Dynamic hydrogen dispenser from lohc

Abstract

A Liquid Organic Hydrogen Carrier (LOHC) dehydrogenation system is described, capable of meeting time-variable hydrogen demands with short dynamics while always ensuring a high degree of dehydrogenation under all operating conditions. The system comprises one or more reactors comprising an LOHC vessel and a recirculation circuit comprising a recirculation pump, a preheater capable of supplying the reaction heat as sensible heat, and a cartridge containing the solid catalyst.

Description

[0001] DYNAMIC HYDROGEN DISPENSER FROM LOHC

[0002] The present invention relates to a continuous Liquid Organic Hydrogen Carrier (LOHC) dehydrogenation system comprising one or more reactant liquid vessels having a free volume for the produced gas, said vessels being connected to external solid dehydrogenation catalyst cartridges via circuits equipped with pumps for recirculating the liquid phase. In particular, the dehydrogenation system comprises a heat exchanger adapted to pre-heat this liquid phase, providing reaction heat.

[0003] State of the Art

[0004] LOHC (Liquid Organic Hydrogen Carrier) technology has established itself as one of the main methods for storing and transporting hydrogen. It offers numerous advantages over other technologies (high-pressure hydrogen gas, hydrogen in the liquid state, hydrogen stored as metal hydrides). In particular, it avoids the use of high-pressure tanks, the need to subject the hydrogen itself to costly compression or liquefaction procedures at very low temperatures, and allows the same infrastructures to be used as for hydrocarbons and to be stored for unlimited times without progressive losses. The cycle of producing, storing and releasing hydrogen on the appropriate media has been the subject of numerous studies, as has the way the released hydrogen is used in the production of thermal or electrical energy. It has been shown that the use of LOHCs is also cost-effective, especially in long-distance transport.

[0005] The LOHC technology cycle essentially consists of four steps: hydrogenation of the medium, or carrier, at average pressures, 10-50 bar, to produce charged LOHC, transport as a liquid at ambient pressure and temperature of the charged LOHC, dehydrogenation of the LOHC to release the hydrogen and obtain the spent LOHC, and recycling of the spent LOHC back to the hydrogenation step. The first two steps and the last are well-established, both in terms of equipment and operating conditions. The dehydrogenation stage is more problematic, however, because the way it is carried out depends very much on the specific use to which the hydrogen is to be put, the location of the dehydrogenator, whether in a fixed location or on a movable support, and the continuity of hydrogen demand from the system of use. In addition, the process is complicated by the fact that the dehydrogenation reaction takes place at high temperatures, between 200°C and 350°C depending on the type of LOHC used, and being endothermic requires the reaction heat to be supplied at these temperatures. It is thus necessary to define the structure that meets the following requirements:

[0006] 1. able to supply hydrogen at variable flow rates that adapt quickly to changes in demand of the system used;

[0007] 2. energy autonomous, i.e. does not require energy supply from an external source;

[0008] 3. ensures a high degree of dehydrogenation of the charged LOHC under all operating conditions.

[0009] Rapid adaptation to changes in demand means transitions from one steady state to another in the order of seconds or tens of seconds; a high degree of dehydrogenation means a conversion of charged LOHC to dehydrogenated LOHC greater than 50%, preferably 80% under all operating conditions. A voluminous storage buffer for the hydrogen produced, inserted between the dehydrogenator and the system used, should also not be required.

[0010] Several types of dehydrogenators have been proposed in open and patent literature. The most studied is based on a plug flow reactor (PFR) split into three areas: a heating section, which brings the temperature of the fed LOHC to the reaction temperature; a section containing the solid catalyst on which the reaction takes place; and a cooling section that brings the temperature of the two-phase gas-liquid flow to a level compatible with the use requirements.

[0011] Patent US 11,383,974 describes different types of dehydrogenation reactors, fixed- and moving-bed, but always of the batch or plug-flow type. It also focuses on the use of a reaction accelerator, a secondary alcohol or polyalcohol, in which the dehydrogenation reaction occurs in two steps, the first by hydrogen exchange between the LOHC and the oxidised form of the accelerator, i.e. the corresponding ketone, and the second in which the formed alcohol is dehydrogenated. The advantage of this solution lies in being able to supply the system with reaction heat at a lower temperature (around 200°C) than that required by direct dehydrogenation of the LOHC). There is no mention of a possible dynamic use.

[0012] Patent US 10,350,566 describes in detail a plug flow reactor consisting of three areas, the first for preheating the LOHC, the second for the reaction, and the third for separating the gaseous flow from the liquid at the outlet. Again, there is no mention of dynamic behaviour.

[0013] Patent DE 10 2021 203 887 describes a plug-flow reactor in which a vibration of the solid catalytic bed takes place, which promotes the disengagement of the hydrogen gas produced by the catalyst pores. No reference is made to dynamic behaviour, but it is interesting that it demonstrates the importance of the fluid-dynamic regime in which the dehydrogenation reaction takes place. US 2022 / 0305469 describes a plug-flow reactor that houses a solid catalyst structured in a bundle of channels that allow good liquid-solid contact through the rapid escape of the gas released through the aforesaid channels. It is also indicated that the system should be subjected to vibration in order to reduce the size of the hydrogen bubbles and facilitate their exit from the channels.

[0014] Patent US 10,840,529 describes a system consisting of one or more dehydrogenation reactors that supplies hydrogen to a fuel cell. This is operated in controlled oxygen deficiency, so that the unconverted hydrogen leaving the cell is used to provide the reaction heat required for dehydrogenation. Hydrogen flow variations are achieved by running all or only some of the dehydrogenation reactors in parallel.

[0015] US 2016 / 0214858 describes a sequence of one or more stirred reactors releasing hydrogen from LOHC (in the examples perhydro-N-ethyl carbazole), each operating under different temperature, pressure and catalyst conditions. To meet the purity and flow requirements, a hydrogen accumulator is provided downstream of the reactors based on the storing the hydrogen itself as a metal hydride, which allows it to accumulate or be released by means of temperature and pressure changes according to demand. The reported flow variation times are in the order of hundreds of minutes, too high for dynamic system behaviour.

[0016] Patent US 10,260,680 claims the use of a dehydrogenation reactor in a hydrogen vehicle refuelling station, and cites as a feature the need to ensure quick refuelling times in the order of tens of minutes. To achieve this, he suggests inserting hydrogen storage between the hydrogenation reactor and the refuelling pump at a higher pressure than the vehicle's own storage pressure, typically comprised between 300 and 700 bar. However, this solution implies the presence of a pressurised hydrogen tank of considerable volume, with the risks this entails especially in a busy area such as a service station. The cited patent gives no indication of the reactor design, nor of the response time to demand, which is highly variable, from no flow rate when there is no customer to 0.6 kg / minute during refuelling. Even this application, although based on a fixed reactor and not on a movable support, requires a highly dynamic dispenser.

[0017] Patent CN115520834 describes a circuit in which an exchanger and a fixed-bed reactor are inserted in a recirculation circuit between the vessel containing the reactant LOHC and the top of the reactor. The exchanger has the function of condensing the LOHC vapours contained in the gaseous hydrogen stream, condensation also favoured by the charged LOHC inlet flow.

[0018] In contrast, patent CN115285936 describes a different strategy for dehydrogenation based on reaction cycles of hydrogen with dimethyl carbonate.

[0019] Several articles in open literature describe LOHC dehydrogenation reactors. They are mainly focused on catalyst morphology with the aim of maximising solid-liquid contact and heat exchange on the reactor wall. Film reactors, where the catalyst is spread onto a surface through which the reaction heat is supplied (Journal of Catalysis 181, 113-123 (1999)), structured reactors using metal foams, ceramic substrates or metal meshes of various morphologies as substrates (Materials 2020, 13, 277), as well as the usual tubular reactors filled with pelleted catalyst have been proposed in this perspective.

[0020] The types of reactors for LOHC dehydrogenation reported in open literature comprise fixed-bed reactors, membrane reactors, spray and pulse reactors, thin-film liquid reactors, heat-exchange reactors and microstructured reactors (Aki Braunschweiler, Catalytic dehydrogenation of liquid organic hydrogen carriers, Aalto Energy School of Chemical Engineering Thesis, 2018). In all cases, the reactor structure is analysed, but the way in which the reactor is operated is always of the plug flow type, in which the fluid dynamics of the reactor, and thus its efficiency, area conditioned by the residence time. There is therefore no dehydrogenation system layout available that ensures reactor fluid dynamics that are independent of operating conditions, in particular the LOHC feed rate, and thus offering an efficiency that is independent of the feed rate and ultimately of the instantaneously required hydrogen production.

[0021] Description of the invention

[0022] A Liquid Organic Hydrogen Carrier (LOHC) dehydrogenation system has now been found capable of meeting time-variable hydrogen demands with short dynamics while always ensuring a high degree of dehydrogenation under all operating conditions.

[0023] The Liquid Organic Hydrogen Carrier (LOHC) continuous dehydrogenation system of the invention, shown in Figure 1, comprises one or more reactors, e.g. of the CSTR (Continuous-flow Stirred-Tank Reactor) type, in series, characterised in that each reactor comprises a container, hereinafter also referred to as a vessel, which contains the reactant LOHC, and a cartridge containing the solid catalyst through which the liquid LOHC is circulated by means of a recirculation pump, at a flow rate independent of the residence time of the LOHC in the reactant complex, while the hydrogen released is sent to be used, after purification from the LOHC vapours.

[0024] As illustrated in Figure 2, a heat exchanger can be advantageously inserted in the recirculation circuit to pre-heat the circulating liquid to a temperature higher than the reaction temperature and sufficient to maintain the temperature of the liquid contained in the vessel at a desired level by compensating for the heat demand of the reaction and the dissipation of heat to the outside.

[0025] Preferably, as illustrated in Figures 1 and 2, the vessel with the reactant LOHC is fed with LOHC liquid (“charged LOHC” in the figures). Specifically, the dehydrogenation process of the so-called charged LOHC takes place inside this vessel, allowing hydrogen to be released and the spent LOHC to be obtained, preferably in the liquid phase.

[0026] Preferably this spent LOHC is circulated in the recirculation circuit through at least one recirculation pump, so that it can be recycled.

[0027] More preferably, as can be seen in Figures 2 to 4, in this recirculation circuit the spent LOHC is pre-heated through at least one heat exchanger.

[0028] Preferably, as illustrated in Figures 2 to 4, the heat exchanger is positioned in the circuit upstream of the external cartridge containing a dehydrogenation catalyst. Specifically, downstream of the heat exchanger, the spent LOHC encounters the cartridge outside the vessel containing the reactant LOHC, which in turn contains the solid catalyst, where the reaction takes place to obtain uncharged LOHC, i.e. partially dehydrogenated, together with the hydrogen produced in the gaseous phase.

[0029] Subsequently, as shown in Figure 2, this mixture is fed into the reactant LOHC vessel, again allowing both hydrogen to be removed and spent LOHC to be recycled.

[0030] In other words, the reactant LOHC from the external cartridge is mixed with the fed LOHC and the resulting mixture is recirculated to the external cartridge containing the catalyst.

[0031] The volume of liquid in the vessel can be controlled via a level controller.

[0032] The recirculation flow rate of the reactant liquid is adjusted according to the desired temperature of the liquid in the vessel, while the LOHC flow rate fed to the system is regulated according to the desired hydrogen withdrawal rate. The hydrogen pressure is regulated by a valve that withdraws the released hydrogen.

[0033] The LOHC liquid fed is preferably hydrogenated dibenzyl toluene.

[0034] The temperature of the liquid in the vessel is comprised between 250°C and 350°C, preferably between 280°C and 320°C. In fact, the vessel is suitable for containing liquid at such a temperature.

[0035] The total residence time depends on the ratio between the total volume of LOHC contained in the vessel and the recirculation circuit, which also comprises the catalyst cartridge, and the flow rate of charged LOHC: LOHC

[0036] T =‘ FL -OHC The recirculation flow rate is an arbitrary parameter to optimise the liquid-solid contact in the cartridge and to promote the release of hydrogen gas from the catalyst itself.

[0037] The recycle flow rate R is greater than the flow rate FL0HCand allows for a much lower volumetric ratio between instantaneous gas and liquid flow rates in the catalyst cartridge than the total volumetric ratio between gas and liquid flow rates at the system outlet. Placing the catalyst in a cartridge separate from the vessel of liquid LOHC also makes possible to achieve a formal concentration of solid catalyst in the system that is not limited by the stirring limits of the usual solid-liquid stirred reactors, which in practice are less than 100-200 grams per litre, and thus to have low liquid volumes and high catalyst quantities. The maximum catalyst concentration that can be achieved is equal to the apparent density of the catalyst (grams of catalyst per unit of apparent volume) usually even higher than 500 grams / 1.

[0038] The higher the recirculation flow rate, the lower the temperature difference between the inlet of LOHC for recycling in the cartridge and the outlet, thus avoiding even local overheating of the liquid. In fact, it is known that at high temperatures LOHCs give rise to decomposition products.

[0039] In a further embodiment of the invention, the gas-liquid flow leaving the solid catalyst cartridge can be sent to a gas-liquid separation unit which sends the gaseous phase for gaseous hydrogen withdrawal and the liquid phase to the LOHC vessel, as shown in Figure 3.

[0040] The use of several reactors in series, each configured as described, makes possible to achieve higher conversions with the same total amount of catalyst used or to achieve the same LOHC conversion with smaller quantities. Figure 4 describes the sequence of 3 reactors.

[0041] Preferably, as illustrated in Figure 4, the system comprises at least two reactors connected in series. In particular, spent LOHC from a first reactor feeds a second reactor following the first reactor.

[0042] In this way, through the configuration described above, hydrogen is released and spent LOHC is obtained.

[0043] More preferably, as illustrated in Figure 4, this spent LOHC, obtained from the first reactor, is fed into the vessel containing reactant LOHC of the second reactor for recycling and conversion.

[0044] In the case of three reactors connected in series, as illustrated in Figure 4, the second reactor is fed with spent LOHC from the first reactor, and similarly, the third reactor is fed with spent LOHC, in the respective reactant LOHC vessel, from the second reactor. Brief description of the figures

[0045] Figure l is a diagram of a reactor unit of the invention;

[0046] Figure 2 represents a diagram of a reactor unit of the invention comprising a heat exchanger for preheating the circulating liquid;

[0047] Figure 3 represents a diagram of a reactor unit of the invention comprising a gasliquid separation unit at the outlet of the cartridge containing the catalyst;

[0048] Figure 4 illustrates the configuration of a dehydrogenation system comprising three reactor units connected in series;

[0049] Figure 5 represents the response times calculated by the dynamic model for an instantaneous variation of the supply flow rate from the steady state value of Example 1 to one tenth thereof;

[0050] Figure 6 shows the outlet pressure and flow rate calculated by the dynamic model for the transition from the final condition of Example 3 to that of Example 1.

[0051] Detailed description of the invention

[0052] The operation of the reaction system summarily described above is explained in detail in the following examples. They were obtained by mathematical modelling of the system from kinetic and thermodynamic data taken from open literature (Durr S. et al., International Journal of Hydrogen Energy, Volume 46, Issue 64, 2021, Pages 32583-32594 e Jorschick H., Preuster P. et al., Hydrogen storage using a swing reactor, Energy Environ. Sci., 2017, 10, 1652-1659), related to perhydro dibenzyl toluene (DBTH) as LOHC. Both articles cited above report results obtained using platinum on alumina as a catalyst. It is well known that DBTH is, among LOHCs, the one with the greatest advantages, due to its energy density (=7479 kJ / kg DBTH), the commercial availability of the precursor dibenzyl toluene (DBT), its physical and chemical characteristics (melting range = -50°C, boiling point = 371 °C), and its compatibility with transport and storage systems for heavy hydrocarbons such as petrol and diesel. Platinum-on-alumina catalysts are generally recognised as the most effective for the DBTH dehydrogenation reaction.

[0053] The reaction scheme used is divided into three successive dehydrogenation stages illustrated in the diagram below:

[0054]

[0055] The equilibrium constants at 291 °C of the three reactions and the kinetic parameters reproducing the time-dependent trends of the degree of hydrogenation at 291 °C reported in the cited references were identified. With these values, a stationary model of CSTR was constructed, which calculates hydrogen production as a function of residence time and the amount of catalyst contained in the cartridge.

[0056] A dynamic model was also built, which, starting from a stationary operating condition obtained from the CSTR model, calculates the time required to reach a different stationary operating condition, characterised by greater or lower hydrogen production.

[0057] With these models, the results shown in the following examples were obtained.

[0058] Example 1: sizing a CSTR reactor to produce 150 kW thermal hydrogen.

[0059] The combustion heat of hydrogen to produce water is -241.8 kJ / mole, so that to obtain 150 thermal kW, 2233 moles / h are required. Operating at a hydrogen pressure of 1 bar at 291 °C with a CSTR reactor containing 6000 g of Pt / Ab Ch catalyst at a formal concentration of 200 g / 1, it is necessary to feed 305.3 mol / h of DBTH with a hydrogenation degree of 100%. Under these conditions, the degree of dehydrogenation of the outgoing DBT is 18.7%. The reactor parameters are summarised in Table 1 below:

[0060] Table 1 Example 2: sizing of three CSTR reactors in series.

[0061] A result practically equal to the one in Example 1 can be obtained with a series of 3 CSTR reactors, the first fed with DBTH at 100% hydrogenation at the same flow rate as in Example 1, the other two with the effluent from the previous reactor. The quantity of catalyst in Example 1 is divided into equal parts in the three reactors, the volume of liquid in each of which is 1 / 3 of the volume in Example 1. The reactor parameters are summarised in Table 2 below:

[0062] Table 2

[0063] Dividing the same amount of catalyst used in Example 1 into 3 reactors allows more complete use of the available hydrogen content and a 15% increase in the energy produced.

[0064] Example 3: calculation of the response time of the CSTR reactor to the instantaneous change of feed rate.

[0065] Figure 5 shows the response times calculated by the dynamic model for an instantaneous variation of the feed rate from the steady-state value of Example 1 to one- tenth thereof. In addition to the volume of liquid, to the reactor is assigned a gas volume of 10 litres. The initial conditions are the steady-state conditions of Example 1, and the graphs in Figures 5 and 6 show the trends in hydrogen production rate and pressure over time.

[0066] Adaptation to the new conditions takes place within a few seconds, during which the excess hydrogen produced with respect to the hydrogen withdrawn is stored in the reactor's gas volume, causing the pressure to increase. This increase reduces the speed of the direct dehydrogenation reactions and increases the speed of the reverse reaction, quickly bringing the system to its new operating conditions. An increase in the demand for H2 takes slightly longer, as the demand cannot exceed the production rate by more than the amount that would cause the system to be depressurised below the limit operating pressure (1 atmosphere in the given example). In fact, in the initial instants, the required hydrogen is supplied by the gas contained in the reactor's free volume and by the gas produced by the dehydrogenation reaction, which takes a few minutes to rise to levels close to those relative to the final steady state. It is therefore necessary to increase the demand gradually while maintaining a pressure above 1 atmosphere in the transient. In the graphs below, a required H2 flow rate was used, which increases according to the formula

[0067] UH2outlet=U°+ (umax-U°) *a*t / (l +a*t) where umaxis the required steady state flow rate and u° that of the initial state. The parameter a is adaptive.

[0068] The graph in Figure 6 shows the outlet pressure and flow rate calculated by the dynamic model for the transition from the final condition of Example 3 to that of Example 1.

[0069] This change of conditions also requires times in the order of tens of seconds to achieve flow rates in the order of magnitude of that desired.

[0070] It was thus demonstrated that the LOHC dehydrogenation system is capable of meeting time-varying hydrogen demands with short dynamics, while still ensuring a high degree of dehydrogenation under all operating conditions.

[0071] In particular, a layout of the dehydrogenation system has been provided that ensures reactor fluid dynamics that are independent of the operating conditions, specifically the LOHC feed rate, and thus offers an efficiency that is independent of the feed rate and ultimately of the instantaneously required hydrogen production.

Claims

CLAIMS1. A continuous Liquid Organic Hydrogen Carrier (LOHC) dehydrogenation system comprising one or more reactors in series, characterised in that each reactor comprises a vessel containing the reactant LOHC, and an external cartridge containing a dehydrogenation catalyst, a circuit equipped with at least one pump for recirculation of the liquid phase from said vessel to said cartridge and a heat exchanger adapted to preheat said liquid phase, the vessel having a free volume for the produced gas.

2. A dehydrogenation system according to claim 1 wherein said heat exchanger is adapted to pre-heat the liquid circulating in said circuit to a temperature higher than the reaction temperature and sufficient to maintain the temperature of the liquid contained in the vessel at a desired level compensating the heat demand of the reaction and the heat dissipation to the outside.

3. A dehydrogenation system according to claim 1 or 2 wherein the vessel comprises a level controller of the volume of present liquid.

4. A dehydrogenation system according to one or more of claims 1-3 wherein the recirculation flow rate of the reactant liquid is adjusted according to the desired temperature of the liquid in the vessel.

5. A dehydrogenation system according to one or more of claims 1-4 wherein the vessel comprises a hydrogen withdrawal valve for regulating the hydrogen pressure.

6. A dehydrogenation system according to one or more of claims 1-5 wherein the LOHC flow rate fed to the system is regulated according to a desired hydrogen withdrawal rate.

7. A dehydrogenation system according to one or more of claims 1-6 wherein the LOHC fed is hydrogenated dibenzyl toluene.

8. A dehydrogenation system according to one or more of claims 1-7 wherein said vessel is adapted to contain liquid at a temperature comprised between 250°C and 350°C, preferably between 280°C and 320°C.

9. A dehydrogenation system according to one or more of claims 1-8 wherein a gasliquid separation unit is inserted between the outlet of the solid catalyst cartridge and the return to the vessel.

10. A dehydrogenation system according to one or more of claims 1-9 comprising three or more reactors connected in series.

11. A dehydrogenation system according to one or more of the preceding claims, wherein LOHC partially dehydrogenated together with the hydrogen produced in the gasphase is present at the outlet from said external cartridge.

12. A dehydrogenation system according to one or more of the preceding claims, wherein said heat exchanger is positioned in said circuit upstream of said external cartridge containing a dehydrogenation catalyst.

13. A dehydrogenation system according to one or more of the preceding claims, wherein said reactant LOHC from said external cartridge is mixed with said fed LOHC and the resulting mixture is recirculated to said external cartridge.

14. A dehydrogenation system according to one or more of the preceding claims, comprising at least two reactors connected in series and wherein spent LOHC from a first reactor feeds a second reactor following said first reactor.

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