Procedure for Modelling a Cstr Continuous Stirred Tank Reactor

Continuous Stirred Tank Reactor

Continuous stirred tank reactors (CSTRs) are used frequently for continuous hydrogen production (Chen and Lin, 2003;

From: Biohydrogen , 2013

Bioreactor and Bioprocess Design for Biohydrogen Production

Kuan-Yeow Show , ... Duu-Jong Lee , in Biohydrogen (Second Edition), 2019

4.2 Continuous Processes

Continuous stirred tank reactors (CSTRs) are frequently investigated in continuous hydrogen production [ 67–69]. In comparison with batch reactors, microbial cultures in a CSTR are evenly suspended in the liquor with lower resistance in mass transfer. Attributable to its intrinsic reactor construction and completely mixed operation, a CSTR is incapable of maintaining high levels of cells inventory. Depending on the HRT, cells concentration between 1 and 4   g-VSS/L is commonly recorded [68,70–72]. Washout of cells could occur at short HRTs resulting in impeded hydrogen production. The production rates were marked restricted by a weak CSTR cell retention and low hydraulic loading [69,73]. The highest production rate of CSTR fed with sucrose with mixed microflora was reported at 1.12   L/L∙h [68].

Granulation of hydrogen-producing microbes can occur spontaneously with reduced HRT in CSTR [35,74,75]. Show et al. [76] and Zhang et al. [54] found that granulation of sludge markedly increased the cells inventory to 16.0   g-VSS/L. With such cells inventory, the CSTR can be operated at higher OLRs (up to 20   g-glucose/L∙h) along with enhanced hydrogen production. In another study, it was reported that granular sludge disintegrated in less than 21   days when CSTRs were operated in static condition instead of agitated mode [77].

A membrane-coupling reactor was employed to enhance biomass growth. It was shown that biomass inventory was increased from 2.2   g/L in a control unit (without membrane coupling) to 5.8   g/L in an anaerobic membrane bioreactor (AMBR) at 3.3   h HRT [78]. This was realized by maintaining sludge retention time (SRT) at 12   h, in line with a marginal rise in hydrogen production rate from 0.50 to 0.64   L/L∙h. Prolonging SRT can increase biomass inventory favoring substrate utilization, but may adversely lead to a decrease in hydrogen production rate. Hydrogen production rates ranging between 0.25 and 0.69   L/L∙h in AMBR systems were reported [57]. Other than enhancement in cells concentration, there is no apparent merit of AMBR over other biohydrogen systems. On the contrary, membrane fouling and excessive operating cost may restrict the application of AMBR in hydrogen production.

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Reactors in Process Engineering

Gary L. Foutch , Arland H. Johannes , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II.C.1 Description

The continuous-stirred tank reactor (CSTR) has continuous input and output of material. The CSTR is well mixed with no dead zones or bypasses in ideal operation. It may or may not include baffling. The assumptions made for the ideal CSTR are (1) composition and temperature are uniform everywhere in the tank, (2) the effluent composition is the same as that in the tank, and (3) the tank operates at steady state.

We traditionally think of the CSTR as having the appearance of a mixing tank. This need not be the case. The previously mentioned assumptions can be met even in a long tube if the mixing characteristics indicate high dispersion levels in the reactor. This is particularly true of gassed liquids where the bubbling in the column mixes the liquid.

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Bioreactor and Bioprocess Design for Biohydrogen Production

Kuan-Yeow Show , Duu-Jong Lee , in Biohydrogen, 2013

Continuous Processes

Continuous stirred tank reactors (CSTRs) are used frequently for continuous hydrogen production ( Chen and Lin, 2003; Majizat et al., 1997; Yu et al., 2002). Compared with batch reactor systems, hydrogen-producing bacteria in a CSTR are well suspended in mixed liquor and suffer less from mass transfer resistance. Because of its intrinsic structure and operating pattern, a CSTR is unable to maintain high levels of biomass inventory. Depending on the operating HRT, biomass inventory in a range between 1 and 4   g-VSS/liter is commonly reported (Chen and Lin, 2003; Horiuchi et al., 2002; Lin and Chang, 1999; Zhang et al., 2006b). Biomass washout is likely at short HRTs, resulting in deterioration in hydrogen production rates. Hydrogen production rates are thus restricted considerably by a low CSTR biomass retention and low hydraulic loading (Lay et al., 1999; Yu et al., 2002). The highest hydrogen production rate of CSTR culture fermenting sucrose with a mixed hydrogen-producing microflora was reported as 1.12 liter/liter   ·   h (Chen and Lin, 2003).

Spontaneous granulation of hydrogen-producing bacteria can occur with shortened HRT in CSTR (Fang et al., 2002a; Oh et al., 2004b; Yu and Mu, 2006). Show et al. (2007) and Zhang et al. (2007c) found that the formation of granular sludge significantly increased overall biomass inventory to as much as 16.0   g-VSS per liter of reactor volume, enabling the CSTR to be operated at higher OLRs of up to 20   g-glucose/liter   ·   h, which hence enhanced hydrogen production. In another study, it was reported that granular sludge disappeared within 3 weeks when CSTRs were incubated statically instead of being shaken (Vanderhaegen et al., 1992).

A reactor coupled with a membrane has been used to increase biomass concentration in the system. At a HRT of 3.3   h, Oh et al. (2004a) demonstrated that biomass concentration increased from 2.2   g/liter in a control reactor (without membrane fitted) to 5.8   g/liter in an anaerobic membrane bioreactor (MBR). This was achieved by controlling sludge retention time (SRT) at 12   h, corresponding to a marginal increase in the hydrogen production rate from 0.50 to 0.64 liter/liter∙h. Increasing SRT can enhance biomass retention, which favors substrate utilization, but may result in a decrease in the hydrogen production rate. Li and Fang (2007) summarized that hydrogen production rates ranged between 0.25 and 0.69 liter/liter·h in the MBR systems reviewed. While the biomass concentration can be enhanced in a MBR, there is no clear advantage of a MBR over other types of efficient hydrogen production systems. In addition, membrane fouling and a high operating cost may limit the use of the MBR process in hydrogen fermentation.

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Multivariate Statistical Process Monitoring Using Multi-Scale Kernel Principal Component Analysis

Xiaogang Deng , Xuemin Tian , in Fault Detection, Supervision and Safety of Technical Processes 2006, 2007

CSTR description

The CSTR with cooling jacket dynamics and variable liquid level is simulated for process monitoring. It is assumed that the classic first order irreversible reaction happens in CSTR. The flow of solvent and reactant A into a reactor produces a single component B as an outlet stream. Heat from the exothermic reaction is removed through cooling flow of jacket. The temperature of reactor is controlled to set-point by manipulating the coolant flow. The level is controlled by manipulating the outlet flow. A schematic diagram of the CSTR with feedback control system is shown in Fig. 2.

Fig.2. A diagram of the CSTR system

The data of normal operating condition and faulty conditions are generated by simulating the CSTR process and Gaussian noise is added to all measurements in simulation procedure. The simulation brings normal operating data and ten kinds of fault pattern data. The applied fault pattern can be seen in Table 1. These faults contain process change, sensor malfunction and valve faults.

Table 1. Process faults for CSTR system

Fault	Description
F1	Step change in feed flow rate.
F2	The feed temperature ramps up or down.
F3	The feed concentration ramps up or down.
F4	The hear transfer coefficient ramps down.
F5	Catalyst deactivation.
F6	The coolant feed temperature ramps up or down.
F7	Set point change for the reactor temperature.
F8	The reactor temperature measurement has a bias.
F9	The coolant temperature measurement has a bias
F10	The coolant valve was fixed at the steady value.

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Environmental Biotechnology and Safety

M.F.M. Bijmans , ... P.N.L. Lens , in Comprehensive Biotechnology (Second Edition), 2011

6.34.5.1 Continuous Stirred-Tank Reactor

The continuous stirred-tank reactor (CSTR) liquor is mixed by a mechanical stirrer (Figure 4 (a)), resulting in a completely mixed system. CSTRs have been used in fundamental studies on the SR process, while the high-energy requirements and the need of an external settler to retain biomass limits its industrial application. For instance, a CSTR was used to study SR at low pH by Kimura et al. [44] that showed SR at pH 3.8 in batch while adding zinc to precipitate the inhibiting sulfide as zinc sulfide.

Figure 4. Reactor types used for sulfate reduction with (a) continuous stirred-tank reactor (CSTR); (b) upflow anaerobic granular sludge bed (UASB) bioreactor with gas production; (c) UASB without gas production; (d) expanded granular sludge bed (EGSB) reactor; (e) fluidized bed reactor (FBR); (f) gas-lift bioreactor (GLB) with internal loop; and (g) membrane bioreactor (MBR).

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Biohydrogen Production From Renewable Biomass Resources

Ganesh Dattatraya Saratale , ... Jo-Shu Chang , in Biohydrogen (Second Edition), 2019

6.1 Continuous Stirred Tank Reactor

Continuous stirred tank reactor (CSTR) is the most generally employed bioreactor for biohydrogen production in continuous mode because of its simplicity in configuration, easy functioning, efficient uniform stirring, and proper maintenance of temperature and pH ( Fig. 10.2A). In these types of bioreactors, biohydrogen generating microbial population is entirely circulated and is in suspension with the mixed liquor in the reactor with the help of stirrer. As a result, the microbes are completely suspended in reactor liquor and contain equivalent biomass load in the effluent. In such circumstances, best inoculum substrate interaction and mass transfer could be obtained. Alternatively, the CSTR is incapable to uphold greater quantity of fermenting microbes due to its hasty mixing operational pattern. Biomass washout may happen at very low HRTs, which lead to lower biohydrogen production [119]. To maintain the elevated concentration of biomass in bioreactors, different processes have been established, such as immobilization of microbes, using upflow reactor, and immobilization on a permeable supporting material, such as loofah sponges, extended clays, activated carbons, and membrane reactors, for better biohydrogen production. The literature survey observed that in CSTRs they also possess certain limitation, such as highly sensitive towards various environmental factors, such as fluctuations in pH and HRT, and processing at an increased dilution rate results in washout of biomass leading to lower biohydrogen production rate [119].

Figure 10.2. Various bioreactors used in fermentative biohydrogen production.

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27th European Symposium on Computer Aided Process Engineering

Piotr Skupin , ... Malgorzata Niedzwiedz , in Computer Aided Chemical Engineering, 2017

1 Introduction

Continuous Stirred Tank Reactor (CSTR) with exothermic reaction is a system that exhibits a highly nonlinear behaviour, for example, multiplicity of steady states or limit cycles ( Henson and Seborg, 1997). As a result, the CSTR is a challenging plant for testing new control algorithms (see, e.g., Wu, 2001), where the process variable is the temperature inside the reactor and the manipulated variable is the flow rate of coolant in the reactor jacket. However, the tests with real reactors are often dangerous (thermal runaway) and expensive in operation. To overcome these difficulties, one can use a hybrid chemical reactor, i.e., the system with a real reactor vessel and simulated reaction heat. The idea of the hybrid reactor is known in the literature and the reaction heat can be simulated either by direct steam injection into the reactor (Kershenbaum and Kittisupakorn, 1994; Afonso et al., 1996; Dua et al., 2004) or by means of electric heaters placed inside the reactor vessel (Metzger, 2002; Cueli and Bordons, 2008; Schubert et al., 2009). In effect, the control algorithms could be tested with a partially simulated plant, but none of these papers has shown that the hybrid system exhibited nonlinear phenomena, such as multiplicity of steady states or limit cycles.

In the presented case, we develop a hybrid reactor with the simulated reaction heat by means of the electric heater inside the reactor vessel. To design a hybrid system that exhibits nonlinear phenomena (e.g., multiplicity of steady states or limit cycles), the electric heater must be controlled by a numerical model of the system. The laboratory plant shown in Figure 1, can be treated as the classical CSTR system with exothermic reaction, provided that the electric heater is controlled based on the classical CSTR model. The nonlinear behaviour of the CSTR system is well-known in the literature and its mathematical model is based on the assumption that the reactor content and the coolant in the reactor jacket are well-mixed. However, this assumption is not always true for real systems, thus the mathematical model of the plant must be modified. Moreover, the power of the electric heater is always constrained by its maximum value in real systems, thus the constraints must also be incorporated in the mathematical model.

Figure 1. Hybrid reactor: a) laboratory plant; b) simplified scheme of the reactor; c) structure of mathematical model.

To intensify the nonlinear effects in the designed hybrid reactor, it is necessary to study the influence of imperfect mixing effects on the dynamical behaviour of the plant. The imperfect mixing can be modelled in different ways and a common approach is to use multi-compartment models (Stebel and Metzger, 2012). In our case, we assume that the reactor vessel can be divided into two compartments of volumes V_r and V_L with an additional flow F_L between the compartments (Figure 1c). This means that the reaction do not take place in the whole volume V, which corresponds to the fact that the electric heater is placed in the centre of the reactor vessel. The content in the real hybrid reactor is mixed by the additional pump in the external recycle, thus we can increase or reduce the imperfect mixing effect in the real system. Moreover, the heating power constraints must also be taken into account, since the mathematical model is used to calculate the reaction heat in the hybrid system.

Hence, the main goal of the paper is to study the influence of model parameters that describe the imperfect mixing effect as well as the heating power constraints on the dynamical behaviour of the hybrid reactor. The results of this study will be helpful in the design of hybrid reactor for testing new control algorithms.

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Ideal Flow Reactors

Shijie Liu , in Bioprocess Engineering (Second Edition), 2017

5.4 Continuous Stirred Tank Reactor and Chemostat

CSTR is an idealized flow reactor such that all the contents inside the reactor are well mixed, just as in a batch reactor. This idealization makes the flow reactor analysis simplified extremely as now one can treat the whole reactor as one simple unit or "black box." Fig. 5.8 shows a schematic of a CSTR. One can observe that the reactor resembles a batch reactor but with inlets and outlets attached to it. Since the contents inside the reactor are well mixed, the concentrations and temperature are identical everywhere inside the reactor and are equal to those at the outlet. The inlet conditions, on the other hand, can be different.

Fig. 5.8. Schematic diagram of a CSTR or chemostat showing one stream flowing in and one stream flowing out of the reactor, with proper volumetric flow rates, temperature, and concentrations.

Mole balance for a given species j over the entire reactor leads to

(5.33) $F_{j0}-F_j+r_{j}V=\frac{d{n}_j}{dt}$

which leads to

(5.34) $F_{j0}-F_j+r_{j}V=V\frac{d{C}_j}{dt}+C_j\frac{dV}{dt}$

or

(5.35) $Q_0{C}_{j0}-Q{C}_j+r_{j}V=V\frac{d{C}_j}{dt}+C_j\frac{dV}{dt}$

which is the general mole balance equation for a CSTR.

Since the total amount of mass cannot be created or destroyed during chemical and bioreactions, we obtain through mass balance over the entire reactor, as shown in Fig. 5.6:

(5.36) ${\rho }_0{Q}_0-\rho Q=\frac{d\left(\rho V\right)}{dt}=\rho \frac{dV}{dt}+V\frac{d\rho }{dt}$

For condensed matters, density is only a function of temperature. In general,

(5.37) $\rho ={\displaystyle \sum _{j=1}^{N_{\mathrm{S}}}{C}_j{M}_j}$

Eqs. (5.33) through (5.37) are equally applicable to batch reactors. Therefore, these equations are more general mole balance equations for any well-mixed reactors. In a general case, Eq. (5.35) needs to be written for every component (species) except one involved. Like the plug flow reactions, momentum balance equation is needed to close the problem. Thus, we would have N _S differential equations to solve even for an isothermal reactor. If temperature changes, the energy balance equation must be solved simultaneously as well. In addition, we need to know how Q would vary with time.

We next consider a more simplified case, where steady state condition has been reached. In fact, most times when we say CSTR or chemostat, we mean CSTR at steady state. At steady state, nothing changes with time. Therefore, Eqs. (5.33) and (5.35) are reduced to

(5.38) $F_{j0}-F_j+r_{j}V=0$

or

(5.39) $Q_0{C}_{j0}-Q{C}_j+r_{j}V=0$

which are algebraic equations.

For a single reaction that is carried out in a steady CSTR, there is only one independent concentration or reaction mixture content variable, and all other concentrations can be related through stoichiometry as shown in Chapter 3. Without loss of generality, let us use component A as the key component of consideration. Eq. (5.38) applied to species A gives

(5.40) $F_{\mathrm{A}0}-F_{\mathrm{A}}+r_{\mathrm{A}}V=0$

or

(5.41) $Q_0{C}_{\mathrm{A}0}-Q{C}_{\mathrm{A}}+r_{\mathrm{A}}V=0$

These algebraic equations can be solved easily.

(5.42) $V=\frac{F_{\mathrm{A}0}-F_{\mathrm{A}}}{-r_{\mathrm{A}}}=\frac{F_{\mathrm{A}0}{f}_{\mathrm{A}}}{-r_{\mathrm{A}}}$

or

(5.43) $V=\frac{Q_0{C}_{\mathrm{A}0}-Q{C}_{\mathrm{A}}}{-r_{\mathrm{A}}}$

Note that the rate of reaction is evaluated at the reactor outlet conditions.

In general, the rate of reaction is a function of concentration and temperature. From Chapter 3, we learned that stoichiometry can be applied to relate the amount of every species in the reaction mixture. The amount change of a component participating in the reaction divided by its stoichiometric coefficient is the universal extent of reaction for a single reaction. The stoichiometry can be written in a flow reactor as

(5.44) $\frac{F_{j0}-F_j}{{\nu }_j}=\frac{F_{\mathrm{A}0}-F_{\mathrm{A}}}{{\nu }_{\mathrm{A}}}=rV$

The total molar flow rate can be computed by adding up all the component (species) flow rates. That is,

(5.45) $F={\displaystyle \sum _{j=1}^{N_{\mathrm{S}}}{F}_j}={\displaystyle \sum _{j=1}^{N_{\mathrm{S}}}\left(F_{j0}+{\nu }_j\frac{F_{\mathrm{A}}-F_{\mathrm{A}0}}{{\nu }_{\mathrm{A}}}\right)}=F_0+\frac{F_{\mathrm{A}}-F_{\mathrm{A}0}}{{\nu }_{\mathrm{A}}}{\displaystyle \sum _{j=1}^{N_{\mathrm{S}}}{\nu }_j}$

Letting ν _Σ be the total stoichiometric coefficients, ie,

(5.46) ${\nu }_{\mathrm{\Sigma }}={\displaystyle \sum _{j=1}^{N_{\mathrm{S}}}{\nu }_j}$

we obtain

(5.47) $F=F_0+\frac{{\nu }_{\mathrm{\Sigma }}}{{\nu }_{\mathrm{A}}}\left(F_{\mathrm{A}}-F_{\mathrm{A}0}\right)=F_0-\frac{{\nu }_{\mathrm{\Sigma }}}{{\nu }_{\mathrm{A}}}{F}_{\mathrm{A}0}{f}_{\mathrm{A}}$

While the preceding derivation is concise, we often tabularize the stoichiometry to gain a thorough understanding of the stoichiometry for every species, whether for those involved in the reaction or those that are not participating in the actual reaction. The stoichiometry is shown in Table 5.3.

Table 5.3. Stoichiometry of a Reaction System With Side Inlets or Outlets

Species	At inlet	Change	At Outlet
A	F A0	F A  − F A0	F A
j	F j0	$F_j-F_{j0}={\nu }_j\frac{F_{\mathrm{A}}-F_{\mathrm{A}0}}{{\nu }_{\mathrm{A}}}$	$F_j=F_{j0}+{\nu }_j\frac{F_{\mathrm{A}}-F_{\mathrm{A}0}}{{\nu }_{\mathrm{A}}}$
…	…	…	…
Total	F 0	${\displaystyle \sum _{j=1}^{N_{\mathrm{S}}}\left(F_j-F_{j0}\right)}={\nu }_{\mathrm{\Sigma }}\frac{F_{\mathrm{A}}-F_{\mathrm{A}0}}{{\nu }_{\mathrm{A}}}$	${\displaystyle \sum _{j=1}^{N_{\mathrm{S}}}{F}_j}=F_0+{\nu }_{\mathrm{\Sigma }}\frac{F_{\mathrm{A}}-F_{\mathrm{A}0}}{{\nu }_{\mathrm{A}}}$

The concentration can be related to the molar flow rate through

(5.48) $C_j=\frac{F_j}{Q}=\frac{F_{j0}-\frac{{\nu }_j}{{\nu }_{\mathrm{A}}}{F}_{\mathrm{A}0}{f}_{\mathrm{A}}}{Q}$

The volumetric flow rate Q can be a function of temperature and pressure (density change). Since the mass flow rate does not change if no side inlets or outlets, we have

(5.49) $Q=\frac{{\rho }_0}{\rho }{Q}_0$

(5.50) $C_j=\frac{\rho }{{\rho }_0}\frac{F_{j0}+\frac{{\nu }_j}{{\nu }_{\mathrm{A}}}\left(F_{\mathrm{A}}-F_{\mathrm{A}0}\right)}{Q_0}=\frac{\rho }{{\rho }_0}\frac{F_{j0}-\frac{{\nu }_j}{{\nu }_{\mathrm{A}}}{F}_{\mathrm{A}0}{f}_{\mathrm{A}}}{Q_0}$

For isothermal operations, Q is constant for reactions involving condensed matter (liquid or solid) only. For ideal gas, the volumetric flow rate can be related to the molar flow rate through ideal gas law

(5.51) $PQ=FRT$

which leads to

(5.52) $Q=\frac{P_{0}T}{P{T}_0}\frac{F}{F_0}{Q}_0=\frac{P_{0}T}{P{T}_0}\left[1+\frac{{\nu }_{\mathrm{\Sigma }}}{{\nu }_{\mathrm{A}}}\frac{F_{\mathrm{A}}-F_{\mathrm{A}0}}{F_0}\right]Q_0=\frac{P_{0}T}{P{T}_0}\left[1-\frac{{\nu }_{\mathrm{\Sigma }}}{{\nu }_{\mathrm{A}}}\frac{F_{\mathrm{A}0}}{F_0}{f}_{\mathrm{A}}\right]Q_0$

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28th European Symposium on Computer Aided Process Engineering

Juraj Oravec , ... Alajos Mészáros , in Computer Aided Chemical Engineering, 2018

1 Introduction

Continuous stirred-tank reactors (CSTRs) are frequently used plants in the chemical and food industries. Control of CSTRs is a challenging problem due to the non-linear behaviour, multiple steady-states, heat effect of the chemical reactions, time delay, and effect of various time-varying uncertainties, see (Luyben, 2007, chap. 1). The model predictive control (MPC) represents the state-of-art in the optimization-based control for complex systems (Maciejowski, 2000). The robust model predictive control optimizes control action according to the bounded uncertain parameters (Bemporad and Morari, 1999). LMI-based robust MPC represents an intensively developed class of control strategies, as it is handled via the convex optimization (Kothare et al., 1996). MPC requires the system model in the proper form. To find a suitable mathematical formulation for a complex system can be a challenging task. The model of monomer semi-batch emulsion copolymerization for MPC design was investigated in Chaloupka et al. (2017). The multi-parametric MPC enables to speed up the online evaluation of the complex controller, see Charitopoulos et al. (2017). Another perspective branch of predictive control, the economic MPC for an uncertain model of a heat exchanger was designed in Pour et al. (2017). The possibility to control a CSTR using the novel algorithm of hierarchical MPC providing the exact, global and multi-parametric solution of bi-level programming was investigated in Avraamidou and Pistikopoulos (2017). In Holaza et al. (2018) an advanced reference-governor-based control of the chemical reactor was analysed. When comes to implementing the advanced control strategy, it is not straightforward to develop the software tool for the different groups of users, e.g., industrial users, technology developers, and researchers, see Varbanov et al. (2017). The software tool MUP for robust MPC design suitable for researchers is proposed in Oravec et al. (2017).

This paper presents particular results of our research focused on the advanced LMI-based robust MPC design for the chemical reactors. The work directly extends the results of previous work Oravec et al. (2017), where robust MPC was designed for the considered neutralization chemical reactor, but in the different control setup. In comparison to Oravec et al. (2017), the main contribution of this work originates in the investigation of the control performance by using two control inputs, i.e., the volumetric flow-rates of acid and base, respectively. The controlled output was pH of the solution in the outlet stream. As only the controlled output was measured, the state observer had to be used to estimate the system states. We redesigned the robust MPC subject to the extended vector of the system states and the integral action. The weighting matrices in the quadratic cost function optimizing the control performance were experimentally tuned. The paper demonstrates the benefits of the proposed robust MPC design strategy for set-point tracking problem.

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Combined Gasification-Fermentation Process in Waste Biorefinery

Konstantinos Chandolias , ... Mohammad J. Taherzadeh , in Waste Biorefinery, 2018

3.9.1 Continuous Stirred Tank Reactor

The CSTR reactor is one of the most commonly used in syngas fermentation. In CSTR, the syngas is continuously fed at the bottom of the vessels, and the size of the gas bubbles is reduced by the impellers. This mechanical size reduction creates better gas-liquid mass transfer because of the higher interfacial surface area [122] and the longer retention time of these smaller gas bubbles in the digester [95]. Another strategy to enhance the gas-liquid mass transfer is to increase the feeding flow rate of the gas. In a study of syngas fermentation in a CSTR, the increase of the specific CO flow rate and of the agitation speed from 0.14 to 0.86   vvm and from 200 to 600   rpm, respectively, caused an increase in the K _La of CO from 10.8 to 155/h. Moreover, the gas flow rate and the agitation are linearly proportional to the power per volume and the superficial gas velocity, respectively [157]. However, in industrial applications, increasing the syngas flow results in unfermented syngas at the gas exhaust, and an increase in the agitation is economically inefficient; therefore, other gas-dispersing techniques, which are presented in Section 3.9.3, have been studied.

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