PROCESS FOR PRODUCING SYNTHESIS GAS WITH REDUCED STEAM EXPORT

Abstract

A process is proposed for producing synthesis gas with reduced steam export by catalytic steam reforming of a hydrocarbonaceous feed gas with steam in a multitude of reformer tubes in a burner-heated reformer furnace to form a steam reforming flue gas. This process includes a configuration of the reformer tubes as reformer tubes with internal heat exchange and the use of a structured catalyst. For amounts of export steam between 0 and 0.8 kg of export steam per m.sub.N.sup.3 of hydrogen produced, these features interact synergistically when particular steam reforming conditions are selected.

Claims

1. A process for producing a synthesis gas containing hydrogen and carbon oxides with reduced steam export by catalytic steam reforming of a hydrocarbonaceous feed gas with steam under steam reforming conditions in a multitude of reformer tubes in a burner-heated reformer furnace, with formation of a steam reforming flue gas, the reformer tubes comprising the following constituents: (a) an outer, pressure-rated shell tube heated by means of the burners and having an inlet for a feed gas stream and an outlet for a crude synthesis gas product stream, (b) at least one structured catalyst which is catalytically active in respect of steam reforming and disposed within the shell tube, having an inlet end and an outlet end, (c) a heat exchanger tube which is disposed within the shell tube and within the structured catalyst, the inlet end of which is in fluid connection with the outlet end of the structured catalyst, and the outlet end of which is in fluid connection with the outlet for the crude synthesis gas product stream, (d) configured such that the feed gas stream is introduced into the shell tube via the inlet and flows first through the structured catalyst and subsequently through the heat exchanger tube in countercurrent, and the crude synthesis gas product stream produced is discharged from the shell tube via the outlet, (e) configured such that the heat exchanger tube and the gas stream that flows through it are in a heat-exchanging relationship with the structured catalyst and the gas stream that flows through it, the process comprising: (f) providing the hydrocarbonaceous feed gas and adding reforming steam and/or carbon dioxide, (g) at least partly catalytically converting the feed gas under steam reforming conditions in the reformer tubes to a crude synthesis gas product containing carbon oxides and hydrogen, wherein the steam reforming conditions comprise a reforming temperature T.sub.ref of at least 900° C., a steam/carbon ratio S/C of not more than 2.8 mol/mol and a normalized space velocity of the feed gas stream at the inlet into the reformer tubes between 2 and 5 m.sub.N.sup.3/(s*m.sub.cat.sup.3), (h) discharging the crude synthesis gas product from the reformer tubes, (i) discharging the reforming flue gas from the reformer furnace, (j) cooling at least a portion of the reforming flue gas and/or of the crude synthesis gas product by indirect heat exchange with cooling water to generate steam which can be discharged at least partly from the process as export steam, wherein the specific amount of export steam is between 0 and 0.8 kg of steam per m.sub.N.sup.3 of hydrogen generated.

2. The process according to claim 1, wherein the steam reforming conditions comprise a reforming temperature T.sub.ref of at least 920° C., a steam/carbon ratio S/C of not more than 2.7 mol/mol and a normalized space velocity of the feed gas stream at the inlet into the reformer tubes between 2.5 and 3.0 m.sub.N.sup.3/(s*m.sub.cat.sup.3), and in that the specific amount of export steam is zero.

3. The process according to claim 1, wherein the steam reforming conditions comprise a reforming temperature T.sub.ref of at least 930° C., a steam/carbon ratio S/C of not more than 2.7 mol/mol, and a normalized space velocity of the feed gas stream at the inlet into the reformer tubes between 3.5 and 4.0 m.sub.N.sup.3/(s*m.sub.cat.sup.3), and in that the specific amount of export steam is between 0 and 0.3 kg of steam per m.sub.N.sup.3 of hydrogen generated.

4. The process according to claim 1, wherein the steam reforming conditions comprise a reforming temperature T.sub.ref of at least 930° C., a steam/carbon ratio S/C of not more than 2.7 mol/mol, and a normalized space velocity of the feed gas stream at the inlet into the reformer tubes between 3.0 and 3.5 m.sub.N.sup.3/(s*m.sub.cat.sup.3), and in that the specific amount of export steam is between 0.3 and 0.8 kg of steam per m.sub.N.sup.3 of hydrogen generated.

5. The process according to claim 1, wherein the shell tube and the heat exchanger tube has a circular cross section and the structured catalyst has a circular ring-shaped cross section, and in that the shell tube, the structured catalyst and the heat exchanger tube are in a coaxial and concentric arrangement, wherein the structured catalyst is arranged with an essentially gastight seal between the inner wall of the shell tube and the outer wall of the heat exchanger tube.

6. The process according to claim 1, wherein the heat flow density between the outer wall and the inner wall of the shell tubes is between 50 and 200 kW/m.sup.2, averaged over the length of the shell tubes.

7. The process according to claim 1, wherein the inlet for the feed gas stream and the outlet for the crude synthesis gas product stream are disposed at the same end of the shell tube.

8. The process according to claim 7, wherein the reformer tubes are arranged within the reformer furnace in an upright manner on a base of the reformer furnace or in a suspended manner from a roof of the reformer furnace, and the end of the shell tube at which the inlet for the feed gas stream and the outlet for the crude synthesis gas product stream are disposed projects out of the reformer furnace, with the opposite end of the shell tube disposed within the reformer furnace.

9. The process according to claim 1, wherein a multitude of reformer tubes and burners are disposed in an interior of the reformer furnace, and in that the longitudinal axes of the flames generated by the burners are aligned parallel to the longitudinal axes of the reformer tubes, wherein the burners are disposed on the roof of the reformer furnace and/or at the base of the reformer furnace.

10. The process according to claim 1, wherein the at least partially catalytic conversion of the feedstock is effected to an extent of at least 50% under steam reforming conditions in the reformer tubes to give the crude synthesis gas product, based on the hydrocarbons present in the feedstock.

11. The process according to claim 1, wherein at east one of the reformer tubes contains more than one kind of structured catalyst, wherein the type of structured catalyst relates to the material, structural or textural characteristics thereof and/or the specific catalytic activity thereof.

12. The process according to claim 1, wherein the structured catalyst(s) comprise at least one element selected from the group of: structured packings, monoliths, honeycombs, open-cell metallic, vitreous or ceramic foams, stacked wire meshes, wherein the elements each have catalytic activity for steam reforming.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0052] Developments, advantages and possible applications of the invention are also apparent from the following description of working and numerical examples and the drawings. The invention is formed by all of the features described and/or depicted, either on their own or in any combination, irrespective of the way they are combined in the claims or the dependency references therein and wherein:

[0053] FIG. 1 an illustrative configuration of a reformer tube with internal heat exchange and structured catalyst for performance of the process according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0054] The reformer tube 1 according to the invention depicted in FIG. 1 is divided into the sections A (reaction chamber), B (outlet chamber) and C (collecting conduit).

[0055] Via an inlet conduit 2, desulfurized natural gas together with reforming steam enters the reaction chamber A arranged in the upper portion of a shell tube 3. The shell tube consists of a nickel-chromium steel for example of the type G-X45NiCrNbTi3525. The inlet temperature of the feed gas is 600° C.; the space velocity based on the catalyst volume is typically 4000 to 5000 m.sub.N.sup.3/(m.sup.3 h).

[0056] In the present working example, the reformer tube is in an upright arrangement with the open tube end of the shell tube 3 in the upper position and is externally heated by means of burners (not shown in FIG. 1). During operation of the reformer tube, the open tube end of the shell tube is sealed with a sealing device 4, for example a flanged lid, which may be opened for inspection operations and for charging and discharging of the catalyst.

[0057] The natural gas and the reforming steam, after entering the shell tube, enter a structured catalyst 5 which is composed of individual packing segments and corresponds in terms of structure to the structured catalyst described in US 2012/0195801 A1. It has a specific surface area of 1100 m.sup.2/m.sup.3 and is equipped with a nickel-containing active layer which is catalytically active for the steam reforming. The structured catalyst is also structured such that a significant proportion of the gas flow is deflected radially. As a result, a portion of the gas flow hits the inner wall of the reaction tube, which improves radial heat transfer. However, the effect is limited, and so a further improvement in radial heat transfer as achieved with the reformer tube according to the invention is advantageous.

[0058] The feedstocks then flow upward through the structured catalyst, in the course of which the endothermic steam reforming reaction takes place. After leaving the structured catalyst, the partially converted natural gas comprising not only carbon oxides and hydrogen but also unconverted methane enters an open space 8 disposed at the sealed tube end 4 of the shell tube. The partly converted feed gas stream subsequently enters the inlet end of a straight heat exchanger tube 9 disposed within the catalyst bed. The gas stream flowing through the heat exchanger tube 9 releases a portion of its sensible heat in countercurrent to the catalyst bed and the feed gas stream flowing through it. The heat exchanger tube is made of materials having good resistance to metal dusting corrosion, for example Alloy 601, 602 CA, 617, 690, 692, 693, HR 160, HR 214, copper-containing alloys or what are called multilayer materials where the tubes are coated with tin-nickel or aluminium-nickel alloys. Alternatively or additionally, the outlet ends of the heat exchanger tube are equipped with an anticorrosion layer on the inside and also on the outside in the portions that are passed through the separation plate. In the present example, this is an aluminium diffusion layer.

[0059] After flowing through the heat exchanger tube, the synthesis gas product stream enters the outlet chamber B. For this purpose, the outlet end of the heat exchanger tube 9 is passed through a separation plate 6 and fixed in this manner. It then opens at the outlet end into an inner tube 10 that constitutes the connection between the heat exchanger tube 9 and a collecting conduit 11. The inner tube is likewise fabricated from one of the abovementioned metallic materials of construction and its inner wall and preferably also its outer wall are provided with an aluminium diffusion layer as a corrosion protection layer. Alternatively, it is also possible to use an inner tube produced from a ceramic material.

[0060] A gas-permeable insulating material 12 is mounted between the outer wall of the inner tube and the inner wall of the shell tube. For this purpose, it is possible to use fibre-based insulation materials, but also intrinsically dimensionally stable ceramic shaped bodies. The latter are particularly advantageous since they can be installed and deinstalled in a particularly simple manner. On account of their dimensional stability, they can be placed into the ring space between shell tube and inner tube in a simple manner on installation, without requiring any particular securing means.

[0061] The collecting conduit 11 is provided on its inside with insulation material 13 and/or a corrosion-resistant, for example ceramic, coating 14, which have elevated resistance to metal dusting corrosion. The synthesis gas product stream is discharged from the reformer tube 1 via the collecting conduit and is sent for further processing. Depending on the intended use of the synthesis gas product, this may comprise a carbon monoxide conversion, a gas scrubbing operation for removal of carbon dioxide, a pressure swing adsorption for hydrogen removal and further processing stages.

EXAMPLES

[0062] The examples shown in the tables that follow are based on an equal production capacity of hydrogen and steam. All examples proceed from “End of Run” conditions (end of a catalyst cycle) with regard to catalyst activity.

[0063] The inventive examples are based on configurations of the reformer with bayonet tubes having structured catalyst. The export of steam is between 0 and 0.8 kg of steam per m.sub.N.sup.3 of hydrogen produced. According to the invention, the reforming temperature is above 900° C. The invention gives some or all of the following advantages compared to processes according to the prior art: higher hydrocarbon conversion, higher reformer efficiency, lower total natural gas consumption, lower S/C ratio.

Configurations of the Process for Steam Export of Zero or Low Steam Export (0 to 0.3 kg of Export Steam per m.SUB.N..SUP.3 .of Hydrogen Produced)

[0064] The prior art recommends providing a reduced number of reformer tubes for configurations of a reformer plant with bayonet tubes and structured catalyst in order to lower capital costs.

[0065] In a reformer plant without steam export, however, total hydrogen production costs are dominated essentially by natural gas consumption, and to a lesser degree by capital costs. Table 1 therefore compares various embodiments with bayonet tubes for the case of a reformer plant without steam export.

[0066] The configuration shown in Table 1, column 2, shows a design according to the prior art with bayonet tubes and catalyst pellets. This configuration has the following disadvantages:

[0067] (1) In order to prevent fluidization, a particular mass flow rate of feed gas must not be exceeded, which requires a high number of reformer tubes and/or a high tube diameter. This increases capital costs, and the lower mass flow rate per reformer tube reduces the coefficient of heat transfer.

[0068] (2) Catalyst pellet beds require a minimum ratio of pellet diameter to ring space in order to obtain an acceptable bed porosity and to avoid bypassing of the catalyst bed. This leads to a higher tube diameter that increases thermomechanical stresses, such that the maximum permissible tube thickness is attained at a lower reforming temperature, which limits performance aspects of the reformer, for example hydrocarbon conversion or reformer efficiency.

[0069] The configuration shown in Table 1, column 3, includes bayonet tubes with structured catalyst, with fewer tubes in the configuration proposed by the prior art than in the case of column 2. However, various adverse effects are observed when the number of tubes is reduced:

[0070] (1) The higher mass flow rate of feed gas per reformer tube leads to a lower dwell time, which is somewhat alleviated by the greater geometric surface area of the structured catalyst. Consequently, the system increasingly approaches the reaction equilibrium, which leads to a higher natural gas consumption for a given hydrogen production.

[0071] (2) For a given reformer output and reduced number of reformer tubes, there is a rise in the maximum wall temperature, which can have an adverse effect on the lifetime of the reformer tubes.

[0072] (3) The high maximum wall temperature leads to a maximum permissible tube wall thickness, which rules out any further increase in the reforming temperature.

[0073] (4) The reduced number of reformer tubes does not lead to significant savings in capital costs, since the structured catalyst is very costly compared to catalyst pellets.

[0074] Even in the case of a reformer plant with low steam export (0 to 0.3 kg of export steam per m.sub.N.sup.3 of hydrogen produced), total hydrogen production costs are dominated essentially by natural gas consumption, and to a lesser degree by capital costs. Table 2 compares various embodiments with bayonet tubes for the case of a reformer plant with low steam export.

[0075] The cases shown in Table 2, column 2 and column 3, are configurations with catalyst pellets or with structured catalyst, with attempts being made in the case of column 3 to achieve savings with a reduced number of reformer tubes. While capital costs in configurations with low steam export influence economic viability to a somewhat greater degree than in the example without steam export, the disadvantages of the cases shown in column 2 and column 3 are essentially the same as in the cases in Table 1, column 2 and column 3.

Configurations of the Process for Moderate Steam Export (0.3 to 0.8 kg of Export Steam per m.SUB.N..SUP.3 .of Hydrogen Produced)

[0076] The prior art suggests, in a steam reforming process with moderate steam export, configuring the reformer tubes as normal tubes with straight pass, i.e. without internal heat recovery, and with catalyst pellets or with structured catalyst. In the latter case, the prior art recommends, for example according to patent publications EP 1944269 B1 or U.S. Pat. No. 7,501,078 B2, a configuration with a reduced number of reformer tubes in order to achieve savings.

[0077] In a plant with moderate steam export, capital costs have a greater share of the total costs for hydrogen production than in a scenario without steam export or with low steam export, but overall economic viability is still unambiguously determined by natural gas consumption.

[0078] Table 3, for the case of a reformer plant with moderate steam export, compares various embodiments with normal tubes having a bed of pellets or structured catalyst and having bayonet tubes with structured catalyst. The case shown in column 2 has the following limitations:

[0079] (1) On account of the limited reformer efficiency of a configuration having normal tubes, the desired amount of export steam is already attained at a moderate reforming temperature. Since other operating parameters, for example the combustion air preheating temperature, have already reached their respective upper limits, the reforming temperature cannot be increased further without producing more steam, which would reduce the overall thermal efficiency of the reforming process.

[0080] The case shown in Table 3, column 3, includes a configuration with normal tubes having straight pass without heat recovery with structured catalyst and a reduced number of reformer tubes. However, this configuration leads to various disadvantages:

[0081] (1) It is not possible to increase the reforming temperature without producing excess steam, which reduces the overall thermal efficiency of the reforming process.

[0082] (2) The reduced number of reformer tubes does not lead to significant savings in capital costs, since the structured catalyst is very costly compared to catalyst pellets.

Configurations According to the Invention

[0083] Inventive configurations of steam reforming processes without steam export or with low steam export (0 to 0.3 kg of export steam per m.sub.N.sup.3 of hydrogen produced) have the following features, the aim of which is to increase the reformer efficiency and reforming temperature as far as possible:

[0084] In the inventive configuration shown in Table 1, column 4, the tube diameter is reduced and the number of reformer tubes is increased to overcome mechanical limitations. This permits increasing the reforming temperature above 900° C. The reduction in the tube diameter is possible by virtue of the use of the structured catalyst, which can be matched to the small diameter and ring space.

[0085] The increased reforming temperature moves the reaction equilibrium in the direction of a higher hydrocarbon conversion in the reformer, and simultaneously increases the efficiency of the reformer (reformer efficiency) and reduces total natural gas consumption, even though the exchange area for heat exchange has decreased compared to the reference case in column 2. The cases shown in columns 2 and 3 are limited on account of the high tube thickness of the reformer tubes with regard to the maximum achievable reforming temperature.

[0086] In the case of inventive configurations of steam reforming processes with moderate steam export (0.3 to 0.8 kg of export steam per m.sub.N.sup.3 of hydrogen produced), it would not be very practicable to provide a bayonet tube with catalyst pellets, since the limits for various process parameters would already be achieved at lower steam export values. This is especially true of the reforming temperature, which is in turn limited by the maximum tube wall thickness. However, the use of structured catalyst even for moderate steam export allows bayonet tubes to be used rather than normal tubes, which achieves the following advantages:

[0087] (1) The significantly higher reforming temperature in the inventive case of Table 3, column 4, leads to a higher hydrocarbon conversion, a higher reformer efficiency and a lower overall natural gas consumption than in the cases with normal tubes, columns 2 and 3, without formation of an excess of steam by virtue of the internal heat recovery.

[0088] (2) On account of the utilization of the sensible heat from the crude synthesis gas produced, this high hydrocarbon conversion and a higher reformer efficiency can be achieved without increasing the average external heat flow from the burners to the reformer tubes and with a reduced number of reformer tubes by comparison with the cases shown in Table 3, columns 2 and 3.

[0089] (3) In addition, it is possible in this way to lower the S/C ratio in the steam reforming, which is advantageous in relation to capital costs and operating costs.

TABLE-US-00001 TABLE 1 Process conditions for steam reforming processes without steam export Bayonet tube, structured catalyst, Bayonet tube, Bayonet tube, reduced number structured catalyst bed of pellets of tubes (invention) Reforming temperature T.sub.ref (° C.) 885 885 930 Steam/carbon S/C (mol/mol) 3.05 3.05 2.65 Heat flow density per tube (kW/m.sup.2) 64.9 80.5 78.4 Normalized space velocity 1.22 3.49 2.89 at reformer inlet (m.sub.N.sup.3/s/m.sup.3) Methane conversion (%) 75.9 75.1 83.5 Reformer efficiency (%) 61.23 57.30 62.09 Exchange area (m.sup.2) 846.9 682.2 800.0 Number of reformer tubes in the 144 116 164 reformer furnace Total natural gas consumption 20660 20699 20181 (feed gas + heating gas) Internal shell tube diameter (m) 0.125 0.125 0.100 Shell tube wall thickness (m) 0.0155 0.0155 0.0147

TABLE-US-00002 TABLE 2 Process conditions for steam reforming processes with low steam export (0 to 0.3 kg of export steam per m.sub.N.sup.3 of hydrogen produced) Bayonet tube, structured catalyst, Bayonet tube, Bayonet tube, reduced number structured catalyst bed of pellets of tubes (invention) Reforming temperature T.sub.ref (° C.) 877 877 935 Steam/carbon S/C (mol/mol) 2.91 2.91 2.36 Heat flow density per tube (kW/m.sup.2) 65.2 95.2 107.8 Normalized space velocity 1.22 3.90 3.89 at reformer inlet (m.sub.N.sup.3/s/m.sup.3) Methane conversion (%) 75.8 75.0 81.6 Reformer efficiency (%) 57.57 54.46 58.35 Exchange area (m.sup.2) 841.4 607.7 637.0 Number of tubes in the reformer 144 104 160 furnace Total natural gas consumption 21469 21497 20721 (feed gas + heating gas) Internal shell tube diameter (m) 0.125 0.125 0.076 Shell tube wall thickness (m) 0.015 0.015 0.0148

TABLE-US-00003 TABLE 3 Process conditions for steam reforming processes with moderate steam export (0.3 to 0.8 kg of export steam per m.sub.N.sup.3 of hydrogen produced) Bayonet tube, Normal tube, Normal tube, structured catalyst bed of pellets structured catalyst (invention) Reforming temperature T.sub.ref (° C.) 890 890 934 Steam/carbon S/C (mol/mol) 2.38 2.38 2.02 Heat flow density per tube (kW/m.sup.2) 85.1 104.2 85.6 Normalized space velocity 1.19 3.03 3.16 at reformer inlet (m.sub.N.sup.3/s/m.sup.3) Methane conversion (%) 78.1 77.5 83.3 Reformer efficiency (%) 51.40 51.40 56.65 Exchange area (m.sup.2) 1338.2 1093.3 1176.2 Number of tubes in the reformer 306 250 240 furnace Total natural gas consumption 33353 33403 32664 (feed gas + heating gas) Internal shell tube diameter (m) 0.100 0.100 0.100 Shell tube wall thickness (m) 0.008 0.008 0.015

LIST OF REFERENCE SYMBOLS

[0090] [1] Reformer tube

[0091] [2] Inlet conduit

[0092] [3] Shell tube

[0093] [4] Sealing apparatus

[0094] [5] Structured catalyst

[0095] [6] Separation plate

[0096] [7] Catalyst bed

[0097] [8] Open space

[0098] [9] Heat exchanger tube

[0099] [10] Inner tube

[0100] [11] Collecting conduit

[0101] [12] Insulating layer

[0102] [13] Insulating layer

[0103] [14] Coating

[0104] [A] Reaction chamber

[0105] [B] Outlet chamber

[0106] [C] Collecting conduit

[0107] 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.