Process and apparatus for producing a mixed feed stream for a steam reforming plant

Abstract

This invention relates to a process and an apparatus for producing a mixed feed stream for a steam reforming plant from a first feed stream containing methane and a second feed stream comprising higher hydrocarbons, olefins and diolefins. According to the invention, the required hydrogenation of the mono- and diolefins and the hydrodesulfurization of the organic sulfur compounds contained in the feed stream are carried out step by step under process conditions optimized in each case. Furthermore, the inlet temperature into the respective reaction zone is controlled such that overheating of the feedstocks is avoided, which otherwise leads to undesired coke deposits, cloggings and the accelerated deactivation of the catalysts used.

Claims

1. A process for producing a mixed feed stream for a steam reforming plant containing at least one reforming stage, the mixed feed stream comprising a first feed stream containing methane and a second feed stream comprising higher hydrocarbons, olefins and diolefins, wherein the first and/or the second feed stream also contains sulfur compounds, and wherein the process comprises the following steps: a) providing the first feed stream and the second feed stream; b) heating the first feed stream in a first heating device; c) combining and mixing at least a part of the first feed stream with the second feed stream, introducing this mixture into a first hydrogenation zone, at least partly converting the diolefins in the first hydrogenation zone under diolefin hydrogenation conditions, discharging a diolefin hydrogenation product stream from the first hydrogenation zone; d) introducing the diolefin hydrogenation product stream into a second hydrogenation zone, at least partly converting the olefins in the second hydrogenation zone under olefin hydrogenation conditions, discharging an olefin hydrogenation product stream from the second hydrogenation zone; e) introducing the olefin hydrogenation product stream into a third hydrogenation zone, at least partly converting the sulfur compounds in the third hydrogenation zone under conditions of the hydrodesulfurization (HDS) to desulfurized hydrocarbons and hydrogen sulfide, discharging an HDS product stream from the third hydrogenation zone; f) introducing the HDS product stream into at least one adsorption zone, filled with an adsorbent selective for hydrogen sulfide, at least partly separating the hydrogen sulfide by adsorption on the adsorbent, discharging an adsorber product stream depleted of hydrogen sulfide from the adsorption zone; and g) introducing the adsorber product stream as mixed feed stream into at least one reforming stage, wherein: i. after heating in the first heating device, the first feed stream is split up into a first hydrogenation partial stream and a second hydrogenation partial system, ii. the first hydrogenation partial stream is introduced into a cooling device, iii. a cooled first hydrogenation partial stream is discharged from the cooling device, iv. the cooled first hydrogenation partial system is combined and mixed with the second feed stream, v. the mixture thus obtained is introduced into the first hydrogenation zone.

2. The process according to claim 1, wherein combining and mixing of at least a part of the first feed stream with the second feed stream is effected such that on entry into the first hydrogenation zone the mixture has a temperature of less than 300 C.

3. The process according to claim 1, wherein the diolefin hydrogenation product stream is combined and mixed with the second hydrogenation partial stream and the mixture thus obtained is introduced into the second hydrogenation zone.

4. The process according to claim 1, wherein an air cooler or a heat exchanger is used as cooling device, wherein in use of a heat exchanger cooling preferably is effected by indirect heat exchange against at least a part of the first feed stream before the first heating device.

5. The process according to claim 1, wherein the cooling capacity of the cooling device and/or the quantity ratio of the first hydrogenation partial stream to the second hydrogenation partial stream is chosen such that on entry into the first hydrogenation zone the mixture has a temperature of less than 300 C.

6. The process according to claim 1, wherein prior to combining and mixing with at least a part of the first feed stream the second feed stream is introduced into an adsorption zone which is filled with an adsorbent selective for metals.

7. The process according to claim 1, wherein the first heating device is designed as heat exchanger, wherein the first feed stream is heated by indirect heat exchange against a hot raw synthesis gas product stream from the steam reforming plant, against a hot flue gas stream from the reformer furnace or against a hot product gas stream from the CO conversion plant (CO shift) downstream of the stream reforming plant.

8. The process according to claim 1, wherein the steam reforming plant comprises a pre-reforming stage in which higher hydrocarbons are at least partly converted to methane under pre-reforming conditions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features, advantages and possible applications of the invention can also be taken from the following description of exemplary embodiments and numerical examples as well as the drawings. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.

(2) In the drawings:

(3) FIG. 1 shows a process for producing a natural gas feed stream and its subsequent conversion in a steam reforming plant according to the prior art,

(4) FIG. 2 shows the schematic representation of the process of the invention and the apparatus of the invention according to a first aspect,

(5) FIG. 3 shows the schematic representation of the process of the invention and the apparatus of the invention according to a second aspect.

DETAILED DESCRIPTION OF THE INVENTION

(6) In FIG. 1 a process for producing a natural gas feed stream and its subsequent conversion to a hydrogen-containing product gas in a stream reforming plant according to the prior art is schematically shown as block flow diagram (Life Cycle Assessment of Hydrogen Production via Natural Gas Steam Reforming, Spath, P. L., Mann, M. K., National Renewable Energy Laboratory Technical Report, NREL/TP-570-27637, February 2001, FIG. 1, page 4).

(7) A hydrocarbonaceous feed stream, for example natural gas (NG), is guided into a hydrogenation stage (Hydr). In the same, the hydrogenation of unsaturated hydrocarbons, for example of mono- and diolefins, to the corresponding alkanes is effected, as well as the hydrodesulfurizaton (HDS) of sulfur-containing organic compounds, for example of mercaptans, to sulfur-reduced organic compounds and hydrogen sulfide (H.sub.2S). The latter is adsorbed or absorbed on zinc oxide (ZnO) in the downstream sorption stage and the sulfur content thus is removed from the reformer feed. The reformer feed subsequently is introduced into the steam reforming stage (R) and in the same converted with likewise supplied steam (V) to obtain a raw synthesis gas. The raw synthesis gas subsequently is charged to a CO conversion stage (shift) in which carbon monoxide (CO) contained in the raw synthesis gas is converted to further hydrogen by using additionally supplied steam (not illustrated). The product stream of the CO conversion stage subsequently is guided into a pressure swing adsorption plant (PSA) in which it is split up into a hydrogen clean product stream and a residual gas stream (F). The residual gas stream still contains combustible components and is re-circulated to the reformer, where it serves for heating the reformer furnace together with additionally supplied natural gas. A part of the hydrogen clean product stream is recirculated to the hydrogenation stage as hydrogenating agent.

(8) FIG. 2 schematically shows the process of the invention and the apparatus of the invention according to a first aspect. Via conduit 10 the first feed stream, in the present case natural gas, is passed to the heat exchanger 30. Via conduit 20 a second feed stream, in the present example a refinery waste gas, is supplied to the adsorber 21 which is filled with an adsorbent selective for metals, for example activated carbon impregnated with sulfur, and thus effects a demetallization of the second feed stream. The adsorber furthermore contains an adsorbent for the deposition of chlorides.

(9) Via conduit 22 the second feed stream depleted of metals is guided to the condenser 23, condensed there and via conduit 24 combined and mixed with the first feed stream in conduit 10. The resulting mixed feed stream is heated in the heat exchanger 30 by indirect heat exchange against the hot product gas stream from the CO conversion plant (CO shift) downstream of the steam reforming plant, which via conduit 32 is guided to the heat exchanger 30.

(10) To limit the increase in temperature when heating the mixed feed stream in the heat exchanger 30, the same is equipped with a bypass, conduit 34 and a control valve 35 arranged in the conduit path 34. In the same way, the exothermicity of the hydrogenation of the unsaturated hydrocarbons can be taken into account and the exit temperature for example from the second hydrogenation zone can be kept constant. In an alternative configuration the conduit path 34 and the control valve 35 arranged therein also can be omitted and instead a cooling device can be integrated into the conduit path 31 (not illustrated). In a further alternative configuration, instead of the bypass of conduit 34, with which a part of the hot product gas stream from the CO conversion plant is guided around the heat exchanger 30, a bypass can be connected with which a part of the cold first feed stream, here natural gas, is guided around the heat exchanger 30 (not illustrated). Combinations of these configurations also are possible.

(11) Via conduit 31 the heated, mixed feed stream is discharged from the heat exchanger 30 and via conduit 40 charged to the diolefin hydrogenation reactor 41 (first hydrogenation zone), which is equipped with a fixed bed of a granular, solid hydrogenation catalyst which is active and selective for the hydrogenation of olefins, in particular diolefins, and at the same time is resistant to poisoning by organic sulfur compounds. Corresponding catalysts are kept in stock by the trade. The reaction conditions in the diolefin hydrogenation reactor, in particular the kind of and the chosen inlet temperature, are chosen such that there above all the diolefins contained in the mixed feed stream, but also a smaller part of the contained monoolefins, are hydrogenated to the corresponding monoolefins or alkanes and the adiabatic increase in temperature is limited. Via conduit 42, the diolefin hydrogenation product stream is discharged from the diolefin hydrogenation reactor.

(12) Conduit 43 branching off from conduit 40, the valve 44 arranged in the conduit path 43, and further non-illustrated shut-off devices and auxiliary conduits substantially serve the connection of a bypass around the diolefin hydrogenation reactor 41, if revision work, for example an exchange of the contained hydrogenation catalyst, must be carried out on the latter in ongoing operation. In normal operation the valve 44 remains closed.

(13) The diolefin hydrogenation product stream depleted of diolefins, which is discharged from the diolefin hydrogenation reactor via conduit 42, is charged to the hydrogenation reactor 51 (second hydrogenation zone) via conduit 50. In the present example, the same is equipped with an upper and a lower fixed catalyst bed. The upper fixed catalyst bed, which forms the second hydrogenation zone, contains a bed of a granular, solid hydrogenation catalyst which is active and selective for the hydrogenation of olefins and at the same time is resistant to poisoning by organic sulfur compounds. The lower fixed catalyst bed forming the third hydrogenation zone on the other hand above all serves the hydrogenating desulfurization (hydrodesulfurization, HDS) of the contained organic sulfur components. Such catalysts also are known per se and commercially available; they are often based on the active components nickel-molybdenum (NiMo) or cobalt-molybdenum (CoMo).

(14) Via conduit 52, the HDS product stream is discharged from the olefin hydrogenation reactor. It now largely only contains hydrogenated and thus saturated hydrocarbons and hydrogen sulfide. Via conduit 60 this material stream is introduced into a first adsorber 61 which in the present example contains two adsorption zones designed as fixed beds. The upper adsorption zone is equipped with a bed of a solid alumina-based adsorbent which is active for the adsorption of chlorides. The lower adsorption zone contains a bed of a solid sorbent on the basis of zinc oxide for the removal of hydrogen sulfide. Both sorbents are known per se and commercially available.

(15) Via conduit 62 a first adsorber product stream depleted of hydrogen sulfide and chlorides is discharged from the first adsorber 61 and via conduit 70 introduced into a second adsorber 71 which is constructed and filled in the same way as the first adsorber 61. When the first and the second adsorber are connected in series, the second adsorber serves for the aftertreatment of the first adsorber product stream, whereby a second first adsorber product stream further depleted of hydrogen sulfide and chlorides is obtained, which is discharged from the second adsorber 71 via conduit 72.

(16) Conduit 63 branching off from conduit 60, the valve 64 arranged in the conduit path 63, and further non-illustrated shut-off devices and auxiliary conduits for example serve the connection of a bypass around the first adsorber 61, if revision work, for example an exchange of the contained sorbents, must be carried out on the latter in ongoing operation.

(17) Via conduit 72 the now completely pre-treated mixed feed stream is discharged from the process or the apparatus. It can now be supplied to a steam reforming plant. As due to its origin and pre-treatment the feed stream now contains a significant amount of higher hydrocarbons, it is recommendable to initially subject the same to prereforming by means of which the higher hydrocarbons are split into methane and/or already are partly converted to synthesis gas constituents.

(18) In the schematic representation of the process of the invention or the apparatus of the invention according to a second aspect as shown in FIG. 3 identical reference numerals in principle correspond to the process steps and apparatus components as they have been described already in the explanation of the first aspect of the invention, FIG. 1. The respective process conditions, working steps, apparatus components and auxiliary substances also are the same, unless described otherwise in the following.

(19) In contrast to the first aspect the first feed stream (natural gas) now is not combined with the second feed stream (refinery waste gas) before heating in the heat exchanger 30, but is heated alone in the heat exchanger. This has the advantage that the risk of coke deposits in the heat exchanger 30 is reduced considerably, as the concentration of unsaturated compounds in the fluid stream guided in conduit 10 is considerably smaller, often even equal to zero. A bypass around the heat exchanger 30, which is meant to reduce the increase in temperature on heating, therefore can be omitted.

(20) The heated first feed stream leaves the heat exchanger 30 via conduit 31 and is split up into a first and a second hydrogenation partial stream, which are guided on in conduits 45 and 43. The valve 44 now is designed as control valve and along with further, partly non-illustrated control devices serves for setting the mass flows of the two hydrogenation partial streams. Via conduit 45 and a valve 46 arranged in the conduit path 45 the first hydrogenation partial steam is introduced into a cooling device 47 which in the present example is designed as air cooler. The cooled first hydrogenation partial stream is discharged from the cooling device 47 via conduit 40 and now combined and mixed with the second feed stream, which is supplied via conduit 24 after it has first been demetallized and condensed in the way described above.

(21) It is very advantageous that the second feed stream, which contains the olefins and diolefins, is added to the cooled first hydrogenation partial stream and thus is introduced into the process at a relatively cold point. By correspondingly setting the cooling capacity of the cooling device and/or the quantity ratio of the first hydrogenation partial stream to the second hydrogenation partial stream it is ensured that on entry into the diolefin hydrogenation reactor 41 (first hydrogenation zone) the mixture has a temperature of less than 300 C., preferably less than 250 C., most preferably less than 230 C. This measure is very helpful to avoid undesired cracking and polymerization reactions of the olefins and diolefins contained in the mixture. It is particularly advantageous that before the conversion of the diolefins in the first hydrogenation zone the second feed stream no longer gets in contact with heat-exchange surfaces of heat exchangers, for example of the heat exchanger 30. As due to heat transfer limitations said heat exchange surfaces always must be hotter than the desired exit temperature of the fluid stream from the heat exchanger, an increased decomposition of unsaturated hydrocarbons to coke occurs at such heat-exchange surfaces.

(22) Via conduit 40 the mixture formed of the cooled first hydrogenation partial stream and the second feed stream is charged to the diolefin hydrogenation reactor 41 as first hydrogenation reactor, which with regard to its features and its operation corresponds to the above-discussed first aspect of the invention. By corresponding adaptation for example of the cooling capacity of the cooling device, the inlet temperature into the diolefin hydrogenation reactor is set to the optimum value.

(23) Via conduit 42 and the valve 48 located in the conduit path 42 a diolefin hydrogenation product stream depleted of diolefins is discharged from the diolefin hydrogenation reactor and combined with the second hydrogenation partial stream supplied via conduit 43. The mixture thus obtained is guided to the olefin hydrogenation reactor 51 (second hydrogenation zone) via conduit 50. From reference numeral 50 the second aspect corresponds to the first aspect of the invention.

NUMERICAL EXAMPLES

(24) In the following numerical examples the mode of action of the invention is illustrated with reference to the results of simulation calculations. Organic sulfur components are not taken into account here, as for the resulting temperature profiles the hydrogenation of the mono- and diolefins is decisive.

(25) Both examples are based on natural gas as first feed stream and on a refinery waste gas as second feed stream. The compositions of the first and the second feed stream are shown in the columns for conduit 10 and conduit 24 of the material stream table indicated below. All further indications in the material stream table only are applicable for Example 2. The hydrogen content of the refinery waste gas is sufficient for the hydrogenation steps according to the invention, so that the recirculation of hydrogen from the hydrogen product stream of the steam reforming plant can be omitted.

Example 1

(26) According to the first aspect of the invention natural gas as first feed stream is guided to the heat exchanger 30 via conduit 10 at room temperature (25 C.). A refinery waste gas as second feed stream initially is demetallized in the adsorber 21 and subsequently condensed in the condenser 23 and then has a temperature of 81 C. Via conduit 24 the second feed stream is combined with the first feed stream. In the heat exchanger 30 the mixed feed streams are heated such that the temperature in conduit 40 is 260 C. before entry into the diolefin hydrogenation reactor 41. Due to the exothermally proceeding hydrogenation of the diolefins and a part of the olefins, the temperature in conduit 50 before entry into the second hydrogenation stage (reactor 51, olefin hydrogenation+HDS) is about 290 C. Due to the again released reaction enthalpy of the hydrogenation of the olefins and sulfur compounds the temperature at the outlet of the reactor 51 is about 360 C. (conduit 52). With a temperature slightly below this value the gas stream enters into the adsorber 61 via conduit 60, in which adsorber the hydrogen sulfide formed during the hydrodesulfurization is bound to zinc oxide.

(27) In the aspect of the invention as shown in Example 1 it is advantageous that at the inlet of the diolefin hydrogenation reactor 41 and of the olefin hydrogenation or HDS reactor 51 a temperature of 300 C. is not exceeded. The upper limit of 343 C. as defined in U.S. Pat. No. 3,477,832 safely is maintained and the deposition of coke on the hydrogenation catalysts and of adsorbent thus is prevented.

Example 2

(28) The composition and properties of important material streams in conjunction with the following explanation of a second example are shown in the indicated material stream table.

(29) According to the second aspect of the invention natural gas as first feed stream is guided to the heat exchanger 30 via conduit 10 at room temperature (25 C.). In the heat exchanger 30 the natural gas stream is heated without admixing the second feed stream, so that the gas temperature in conduit 31 before branching into conduits 43 and 45 is 368 C. The first hydrogenation partial stream branched off via conduit 45 is introduced into the cooling device 47, cooled there. and via conduit 40 combined with the second feed stream, which is supplied via conduit 24 and has a temperature of 81 C. The mixture obtained then enters into the diolefin hydrogenation reactor 41 with a temperature of 222 C. and leaves said reactor in conduit 42 with a temperature of 229 C. After combining and mixing with the second hydrogenation partial stream (conduit 43) the mixture thus obtained enters into the olefin hydrogenation or HDS reactor 51 with a temperature of 313 C. and leaves the same in conduit 52 with a temperature of 354 C.

(30) Material Stream Table, Example 2, Second Aspect of the Invention

(31) TABLE-US-00001 Conduit No. 10 24 31 40 42 50 52 Designation first feed second feed heated first Feed - first Product - first Feed - second Product - second stream (natural stream (refinery feed stream hydrogenation hydrogenation hydrogenation hydrogenation gas) waste gas) (natural gas) zone zone zone (olefin zone (olefin (Diolefin hydr.) (Diolefin hydr.) hydr. + HDS) hydr. + HDS) Molar flow rate (kmol/h) 1519 428 1519 808 805 1945 1910 Mass flow rate (kg/h) 27610 7605 27610 14508 14508 35216 35216 Temperature ( C.) 25 81 368 222 229 313 354 Mole fraction (%) H.sub.2 0.00 21.20 0.02 11.24 10.91 4.53 2.67 CO 0.00 1.30 0.00 0.69 0.62 0.26 0.13 CO.sub.2 1.07 0.10 1.07 0.56 0.56 0.86 0.87 H.sub.2O 0.00 0.00 0.00 0.00 0.07 0.03 0.16 N.sub.2 2.00 3.50 2.00 2.79 2.80 2.33 2.37 O.sub.2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ar 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Methane 89.20 49.25 89.18 68.02 68.29 80.53 82.12 Ethane 5.35 15.40 5.35 10.67 10.83 7.62 8.78 Ethene 0.00 4.80 0.00 2.54 2.42 1.00 0.00 Ethyne 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Propane 1.52 1.60 1.52 1.56 1.57 1.54 1.97 Propene 0.00 1.80 0.00 0.95 0.96 0.40 0.00 n-Butane 0.32 0.40 0.32 0.36 0.36 0.34 0.34 i-Butane 0.26 0.00 0.26 0.12 0.12 0.20 0.33 Butene 0.00 0.50 0.00 0.26 0.29 0.12 0.00 Butadiene 0.00 0.05 0.00 0.03 0.00 0.00 0.00 n-Pentane 0.08 0.10 0.08 0.09 0.09 0.08 0.09 i-Pentane 0.08 0.00 0.08 0.04 0.04 0.06 0.06

(32) All in all, the aspect of the invention explained in Example 2 has the same advantages as explained in conjunction with Example 1. A further advantage consists in that the second feed stream, which contains by far the most part of the olefins and diolefins, is introduced into the process at a relatively cold point and in particular is not heated in the heat exchanger 30 together with the first feed stream. By correspondingly setting the cooling capacity of the cooling device and/or the quantity ratio of the first hydrogenation partial stream to the second hydrogenation partial stream it is ensured that on entry into the diolefin hydrogenation reactor 41 the mixture has the desired temperature of below 230 C.

(33) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(34) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(35) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

(36) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(37) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(38) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(39) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.

INDUSTRIAL APPLICABILITY

(40) The invention provides a process and an apparatus for producing a mixed feed stream for a steam reforming plant. As feed streams natural gas and higher hydrocarbon streams can be used, which include a significant amount of unsaturated hydrocarbons and organic sulfur compounds. Due to the stepwise hydrogenation according to the invention and the optimized temperature profile overheating of the feedstocks is avoided, which leads to thermal cracking in particular of the contained unsaturated hydrocarbons, to undesired coke deposits and thus to cloggings of the apparatus and downstream systems and to the accelerated deactivation of the catalysts used. The availability of the plant and therefore the economy of the downstream steam reforming plant thereby is increased.

LIST OF REFERENCE NUMERALS

(41) 10 conduit 20 conduit 21 adsorber (demetallization) 22 conduit 23 condenser 24 conduit 30 heat exchanger 31 conduit 32 conduit 33 conduit 34 conduit 35 valve 40 conduit 41 diolefin hydrogenation reactor (first hydrogenation zone) 42 conduit 43 conduit 44 valve 45 conduit 46 valve 47 cooling device 48 conduit 50 conduit 51 olefin hydrogenation+HDS reactor (second+third hydrogenation zones) 52 conduit 60 conduit 61 adsorber 62 conduit 63 conduit 64 valve 70 conduit 71 adsorber 72 conduit