and caproic acid and their biochemical,physicochemical reactions in which CO participates.(2)Only recommended parameter values under specific process conditions are given,and the general application of synthetic gas acid production system is low.In addition,the parameter values of mechanism model change with the change of experimental conditions,so it is necessary to update the existing parameter reference values.(3)The framework structure of the existing acid production model is relatively simple,and the simulation results cannot evaluate the contribution of specific equations.Therefore,based on the synthetic gas acid production system,this study referred to the existing modeling methods of anaerobic mechanism models,including the description of biochemical process,description of physicochemical process,description of inhibiting factors,model simulation algorithm,model implementation platform,etc.The SAFM(Syngas Anaerobic Model)is put forward in three stages.(1)Modeling and simulation of short-chain fatty acids produced by glucoseIn this chapter,SAFM1 model was built based on the biochemical,physicochemical processes of glucose anaerobic digestion system,and was used to simulate the process of glucose to acetic acid under different temperature and p H conditions.The model parameters were modified and solved according to the experimental data of 6 groups of reactors(p H4,5,6,9,10,11)at medium temperature.The modified model was verified according to the experimental data of 6 groups of reactors(p H 4,5,6,9,10,11)at normal temperature.The concentration of acid and alcohol in the steady-state process of glucose production of acetic acid was successfully simulated.Goodness of fit is above 0.8.According to the results of parameter sensitivity analysis,the sensitivity parameters of SAFM1 model were selected as km(1),ks(1),km(2),ks(2),km(3),ks(3),km(16),ks(16),Y(16).The verified SAFM1 model further extends the original anaerobic digestion mechanism model and updates the recommended values of related parameters of anaerobic digestion.From the perspective of inhibition function kinetics,the values of Iac and Iglu of p H on acetic acid and glucose increased with the increase of p H value.It was proved that under acidic conditions,p H inhibited the production of acetic acid mainly by inhibiting the degradation rate of glucose.At the same time,the values of Ih2 inhibition function of p H on hydrogen and Ico3 inhibition function of p H on carbonate are about 0.4 and0.15 respectively when p H is greater than 6,which proves that under alkaline conditions,p H can inhibit acetic acid production mainly by inhibiting isoacetic acid producing bacteria.From the perspective of microorganisms,the microbial concentration related to acetic acid production processes such as glucose consumption to acetic acid production,propionic acid consumption to acetic acid production,isoacetic acid production,etc.in Z9 reactor,the sum of microbial relative concentration Xglu,Xpro,Xco2 was the largest(nearly90 %)in all reactors.It is proved that Z9 reaction conditions can promote the yield of acetic acid in the process of glucose to acetic acid,and it is verified that p H is the main influencing factor in the process of glucose to acetic acid.(2)Modeling and simulation of short-chain fatty acids produced by syngasIn this chapter,CO is defined on the basis of SAFM1 model,and descriptions of biochemical,physicochemical processes of CO in the process of producing short-chain fatty acids from syngas are added,so as to further expand and build the SAFM2 model,which is used to simulate the process of producing short-chain fatty acids from syngas under different temperature and p H conditions.The model parameters were modified and solved according to the experimental data of 6 groups of reactors(p H 4,5,6,9,10,11)at medium temperature,and the revised model was verified according to the experimental data of 6 groups of reactors(p H 4,5,6,9,10,11)at normal temperature.According to the results of parameter sensitivity analysis,the parameters of the model were determined and the sensitivity parameters of the SAFM2 model were selected as km(2),ks(2),km(3),km(15),ks(15),km(21),km(24),ks(24).The kinetics of p H inhibition function and the non-competitive inhibition function of CO show that p H affects the acetic acid concentration of each reactor in the stable stage by affecting the absorption efficiency of syngas.From the perspective of microorganisms,with the increase of initial p H value,the relative microbial concentration of Co-related bacteria Xco increased and exceeded 30 % when p H value was 9,which verified that the initial p H value of 9 was conducive to the growth of Co-consuming bacteria.(3)Model building and simulation of middle chain fatty acids produced by different electron acceptorsBased on the SAFM2 model,this study extended the process of carbon chain extension by CO as an electron donor to produce mid-chain fatty acids,and defined caproic acid and the biological,physicochemical processes in which caproic acid participated.The model parameters were modified according to the experimental data of four groups of reactors with acetic acid as electron acceptor(initial concentration of acetic acid was 20,40,80 and160 m M,respectively),and the model was verified according to the experimental data of four groups of reactors with butyric acid as electron acceptor(initial concentration of butyric acid was 20,40,80 and 160 m M,respectively).The steady-state concentrations of acid and alcohol during the production of medium-chain fatty acids by CO and different electron acceptors were successfully simulated,and the goodness of fit was more than 0.8.According to the results of parameter sensitivity analysis,the parameters of the model were determined and the sensitivity parameters of SAFM model were screened out: km(5),km(6),km(7),km(8),km(11),km(18),km(32),km(39).Further using the function of the model,the carbon flux distribution of the carbon chain extension system under optimal experimental conditions(reactor 80A)was analyzed.For the products,the carbon utilization of caproic acid accounted for 28 %,acetic acid and propionic acid accounted for 7 % and14 %,respectively.In terms of substrate consumption,the CO consumed through the carbon chain extension process accounts for about 50 % of the total CO consumption,among which the CO consumed for producing medium chain acid(caproic acid)accounts for about 20 % of the total CO consumption.The carbon contribution and carbon loss of caproic acid production by different electron acceptors were investigated from the perspective of product utilization and substrate consumption. |
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