METHOD FOR PREVENTING THE FLUIDIZATION OF A CATALYTIC FIXED BED IN A TUBULAR UPWARD-FLOW REACTOR OF A STEAM METHANE REFORMER

Abstract

The present invention relates to a method to prevent the fluidization of a catalytic fixed bed present in a tubular reactor operated in upward-flow configuration by estimating a pressure drop margin remaining before fluidization of the catalytic bed and adjusting the reactant gas flow in response. It relates also to a method to operate safely a furnace suitable for performing endothermic reactions containing a plurality of catalytic fixed bed reactors operated in upward-flow configuration, and to a method to debottleneck safely a catalytic fixed bed reactor involving a gas flowing in up flow direction.

Claims

1-8. (canceled)

9. A method to prevent the fluidization of a catalytic fixed bed present in a tubular reactor of a furnace suitable for performing endothermic reactions, where the reactant gas flows upwardly, comprising: a) estimating a pressure drop margin remaining before fluidization of the catalytic bed, with the pressure drop at fluidization of the catalytic bed being:
DP.sub.critical=M.sub.cat×g/S with M.sub.cat being the mass of the catalyst in the catalytic bed, g being the gravitational constant, S being the gas-cross-sectional area through which the reactant gas flows in the catalytic bed, comprising: i) calculating the pressure drop at fluidization of the catalyst bed, ii) measuring the pressure drop DP.sub.bed between the top and the bottom of the catalytic bed, iii) determining a pressure drop margin before fluidization, b) adjusting the reactant gas flow in response to the pressure drop margin.

10. The method according to claim 9 where DP.sub.bed is measured by means of a Differential Pressure Transmitter.

11. The method according to claim 9 where DP.sub.bed is measured by means of two Pressure Transmitter, one installed at the catalytic bed inlet and the other installed at the catalytic bed outlet, and the pressure drop is calculated in the plant Distributed Control System.

12. The method according to claim 9, wherein the pressure drop margin before fluidization is expressed as a consumed margin, defined as the percentage (DP.sub.bed/DP.sub.critical)×100, the reactant gas flow being adjusted to keep the consumed margin below 90%.

13. The method according to claim 9, wherein the pressure drop margin is expressed so as to give a remaining margin before fluidization, defined as the percentage (1−DP.sub.bed/DP.sub.critical)×100, the reactant gas flow being adjusted to keep the reactant margin above 10%.

14. A method to operate safely a furnace suitable for performing endothermic reactions, containing a plurality of catalytic fixed bed reactors operated in upward-flow configuration comprising the prevention of the fluidization of the catalyst fixed bed present in said reactors by applying the method of claim 9 to at least one of said reactors.

15. The method according to claim 14, wherein the furnace is a steam methane reformer furnace.

16. A method to debottleneck safely a catalytic fixed bed present in a tubular reactor of a furnace suitable for performing endothermic reactions, with the reactant gas flowing upwardly, where the method includes increasing the reactant gas flow in the reactor characterized in that the risk of fluidization of the catalyst bed present in the reactor is simultaneously prevented by applying the method of claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The invention and its advantages will be described in more details in the following examples and on the basis of the drawings, where:

[0075] FIG. 1 shows the four main configurations of the industrial practice for the steam methane reforming;

[0076] FIG. 2 shows a bayonet tube in a bottom-fired furnace, in an upward-flow configuration, suitable for the implementation of the invention;

[0077] FIG. 3 shows two conventional tubes in an upward-flow configuration, suitable for the implementation of the invention;

[0078] FIG. 4 shows schematically the behaviour of a catalyst fixed bed submitted to the force of gravity and the drag force induced by gas flowing up through the catalyst bed;

[0079] FIG. 5 shows an example of application of the invention applied to a conventional tube similar to the tubes of FIG. 3;

[0080] FIG. 6 shows an example of application of the invention applied to a bayonet tube similar to the tubes of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0081] Reading the following more detailed description of the figures will help understanding the invention. Note that in the figures, analogous items (either apparatus or process step) are identified by reference numerals identical except for the left digit, which refers to the number of the figure.

[0082] FIG. 1 shows schematically four conventional designs where a furnace 101 contains tubes 102 heated by burners 103. Reactant gas 105 are introduced at the top of the tubes and flow down through the catalyst bed 104 installed in the tubes for the reforming of the reactant gas, and the product gas 106 leave the tube at the bottom end, opposite to the entrance.

[0083] In FIG. 2, the tube 202 presented is a bayonet tube; installed in the furnace 201, the tube is heated by burners 203 mounted at the floor of the furnace. A bayonet tube is composed of one external tube where the reforming takes place and one or more internal tubes installed in the catalyst bed for the transportation of the hot gas produced gas. The reactant gas 205 is introduced in the catalytic bed 204 (the catalytic bed appears in grey in this figure and in the other figures as well) at the bottom of the tube (upward-flow). Two inner tubes 207 are placed within the tubes 202 of the furnace 201. The reactants 205 are introduced at the bottom of the catalytic bed 204. The upper end of the tube 202 is a closed end; therefore, above the end of the catalytic bed, at a location identified as “reverse point” 208, the product gas is forced to enter into the inner tubes 207 and to flow counter-currently to the reactant gas. The product gas 206 then exits the tube at the same end where the reactant gas 205 is introduced in the tube 202. The upward-flow configuration coupled to the use of bayonet tubes as presented on the figure allows advantageously to install heavy equipment like the product gas manifold on the ground and thereby decreases in an important manner the cost of the furnace structure.

[0084] FIG. 3 presents conventional tubes 302 in an upward-flow configuration; tubes are installed in the furnace 301 and are heated by burners 303 mounted at the bottom of the furnace. The reactant gas 305 is fed at the bottom of the tubes filled with catalyst 304 and the gas produced 306 is collected at the top of the tubes.

[0085] FIG. 4, as already explained above, presents the forces F and W acting on the catalyst fixed bed 404 which can initiate the phenomenon of fluidization if the upward drag force F induced by the gas flowing up through the catalyst bed (the direction of the flow is represented by arrow 409) is higher than the force of gravity W; FIG. 4(a) illustrates the case of a fixed bed 404 with the intensity of the Drag Force F being lower than the intensity of the Force of Gravity W, FIG. 4(b) shows the fluidization of the bed resulting from the intensity of the Drag Force F induced by the gas flow rate being higher than the intensity of the Force of Gravity W of the catalyst bed, with a risk of destruction by attrition or grinding. By construction, a support 410 is of course present at the bottom of the bed to maintain it in place.

[0086] Thanks to the present invention, simple method and apparatus are proposed to monitor and control the operation of an upward-flow reactor to remain below the fluidization limit. From the fluidization theory as exposed above, the pressure drop at fluidization DP.sub.critical (which is the pressure drop value corresponding to fluidization limit, i.e. when the Drag Force F is equal to the Force of Gravity W) is easily calculated from Eq.1=Eq.2, leading to:

DP.sub.critical=M.sub.cat×g/S with:  (Eq.3)

[0087] g is the gravitational constant;

[0088] S is the gas-cross-sectional area through which the reactant gas flows in the tube, its value is calculated from the tube dimensions;

[0089] M.sub.cat can be estimated from the data coming from the loading operations (the average mass M.sub.Cat of the catalyst filled in tubes during the loading operation is recorded). It appears thus from the above that this pressure drop at fluidization DP.sub.critical is independent of the operating conditions.

[0090] DP.sub.bed can be immediately obtained from simple measurements.

[0091] By making said measurements continuously—for example by installing a Differential Pressure Sensor/Transmitter (DPT) that will give an on-line and continuous access to the pressure drop of the catalytic bed DP.sub.bed—and as the pressure drop at fluidization DP.sub.critical is known, the margin between the actual pressure drop DP.sub.bed and the critical one DP.sub.critical is easily monitored.

[0092] The choice of the DPT and its installation have to be made carefully to ensure that: [0093] the sensor technology will not generate pressure drop and disturb the gas stream flow; [0094] there is no other equipment installed between the pressure taps located upstream and downstream that could generate a pressure drop.

[0095] Implementing a critical margin monitoring method in the Distributed Control System (DCS) of the plant is easy; the monitoring criterion will depend on DP.sub.Bed which is measured and DP.sub.Critical, which is calculated according to Eq.3.

[0096] This criterion can be expressed by different ways, for instance: [0097] it can be expressed as (DP.sub.bed/DP.sub.critical)×100 according to Eq.4; In that case, it is representative of the percentage of the margin that is consumed: when this criterion is equal to 100%, it means that the margin is fully consumed; the critical load is reached, the load of the plant cannot be increased without inducing a risk of fluidization of the catalyst fixed bed; [0098] alternatively, by expressing the criterion as (1−[DP.sub.Bed/DP.sub.Critical])×100) according to Eq.5, it is representative of the percentage of remaining margin before reaching the critical load of the plant; [0099] other expressions can of course be imagined for this criterion while remaining within the scope of the invention.

[0100] Two examples of application of the invention are presented hereafter in relation with FIG. 5 and FIG. 6. They illustrate the present invention, but must not be considered as limiting its scope of application.

Examples

[0101] Example 1 is based on the design of the tube of FIG. 5, which is itself a conventional tube similar to the tube of FIG. 4. The tube 502 is a conventional tube installed in a bottom-fired furnace 501. The reactants 505 are introduced at the bottom the tube, at the inlet of the catalytic bed 504, the product gas mixture 506 is collected at the outlet of the catalytic bed, at the top of the tube. The burners 503 are placed at the furnace floor. To implement the invention, a differential pressure sensor 511 equipped with a transmitter (referred as DPT) is installed to measure the pressure drop DP.sub.bed between the catalytic bed inlet 510 and the catalytic bed outlet 508.

[0102] The tube 502 is filled with 100 kg of catalyst, it has an inner diameter of 0.1 m. [0103] pressure drop at fluidization is calculated by applying Eq.3: DP.sub.Critical=M.sub.Cat×g/S:

DP.sub.Critical=100×9.81/(3.14×(0.1/2).sup.2)=1.25.Math.10.sup.5 Pa. (i.e. 1.25 bar). [0104] DP.sub.bed is given by the DPT. [0105] The criterion defining the margin remaining left between DP.sub.bed and DP.sub.Critical is calculated either by applying Eq.4 or Eq.5; assuming that DP.sub.bea as measured by the DPT is 1 bar, then it means that according to Eq.4, 80% of the margin is consumed; expressed otherwise according to Eq.5, the margin remaining before fluidization is 20%.

[0106] Example 2 is based on the design of the tube of FIG. 6 which presents a bayonet tube 602 installed in a bottom-fired furnace 601, the reactants 605 are introduced at the inlet 610 of the catalytic bed 604—at the bottom of the tube—and, at the outlet of the catalytic bed 608—also known as reverse point—, the gas produced 606 enters the inner tubes 607 and is then collected at the bottom end of the tube 602. The burners 603 are placed at the furnace floor. To implement the invention, a differential pressure sensor equipped with a transmitter (referred as DPT) 611 is installed to measure the pressure drop DP.sub.bed between the catalytic bed inlet and the reverse point 608.

[0107] The tube 602 has an inner diameter of 0.125 m, it contains two inner tubes 607 having an outer diameter of 0.025 m. The tube 602 is filled with 95 kg of catalyst. [0108] pressure drop at fluidization is calculated by applying Eq.3: DP.sub.Critical=M.sub.Cat×g/S; in that case, it is necessary to take into account the 2 inner tubes and to remove their section for the calculation of S, which leads to S=3.14×(0.125/2).sup.2−2×3.14×(0.025/2).sup.2; the pressure drop at fluidization is therefore: DP.sub.Critical=M.sub.Cat×g/S=95×9.81/((3.14×(0.125/2).sup.2−2×3.14×(0.025/2).sup.2)=8.25.Math.10.sup.4 Pa. (i.e. 0.825 bar). [0109] DP.sub.bed is given by the DPT. [0110] The criterion defining the margin remaining left between DP.sub.bed and DP.sub.Critical is calculated either by applying Eq.4 or Eq.5; assuming that DP.sub.bed as measured by the DPT is 0.6 bar, then it means that according to Eq.4, 73% of the margin is consumed; expressed otherwise according to Eq.5, the margin remaining before fluidization is 27%.

[0111] Thereby, based on the method set forth above, the operator can permanently know the actual fluidization margin of the plant. This information is of prime importance when a the plant is operated at a higher load than its nominal capacity to meet a customer's need for more hydrogen.

[0112] Steam reformers and other externally fired reactors can contain from ten to several hundred of tubes filled with catalyst. The filling procedures are well established and can lead to a small variation of the amount of catalyst in each tube. This variation is usually kept below +/−5% and recorded during the filling procedure. Thus, the operator knows which tube contains the lowest amount of catalyst and can use this lowest amount in the fluidization margin calculation. In operation, the feed gas flow will be distributed amongst the tubes so that the pressure drop is the same in all tubes; therefore equipping only one tube with pressure sensors may be enough in theory.

[0113] In practice, it might be advantageous to equip several tubes. Indeed, if only one tube is equipped and there is a sensor failure due to breakage, dust or water clogging, then the information is lost whereas if sensors are installed on several tubes, then the system is more reliable. In the case of a single reactor, it might be advantageous to install several sensors to prevent any information loss in case of breakage, dust or water clogging.

[0114] As this monitoring method is easy to implement, reliable and low cost, it will be possible to select and equip the tubes being filled with the lowest amount of catalyst for example, or to select and equip tubes in different rows and according to their position in a row (i.e. at the beginning, the middle or the end), or any other selection based on specific behaviour of certain tubes in the furnace.

[0115] The method can be implemented in a plant when a debottlenecking is scheduled in order to control the reactants gas flow so as to prevent the risk of fluidization.

[0116] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.