WO2014131435A1

WO2014131435A1 - Reactor for an auto-poisoning proces

Info

Publication number: WO2014131435A1
Application number: PCT/EP2013/053860
Authority: WO
Inventors: Christian Wix; Skov Martin SKJØTH-RASMUSSEN; Thoa Thi Minh NGUYEN
Original assignee: Haldor Topsøe A/S
Priority date: 2013-02-27
Filing date: 2013-02-27
Publication date: 2014-09-04
Also published as: CN105163844B; CN105163844A

Abstract

The present invention relates to a reactor for a transport limited reaction comprising an inlet zone comprising particles of a material catalytically active in said reaction and a main zone comprising particles of a material catalytically active in said reaction characterized by the external geometric surface area of the particles of said main zone being lower than the external geometric surface area of the particles of said inlet zone.

Description

Title: Reactor for an auto-poisoning process

The present invention relates to a reactor for an auto- poisoning process, and the optimal reactor loading for such a process.

Many catalytic processes are prone to deactivation at the inlet of a reactor where poisons typically deposit on the top-layer catalyst. The common solution is to install a guard material in form of excess catalyst volume or materi^¬ als targeted towards specific adsorption or absorption of the poison.

However, in some cases the poison or deactivating species may constitute all or be part of the reactant feedstock, in other words be an auto-poisoning reactant.

A first example of an auto-poisoning reaction is the case in adiabatic steam prereforming of hydrocarbons

(C_nH_m + n¾0 = nCO + {n+m/2}¾) where particularly higher hydrocarbons adsorbed for reac^¬ tion on the catalytically active phase are prone not to de- sorb from the active phase, if the temperature is insuffi^¬ cient or the catalyst has insufficient activity. Conse^¬ quently the higher hydrocarbons are dehydrogenated and cause the formation of an encapsulating layer of carbon- containing species around the active phase in the catalyst ultimately resulting in deactivation. The practical solu^¬ tion has been to use a sufficiently active catalyst and raise the operation inlet temperature to a level where the risk of auto-poisoning is eliminated or reduced to an ex^¬ tent where it does not reduce the life time of the cata^¬ lyst. A second example of an auto-poisoning reaction is high- temperature methanation (CO+3¾ = CH₄ + ¾0) in e.g. substi^¬ tute natural gas (SNG) processes, where a very reactive gas with concentrated carbon monoxide and hydrogen is fed to a first reactor, often with the feed gas having a module, i.e. a molar ratio M= (H₂-C0₂) / (CO+C0₂) , close to 3 Typical^¬ ly the gas contains mainly ¾ and CO, but also other com^¬ pounds such as C0₂, CH₄ and higher hydrocarbons, as well as inerts may be present, and furthermore additional steam may be added to suppress an excessive adiabatic temperature rise in the reactor. Deactivation of the catalyst may occur at low inlet temperatures, e.g. below 330°C, and this is believed to be caused by carbon monoxide dissociated on the active phase in the catalyst, where it forms surface car^¬ bon-containing intermediates which instead of reacting with surface bound hydrogen to methane, recombines and creates an encapsulating layer of carbon-containing species, around the active phase in the catalyst which ultimately causes deactivation. A practical solution has been to operate at increased inlet temperatures (H. H. Gierlich, M. Fremery, A. Skov, J. R. Rostrup-Nielsen : Studies in Surface Science and Catalysis (Elsevier) Vol. 6, P. 459-469), but this will increase the outlet temperature, which is undesired as this shifts the equilibrium of the reaction away from the de^¬ sired product methane, and furthermore the increased tem- perature may be damaging to the stability of the catalyst, unless e.g. the adiabatic temperature increase is reduced by either controlling the equilibrium, e.g. by reducing the partial pressure of reactants or increasing the partial pressure of products.

Now according to the present invention a solution for such problems is proposed. It has been identified that for reac^¬ tions which are transport limited, an increase in the availability of active surface area for an initial zone will allow a rapid increase to a sufficient reaction tem^¬ perature, which will reduce the risk of auto-poisoning e.g. by carbon formation. If a main zone is then maintained at a lower active surface area, the balance between high inlet reactivity and moderate pressure drop is maintained.

A transport limited reaction or partly transport limited reaction shall for the present application be construed as a reaction which is limited by external mass transfer of reactants from the bulk gas phase to the catalyst surface or is limited by internal mass transfer of reactants from the surface to the interior of the catalyst particles, ra- ther than limited by the actual chemical rate of reaction.

Often the term diffusion limited may alternatively be used for such reactions.

An exothermal inlet zone shall for the present application be understood as a zone proximate to the inlet of a reac^¬ tor, in which an exothermic reaction occurs releasing heat.

An endothermic inlet zone shall for the present application be understood as a zone proximate to the inlet of a reac- tor, in which an endothermic reaction occurs consuming heat which may be followed by a downstream zone which is exo^¬ thermic. The endothermal reaction may be an activation of the primary reactants, or it may be related to consumption of impurities in the feed.

The maximum temperature increase shall be understood as the difference between the minimal reactor temperature and the outlet temperature.

Auto-poisoning shall be understood as the process where a catalyst is poisoned or deactivated by a species which may constitute all or be an inseparable part of the reactant feedstock .

The external geometric surface area shall be construed as the area of the bulk surface relative to the bulk volume in the reactor.

The bulk surface shall be understood as the catalyst exte^¬ rior surface and shall not include internal pore surface. The bulk volume shall be understood as the reactor volume taken up by catalyst particles, and shall be understood to include the void between particles.

An adiabatic reactor shall be understood as a reactor in which no deliberate heat exchange to a heat exchange medium takes place. However, a reactor with moderate loss of heat to the surroundings is still considered adiabatic.

In a broad form the present invention relates to a reactor for a transport limited reaction comprising an inlet zone comprising particles of a material catalytically active in said transport limited reaction and a main zone comprising particles of a material catalytically active in said transport limited reaction characterized by the external geometric surface area of the particles of said main zone being lower than the external geometric surface area of the particles of said inlet zone with the benefit of providing a reactor for said reaction having reduced transport limitation, and limited pressure drop.

In a further embodiment said transport limited reaction is an exothermal reaction having an outlet temperature 5 to 450 °C above the inlet temperature.

In a further embodiment said transport limited reaction is methanation of a carbon oxide by reaction with hydrogen, with the benefit of producing methane from a carbon oxide and hydrogen.

In a further embodiment said transport limited reaction is steam prereforming of hydrocarbons with the benefit of pro- ducing hydrogen from hydrocarbons.

In a further embodiment the external geometric surface area of the particles of the inlet zone is 700-2000 m²/m³ and the external geometric surface area of the particles of the main zone is 50-90% of the external geometric surface area of the particles of the inlet zone with the associated ben^¬ efit of providing a set of highly reactive process condi^¬ tions . In a further embodiment the particles of the transport lim^¬ ited inlet zone have a diameter of 2 to 6 mm with the bene- fit of said particles having a high surface area, while be^¬ ing physically stable and convenient to produce.

In a further embodiment at least one of the particles of the inlet zone and the main zone has a geometry taken from the group consisting of cylinders, rings, spheres, multi^¬ ple-hole rings, including 7-hole rings, dayisies and quad- rolobes with the benefit of said particles having a favora^¬ ble surface area, while being convenient to handle, com- pared to smaller particles having the same surface area.

In a further embodiment the bulk volume of the particles of the inlet zone is less than 50%, preferably less than 25% and most preferably less than 15% of the reactor volume, upstream the main zone with the benefit of said particles providing an reactive inlet zone, sufficient for raising the temperature to a level where the particles of the main zone are sufficiently reactive. In a further embodiment an exothermic inlet zone is down^¬ stream an endothermic inlet zone with the benefit of said endothermic reaction zone accommodating initial reactions prior to the main exothermic reaction. A further aspect of the present invention relates to a re^¬ actor according to any claim above which is adiabatic, with the associated benefit of such a reactor being simple and having a moderate cost, compared to cooled reactors. According to the present disclosure new solution to auto- poisoning deactivation phenomena has been found, which allows a more flexible operation in processes which are fast catalytic reactions, which are transport limited or partly transport limited by external mass transfer of reactants from the bulk gas phase to the catalyst surface and which are deactivated by reactants, such as prereforming and methanation. Thus, if the transport of reactants from the bulk gas phase to the catalyst surface is slowed down, by decreasing the mass transfer number by a reduction of the Reynolds number, more time will be available for reaction and desorption, however for the desorption to work it is critical that the temperature is also raised by reaction.

Thus, to increase the availability of active surface area a higher geometric surface area is needed. Both requirements point towards usage of smaller catalyst particles. Unfortu^¬ nately, small catalyst particles also cause high pressure drop and consequently an increase in power consumption.

The solution is to balance the use of catalyst particles with a high external geometric surface area (often small particles) in the inlet of the reactor where rapid reaction is required to avoid deactivation, and particles with a lower external geometric surface area (often larger parti^¬ cles) in the remaining part of the reactor. The shift from high to low external geometric surface area particles should be where reaction has been initiated and the temper- ature is above a critical value. Preferably the high exter^¬ nal geometric surface area catalyst particles are applied in the inlet zone where 10-50%, preferably 20-30% of the maximum temperature increase has occurred. The temperature may in some cases dip initially, followed by an increase. In such a case the relative size of the inlet zone and the main zone shall be defined from the minimal temperature to the reactor outlet. The zone upstream the minimal tempera- ture may comprise particles with either high or low geomet^¬ ric surface area.

The required temperature increase may typically be achieved within the first 50% of the relative loading height at the start of run, often even within the first 15% of the rela^¬ tive loading height. Thus, preferably the small catalyst particles are applied in the first 50% of the relative loading height, more preferably in the first 25% of the relative loading height and most preferably in the first 15% of the relative loading height.

The invention is described in greater detail below with reference to the accompanying drawings, in which

Fig. 1 shows a sketch of a cylindrical catalyst particle, Fig. 2 shows a sketch of a ring shaped catalyst particle, Fig. 3 shows a sketch of a 7-hole catalyst particle,

Fig. 4 shows a sketch of a quadrulobe catalyst particle, Fig. 5 shows a sketch of a daisy catalyst particle,

Fig. 6 shows an embodiment of the invention,

Fig. 7 shows an embodiment of the prior art, and Fig. 8 shows data from 3 examples with different process configurations . Figs. 1-5 show examples of catalyst particles which may have the shape of cylinders, cf. Fig. 1, rings, cf. Fig. 2, shape optimized particles such as 7-hole rings, cf . Fig. 3, quadrulobes, cf. Fig.4, and daisies, cf. Fig.5, but many other common catalyst shapes exist. Typically the external geometric surface area is increased in the shape optimized particles by adding holes or modifying the surface shape.

Catalysts particles are characterized by an external geo- metric surface area typically ranging from 500 to 2000 m²/m³. This may be obtained using 3 to 4.5 mm diameter cyl^¬ inders with heights varying from 2 to 6 mm, 5 to 9 mm diameter 7-hole cylinders with heights varying from 3 to 6 mm and hole diameters from 1 to 2 mm. Other variations can easily be achieved from common catalyst shapes. The pres^¬ sure drop will vary around 10 to 180% relative to a 4.5x4.5 mm cylinder. These principles may also be applied by the person skilled in the art for even larger particles with lower pressure drops, as well as smaller particles with higher external geometrical surface area.

For the purpose of the present disclosure particles of cat- alytically active material are classified according to their external geometric surface area. Catalyst loadings of large particles will have a low external geometric surface area, while loadings of smaller particles or particles with holes or optimized shapes may have a higher external geo^¬ metric surface area. Table 1 shows examples of 3 catalyst shapes, and the relat^¬ ed parameters for these catalysts. Table 1

In Fig. 6 an embodiment of the prior art is shown. It re^¬ lates to a process for the methanation of synthesis gas, and a process plant implementing this process. In such a process plant a stream of synthesis gas 10 is directed to a first reactor 12 comprising a single catalytic bed 14. To control the temperature of the first reactor, a portion of the product is recycled. In a second reactor 22 the

methanation reaction is further completed, before condensa^¬ tion of process condensate, and the methane rich gas 26 is directed to final methanation.

In one embodiment according to the prior art, the catalytic bed of the first reactor contains catalytic particles with a high geometric surface area, e.g. small particles, such as cylinders having a diameter of 4.5 mm and a height of 4.5 mm .

In Fig. 7 a process according to the present disclosure is shown. In such a process plant a stream of synthesis gas 10 is directed to a first reactor 12 comprising two catalytic beds 16 and 18. To control the temperature of the first re^¬ actor, a portion of the product is recycled. In a second reactor 22 the methanation reaction is further completed, before condensation of process condensate, and a methane rich gas 26 is directed to final methanation.

In one embodiment, the first catalytic bed of the first re^¬ actor contains catalytic particles with a high geometric surface area, e.g. small cylindrical particles with a diam- eter of 4.5 mm and a height of 4.5 mm, while the second catalytic bed contains catalytic particles with a smaller geometric surface area and lower pressure drop factor, e.g. 7-hole catalytic particles with a diameter of 16 mm and a height of 10 mm. In this manner the overall reactivity in the first catalytic bed is the same as that of the second bed, while a significant lower pressure drop is made possi^¬ ble. The effect of this is that rapid initiation of the re^¬ action may take place at the cost of only moderate increase in pressure drop.

Examples

In an evaluation of the effect of the present invention on the methanation process, three process configurations are compared. The temperature vs. reactor height according to the three process configurations is shown in Fig. 8.

All examples are based on an inlet gas composition as shown in Table 2. Table 2

Example 1 :

A first example according to the prior art demonstrates the operation of a methanation process in the presence of cata^¬ lyst particles having a high surface area. The temperature vs. reactor height is shown in Figure 8, with example 1 be- ing indicated as a dotted line. This example shows a rapid reaction, with 98% conversion after 0.6 m reactor, and a while the length of the reaction zone in the temperature interval 310-360°C is only about 0.45 m. However, the pres^¬ sure drop according to this operation is an excessive

0.0261 kg/cm².

Example 2 :

A second example according to the prior art demonstrates the operation of a methanation process with a reduced pres^¬ sure drop, in the presence of catalyst particles having a low surface area. The temperature vs. reactor height is shown in Figure 8, with example 2 being indicated as a sol^¬ id line. This example shows a slow reaction, with 98 % con version after 0.9 m reactor, and while the length of the reaction zone in the temperature interval 310-360°C is about 0.6 m, which increases the risk of carbon deposition. However the pressure drop according to this operation is only 0.0031 kg/cm². Since the particle structure is more open, the reactor bulk volume is also higher, but the asso^¬ ciated mass of catalyst is lower.

Example 3 :

A third example according to an embodiment of the present disclosure art demonstrates the possibility to achieve a methanation process with satisfactory ignition and a reduced pressure drop, by the combined use of two types of catalyst particles. The temperature vs. reactor height is shown in Fig. 8, with example 3 being indicated as a dot/dashed line. This example shows the favourable fast in- itial reaction as in the first example, with 98 ~6 conversion after 0.6 m reactor, and while the length of the reaction zone in the temperature interval 310-360°C is only about 0.45 m while keeping the pressure drop at only 0.0107 kg/cm². In this case the reactor bulk volume is also inter- mediate, and so is the associated mass of catalyst.

These three examples demonstrates that by designing a methanation process according to the present disclosure the operational characteristics can reduce the risk of catalyst deactivation, and reduce the pressure drop over the reac^¬ tor . Where the inlet temperature is below 310°C the reaction is also possible, but a lower limit for the reaction tempera^¬ ture may be defined by the requirement of sufficient cata^¬ lytic activity, or the risk of formation of nickel carbon- yls from the synthesis gas and the catalyst nickel.

Claims

1. A reactor for a transport limited reaction comprising an inlet zone comprising particles of a material catalyti- cally active in said reaction and a main zone comprising particles of a material catalytically active in said reac^¬ tion characterized in the external geometric surface area of the particles of said main zone being lower than the ex^¬ ternal geometric surface area of the particles of said in- let zone.

2. A reactor according to claim 1 in which said transport limited reaction is an exothermal reaction having an outlet temperature 5 to 450 °C above the inlet temperature.

3. A reactor according to claim 2 wherein said reaction is methanation of a carbon oxide by reaction with hydrogen.

4. A reactor according to claim 1 wherein said transport limited reaction is steam prereforming of hydrocarbons

5. A reactor according to claim 1, 2, 3 or 4 wherein the external geometric surface area of the particles of the in^¬ let zone is 700-2000 m²/m³ and the external geometric sur- face area of the particles of the main zone is 50-90% of the external geometric surface area of the particles of the main zone

6. A reactor according to any claim above wherein the particles of the inlet zone have a diameter of 2 to 6 mm

7. A reactor according to any claim above wherein at least one of the particles of the inlet zone and the main zone has a geometry taken from the group consisting of cylinders, rings, spheres, multiple-hole rings, including 7- hole rings, daysies and quadrulobes.

8. A reactor according to any claim above wherein the bulk volume of the particles of the inlet zone is less than 50%, preferably less than 25% and most preferably less than 15% of the reactor volume, upstream the main zone.

9. A reactor according to any claim above wherein having an exothermal inlet zone is downstream an endothermal inlet zone .

10. A reactor according to any claim above which is adia- batic .