APPARATUS AND PROCESS FOR WORKING UP A HYDROGEN- AND METHANE-COMPRISING STREAM

- BASF SE

The invention relates to an apparatus (100) for working up a hydrogen- and methane-comprising stream (1.1), which comprises the following components: (i) at least one heat exchanger (KS1) for cooling a stream (1.1) to be worked up; (ii) at least one separation unit (A, A1, A2, A2′) for purifying the stream (3) to be worked up to give a stream (5) rich in hydrogen and methane; (iii) at least one cooling unit (KS2) for cooling the stream (5) rich in hydrogen and methane; and (iv) at least one cryogenic gas separation unit (KS3) for separating the stream (6) rich in hydrogen and methane into at least one hydrogen-rich stream (7) and at least one methane-rich stream (8, 9). The invention further relates to a process for working up a stream of material.

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Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit, under 35 U.S.C. §119(e), to U.S. Provisional Application No. 61/579,663, filed Dec. 23, 2011, which is incorporated herein by reference.

BACKGROUND

The invention relates to an apparatus for working up a hydrogen- and methane-comprising stream, wherein the apparatus comprises at least one heat exchanger, at least one separation unit, at least one cooling unit and at least one cryogenic gas separation unit. The invention further relates to a process for working up a stream of material and the use of the apparatus or of the process for working up a stream extracted from a hydrodealkylation plant.

Aromatic hydrocarbons, in particular benzene, toluene, xylene and ethylbenzene (also referred to as BTXE fraction), are among the industrially most important bulk products in the chemical industry and are raw materials for plastics and other bulk chemicals.

Aromatic hydrocarbons are present, for example, in crude oil and a predominant part of the industrially important compounds are synthesized by petrochemical processes such as steam cracking. Here, long-chain hydrocarbons are cracked in the presence of steam at residence times in the millisecond range and temperatures in the range from 800 to 850° C. The objective here is to obtain short-chain alkenes, such as ethylene or propylene. Aromatic hydrocarbons are firstly obtained as by-products and separated by means of complicated separation processes into the individual components such as benzene or toluene.

To cover the large demand for benzene, transformation processes which allow alkyl-substituted aromatic hydrocarbons to be dealkylated, in particular to benzene or toluene, have been developed. This makes flexible production matched to market conditions of aromatic hydrocarbons possible.

An important transformation process is, apart from isomerization, hydrodealkylation. Here, an aromatic hydrocarbon such as toluene is converted in the presence of hydrogen into a simpler aromatic hydrocarbon such as benzene. This chemical process is described, for example, in WO 2007/051851 and is generally carried out at high temperatures, under high pressure or in the presence of a catalyst.

In hydrodealkylation, it is necessary to use a large excess of hydrogen, which means that hydrogen recycling and in particular separation of hydrogen from the resulting product gas mixture have to meet demanding requirements. One possibility is cryogenic separation, in which condensable impurities are separated off from the product gas mixture in various cooling stages in a cryogenic gas separation unit. DE 20 55 507 A1 discloses a process for purifying a feed gas composed of crude hydrogen, in which a crude hydrogen feed gas comprising condensable impurities is subjected to stepwise cooling. For this purpose, the feed is passed through various cooling stages, the condensate is separated off from the feed after passage through each cooling stage, each condensate is depressurized and then passed in a recycle stream through each preceding cooling stage for autogenous cooling. To maintain the correct heat balance of the process, the heat released in the last cooling stage is given off to the outside, as a result of which temperatures down to −165° C. can be achieved. This cryogenic purification plant enables hydrogen having a purity of more than 90% to be obtained.

U.S. Pat. No. 3,371,126 describes a process for producing benzene and heating gas, in which, after a dealkylation unit, a stream rich in hydrogen and methane is purified in a cryogenic purification zone. Here, the stream coming from the dealkylation unit is firstly fractionated to give a benzene-rich fraction and a fraction rich in hydrogen and methane. The fraction rich in hydrogen and methane comprises, for example, 55.6 mol % of hydrogen, about 40.7 mol % of methane and about 4 mol % of ethane. After the low-temperature fractionation, a hydrogen-rich stream comprising more than 80% by volume of hydrogen is provided.

Known plants for the work-up and reuse of hydrogen use complex and corrosion-sensitive components which have to meet highly demanding requirements in respect of their efficiency, stability and safety.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an illustrative embodiment of the invention as a process flow diagram.

 DESCRIPTION

It is an object of the present invention to provide an apparatus and a process in order to simplify the work-up of product gas mixtures and provide an inexpensive alternative which gives a high purity of the worked up hydrogen and meets high safety standards.

The object is achieved by an apparatus for working up a hydrogen- and methane-comprising stream, which comprises the following components:

  • (i) at least one heat exchanger for cooling a stream to be worked up;
  • (ii) at least one separation unit for purifying the stream to be worked up to give a stream rich in hydrogen and methane;
  • (iii) at least one cooling unit for cooling the stream rich in hydrogen and methane; and
  • (iv) at least one cryogenic gas separation unit for separating the stream rich in hydrogen and methane into at least one hydrogen-rich stream and at least one methane-rich stream.

In addition, the object is achieved by a process for working up a hydrogen- and methane-comprising stream, which comprises the following steps:

  • (a) cooling of a stream to be worked up in at least one heat exchanger;
  • (b) purification of the stream to be worked up to give a stream rich in hydrogen and methane in at least one separation unit;
  • (c) cooling of the stream rich in hydrogen and methane in at least one cooling unit;
  • (d) separation of the stream rich in hydrogen and methane into at least one hydrogen-rich stream and at least one methane-rich stream in at least one cryogenic gas separation unit.

The invention makes it possible to recover hydrogen and methane in high purity and reuse them in a simple way. This can be achieved using simply and robustly constructed components which increase the life of the apparatus and minimize the outlay for monitoring and maintenance. Thus, in particular, the heat exchanger for precooling the stream to be worked up can be constructed simply and robustly since the requirements for efficiency of the heat exchanger are less demanding in the case of the apparatus of the invention and the process of the invention than in the case of the plants known from the prior art. In addition, sensitive components such as the cryogenic gas fractionation unit can adequately be protected against damage by separating off corrosive and freezable materials from the stream. The removal of impurities thus leads to recovery of hydrogen and methane in high purity, which makes the subsequent operation easier.

Furthermore, the invention provides for two-stage cooling of the stream to be worked up using the heat exchanger and the cooling unit in a first stage and the cryogenic gas separation unit in a second stage, which increases the stability of the work-up process.

This results, in particular, from the cooling unit being located upstream of the cryogenic gas separation unit and thus providing a stream having an essentially constant temperature level before entry into the cryogenic gas fractionation unit. For the present purposes, an essentially constant temperature level means temperature fluctuations in the range +/−2° C.

In the following, values in % by volume are a measure of the proportion of a material in a mixture based on the total volume of the mixture. For the purposes of the invention, ppm based on the total volume of the mixture is one part by volume per million. ppt based on the total volume of the mixture refers to one part by volume per trillion. Standard m3/h refers to standard cubic meters per hour, with the standard volume being given for a pressure of 1 bar and a temperature of 20° C.

In an embodiment, the apparatus of the invention is used in the work-up of a stream taken from a dealkylation of alkyl-substituted aromatic hydrocarbons. For this purpose, the apparatus of the invention can be installed downstream of a plant for dealkylation of alkyl-substituted aromatic hydrocarbons. The individual components of the plant are preferably arranged in the abovementioned order and, correspondingly, the steps of the process of the invention are likewise carried out in the stated order.

In the plant for dealkylation of alkyl-substituted aromatic hydrocarbons, nonaromatic hydrocarbons having six or more carbon atoms are, for example, aromatized in the presence of steam and a catalyst in a first step. In a second step, at least part of the product stream obtained, which comprises alkyl-substituted aromatic hydrocarbons, is reacted with the aid of hydrogen, optionally in the presence of a catalyst, to dealkylate the alkyl-substituted aromatic hydrocarbons.

After the dealkylation, the reaction product is rich in hydrogen, impurities and dealkylated aromatic hydrocarbons such as benzene or toluene. The dealkylated aromatic hydrocarbons formed and a hydrogen-comprising gas phase can be separated off from the reaction product by conventional methods.

The reaction product of the dealkylation can be conveyed from the reactor into a heat exchanger and be cooled there, preferably to from 20 to 100° C. It is advantageous to integrate the heat liberated here into the process in order, for example, to heat the feed stream to the dealkylation or other streams to be heated, for example in a vaporizer of a column. A liquid phase and a hydrogen-comprising gas phase are formed in the heat exchanger.

The liquid phase formed, which comprises the dealkylated aromatic hydrocarbon such as benzene and also excess water from the reactions, is fed to a phase separator and the organic phase is separated from the aqueous phase. The organic phase, which comprises the dealkylated aromatic hydrocarbon, can optionally be purified further, for example by distillation. The products obtained in the distillation can optionally be recirculated to the reaction steps.

Before the hydrogen-comprising gas phase is fed to the apparatus of the invention, it can be introduced into, for example, a separator or a stripping column where high-boiling hydrocarbons, preferably hydrocarbons having 4 and more carbon atoms and particularly preferably hydrocarbons having 6 and more carbon atoms are separated off. This step can, for example, be carried out in a phase separator by adiabatic depressurization. The bottom fraction obtained here, which comprises high-boiling hydrocarbons, in particular hydrocarbons having 6 and more carbon atoms, can optionally be recirculated to the dealkylation process.

The overhead fraction obtained here, which comprises essentially hydrogen and methane, forms the stream to be worked up and is, except for minor impurities, separated into at least one hydrogen-rich stream and at least one methane-rich stream in the apparatus of the invention.

The stream to be worked up preferably comprises at least 40% by volume of hydrogen and at least 15% by volume of methane. The stream to be worked up more preferably comprises from 45 to 75% by volume, particularly preferably from 50 to 70% by volume, of hydrogen and from 15 to 45% by volume, particularly preferably from 20 to 40% by volume, of methane. The stream to be worked up can additionally comprise up to 10% by volume of impurities. Impurities which may be comprised are from 1 to 5% by volume of ethane, from 0.2 to 2% by volume of nitrogen and from 0 to 7% by volume of hydrocarbons, for example from 0.3 to 4% by volume of hydrocarbons having 6 and more carbon atoms, in particular aromatic hydrocarbons, and up to 2% by volume of C1-C5-hydrocarbons. Here, the individual components of the stream to be worked up are selected so that the sum of the volumes of the individual components is not greater than 100%.

The stream to be worked up is fed to the apparatus of the invention at a pressure of up to 100 bar, preferably up to 70 bar and particularly preferably up to 60 bar. The volume flow of the stream which is fed in and is to be worked up is up to 50 000 standard m3/h, preferably up to 40 000 standard m3/h and particularly preferably up to 35 000 standard m3/h. The temperature of the stream to be worked up can be up to 100° C. and is preferably in the range from 20 to 50° C. The stream to be worked up particularly preferably has a temperature in the range from 30 to 40° C.

The apparatus of the invention firstly provides a first cooling stage for the work-up of the stream to be worked up. This cooling stage comprises at least one heat exchanger which comprises at least one feed line for introduction of a stream to be worked up and to be cooled and at least one outlet for discharging a cooled stream to be worked up. In a preferred embodiment, the heat withdrawn from the stream to be cooled and to be worked up is utilized in the work-up process by heating at least one stream to be heated before exit from the apparatus. In this way, worked up streams, in particular, can be heated before leaving the apparatus and thus be reused directly in process steps, for example for dealkylation or for heating.

The heat exchanger can be configured as a plate, helical or shell-and-tube heat exchanger. The stream to be cooled and worked up can be conveyed in cocurrent, countercurrent, cross-cocurrent or cross-countercurrent relative to the stream to be heated. Here, cross-countercurrent refers to a mode of operation in which the materials overall flow toward and past one another but cross at least once on their way. Analogously, cross-cocurrent refers to a mode of operation in which streams flowing in the same direction cross at least once. Preference is given to the countercurrent mode of operation.

The heat transfer power of the heat exchanger can be in the range from 100 to 600 kW, preferably from 200 to 500 kW and particularly preferably from 250 to 450 kW. The heat transfer power of the heat exchanger depends critically on the heat transfer coefficient of the transfer surface provided, the volume flow of the streams and the desired average temperature difference between the cooled stream to be worked up and the stream to be heated.

The heat transfer coefficient is determined, inter alia, by the thermal conductivity of the material used. Here, a high thermal conductivity of the material used has a positive effect on the heat transfer power. The heat exchanger or at least the surfaces participating in heat transfer can be made of metals, preferably steel such as an unalloyed or low-alloy, ferritic steel, in particular stainless steel, copper, aluminum, and also glass, plastic, enamel, silicon carbide or combinations thereof. In addition, an increased transfer area leads to a better heat transfer power. For example, the plates of a plate heat exchanger or tubes of a shell-and-tube heat exchanger can have fins in order to provide an increased heat transfer power in the smallest possible volume.

In a preferred embodiment of the apparatus proposed according to the invention, the heat exchanger is configured so that the average temperature difference between the cooled stream to be worked up and the stream to be heated is from 0.5 to 10° C., preferably from 1 to 9° C., particularly preferably from 2 to 8° C. Thus, the stream to be worked up can be cooled from an inlet temperature of, for example, 35° C. to an outlet temperature of, for example, 5° C., while the at least one stream to be heated, in particular at least one worked up stream, can be heated from, for example, 0° C. to 40° C.

To achieve average temperature differences in the abovementioned range, a heat exchanger having a moderate heat transfer power can be sufficient. Preference is given to using a plate heat exchanger made of stainless steel. As an alternative, it is also possible to use a heat exchanger made of, for example, aluminum which has a higher thermal conductivity than stainless steel. In this case, a smaller transfer area compared to a stainless steel heat exchanger can be sufficient, although aluminum is more sensitive to corrosion because of, for example, ammonia- or sulfur-comprising residues in the stream to be worked up. Relatively large heat transfer areas can be achieved more cheaply in the case of materials such as aluminum than in the case of stainless steel.

In a first step of the process of the invention, the stream to be worked up is introduced into a first cooling stage in order to cool it. Here, the stream to be worked up is firstly cooled by means of a heat exchanger of the above-described type. In an embodiment of the process proposed according to the invention, the stream to be worked up is cooled from an inlet temperature of not more than 100° C., preferably in the range from 20 to 50° C. and particularly preferably in the range from 30 to 40° C., to a temperature of not more than 15° C., preferably in the range from −5 to 10° C. and particularly preferably in the range from 0 to 7° C. In return, at least one stream to be heated, in particular at least one worked up stream, can be heated from, for example, 0° C. to 40° C.

In the apparatus proposed according to the invention, it is advantageous for the heat exchanger to be followed by a separation unit in order to separate off corrosive and high-boiling materials from the stream to be worked up. For example, ammonia-comprising materials, sulfur-comprising materials, in particular hydrogen sulfides, or both can be comprised in the ppm range in the stream to be worked up and can damage the subsequent components, in particular the cryogenic gas separation unit. In addition, high-boiling materials such as water and hydrocarbons having 4 and more carbon atoms can freeze in the cryogenic gas separation unit at temperatures below −100° C. and damage the cryogenic gas separation unit by formation of ice.

The separation unit can be made up of one part or a plurality of parts. In a preferred embodiment, the separation unit comprises at least one phase separator and/or at least one gas purification unit.

To separate off condensed components of the cooled stream to be worked up after passage through the heat exchanger, the heat exchanger can be followed by at least one phase separator. In this, the condensed fraction of the gas to be worked up can, after cooling by means of the heat exchanger, be separated by, for example, adiabatic depressurization from the gas phase of the stream to be worked up. In this way, high-boiling components, in particular high-boiling hydrocarbons, preferably hydrocarbons having 4 and more carbon atoms, particularly preferably hydrocarbons having 5 and more carbon atoms and very particularly preferably hydrocarbons having 6 and more carbon atoms, and also corrosive materials, in particular ammonia-comprising materials and sulfur-comprising materials, can be removed from the stream to be worked up.

To achieve improved removal of corrosive components in the phase separator, a scrubbing liquid which binds constituents of the stream to be worked up can be introduced into the stream to be worked up before it enters the heat exchanger. The constituents of the stream to be worked up which go over can be solid, liquid and gaseous components. As scrubbing liquid which takes up, in particular, corrosive components of the stream to be worked up can be a pure solvent such as water or steam or a suspension such as calcium hydroxide solution Ca(HO)2.

In addition or as an alternative, a gas purification unit can be provided within the separation unit. This serves to separate off high-boiling constituents, in particular high-boiling hydrocarbons, preferably hydrocarbons having 4 and more carbon atoms, for example toluene, and corrosive constituents such as water from the stream to be worked up. The gas purification unit can be configured as an adsorptive gas purification unit.

The adsorptive gas purification unit can be designed as a temperature- or pressure-swing adsorption. In contrast to temperature-swing adsorption, in which the regeneration of the adsorber (desorption) is carried out by means of a temperature increase, the regeneration in pressure-swing adsorption takes place at a reduced pressure. To operate the adsorptive separation process (pseudo)continuously, it is possible to provide at least two adsorbers which are operated in parallel and of which at any time at least one is in the adsorption phase and at least one is in the regeneration phase.

In the apparatus proposed according to the invention, the gas purification unit can be configured as an adsorptive gas purification unit which is, in particular, designed for carrying out a continuous temperature-swing adsorption. Examples of materials to be adsorbed in the temperature-swing process are water, hydrogen sulfide and/or hydrocarbons, in particular hydrocarbons having 4 and more carbon atoms. Adsorbents can be molecular sieves, in particular zeolites, silica gel and/or aluminum oxide, with different molecular sieves being able to be combined depending on the use.

In the process proposed according to the invention, the heat exchanger for cooling the stream to be worked up is followed by a separation unit in order to obtain a stream rich in hydrogen and methane. For example, corrosive and/or high-boiling components can be separated off in the separation unit from the stream to be worked up. The removal of corrosive components and/or high-boiling components can, as described above, be carried out in one step or a plurality of steps.

In a preferred embodiment of the process of the invention, it is possible, as described above, to use at least one phase separator and/or at least one gas purification unit for separating off corrosive components and/or high-boiling components. In the phase separator, high-boiling constituents, in particular high-boiling hydrocarbons, preferably hydrocarbons having 4 and more carbon atoms, particularly preferably hydrocarbons having 5 and more carbon atoms and very particularly preferably hydrocarbons having 6 and more carbon atoms, can, for example, be removed from the stream to be worked up. In addition, corrosive components in the stream to be worked up, e.g. ammonia-comprising materials and sulfur-comprising materials, can be bound by introduction of a scrubbing liquid before entry into the heat exchanger. The corrosive components of the cooled stream to be worked up can, after the stream has passed through the heat exchanger, be separated off in a phase separator. To effect further purification of the stream to be worked up, it is possible to use a continuously operated temperature-swing adsorption in order, for example, to separate water vapor from the stream to be worked up. For example, water, hydrogen sulfide and/or hydrocarbons, in particular hydrocarbons having 4 and more carbon atoms, are removed adsorptively in the adsorption operated by the temperature-swing method using molecular sieves, in particular zeolites, silica gel and/or aluminum oxide as adsorbents.

The stream rich in hydrogen and methane which is extracted from the separation unit is preferably essentially free of corrosive components and/or high-boiling components such as high-boiling hydrocarbons, in particular hydrocarbons having 4 and more carbon atoms, ammonia-comprising materials, sulfur-comprising materials and water, with the stream rich in hydrogen and methane comprising only traces of a few ppm of corrosive and freezable impurities.

In an embodiment of the apparatus proposed according to the invention, the cooling unit comprises at least one refrigeration unit. In the refrigeration unit, indirect cooling of the stream to be worked up occurs, with the refrigeration unit taking up heat at below ambient temperature and giving it off at a higher temperature. Compression and/or sorption refrigeration machines to which the required energy is supplied entirely as mechanical work or in the form of heat are typically used for this purpose. Electrically operated refrigeration units are also conceivable.

The cooling unit can have a cooling power of from 10 to 500 kW, preferably from 50 to 300 kW, particularly preferably from 80 to 200 kW. By means of these powers, the hydrogen- and methane-rich stream which has been precooled in the heat exchanger and after passing through the separation unit can have a temperature in the range from 0 to 15° C. can be cooled to a constant temperature level in the range from −10 to 5° C. Here, the constant temperature level comprises small deviations of less than 2° C.

In a particularly preferred embodiment of the apparatus of the invention, the cooling unit is located directly upstream of the cryogenic gas separation unit. This is particularly advantageous since the first cooling stage ensures an essentially constant temperature before entry into the cryogenic gas separation unit and thus stabilizes the process of cryogenic gas fractionation. Furthermore, a simply constructed cooling unit having moderate power is sufficient to achieve stabilization of the process.

In an embodiment, the refrigeration unit comprises a compression refrigeration machine which is equipped with at least one compression element, at least one expansion element and at least two heat exchangers. Here, the compression element can be realized by means of a mechanical compressor, for example a conventional compressor. The expansion element can be configured as throttle device, for example as an expansion valve. For the operating circuit, a compression element and an expansion element and two heat exchangers can be connected in a circuit so that the heat exchangers are located on both sides between compression element and expansion element. As refrigerant for the thermodynamic cycle, it is possible to use, for example, carbon dioxide (CO2) or ammonia (NH3).

In an embodiment of the process of the invention, a cooling unit of the above-described type is used. The stream rich in hydrogen and methane can be cooled by means of the cooling unit having a cooling power in the range from 10 to 200 kW to an essentially constant temperature level. For the present purposes, an essentially constant temperature level is a temperature level of the stream which is constant to within small deviations of less than 2° C. The cooling by means of the cooling unit is preferably carried out directly before the separation of the stream rich in hydrogen and methane in the cryogenic gas separation unit. In this way, the hydrogen- and methane-rich stream which has been precooled by the heat exchanger can be cooled by up to 30° C., preferably by from 5 to 10° C., with a simply constructed cooling unit having a moderate power being able to be used. In addition, the first cooling stage ensures an essentially constant temperature level before entry into the cryogenic gas separation unit, which stabilizes the work-up process, in particular the cryogenic gas separation.

Preference is given to using a compression refrigeration unit for cooling the stream rich in hydrogen and methane. As refrigerant in the compression refrigeration unit, it is possible to use, for example, carbon dioxide (CO2) or ammonia (NH3).

The separation into at least one hydrogen-rich stream and at least one methane-rich stream in the present invention takes place in the cryogenic gas separation unit. The synthesis gas is typically cooled, by means of expansion units and by indirect heat exchange with streams to be heated, to such an extent that partial condensation occurs to form at least one methane-rich liquid fraction and at least one hydrogen-rich gas fraction which are subsequently separated in a phase separator. The methane-rich liquid fraction can be revaporized and heated and taken in gaseous form from the cryogenic gas separation unit.

In an embodiment, the cryogenic gas separation unit comprises a cascade of heat exchangers and/or expansion units which are enclosed by a thermally insulated housing. The cascade of heat exchangers and/or expansion units necessary for carrying out the condensation process is usually arranged in a steel housing and insulated with perlite in order to minimize input of heat. Furthermore, to prevent synthesis gas components leaking from the cryogenic gas separation unit from accumulating within the housing and to prevent ice formation due to intrusion of surrounding air, the interior of the housing can be continuously flushed with a stream of nitrogen at a somewhat elevated pressure.

The cryogenic gas separation unit is usually operated under cryogenic conditions and exploits the Joule Thomson effect and low-temperature cooling in order to separate hydrogen from methane. In this way, the stream rich in hydrogen and methane can be cooled in the cryogenic gas separation unit to very low temperatures of less than −100° C., with cooling down to −180° C., preferably in the range from −150 to −165° C., being able to be achieved. At these temperatures, methane condenses from the gaseous stream and the methane fraction can be taken off as a liquid fraction in a phase separator.

In a preferred embodiment, a plurality of expansion stages, preferably two expansions, are provided in the cryogenic gas separation unit. Methane-rich streams having different pressures can be formed as a result. Thus, the methane-rich liquid fraction formed in the first expansion can be discharged and subsequently be revaporized, warmed and discharged in gaseous form from the gas separation unit. This methane-rich stream can have a pressure in the range from 5 to 10 bar. In a second expansion stage, the stream rich in hydrogen and methane can be cooled to a lower temperature than in the first expansion stage. This gives a second methane-rich stream which has a pressure in the range from 0 to 4 bar. Finally, the gas fractions from the two expansion stages are combined and form the hydrogen-rich stream.

Overall, at least one hydrogen-rich stream and at least one methane-rich stream are obtained at the outlet of the cryogenic gas separation unit. In a preferred embodiment, these streams are heated in the heat exchanger of the first cooling stage by means of the stream to be worked up. In this way, the worked up hydrogen-rich stream and the worked up methane-rich stream can be heated before recirculation to processes such as the dealkylation.

The methane-rich stream comprises at least 70% by volume, preferably in the range from 75 to 95% by volume, of methane. In addition, up to not more than 10% by volume, preferably up to not more than 5% by volume, of hydrogen can be comprised in the methane-rich stream. Further impurities, in particular ethane, ethene, propene, propane, nitrogen, oxygen and carbon monoxide can be comprised in a proportion of not more than 20% by volume, preferably not more than 15% by volume, in the methane-rich stream.

In an embodiment of the proposal according to the invention, a plurality of methane-rich streams which can have different pressures in the range from 0 to 20 bar and in each case come from one of the plurality of expansion stages of the cryogenic gas separation unit are obtained. Thus, for example, a methane-rich stream having a pressure of from 0 to 4 bar (low-pressure stream) and a methane-rich stream having a pressure of from 5 to 10 bar (high-pressure stream) can be produced. In such a combination, the composition of the high-pressure stream and of the low-pressure stream can differ in that the low-pressure stream is obtained in the cryogenic gas separation unit from a condensate having a relatively low temperature. Thus, the low-pressure stream can have proportions of gases, for example oxygen, which condense in a temperature range which is, for example, achieved only after the second expansion.

The work-up according to the invention makes it possible to obtain a hydrogen-rich stream having a purity of at least 85% by volume, preferably in the range from 90 to 95% by volume, from the stream to be worked up. In addition, up to 10% by volume, preferably up to 8% by volume, of methane can be comprised in the hydrogen-rich stream. Further impurities, in particular nitrogen and carbon monoxide, can be comprised in a proportion of not more than 5% by volume, preferably not more than 2% by volume, in the hydrogen-rich stream.

After leaving the cryogenic gas separation unit, the hydrogen-rich stream can have an exit pressure corresponding to the inlet pressure upstream of the cryogenic gas separation unit. The pressure can consequently be up to 100 bar, preferably up to 70 bar and particularly preferably up to 60 bar.

The at least one methane-rich stream can be utilized as heating gas, for example for firing a steam cracking furnace. The joule value of the at least one methane-rich stream can be from 35 000 to 50 000 kJ/m3, preferably from 40 000 to 45 000 kJ/m3. The joule value is a measure of the specifically useable quantity of heat obtainable from a fuel, without taking into account any heat of condensation. The at least one hydrogen-rich stream typically has a lower joule value of from 10 000 to 20 000 kJ/m3 and is preferably reused in the hydrodealkylation of alkyl-substituted hydrocarbons.

EXAMPLES

An illustrative embodiment of the invention is shown as a process flow diagram in FIG. 1. The process of the invention and the associated apparatus 100 for working up a stream of material are, in the present example, described in the form of a product stream 1.1 from a hydrodealkylation plant. After the dealkylation, the reaction product is typically rich in dealkylated aromatic hydrocarbons, hydrogen and methane.

The aromatic hydrocarbons and a recycle gas to be worked up are therefore firstly separated off from the reaction product of the hydrodealkylation. After a prepurification stage in which high boilers having a boiling point of more than 50° C. are separated off, the recycle gas 1.1 to be worked up can, with addition of water 2, firstly be introduced into the apparatus 100 according to the invention. The addition of water makes it possible to bind corrosive compounds such as ammonia and sulfur-comprising materials in water and, after going through a heat exchanger KS1, separate them off in a restrictor A1.

A volume flow of from about 30 000 to 35 000 standard m3/h of recycle gas 1.2 is fed at a pressure of from about 50 to 60 bar and a temperature of from 30 to 40° C. to the apparatus 100 according to the invention. An illustrative composition of the recycle gas 1.2 before entry into the apparatus 100 is shown in Table 1. This comprises essentially hydrogen and methane together with relatively low concentrations of, for example, ethane, ethene, propane, C4- and C6-hydrocarbons (in Table 1, abbreviated as C4—HC and C6—HC, respectively), oxygen, nitrogen and carbon monoxide as impurities.

TABLE 1 Recycle gas (% by volume) i- n- 1,3- Hydrogen Methane Ethane Ethene Propane Propene Butane Butane Butadiene Min. 53.70 31.01 1.61 0.01 0.03 0.00 0.00 0.00 0.01 value Average 60.43 35.20 2.99 0.03 0.05 0.00 0.00 0.00 0.01 value Maxim. 66.89 40.84 4.68 0.04 0.09 0.00 0.00 0.00 0.01 value Carbon Gas Lower Upper joule C4HC C6HC Oxygen Nitrogen monoxide density joule value value Min. value 0.00 0.42 0.34 0.34 0.09 0.3132 19435 22068 Average 0.00 0.91 0.40 0.64 0.17 0.3789 21954 24774 value Maxim. 0.00 3.47 0.46 1.83 0.20 0.5141 27403 30620 value

The first cooling stage of the apparatus 100 according to the invention comprises a stainless steel plate heat exchanger KS1 having a transfer power of at least 300 kW, in order to cool the recycle gas 1.2 to a temperature in the range from 3 to 7° C. In a subsequent restrictor A1, components of the recycle gas 2 condensed by cooling are separated off. Here, it is first and foremost corrosive components and high-boiling hydrocarbons such as hydrocarbons having 4 and more carbon atoms, preferably 5 and more carbon atoms and particularly preferably 6 and more carbon atoms, which are condensed and can optionally be recirculated to the dealkylation process.

Within the first cooling stage of the apparatus 100 according to the invention, further purification steps A which filter high-boiling components such as water from the recycle gas 4 can be carried out. This is of particular interest in respect of the cryogenic gas separation unit KS3 used for the low-temperature fractionation and the components of the cryogenic gas separation unit in order to increase the life and reduce the need for maintenance. The recycle gas 4 is dried in an adsorption unit A2, A2′ in which water is adsorbed using molecular sieves. In addition, high-boiling hydrocarbons, in particular hydrocarbons having 4 and more carbon atoms, are adsorbed in the adsorption unit A2, A2′ using further adsorbents known to those skilled in the art. The regeneration of the adsorbents is effected by desorption by means of hydrogen at elevated temperature. To achieve a continuous process in the apparatus 100, two adsorption units A2 and A2′ are therefore operated in parallel so that a functional adsorption unit A2, A2′ is available during the regeneration phase. After going through the individual separation stages A, which can also be configured differently from the embodiment shown in FIG. 1, the recycle gas 5 comprises essentially hydrogen and methane with less than 10% by volume of other impurities.

As last step of the first cooling stage, the recycle gas 5 is cooled by means of a refrigeration unit KS2 to a temperature level in the region of 0° C. and thus fixes the temperature of the recycle gas 6 at the inlet to the cryogenic gas separation unit KS3. This makes it possible to ensure that the temperature of the recycle gas 6 at the inlet into the cryogenic gas separation unit KS3 is essentially constant and any temperature fluctuations due, for example, to the temperature change in the regeneration in the adsorption unit A2 or A2′ can be prevented.

In the cryogenic gas separation unit KS3, which represents the second cooling stage of the work-up process of the invention, the low-temperature fractionation of the recycle gas 6, which comprises essentially hydrogen and methane, is carried out. The recycle gas after prepurification by means of the units A typically comprises somewhat more than 60% by volume of hydrogen and somewhat more than 35% by volume of methane. In the cryogenic gas separation unit KS3, the recycle gas 6 is cooled by means of cascades of heat exchangers and expansion stages to very low temperatures in the range from −150 to −165° C., with the dew point of methane being attained at these low temperatures. The very low temperatures in the cryogenic gas separation unit KS3 are typically reached in two expansions. Details of the cryogenic gas separation unit are not shown in FIG. 1. In a first expansion to very low temperatures, a first liquid fraction of methane is separated off from the recycle gas 6. This first liquid fraction is, before leaving the cryogenic gas separation unit KS3, preferably reheated in heat exchangers against streams to be cooled and fed in gaseous form under a pressure in the range from 5 to 10 bar back into the heat exchanger KS1 where further heating against the feed stream 1.2 to be cooled takes place.

TABLE 2.1 Methane-rich stream (high-pressure) (% by volume) n- 1,3- Hydrogen Methane Ethane Ethene Propane Propene i-Butane Butane Butadiene Min. 4.14 80.10 5.38 0.04 0.03 0.01 0.01 0.00 0.01 value Average 4.61 85.53 8.94 0.08 0.15 0.01 0.01 0.00 0.01 value Maxim. 4.85 88.70 14.68 0.13 0.22 0.01 0.01 0.00 0.01 value Carbon Gas Lower Upper joule C4HC C6HC Oxygen Nitrogen monoxide density joule value value Min. value 0.00 0.00 0.00 0.24 0.13 0.7543 37197 41174 Average 0.00 0.00 0.00 0.49 0.19 0.7724 38091 42140 value Maxim. 0.00 0.00 0.00 0.70 0.23 0.8013 39489 43644 value

The gaseous fraction formed after the first expansion is cooled further in a second expansion in order to obtain even lower temperatures and separate off further amounts of methane from the gaseous fraction. After the second expansion, a second liquid fraction of essentially methane is again formed. The second liquid fraction is also warmed again by means of streams to be cooled in the heat exchanger cascades of the cryogenic gas separation unit KS3 before leaving the cryogenic gas separation unit KS3. The gaseous fraction formed after the second expansion comprises essentially hydrogen and thus forms the stream of reusable pure gas 7 which is likewise warmed by means of streams to be cooled before leaving the cryogenic gas separation unit KS3.

After the low-temperature fractionation, a total of three worked up streams thus leave the cryogenic gas separation unit KS3: two methane-rich streams 9 and 8 at differing pressures and a hydrogen-rich stream 7. These are present in the gaseous phase and all have a temperature in the region of the inlet temperature of about 0° C. as has been achieved by means of the refrigeration unit KS2 before entry into the cryogenic gas separation unit KS3.

The worked up methane-rich stream 9 resulting from the first expansion is typically under a pressure in the range from 5 to 10 bar. The worked up methane-rich stream 8, on the other hand, comes from the second expansion and therefore has a lower pressure in the range from 0 to 4 bar. Illustrative compositions of the streams 8 and 9 are shown in Tables 2.1 and 2.2.

TABLE 2.2 Methane-rich stream (low-pressure) (% by volume) i- n- 1,3- Hydrogen Methane Ethane Ethene Propane Propene Butane Butane Butadiene Min. 4.14 81.02 4.94 0.02 0.02 0.01 0.01 0.00 0.00 value Average 4.65 85.69 8.72 0.08 0.15 0.01 0.01 0.00 0.00 value Maxim. 4.95 89.32 13.96 0.12 0.23 0.01 0.01 0.00 0.00 value Carbon Gas Lower joule Upper joule C4HC C6HC Oxygen Nitrogen monoxide density value value Min. value 0.00 0.00 0.12 0.35 0.14 0.7458 36883 40845 Average 0.00 0.00 0.16 0.53 0.19 0.7718 38044 42087 value Maxim. 0.00 0.00 0.19 1.03 0.22 0.7970 39284 43426 value

The worked up methane-rich streams 8, 9 are, before further use, recirculated as heating gas to the heat exchanger KS1 and heated against the stream 1.2 to be cooled. The heated streams 12 and 13 are subsequently combined to form a total stream 15, with the pressure conditions of the stream 13 being matched to those of the stream 12.

Table 3 lists an illustrative composition of the hydrogen-rich stream 7 which forms the reusable pure gas 7. This stream has a temperature and a pressure which correspond to the conditions of the hydrogen-rich stream 6 before entry into the cryogenic gas separation unit KS3. This hydrogen-rich stream 6 typically has a pressure in the range from 40 to 80 bar.

TABLE 3 Pure gas (% by volume) 2-Me- Propane (i- n- 1,3- Hydrogen Methane Ethane Ethene Propane Propene Butane) Butane Butadiene Min. 90.64 6.33 0.00 0.00 0.00 0.00 0.00 0.00 0.00 value Average 91.68 7.68 0.00 0.00 0.00 0.00 0.00 0.00 0.00 value Maxim. 93.47 8.74 0.00 0.00 0.00 0.00 0.00 0.00 0.00 value Lower Carbon Gas joule Upper joule C4HC C6HC Oxygen Nitrogen monoxide density value value Min. 0.00 0.00 0.00 0.06 0.01 0.1318 12344 14424 value Average 0.00 0.00 0.00 0.50 0.14 0.1454 12657 14763 value Maxim. 0.00 0.00 0.00 0.73 0.18 1.1525 12929 15057 value

After separation of the recycle gas 1.1 into two methane-rich streams and a hydrogen-rich stream, the streams 7, 8, 9 are recirculated to the heat exchanger and heated against the stream 1.2 to be cooled. The heated pure gas 11 is subsequently introduced in the total stream 14 into the plant for dealkylation of alkyl-substituted aromatic hydrocarbons.

Furthermore, part of the reusable pure gas 7 in the embodiment shown in FIG. 1 is branched off before heating in the heat exchanger KS1 and is used for regeneration of the adsorption unit A2 or A2′. To regenerate the adsorption unit A2 or A2′, the substream 10.1 of the pure gas 7 is fed into a heater A3, for example an electric heater, and heated to a temperature of from 150 to 350° C., preferably from 200 to 260° C. In other embodiments, the substream of the reusable pure gas can also be branched off from stream 11 after heating in the heat exchanger KS1 and be fed to a heater.

The stream 10.2 is subsequently fed into the adsorption unit A2 or A2′ to be regenerated. In addition, it can be provided for part of the stream 10.2 to be added to the total stream of the heating gas 15. After regeneration, the stream 10.3 is added to the heated stream of reusable pure gas 11 and the total stream 14 is fed to the plant for dealkylation of alkyl-substituted aromatic hydrocarbons.

To be able to reuse the three worked up streams 7, 8, 9 as directly as possible after the apparatus 100, these are in each case heated in the heat exchanger KS1 of the first cooling stage against the stream 1.2 to be cooled and the methane-rich streams 12, 13 and the hydrogen-rich stream 11 leave the apparatus at temperatures of more than 20° C. In this way, the apparatus 100 according to the invention can be operated efficiently in the recycle mode, with virtually all resources being reused. In particular, the hydrogen-rich stream 11 can be recirculated to the hydrodealkylation (not shown). The two methane-rich streams 12 and 13 can be used as heating gas, for example for firing a steam cracking furnace (not shown).

LIST OF REFERENCE NUMERALS

  • 1.1, 1.2 stream to be worked up
  • 2 water
  • 3 stream after heat exchanger
  • 4 stream after restrictor
  • 5 stream after separation stage
  • 6 stream after cooling unit
  • 7 hydrogen-rich pure gas after cryogenic gas separation unit
  • 8 methane-rich heating gas (low-pressure)
  • 9 methane-rich heating gas (high-pressure)
  • 10.1 regeneration stream of pure gas
  • 10.2 heated regeneration stream of pure gas
  • 10.3 recycle stream of pure gas after regeneration
  • 11 hydrogen-rich pure gas after heat exchanger
  • 12 methane-rich heating gas (low-pressure) after heat exchanger
  • 13 methane-rich heating gas (high-pressure) after heat exchanger
  • 14 total stream of pure gas
  • 15 total stream of heating gas
  • 100 apparatus for working up a stream of material
  • KS1 heat exchanger
  • KS2 refrigeration unit
  • KS3 cryogenic gas separation unit
  • A separation stage
  • A1 phase separator
  • A2, A2′ adsorptive separation unit
  • A3 heater

Claims

1. An apparatus (100) for working up a hydrogen- and methane-comprising stream (1.1), which comprises the following components:

(i) at least one heat exchanger (KS1) for cooling a stream (1.1) to be worked up;
(ii) at least one separation unit (A, A1, A2, A2′) for purifying the stream (3) to be worked up to give a stream (5) rich in hydrogen and methane;
(iii) at least one cooling unit (KS2) for cooling the stream (5) rich in hydrogen and methane; and
(iv) at least one cryogenic gas separation unit (KS3) for separating the stream (6) rich in hydrogen and methane into at least one hydrogen-rich stream (7) and at least one methane-rich stream (8, 9).

2. The apparatus (100) according to claim 1, wherein the stream (1.1, 1.2, 3) to be worked up comprises at least 40% by volume of hydrogen and at least 15% by volume of methane.

3. The apparatus (100) according to claim 1, wherein the at least one heat exchanger (KS1) is configured as a plate, helical or shell-and-tube heat exchanger.

4. The apparatus (100) according to claim 1, wherein the heat exchanger (KS1) is made of steel, copper, aluminum, glass, plastic, enamel and/or silicon carbide.

5. The apparatus (100) according to claim 1, wherein the separation unit (A, A1, A2, A2′) comprises at least one phase separator (A1) and/or at least one gas purification unit (A2, A2′).

6. The apparatus (100) according to claim 5, wherein the gas purification unit (A2, A2′) is configured as an adsorptive gas purification unit.

7. The apparatus (100) according to claim 5, wherein the gas purification unit (A2, A2′) is configured as a continuously operated temperature-swing adsorption.

8. The apparatus (100) according to claim 1, wherein the cooling unit (KS2) is located directly upstream of the cryogenic gas separation unit (KS3).

9. A process for working up a hydrogen- and methane-comprising stream (1.1, 1.2), which comprises the following steps:

(a) cooling of a stream (1.1, 1.2) to be worked up in at least one heat exchanger (KS1);
(b) purification of the stream (3) to be worked up to give a stream (5) rich in hydrogen and methane in at least one separation unit (A, A1, A2, A2′);
(c) cooling of the stream (5) rich in hydrogen and methane in at least one cooling unit (KS2);
(d) separation of the stream (6) rich in hydrogen and methane into at least one hydrogen-rich stream (7) and at least one methane-rich stream (8, 9) in at least one cryogenic gas separation unit (KS3).

10. The process according to claim 9, wherein the stream (1.1, 1.2) to be worked up is cooled by means of the at least one heat exchanger (KS1) from an inlet temperature of not more than 100° C. to a temperature of not more than 15° C.

11. The process according to claim 9, wherein corrosive and/or high-boiling components are separated off in the separation unit (A, A1, A2, A2′) from the stream (1.1, 1.2, 3) to be worked up.

12. The process according to claim 9, wherein the cooling unit (KS2) cools the stream (5) rich in hydrogen and methane to an essentially constant temperature level.

13. The process according to claim 9, wherein the cooling by means of the cooling unit (KS2) is carried out directly before the separation of the stream (6) rich in hydrogen and methane in the cryogenic gas separation unit (KS3).

14. The process according to claim 9, wherein the stream (6) rich in hydrogen and methane is cooled to a temperature of less than −100° C. in the cryogenic gas separation unit.

15. The process according to claim 9, wherein the hydrogen-rich stream (7) is reused in a process for the dealkylation of alkyl-substituted aromatic hydrocarbons.

16. The process according to claim 9, wherein the methane-rich stream (8, 9) is utilized as heating gas.

17. The process according to claim 9, wherein the hydrogen- and methane-comprising stream (1.1, 1.2) is taken from a process for the dealkylation of alkyl-substituted aromatic hydrocarbons.

Patent History
Publication number: 20130174604
Type: Application
Filed: Dec 20, 2012
Publication Date: Jul 11, 2013
Applicant: BASF SE (Ludwigshafen)
Inventor: Basf SE (Ludwigshafen)
Application Number: 13/721,334
Classifications
Current U.S. Class: Natural Gas (62/618)
International Classification: F25J 3/08 (20060101);