Process comprising exothermal catalytic reaction of a synthesis gas and related plant

Abstract

A synthesis process comprising steam reforming a gaseous hydrocarbon feedstock; exothermically reacting the resulting synthesis gas; removing heat from said exothermal reaction by producing steam; using said steam as heat input to the steam reforming, wherein the steam reforming comprises: a) forming a mixture containing steam and hydrocarbons by at least the step of adding a first stream of water to the hydrocarbon feedstock; b) heating said mixture by indirect heat exchange with synthesis gas; c) reforming said mixture after said heating step b).

Claims

1. A synthesis process, comprising: steam reforming a gaseous hydrocarbon feedstock, thereby obtaining a synthesis gas; exothermically reacting said synthesis gas in the presence of a catalyst, thereby obtaining a synthesis product; removing heat from said exothermal reaction by producing steam, wherein at least part of said steam provides a heat input to the reforming of said hydrocarbon feedstock; wherein the steam reforming of the hydrocarbon feedstock includes: a) forming a mixture containing steam and hydrocarbons by at least a step of adding a first stream of water to the hydrocarbon feedstock in a saturating tower, said stream of water being pre-heated by indirect heat exchange, prior to admission into said tower, with at least a portion of the steam produced by removing heat from the exothermal synthesis reaction; b) heating said mixture by indirect heat exchange with at least part of said synthesis gas; and c) reforming said mixture after said heating step b).

2. The synthesis process of claim 1, wherein the formation of said mixture further includes mixing an effluent of said tower with a second stream of water, and said second stream is pre-heated by indirect heat exchange with said synthesis gas.

3. The synthesis process of claim 2, wherein a stream of synthesis gas transfers heat to said mixture during the heating step b), and the synthesis gas effluent of said heating step b) transfers heat to said second stream of water.

4. The synthesis process of claim 1, wherein excess water is drawn off from the saturating tower during said step a) and at least a portion of said excess water is recirculated into the tower, said portion being pre-heated, prior to admission into said tower, with a portion of the steam produced by removing heat from the exothermal synthesis reaction.

5. The synthesis process of claim 2, wherein excess water is drawn off from the tower during said step a) and said second stream of water includes a portion of said excess water.

6. The synthesis process of claim 1, wherein excess water is drawn off from the step b) and at least a portion of said excess water is added to the hydrocarbon feedstock in said saturating tower.

7. The synthesis process of claim 1, wherein said synthesis product includes methanol.

8. The synthesis process of claim 7, wherein the step of reacting the synthesis gas includes an isothermal reactive step, providing a methanol-containing stream, and wherein a stream of boiling water acts as cooling medium that removes heat of the exothermic reaction of the synthesis gas and generates said steam.

9. The synthesis process of claim 8, wherein said step of reacting the synthesis gas further includes subjecting said methanol-containing stream to a further isothermal reactive step, providing said methanol product, and wherein a stream of fresh synthesis gas acts as cooling medium.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the front-end section of a plant for the synthesis of methanol, according to an embodiment of the invention.

(2) FIG. 2 shows the synthesis section of a plant for the synthesis of methanol according to an embodiment of the invention.

DETAILED DESCRIPTION

(3) According to FIG. 1, the front-end section 100 of a methanol plant basically comprises a saturating section 101 and a reforming section 102. The saturating section 101 essentially comprises a saturating tower 1 and a saturating heat exchanger 2. The reforming section 102 essentially comprises a pre-reformer 3, a primary steam reformer 4 and a secondary reformer 5, which is for example an auto-thermal reformer (ATR). According to the example of the figure, the front-end section 100 also comprises a boiler 6 and a steam super-heater 7, which are located in series downstream of the secondary reformer 5.

(4) Said saturating section 101 also comprises a first heat exchanger 8 and a second heat exchanger 9, wherein water is heated by steam recovered from the synthesis section of the plant (shown in FIG. 2) to form at least part of the input stream to the saturating tower 1.

(5) According to the example shown in FIG. 1, the saturating section 101 further comprises a pre-heater 10 as will be better explained below.

(6) The operation of the plant is substantially the following.

(7) A stream 10 of natural gas is supplied to the front-end section 1, wherein contacts a first stream of hot water fed to said saturating tower 1 via line 12, providing an output stream 13 containing steam and natural gas, which furnishes around 45% of the total steam required in the downstream reforming section 102.

(8) Said output stream 13 mixes with a second stream of hot water 29 and the resulting mixture 30 enters the saturating heat exchanger 2, providing the input stream 14 of the reforming section 102. Said input stream 14 has preferably a steam-to-carbon (SC) ratio comprised between 1.8 and 2.8.

(9) The saturating heat exchanger 2 is preferably of the shell-and-tube type, with said mixture 30 flowing in the tube-side thereof with a falling-film flow.

(10) The saturating heat exchanger 2 provides around 50% of the total steam required in the reforming section 102. The balance of the process steam (i.e. around 5%) is supplied directly to the input stream 14 of the reforming section 102 (not shown).

(11) More in detail, the stream 14 is supplied to the pre-reformer 3, wherein reacts to provide an effluent 15. Said effluent 15 splits into a first portion 15a and a second portion 15b. Said first portion 15a is supplied to the primary steam reformer 4, providing a partially reformed gas 16. Said second portion 15b bypasses the primary steam reformer 4 and mixes with the partially reformed gas 16 forming the input stream 17 of the secondary reformer 5, wherein it further reacts providing a reformed gas 18.

(12) According to the example of the figure, said reformed gas 18 passes through the above mentioned boiler 6 and subsequently through the steam super-heater 7.

(13) The reformed gas 18 is supplied to the shell-side of the saturating heat exchanger 2, wherein it acts as heating medium to evaporate at least part of the water contained in the tube-side circulating mixture 30, ultimately providing said input stream 14.

(14) Accordingly, said saturating heat exchanger 2 discharges a cooled stream 19 of reformed gas which enters the pre-heater 10, wherein it acts as heating medium for a water stream 28, providing the second stream of hot water 29 and a reformed gas 20 with lower temperature.

(15) Said second stream of water 29 mixes with the output stream 13 of the saturating tower 1 to form the above mixture 30.

(16) Said reformed gas 20 has advantageously a molar ratio (H.sub.2—CO.sub.2)/(CO+CO.sub.2) close to 2, and is pressurized to about 80-150 bar in a suitable gas compressor 33, thus providing synthesis gas 40 directed to the following synthesis section of the plant (FIG. 2), which produces methanol 47. Within the synthesis section, heat of reaction is removed by producing steam 32, at least a portion of which is recycled back to the front-end section 100, in particular to act as heating medium in the first and second heat exchangers 8, 9.

(17) An excess of water is drawn off from the bottom of the saturating tower 1 via line 21 and splits into a first portion 21a and a second portion 21b. Said first portion 21a is exported from the front-end section 1 and sent to a waste water treatment section (not shown), while said second portion 21b is recirculated into the front-end section 100 through a pump 22 furnishing a pressurized stream 23.

(18) According to the example of FIG. 1, the first stream of hot water feeding the saturating tower 1 via line 12 is obtained by mixing together a first and a second stream of water 24, 25.

(19) Said first stream 24 is obtained by heating the effluent 26 of a distillation section (not shown) of the plant in the heat-exchanger 8 by means of a portion 32a of steam recovered from the synthesis section of the plant (shown in FIG. 2). Similarly, said second stream 25 is obtained by heating a portion 23a of the above mentioned pressurized stream 23 in the heat-exchanger 9 by means of a portion 32b of said steam.

(20) A further portion 23b of the pressurized stream 23 mixes with a process condensate 27, providing the water stream 28 entering the pre-heater 10.

(21) An excess of water 31 is also withdrawn from said saturating heat exchanger 2 and mixes with the first and second streams 24, 25 to form the input stream 12 feeding the saturating tower 1.

(22) FIG. 2 shows the synthesis section 200 of a methanol plant, wherein an input stream 40 of synthesis gas is converted into methanol. The synthesis section 200 essentially comprises a heat exchanger 201, a first isothermal reactor 202 and a second isothermal reactor 203. Said first and second isothermal reactors 202, 203 are located in series.

(23) A first portion 40a of said input stream 40 of synthesis gas is heated in the heat exchanger 201 by heat-exchange with a methanol-containing stream 48 providing a pre-heated stream 40c, while a second portion 40b bypasses the heat exchanger 201 and merges with said pre-heated portion 40c forming a stream 41 of synthesis gas.

(24) Said stream 41 splits into three portions, namely a first portion 41a, a second portion 41b and a third portion 41c. Said portions 41a-41c have the same composition, but may have different molar flows.

(25) The first isothermal reactor 202 contains heat exchange plates 204 immersed in the catalytic bed 205 and traversed by a stream 42 of boiler feed water, which removes the heat generated in said catalytic bed 205. The water leaves the heat exchange plates 204 as steam 32 which is recycled back to the front-end section 100.

(26) The second isothermal reactor 203 contains heat exchange plates 206 immersed in the catalytic bed 207 and traversed by said first portion 41a of synthesis gas which acts as cooling medium, thus generating a preheated stream 43 of reformed gas.

(27) The third portion 41c of reformed gas is mixed with said preheated stream 43 to form the input stream 44 to the first isothermal reactor 202, wherein it partially reacts providing an output stream 45 containing methanol and unreacted gas.

(28) Said output stream 45 is subsequently mixed with the second portion 41b of synthesis gas to form the input stream 46 to the second isothermal reactor 203, wherein the synthesis gas is further converted providing a methanol-containing product stream 47.

(29) Said product stream 47 is pre-cooled in a boiler feed water pre-heater 204 to form the stream 48 which is used as heating medium in the heat exchanger 201. A methanol-containing stream 49 with decreased temperature leaves the heat exchanger 201 and is subjected to purification in a suitable purification section (not shown).

(30) The presence of said heat exchanger 201 is advantageous because allows to modulate the temperatures of the portions 41a, 41b, 41c of reformed gas.