Methods for producing aromatic hydrocarbons from natural gas and processing unit for implementing same

Abstract

The invention relates to the field of gas chemistry and, more specifically, to methods and devices for producing aromatic hydrocarbons from natural gas, which involve producing synthesis gas, converting same into methanol producing, from the methanol, in the presence of a catalyst, a concentrate of aromatic hydrocarbons and water, separating the water, air stripping hydrocarbon residues from the water, and separating-out the resultant concentrate of aromatic hydrocarbons and hydrogen-containing gas, the latter being at least partially used in the production of synthesis gas to adjust the ratio therein of H.sub.2:CO 1.8-2.3:1, and can be used for producing aromatic hydrocarbons. According to the invention, the production of aromatic hydrocarbons from methanol in the presence of a catalyst is carried out in two consecutively-connected reactors for synthesizing aromatic hydrocarbons: in a first, low-temperature isothermal reactor for synthesizing aromatic and aliphatic hydrocarbons, and in a second, high-temperature adiabatic reactor for synthesizing aromatic and aliphatic hydrocarbons from aliphatic hydrocarbons formed in the first reactor, and the subsequent stabilization thereof in an aromatic hydrocarbon concentrate stabilization unit. At least a portion of the hydrogen-containing gas is fed to a synthesis gas production unit and is used for producing synthesis gas using autothermal reforming technology. The installation carries out the method. The achieved technical result consists in increasing the efficiency of producing concentrates of aromatic hydrocarbons.

Claims

1. A method for producing a concentrate of aromatic hydrocarbons from natural gas, the method comprising: a) producing synthesis gas from natural gas; b) converting the synthesis gas into methanol; c) producing from the methanol, in the presence of a catalyst, a concentrate of aromatic hydrocarbons and water; d) separating the water from the concentrate of aromatic hydrocarbons and water; e) air stripping hydrocarbon residues from the water, thereby providing a concentrate of resultant aromatic hydrocarbons and a hydrogen-containing gas; f) using at least a portion of the hydrogen-containing gas in the production of the synthesis gas to adjust a ratio therein of H.sub.2:CO to between 1.8-2.3:1; wherein the production of the concentrate of aromatic hydrocarbons and water from methanol in the presence of a catalyst, is carried out in two consecutively-connected reactors for synthesizing aromatic hydrocarbons: (i) in a first isothermal reactor for synthesizing aromatic and aliphatic hydrocarbons; (ii) in a second adiabatic reactor for synthesizing aromatic and aliphatic hydrocarbons from aliphatic hydrocarbons formed in the first reactor; and, (iii) wherein the temperature of the second reactor is higher than the temperature of the first reactor; wherein in a subsequent stabilization of the concentrate of resultant aromatic hydrocarbons in a stabilization unit there is provided a concentration of stabilized aromatic hydrocarbons; and, wherein the at least a portion of the hydrogen-containing gas is fed to a synthesis gas production unit and is used for producing synthesis gas, wherein the synthesis gas production unit comprises: an autothermal reforming unit with a pre-reforming unit; the use of non-catalytic partial oxidation using oxygen or oxygen-air mixtures as the oxidant; or both, to adjust the molar ratio therein according to the relationship (the MF of H.sub.2−the MF of CO.sub.2)/(the MF of CO−the MF of CO.sub.2)>2, where MF stands for the mole fraction of the component in the synthesis gas.

2. The method according to claim 1, wherein the reaction zone of the first reactor for synthesizing aromatic and aliphatic hydrocarbons is thermally stabilized by heating an external agent, wherein the heat is removed from the external agent by evaporating methanol in a methanol evaporator mounted between the unit for synthesizing methanol and the first isothermal reactor for synthesizing aromatic and aliphatic hydrocarbons.

3. The method according to claim 1, wherein aliphatic hydrocarbons removed from the concentrate of resultant aromatic hydrocabons in a conversion product separation unit are at least partially fed to a reaction zone of the first isothermal reactor for synthesizing aromatic and aliphatic hydrocarbons and/or to the reaction zone of the second adiabatic reactor for synthesizing aromatic and aliphatic hydrocarbons, which provides for an increase in the yield of the concentrate of aromatic hydrocarbons.

4. The method according to claim 1, wherein a catalyst based on a pentasil-type crystalline aluminosilicate with SiO.sub.2/A1.sub.2O.sub.3=25-100 is used as the catalyst of the first isothermal reactor for synthesizing aromatic and aliphatic hydrocarbons, wherein the catalyst comprises 0.05-0.1 wt. % sodium oxide, 0.5-3.0 wt. % zinc oxide, 0.1-5.0 wt. % rare earth oxides, 0.05-2.5 wt. % cobalt oxide, 63.0-70.0 wt. % crystalline aluminosilicate, and a remainder being a binding component.

5. The method according to claim 1, wherein a catalyst based on a pentasil-type crystalline aluminosilicate with SiO.sub.2/Al.sub.2O.sub.3=25-120 is used as the catalyst of the second adiabatic reactor for synthesizing aromatic and aliphatic hydrocarbons, wherein the catalyst comprises 0.05-0.1 wt. % sodium oxide, 0.5-3.0 wt. % zinc oxide, 0.12-0.30 sodium oxide, 0.1-3.0 wt. % rare earth oxides, 0.05-2.5 wt. % cobalt oxide, 63.0-70.0 wt. % crystalline aluminosilicate, and a remainder being a binding component; wherein the composition of the rare earth oxides is: Nd.sub.2O.sub.3: 21.%, and Pr.sub.6O.sub.11: the remainder.

6. The method according to claim 1, wherein a portion of the hydrogen-containing gas produced in the process of synthesizing and separating the concentrate of aromatic hydrocarbons is used in a process of hydrogenating sulfur-containing compounds to hydrogen sulfide.

7. The method according to claim 6, wherein olefins which are contained in the produced hydrogen-containing gas are also hydrogenated together with the sulfur-containing compounds.

Description

A SHORT DESCRIPTION OF THE DRAWINGS

(1) The other distinguishing features and advantages of the invention clearly follow from the description given above as a form of illustration and are not limiting with references to the attached drawings in which:

(2) FIG. 1 is a schematic perspective view of the processing unit for producing aromatic hydrocarbons from natural gas, according to the invention, and

(3) FIG. 2 schematically shows the steps of the method for producing aromatic hydrocarbons from natural gas, according to the invention.

(4) According to FIG. 1, the processing unit for producing a concentrate of aromatic hydrocarbons from natural gas includes the following consecutively connected elements: a unit (1) for producing synthesis gas, a unit (2) for producing methanol, a unit (3) producing a concentrate of aromatic hydrocarbons and a unit (4) for separating the conversion product into reaction water, discharge hydrogen-containing gas, a fraction of light aliphatic hydrocarbons, and a stable concentrate of aromatic hydrocarbons, which is connected to the unit (1) for producing synthesis gas and the unit (3) for producing a concentrate of aromatic hydrocarbons.

(5) The unit (3) for producing a concentrate of aromatic hydrocarbons comprises two consecutively-connected reactors for synthesizing aromatic hydrocarbons: a first, low-temperature isothermal reactor (31) for synthesizing aromatic and aliphatic hydrocarbons, and a second, high-temperature adiabatic reactor (32) for synthesizing aromatic and aliphatic hydrocarbons from aliphatic hydrocarbons formed in the first reactor (31).

(6) The unit (4) for separating the conversion product into reaction water, discharge hydrogen-containing gas, a fraction of light aliphatic hydrocarbons, and a stable concentrate of aromatic hydrocarbons comprises a unit (41) for condensing the hydrocarbons and reaction water, disposing of the waste water and separating out the hydrogen-containing gas and unstabilized concentrate of aromatic hydrocarbons, and a unit (42) for stabilizing the concentrate of aromatic hydrocarbons (connected by line (63) to unit (41)).

(7) The unit (3) for producing a concentrate of aromatic hydrocarbons additionally comprises a recuperative evaporator (33) for recovering the methanol obtained in the methanol synthesis reactor (2) and a recuperative heater (not shown on FIG. 1) for the crude methanol vapors of the reaction medium from unit (32), which is situated on line (58) between the crude methanol evaporator (33) and the first, low-temperature synthesis reactor (31).

(8) The first, low-temperature isothermal reactor (31) for synthesizing aromatic and aliphatic hydrocarbons additionally comprises a circuit (34) with an external heat carrier connected to the methanol evaporator (33) mounted between the methanol synthesis unit (2) and the first, low-temperature isothermal reactor (31) for synthesizing aromatic and aliphatic hydrocarbons.

(9) The unit (3) for producing a concentrate of aromatic hydrocarbons additionally comprises an additional flow heater (positioned on the line 59) which is needed to initialize the conversion process of propane and other aliphatic hydrocarbons in the conversion product (not shown in FIG. 1).

(10) The outlet of the unit (42) (line 65) for stabilizing the concentrate of aromatic hydrocarbons, through which the light aliphatic hydrocarbons are discharged, is connected to the reaction zone of the first low-temperature isothermal reactor (31) for synthesizing aromatic and aliphatic hydrocarbons (line 68) and/or to the reaction zone of the second high-temperature adiabatic reactor (32) for synthesizing aromatic and aliphatic hydrocarbons (line 67).

(11) The unit (1) for producing the synthesis gas consists of an autothermal reforming unit (12) with an adiabatic pre-reforming unit (11) and a heat recovery unit (13). These are found on lines (52), (53), (54) and (56).

(12) The adiabatic pre-reforming unit (11) is an adiabatic reactor in which processes of destructive hydrogenation and steam conversion of hydrocarbons C.sub.2+ occur to form thermally stable compounds C.sub.1, CO, and CO.sub.2, which makes it possible to increase the heating temperature of the steam-gas mixture before feeding same to the autothermal reforming unit (12), and to reduce the steam/carbon ratio in the steam-gas mixture. A mixture composed of natural gas (fed along line (51)), hydrogen-containing gas (fed along line (66)) and superheated steam (fed along line (70)) is fed to the inlet of unit (11). Before being fed to unit (11), the mixture is superheated to a temperature of 300-450° C.

(13) The autothermal reforming unit (12) is a separate apparatus which blows oxygen along line (61). The main advantage of autothermal reforming is to exclude the step of tubular steam conversion and the possibility of operating with a very low steam/carbon ratio. This enables the production of crude methanol with a methanol concentration of at least 94%, which is suitable for converting into a concentrate of aromatic hydrocarbons without a concentration step. In addition, the lack of hydrogen (in a ratio required for synthesizing methanol) in synthesis gas produced by autothermal reforming is fully compensated by the addition, to the raw material to be converted, of hydrogen-containing discharge gas from the synthesis of the concentrate of aromatic hydrocarbons, and also, when necessary, by the addition, to the synthesis gas (line (56)) of hydrogen removed from the discharge gases from synthesizing methanol (line (69)).

(14) Recycling hydrogen-containing gas from the conversion product separating unit (41) makes it possible to increase the function:
f=(the MF of H.sub.2−the MF of CO.sub.2)/(the MF of CO+the MF of CO.sub.2)
from 1.7-1.9 to the required 2.05-2.1 with a low steam/carbon ratio which reduces the amount of energy lost and the size of the heat exchangers used.

(15) The hydrogen-containing gas is blown from the methanol synthesis unit (2) through line (69).

(16) Water is fed from the conversion product separating unit (41) through line (62) to the heat recovery unit (13). The superheated steam is drawn from the heat recovery unit (13) along line (55).

(17) The superheated steam is fed along line (70) for the production of the steam and gas mixture to be fed to unit (11).

Implementation of the Invention

(18) According to the invention, aromatic hydrocarbons are produced from natural gas in the following manner.

Step A1. The Production of Synthesis Gas

(19) Natural gas (denoted as NG in FIG. 1) is fed to a synthesis gas production unit (1) along lines (51) and (52). In order to produce synthesis gas with the stoichiometric ratio between H.sub.2, CO and CO.sub.2 required for methanol synthesis, a scheme for producing synthesis gas is used in a process of autothermal reforming or oxygen blowing, in which oxygen is used for the partial and complete oxidation of the carbon in natural gas, wherein, as is known, during the conversion of natural gas by said gases, it is possible to obtain synthesis gas with the stoichiometric ratio:

(20) f=(the MF of H.sub.2−the MF of CO.sub.2)/(the MF of CO+the MF of CO.sub.2) less than 1.8, and synthesizing methanol through a circulation scheme requires synthesis gas with f≥2.05.

(21) Due to the fact that hydrogen-containing gas (denoted in FIG. 1 as HRG) is additionally fed to the unit for producing synthesis gas from the step for synthesizing the concentrate of aromatic hydrocarbons from unit (4) along line (66), synthesis gas with the required stoichiometric ratio f≥2.05 would be at the outlet of the unit for producing synthesis gas (1), wherein the discharge hydrogen-containing gas from the synthesis of the concentrate of aromatic hydrocarbons is fully recycled, which increases the thermal efficiency coefficient of the integrated process.

(22) Since a steam reformer is not used to correct the composition of the synthesis gas, the content of CO.sub.2 in the synthesis gas is small, which enables the production of crude methanol with a concentration of at least 94 wt. %, which makes it possible to eliminate the step of concentrating the crude methanol.

Step A2. Synthesizing Methanol

(23) Afterwards the synthesis gas is fed along line (56) to the methanol synthesis unit (2) where same is converted to crude methanol and discharge synthesis gases which are not shown in the figure and is sent to the methanol evaporator (33) along line (57).

Step A3. Synthesizing the Concentrate of Aromatic Hydrocarbons

(24) The crude methanol from the methanol synthesizing unit (2) enters lines (57), (58), (59) and (60) of unit (3) for synthesizing the concentrate of aromatic hydrocarbons, which is composed of units for evaporating methanol (33), a recuperative heater for the crude methanol vapors (not shown in FIG. 1), the first, low-temperature isothermal reactor (31) for the synthesis of aromatic and aliphatic hydrocarbons, and an additional heater for the conversion product which initiates the conversion of propane and other aliphatic hydrocarbons in the second, high-temperature adiabatic reactor (32) of crude methanol (not shown in FIG. 1) and the second, high-temperature adiabatic reactor (32) for the synthesis of aromatic and aliphatic hydrocarbons from the aliphatic hydrocarbons formed in the first reactor (31).

(25) In addition, the temperature in the first reactor is kept within the range 450°+/−30° C., and in the second reactor the temperature is kept within the range 500°+/−50° C. The pressure in reactors (31) and (32) is from 0.7 to 3 MPa.

Step A4. Heat Removal

(26) Heat is removed from reactor (31) by evaporating methanol in unit (33), and due to the presence of the circuit (34). It is not possible to directly use a methanol evaporator because the critical temperature for methanol is 242° C. An intermediate heat carrier is used for heat removal, the composition of which depends on the type of reactor equipment used in unit (31). Thus, when using a tube reactor cooled by a liquid medium, the heat carrier can be a high-temperature heat carrier based on, for example, dialkylbenzenes or heat carriers based on silicon compounds, molten salts, etc. Furthermore, some of the heat is removed from the heat carrier in the methanol evaporation unit (33).

(27) When using a tube reactor of a eutectic composition to be cooled by a lead-bismuth alloy, heat exchange takes place due to the convective motion of the melt between the reaction tubes and the tubular methanol evaporator.

(28) When using a reactor with an immovable catalyst layer with heat removal using heat tubes, both sodium and heat resistant hydrocarbons can be used as an intermediate heat carrier.

(29) Methanol in a gaseous state can also be used as an intermediate heat carrier.

(30) The circuit for circulating the intermediate heat carrier is denoted by a dotted line in the scheme and by the number 34.

(31) Converting methanol into a concentrate of aromatic hydrocarbons results in the formation of aliphatic hydrocarbons C.sub.2+ in addition to said aromatic hydrocarbons, wherein a portion of said aliphatic hydrocarbons C.sub.2+ (propane) is converted to aromatic hydrocarbons at high temperatures. In order to convert same, the steam-gas mixture of reactor unit (31) is heated to a temperature of 480-530° C. and fed to reactor (32).

Step A5. Removing Water and Hydrogen-Containing Gas

(32) Afterwards, the mixture from the reactor (32) is fed along line (60) to the unit (41) for separating the conversion product, where the reaction water from the discharge gases is condensed and removed through line (62), hydrogen-containing gas is removed through line (66) and the concentrate of aromatic hydrocarbons is removed through line (63), which enters the stabilization unit (42).

(33) The reaction water to be condensed and separated in the conversion product separation unit (41) is fed to a heat recovery boiler (13). The reaction water is recycled in this manner.

(34) The additional two-stage conversion in reactors (31) and (32) and the reduced temperature in reactor (31) make it possible to reduce the methanol conversion to carbon oxides, which makes it possible to use the discharge hydrogen-containing gas without removing CO.sub.2 therefrom to hydrogenate the sulfur-containing compounds in natural gas, thereby improving the functional of synthesis gas to the optimal values. Hydrogen-containing gas contains very little CO or CO.sub.2, which partially methanate sulfur-containing compounds on a hydrogenating catalyst, which leads to the formation of steam that reduces the chemisorption ability of zinc oxide or iron oxide.

Step A6. Stabilizing the Concentrate of Aromatic Hydrocarbons

(35) The concentrate of aromatic hydrocarbons is fed through line (63) from the unit (41) for separating the conversion product to a stabilization unit (42), where the gases from the stabilization of the concentrate of aromatic hydrocarbons are also used to convert to a concentrate of aromatic hydrocarbons, wherein said gases are sent through lines (65), (67) and (65), (68) to reactors (31) and (32). The product (denoted in FIG. 1 as Product) is discharged through line (64).

(36) The sequence of steps is an example and makes it possible to rearrange, add or produce various operations at the same time without losing the ability to produce a concentrate of aromatic hydrocarbons from natural gas.

INDUSTRIAL APPLICABILITY

(37) The proposed processing unit for producing a concentrate of aromatic hydrocarbons from natural gas can be implemented by a person skilled in the art in practice, and when implemented, provides for the execution of the claimed purpose, which makes it possible to conclude that the invention satisfies the criterion of industrial applicability.

(38) According to the proposed invention, calculations were made concerning the method for operating the processing unit for producing a concentrate of aromatic hydrocarbons from natural gas.

(39) The calculations are shown below. Recycling hydrogen-containing gas in the pre-reforming stage makes it possible to raise the heat efficiency coefficient to 65%, which is a much higher than during conversion without recycling the hydrogen-containing gas, in which case the coefficient is 35-40%. Therefore, integrating the processes makes it possible to raise the heat efficiency coefficient by no less than 20%, which makes it possible to increase the yield by at least 40-50% of hydrocarbon raw material as a result of the conversion. Recycling reduces the steam/carbon ratio (because of the presence of H.sub.2 as well as H.sub.2O in the mixture to be converted, both of which hinder the formation of soot in reactors). The scheme of the processing unit is simplified, wherein there is no need to mount an additional steam reformer or to use other schemes for correcting the composition of synthesis gas. There is no need in the additional processing unit to separate hydrogen from hydrogen rich gas for the hydrogenation of sulfur-containing compounds in the desulfurization stage because the discharge gases from the synthesis of the concentrate of aromatic hydrocarbons can be used for this purpose. Gases with a high olefin content are hydrogenated at the hydrogenation step to H.sub.2S. The production yield of the concentrate of aromatic hydrocarbons is increased. It is possible to synthesize highly concentrated crude methanol, which makes it possible to eliminate the step of concentrating same.

(40) Additional Technical Result: to increase the content of methylbenzenes including xylenes because, together with aromatization, a reaction occurs and the benzene rings undergo alkylation using methanol.

(41) Thus, this invention solves the problem of interest which is to increase efficiency in the production of concentrates of aromatic hydrocarbons.

EXAMPLES

(42) A mixture of desulfurized natural gas and discharge hydrogen-containing gas from the unit for synthesizing a concentrate of aromatic hydrocarbons from methanol is fed to the inlet of the unit for producing synthesis gas and recovering heat. The mixture is hydrogenated to remove the olefins and then mixed with superheated steam and heated to a temperature of 450-650° C. and fed to the autothermal reforming reactor (11). (T≈950° C., P—2-3 MPa). The steam-gas mixture from the reforming reactor (12) outlet is cooled and the condensed water therefrom is separated.

(43) The unit for producing hydrogen from the discharge hydrogen-containing gas from the methanol synthesis: This unit can be composed of a processing unit for concentrating hydrogen which functions according to the principle of short-cycle absorption. The degree of extraction of hydrogen from the hydrogen-containing gas is 60%.

(44) The crude methanol is synthesized along a circulation scheme from a compressed mixture of synthesis gas obtained in unit 1 with the addition of hydrogen obtained in the unit for producing hydrogen from discharge hydrogen-containing gas from synthesizing methanol.

(45) The unit for synthesizing a concentrate of aromatic hydrocarbons from crude methanol.

(46) The Unit for Producing Synthesis Gas by Means of the Technique of Autothermal Reforming and Heat Recycling.

(47) Raw Material (Thousands of Metric Tons Per Year).

(48) TABLE-US-00001 Natural gas with methane equivalent 1.03 −1000 The discharge hydrogen-containing gas from −378 synthesizing the concentrate of aromatic hydrocarbons Water (steam) −800 Oxygen 99.5% −1314 Total: −3491

(49) Produced:

(50) TABLE-US-00002 Synthesis gas −2834 Condensed water −657 Total: −3491
The Unit for Producing Hydrogen from Discharge Gases from Synthesizing Methanol

(51) Raw Ingredient:

(52) TABLE-US-00003 Discharge gas from synthesizing methanol −356 (H.sub.2 −55 vol. %)

(53) Produced:

(54) TABLE-US-00004 Fuel gas from the unit for separating hydrogen −336 99% hydrogen from the discharge gases from −20 synthesizing methanol with extraction of 60% of the hydrogen Total: −356
The Unit for Synthesizing Crude Methanol (Circulation/Purge Ratio=20)

(55) Raw Material:

(56) TABLE-US-00005 Synthesis gas −2834 99% hydrogen −20 Produced: −2854

(57) TABLE-US-00006 Crude methanol 94.5% −2498 Discharge hydrogen-containing gas −356 Total: −2854
The Units for Synthesizing the Concentrate of Aromatic Hydrocarbons and for Separating the Conversion Product.

(58) Raw Material:

(59) TABLE-US-00007 Crude methanol, 94.5 wt. % −2498

(60) Produced:

(61) TABLE-US-00008 Stabilized product (arene concentrate) −663 Reid vapor pressure of 70 kPa Reaction water, with 1.6 wt. % methanol −1457 Discharge hydrogen-containing gas −378 Total: −2498
The Composition of Stabilized Product (Concentrate of Aromatic Hydrocarbons)

(62) TABLE-US-00009 Methanol −0.04% Aliphatic compounds −2.64% Benzene −8.69% Toluene −35.69% Aromatic hydrocarbons C.sub.8+ −40.10% Aromatic hydrocarbons C.sub.9+ −12.84% Total: 100.00%