Optimization of forced periodic operations in milli-scale fixed bed reactor for Fischer-Tropsch synthesis

Highlights

• Forced periodic operations in milli-fixed-bed FTS reactor can be beneficial.

• High amplitude modulations are optimal for maximization of C₅₊ productivity.

• Optimal forcing of coolant temperature resulted in 30% gain of productivity.

• Three inputs forcing resulted in highest C₅₊ productivity improvement of 52%.

• Methane selectivity increased relatively less than C₅₊ productivity in optimal FPO.

Abstract

One-dimensional pseudo-homogenous dynamic reactor model, incorporating detailed Fischer-Tropsch kinetics, was applied in a theoretical analysis of forced periodic operations. A milli-scale fixed-bed reactor was analyzed, using design and operation parameters, obtained previously in a steady-state optimization. Dynamic optimization and NLP methods were utilized to obtain optimal values of amplitude(s), frequency and phase shift(s) of sine-wave variation of inputs, around the corresponding optimal steady-state values, which maximize the productivity of C₅₊ hydrocarbons. Inlet variables that were modulated are: coolant temperature, reactants molar ratio, mass flow rate and pressure. In addition to the single input forcing, simultaneous modulations of multiple inputs were also considered, with combinations of the listed inlet variables. Among the single input cases, periodic variation of the coolant temperature resulted in the highest relative improvement of C₅₊ productivity by 30%. Multiple inputs forcing showed additional potential for improvement, resulting in relative C₅₊ productivity increase of 52% for synchronized modulation of the coolant temperature, reactants molar ratio and mass flow rate. However, the increase in C₅₊ productivity is accompanied with relative increase in methane selectivity of 22–33% (relative to the steady-state value). The results suggest that, in the case of multiple input variations with high amplitudes, modulation of the inlet reactants molar ratio mainly contributes to the increase of CO conversion (e.g. reaction rate), the coolant temperature forcing slightly increases selectivity towards the desirable higher hydrocarbons (C₅₊), while the variation of the inlet mass flow rate enables better reaction temperature control and prevents a thermal runway.

Introduction

Successful long-term transition to a more environmentally and climate-friendly energy production and utilization, based on renewable resources, largely depends on chemicals based energy systems and their efficiency. Effective energy storage via chemicals, supporting unsteady production from renewables (power-to-X), as well as efficient chemical carriers of hydrogen, are some of the concepts under consideration [1,2]. Nonetheless, liquid hydrocarbon fuels, preferably the “clean” ones (i.e. synthetic), will complement the energy demand and play an important role in the above mentioned concepts for the foreseeable future. In possibly unstable future environments (both politically and climate-wise) distributed (un-centralized) and flexible energy production and “smart” distribution grids, will be advantageous, if not indispensable. Thus, demand for small to moderate-scale capacity facilities for conversion of natural gas or coal or biomass into synthetic liquid fuels or value-added chemicals (X-to-L) will most probably grow in the forthcoming decades. This elevates the importance of small-scale reactors for Fischer-Tropsch (FT) synthesis, as an essential part of the X-to-L process [3].

In addition to demands for a compact design (e.g. small-scale), due to lack of space and/or small capacity of future energy systems, micro- and milli-scale Fischer-Tropsch fixed-bed reactors offer other advantages that are of physical and technical nature [[4], [5], [6], [7], [8]]. Namely, the crucial aspect of temperature control, associated with exothermal Fischer-Tropsch reaction and removal of excess heat, can be efficiently realized in milli- and micro- reactor designs, either as wall-coated micro-channel, or milli-fixed bed reactors [6,[9], [10], [11], [12]]. Arzamendi et al. [12] showed through CFD simulations that, for a wide range of syngas space velocities (5000-30000 h⁻¹) and temperatures (483–523 K), a good heat management was observed in a micro-channel reactor with catalyst coated walls. Similar findings were presented by Chabot et al. [13] for milli-fixed bed reactors.

Furthermore, use of milli-scale reactors significantly reduces intra-particle diffusion resistances, that are present in conventional scale FTS reactors [5,11,[13], [14], [15]]. This is due to short diffusion paths in coated wall or small particles catalysts. Myrstad et al. [5] experimentally compared a wall-coated micro-structured reactor with a lab scale commercial fixed bed reactor, both featuring highly active cobalt catalyst. With the micro-structured reactor, they obtained good heat removal, as well as low pressure drop and low mass transfer resistance. The downside of micro-structured reactors is potential blistering of the catalyst from the walls and low catalyst/reactor volume ratio [16] which leads to decrease in the reactor performance. By using milli-fixed bed reactors filled with small catalyst particles, these issues can be resolved. Cao et al. [11] used micro-channel reactor filled with Co-alumina powder with 45 and 150 μm in diameter. They demonstrated excellent heat and mass transfer capabilities of milli-fixed bed reactors. A comparison between conventional and milli- fixed bed reactors was also reported by Chambrey et al. [4]. They found that the milli- reactor was more stable during startup, protecting the catalyst from mechanical deactivation. One of the main concerns with regard to milli-fixed bed catalytic reactors, containing micro-scale catalyst particles, is a large pressure drop per meter of packing. However, compact milli-scale reactors usually have relatively short bed lengths, thus the total pressure drop is still rather low [15]. Overall, milli-scale FT fixed-bed reactors alleviate the main problems associated with large-scale conventional reactors, such as high intraparticle diffusion resistance and related low catalyst effectiveness and low selectivity towards the desired products, as well as poor heat removal.

In addition to the above discussed enhancements that can be achieved by using milli-fixed bed Fischer-Tropsch reactors, one may investigate the possibilities for further intensification via deliberate dynamic operations. System non-linearity, arising from complex reaction kinetics and coupling of different phenomena, provides the foundation for such investigation [17,18]. However, the complex non-linear dynamics poses challenges for efficient control. Thus, advanced model-based control systems, such as non-linear model predictive control are recommended [18,19]. In case such control systems are installed, along with a state-of-the art sensing and actuation devices, the realization of deliberate periodic operations, with forced input modulations, is more viable technically. In power-to-chemicals concepts, fluctuating electricity coming from renewable sources (wind, solar) is first converted into hydrogen, via electrolysis. This fluctuating hydrogen source can then be used for different chemical energy storage processes, for example via Fischer-Tropsch synthesis. This creates unsteady state condition for syngas input to the FT reactor that needs to be properly handled [20,21]. Nevertheless, instead of being treated as disturbances, the fluctuations of the inputs can be seen as an opportunity for performing forced periodic operations, with proper actuation equipment to deliver the desired modulations. Of course, before making any decisions on process design, it is advisable to perform feasibility studies of technically demanding forced periodic operation in Fischer-Tropsch synthesis reactor.

Forced periodic operations (FPO) of Fischer-Tropsch reactors have been investigated for the last 30 years. A pioneering experimental work of Silveston’s group was published in a series of publications [[22], [23], [24], [25], [26], [27], [28]]. They conducted experiments with forced periodic modulation of the feed composition in laboratory scale reactors packed with iron, cobalt or ruthenium based catalyst particles. The composition forcing strategy mainly corresponded to periodic switching between nearly pure H₂ and CO streams. The results showed that forced periodic operation could considerably increase catalyst activity for all catalysts, as the time averaged total hydrocarbon production rate (CO consumption rate) was higher than the one in the steady-state at specific temperature and pressure [28]. It was also shown that the product distribution could be also altered by composition forcing, but the relative change was smaller than the one for activity, and it was dependent on the catalyst used. For an iron catalyst, the methane production was significantly increased under the periodic regime, much more than the production of other hydrocarbons [[22], [23], [24], [25]]. For the FPO with a Co catalyst a decrease in the olefin/paraffin ratio and increase of light hydrocarbons were observed [27,28].

Gulari and co-workers [[29], [30], [31]] performed FPO experiments with Ru and Mo based catalysts and investigated the influence of the time split ratio and pulsating period. For the Ru catalyst with switching between H₂ and synthesis gas containing 11% CO, they found that high split numbers (fraction of the cycle period exposed to H₂ stream) around 0.8, provided a maximal relative increase of CO consumption by 220% w.r.t. consumption in steady-state. [29]. Regarding the effect of the pulsating period, the authors showed that for shorter pulsation periods (0.5–40 min), the production of lighter paraffins was significantly increased, while the light olefins production was suppressed [29]. Interestingly, higher hydrocarbons production was nearly independent on the period length. Similar observations for the influence of pulsation period length were reported for catalysts investigated by Silveston and coworkers [28].

Later on, in the 2000s, the research group of Kapteijn focused on possibilities for selectivity enhancement by forced periodic operation of a FT reactor. Meeuse et al. [32] and De Deugd et al. [33,34] performed a theoretical study using numerical optimization with a goal to maximize the production of diesel fraction (C₁₀-C₂₀). The simple CSTR model, assuming isothermal and isobaric conditions and no intra-particle diffusion resistance, was used. In the optimization study, the feed composition was periodically alternated between CO- and H₂-rich mixtures in the form of rectangular pulses. The optimal periodic operation was achieved using a long cycle period of 159 min, which included H₂-rich phase (99% H₂) for 10.8% of the time, while during the remaining time the feed rich in CO (95.2% CO) was used. They found that, under optimal process conditions, the diesel selectivity could be increased up to 27% (mean diesel fraction of 0.494 vs. 0.394 in the steady-state). However, the reported CO conversion was only 5.1%, which was a consequence of a low time-averaged syngas ratio of 0.19. These results were consistent with the previous work of Peacock-Lopez and Lindernberg [35,36], who showed via modeling that selectivity could be shifted towards the desired higher hydrocarbons, by employing a proper pulsating strategy.

However, until now, there have not been any studies of FPO of FT reactors that would focus on increasing the total productivity of higher hydrocarbons (C₅₊) (a performance criterion which actually combines activity and desired selectivity). Furthermore, periodic changes of inputs other than the inlet composition have not been considered. It can be expected that the reactor temperature has very prominent influence on the FT reactor performance, due to non-linear coupling of transport phenomena and reaction rates. The use of higher temperatures results in increased CO conversion, but also increased selectivity towards undesired methane. Our previous dynamic analysis of the FT milli-fixed bed reactors [18], showed that even large feed temperature changes have small effects on the reactor performance, due to very efficient heat removal. On the other hand, it was found that the milli-scale FT reactor is very sensitive to changes in the coolant temperature. Moreover, it was revealed [18] that in milli-fixed-bed FT reactors the temperature fronts were moving slower through the reactor, than the concentration ones. These findings suggest that the coolant temperature could be a good candidate for periodic modulation in FPO of milli-fixed bed reactors for FTS. In addition, other input variables, such as the flow rate and pressure, may also be considered as forcing input variables. Moreover, some general studies on FPO of chemical reactors [37,38] have demonstrated that enhancements of product yield or reactant conversions can be higher, if synchronized modulation of two inputs is employed.

In this work we have investigated the potential of forced periodic operations of millimeter scale fixed-bed reactors for FTS, through a rigorous dynamic optimization study, using a previously developed dynamic model [18] and initial optimal values of steady state parameters [39]. We have maximized the time-averaged productivity of C₅₊ components, and have considered periodic change of all relevant inputs, such as: reactants feed ratio, inlet coolant temperature, inlet flow rate and pressure. Moreover, using the optimization, we have examined periodic operations with multiple inputs modulations, for several combinations of 2 and 3 input variables. Such a wide and detailed feasibility study of FPO for FTS has not been done so far. The results of this study provide the magnitude of possible enhancements and guidance as to which inputs should be modulated and in what manner, and this is very valuable for planning future experimental studies.