Dynamic operation

Dynamic operation

The effective control and use of dynamic reactor behavior is essential to enhance the desired performance parameters. Dynamic operation can be either systemic, i.e. catalyst deactivation, or forced, modulation of inlet parameters. With the application of suitable tools, it is possible to determine process parameters that make a dynamic operation more efficient than a steady state process. A dynamic operation of multifunctional reactors can be used to reduce downtimes and lead to higher efficiencies compared to conventional reactor concepts.

Simultaneous Modulation of Multiple Reactor Inlet Parameter

In chemical processes it is usually desired to prevent dynamic behavior of the system. Such dynamics can be seen during the startup of a reactor or due to fluctuation of the inlet parameters. Even though, it was proven that with proper choice of the dynamic parameters it is possible to obtain beneficial performance, the application of theoretical results has been limited.  

A relatively new and fast method to evaluate dynamic reactor operation is the nonlinear frequency response (NFR). This method, based on control theory, predicts the output of forced periodically operating single and simultaneous input parameter, such as inlet concentration, total flow and inlet temperature (Markovic2008, Petkovska2013). In a collaboration with the University of Belgrade sinusoidal changes and more general waveforms of these inputs are being validated (Nikolic, 2017).

Fig. 1: Schematic illustration inputs modulating resulting in a fluctuating output, which is beneficial when the time-average is higher compared to the steady state case for the product (Nikolic, 2014).

Nonlinear Frequency Response Method

The NFR method is based on the concept of higher order frequency response functions (FRFs) and applicable for weakly nonlinear systems [3]. Frequency response of a weakly nonlinear system, in addition to the basic harmonic, contains a non-periodic (DC) term and, theoretically, an infinite sequence of higher harmonics. The DC component of the output is responsible for the average performance of the periodically operated reactor, and its sign and value define whether, and to which extent, the periodic operation leads to process improvement. Using the NFR method, this DC component can be approximately estimated from a single asymmetrical second order FRF (for modulation of a single input) or from several single input and cross- asymmetrical second order FRFs (for multiple-input modulation, see Figure 2). For the case of multiple modulated inputs, the optimal phase difference between the modulated inputs, which is an essential parameter, can be directly determined [4,5].

Fig. 2: Single and cross term influence of the periodically operated inlet parameters.

As an example reaction the hydrolysis of acetic anhydride in an adiabatic CSTR has been chosen. In order to implement single and multiple input parameter perturbations and experimental setup was build (Figure 3).

Fig. 3: Experimental setup for the investigation of the dynamic operation for the hydrolysis of acetic anhydride.

It was possible to show that the single input modulation does not change the time-average output, in case of inlet concentration, or results in detrimental output, in case of total flow perturbation. In case of simultaneous modulation of inlet concentration and total flow the phase difference between the two inlets in crucial. This is presented in Figure 4 and Figure 5. A clear maximum is observed at the optimal phase difference.

Fig. 4: Experimental and simulative representation of sinusoidal variation of the inlet concentration flow rates of the feed streams for Acetic Acid and Acetic Anhydride.

Fig. 5: Experimental and simulative representation of sinusoidal variation of the inlet concentration, plot of the measured concentration change over time for Acetic Acid and Acetic Anhydride.

In further investigations, the implementation of different waveforms will be of interest. From Figure 4 it can be seen that the application of a square wave will result in higher optimization, compared to the sinusoidal case. This is based on the higher nonlinearities involved for this function type. Additionally, other combinations of inlet parameter variations will be experimentally validated.


Membrane Reactors for Integrated Coupling of Oxidative and Thermal Dehydrogenation of Propane

Besides forced dynamic behavior of chemical reactors, a lot of catalytic chemical processes show unwanted dynamic behavior due to catalyst deactivation [11]. One way to enhance the performance of such processes is to use other, more advanced catalysts. Another way is to develop integrated multifunctional reactor concepts [12] and strategies for a dynamic operation that includes catalyst regeneration and avoid reactor down times.

An industrial relevant reaction that shows rapid catalyst coking is the thermal dehydrogenation of propane (TDH, Fig. 1, r1). This well-established reaction for on-purpose production of propene is especially interesting since the feedstock for crackers shifts to light hydrocarbons due to the abundance of shale gas [13]. The reaction is used in different commercialized processes (CATOFIN®/STAR®/Oleflex® process) and shows a high selectivity towards propene. An alternative reaction is the oxidative dehydrogenation (ODH, Fig. 1, r2) which is less efficient due to unwanted side reactions, e.g. total oxidation, but shows no coking.

Fig. 1: Reaction network of the ODH and TDH.

The combination and thermal integration of oxidative and thermal dehydrogenation in one multifunctional membrane reactor can help to overcome these disadvantages and can lead to higher yields and a more effective process. This integration is investigated in this project, supported by the German Research Foundation (Project: „Kontrolle und Intensivierung von Reaktionen durch Einsatz zyklisch betriebener Distributoren“ SE 568/23-1 / HA 6762/2-1).

However, the optimization of conversion and selectivity is an ambitious as well as complex challenge and several process designs and process integrations are thinkable [14]. Regardless of the process setup, coking of TDH catalysts lead to a dynamic operation of the integrated reactor (Fig. 2). This reactor concept allows to regenerate the catalyst during the process, while the dehydrogenation takes place in an integrated second catalyst bed.

Fig. 2: Coupling of TDH and ODH in a membrane reactor including operando regeneration by flow reversal.

For both TDH and ODH a VOx catalyst (Al2O3 support) has been used [15, 16]. Experiments with different types of integrated reactors have shown a better performance than established fixed bed reactors (Fig. 3). Especially the use of CO2 as a reactive sweep gas seems to be promising to shift the chemical equilibrium in the water gas shift reaction (Fig. 1, r3).

Fig. 3: Comparison of different membrane reactor setups (extractor vs. distributor) at different temperatures (WHSV =400 kg s m-3, xC3H8 in = 1%).

For a better understanding of TDH and ODH in one apparatus the developing concentration and temperature fields have to be studied simultaneously. Due to coupling of heat and mass balances, a simulation of integrated reactors is not trivial. Kinetic parameters for the desired and undesired reactions have to be available. This includes deactivation kinetics and regeneration kinetics, respectively. To investigate the reaction network experiments in a PFTR have been conducted. The deactivation and regeneration kinetics has been studied in a TGA. Based on these experiments simulations of the reactor setup are possible to evaluate the potential of the integrated reactor concepts (Fig. 4).

Fig. 4: 2-D-Simulation of an integrated reactor concept in steady state (without deactivation, W/F = 100 (kg s)/m³, C3H8in = 1 %, O2/C3H8 = 2, Tin = 430 °C, Comsol). 

References

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