Patent Application
20170152219
The synthesis of urea (H2N—CO—NH2) requires two important reactants, namely CO2 and ammonia (NH3).
The synthesis of ammonia for manufacturing the required ammonia, and the synthesis of urea can in this case be based on a steam reformation that provides the required hydrogen for the synthesis of ammonia or CO2 for the synthesis of urea. CO2 manufactured in this case is customarily removed by means of a scrubbing method, wherein the CO2 is generally manufactured during the regeneration of the scrubbing medium loaded with CO2. For this purpose, the loaded scrubbing medium is typically heated at relatively low pressure, in such a manner that there is a high energy requirement for compressing the CO2 required in the synthesis of urea.
Ammonia and CO2 are the main reactants for the synthesis of urea. Ammonia is usually produced by means of air-fed ATR reformers, wherein manufactured H2 and N2 are mixed, followed by a shift reactor and a methanization reactor for converting all of the CO to methane. CO2 and methane are thereafter separated from the H2—N2 mixture. The H2—N2 mixture is then compressed and converted in the ammonia reactor. In an alternative layout, ammonia can be manufactured by reacting hydrogen from a steam reformer with nitrogen from an air separation unit. This layout requires, in addition to an air separation unit, a conventional hydrogen system. Hydrogen and nitrogen are mixed before being fed into the ammonia reactor and compressed. The advantage of the second layout is the low content in inerts of the ammonia synthesis gas.
The ammonia that is manufactured is then converted to urea by adding CO2. The CO2 manufactured in both layouts is regularly not sufficient in order to completely react the ammonia that is manufactured. Therefore, in each case CO2 is imported from external sources, where present.
The problem addressed by the invention, against this background, is to improve a method of the type cited at the outset.
This problem is solved by a method having the features described herein.
Accordingly, it is provided according to the invention that the method for producing urea (H2N—CO—NH2) comprises the steps:
For the manufacture of synthesis gas, accordingly, preferably partial oxidation (POX) is used. In this case, either a catalyst-based POX or a POX can be used which succeeds without a catalyst.
The feed gas stream preferably comprises one or more of the following components or hydrocarbons that are reacted in the synthesis gas manufacturing step to form the synthesis gas which comprises H2 and CO; natural gas, CH4, H2O, CO2.
In the partial oxidation, the preferably prepurified, in particular desulphurized (see also below) feed gas stream which comprises, e.g. natural gas or CH4, or higher hydrocarbons such as naphtha, LPG, oil or else coal, is reacted, in particular, substoichiometrically in an exothermic process. Reaction products are primarily the materials hydrogen and carbon monoxide that form the synthesis gas and are obtained according to
CnHm+(n+x)/2O2=(n−x+y/2)CO+(m−y)/2H2+xCO2+y/2 H2O
In the partial oxidation, steam can also be added as a reactant.
In said water gas-shift reaction, to which the synthesis gas stream that is manufactured by POX is subjected, according to
CO+H2O<−>CO2+H2
CO that is present in the synthesis gas is reacted with water to form carbon dioxide and hydrogen, which here is particularly advantageous, since firstly hydrogen is required for the synthesis of ammonia and CO2 is required for the synthesis of urea.
According to an embodiment of the invention, it is further provided that the carbon dioxide that is separated off in the separation is provided at a high pressure of at least 20 bar, preferably at least 30 bar, most preferably at least 50 bar.
According to an embodiment of the invention, it is further provided that the carbon dioxide that is separated off is provided at least stoichiometrically for the reaction of ammonia to form urea, in such a manner that the ammonia (NH3) is completely reacted to form urea.
According to an embodiment of the invention, it is further provided that in the temperature-swing adsorption for separating off the CO2, during one cycle time, CO2 is adsorbed from the second synthesis gas substream on an adsorber and is then desorbed, wherein the cycle time is preferably less than 360 min, preferably less than 240 min, most preferably less than 180 min.
According to an embodiment of the invention, it is further provided that for separating off the hydrogen from the first synthesis gas substream, CO2 and CO (and also, in particular CH4) present in the pressure-swing adsorption in the first synthesis gas substream are adsorbed on an adsorber at a first pressure, wherein, preferably, the adsorber is regenerated at a second pressure that is lower than the first pressure, wherein adsorbed CO2 and CO (and also, in particular CH4) are desorbed, and wherein the adsorber, for removing the desorbed CO2 and CO (and also, in particular CH4) is purged e.g. with hydrogen, with production of an corresponding off-gas.
According to an embodiment of the invention it is further provided that the off-gas of the pressure-swing adsorption is used as fuel, wherein, preferably the off-gas is burnt for heating the feed gas stream and/or for producing and/or superheating steam. In addition, off-gases can be burnt for producing energy, or optionally compressed once more and returned to the POX.
According to an embodiment of the invention, it is further provided that an off-gas produced in the temperature-swing adsorption and comprising H2 and CO (and also in particular CH4) is likewise subjected to a pressure-swing adsorption in order additionally to provide hydrogen for manufacture of the ammonia, wherein, preferably, the off-gas from the temperature-swing adsorption is subjected to said pressure-swing adsorption together with the first synthesis gas substream, and/or in that the off-gas from the temperature-swing adsorption is mixed with the off-gas (comprising CO2 and CO and, in particular, CH4) arising in the pressure-swing adsorption and used as fuel.
According to an embodiment of the invention, it is further provided that impurities (e.g. in the form of H2, CH4 and/or CO) present in the CO2 (separated off in the temperature-swing adsorption) are removed in a purification step, preferably by means of catalytic oxidation, upstream of the reaction of the CO2 with the ammonia to form urea.
According to an embodiment of the invention, it is further provided that the synthesis gas stream is cooled upstream and/or downstream of the water gas-shift reaction, wherein the synthesis gas stream is preferably cooled with water, with manufacture of process steam.
According to an embodiment of the invention, it is further provided that heat arising during the cooling is used for regenerating an adsorber in the temperature-swing adsorption.
According to an embodiment of the invention, it is further provided that the oxygen required for the POX is manufactured by cryogenic separation of air, wherein during each separation nitrogen is further manufactured which is reacted with the hydrogen to form ammonia.
According to an embodiment of the invention, it is further provided that the feed gas stream is conducted upstream of the partial oxidation through an adsorber unit, wherein one or more sulphur compounds that are still present in the feed gas stream are adsorbed in the adsorber unit and in this case removed from the feed gas stream.
According to an embodiment of the invention, it is further provided that the synthesis gas stream or the two synthesis gas substreams are dried downstream of the water gas-shift reaction and also upstream of the pressure-swing adsorption and also temperature-swing adsorption.
According to a further aspect of the invention, a plant for producing urea is proposed which has the features described below.
Accordingly, the plant for producing urea comprises:
The plant according to the invention is, furthermore, in further embodiments, characterized by the corresponding embodiments of the method according to the invention. In this respect, the plant is preferably configured in each case to carry out the corresponding method steps of the respective embodiment of the method according to the invention.
Further features and advantages of the invention will be explained hereinafter in the description of the figures of exemplary embodiments of the invention with reference to the figures.
In this case, a feed gas stream NG comprising, e.g., CH4 (e.g. in the form of natural gas), before a reaction to form synthesis gas (comprising H2 and CO) S by partial oxidation 20 is subjected to a desulphurization 30 and then, by means of partial oxidation 20, in the presence of oxygen, and also, in particular steam W, is reacted to form a synthesis gas stream S that comprises H2 and CO, and also further, in particular CH4, H2O and CO2.
The synthesis gas stream S is hereafter subjected to a water gas-shift reaction 40 (see above) and cooled with water, wherein said steam W can be manufactured. In principle, heat arising during the cooling of the synthesis gas S can also be used for regenerating the adsorbers in the temperature-swing adsorption 51 described further below (cf.
The synthesis gas stream S is in addition dried, wherein, hydrogen and carbon dioxide of the synthesis gas stream S are separated. (50), wherein the hydrogen is reacted (60) with nitrogen to form ammonia, and wherein the carbon dioxide is finally reacted with the ammonia that is manufactured to form urea.
The oxygen for the POX 20 is manufactured by cryogenic separation 10 of air L, wherein, also the nitrogen is obtained that is required for the ammonia synthesis 60.
According to
In the pressure-swing adsorption 51 for separating off the hydrogen from the first synthesis gas substream S′, CO2 and CO and also possibly further components (such as, e.g. CH4) that are present in the first synthesis gas substream are adsorbed on an adsorber at a first pressure, wherein, preferably the adsorber is regenerated at a second pressure which is lower than the first pressure, wherein the adsorber components are desorbed, and wherein the adsorber, for removing the desorbed components, is purged, with manufacture of an off-gas A. Preferably, a plurality, in particular two or four, adsorbers are used in the pressure-swing adsorption 51, in order that as far as possible one adsorber can always be operated in the adsorption mode in such a manner that hydrogen can be released semi-continuously.
The off-gas A from the pressure-swing adsorption 51 can be used, e.g. as fuel, wherein, e.g. the off-gas A can be burnt for heating the feed gas stream NG and/or for producing and/or superheating steam.
In the temperature-swing adsorption 52, CO2 is adsorbed at a low first temperature on an adsorber and desorbed at a higher second temperature, for which the necessary energy E is provided. The residual gas arising in the adsorption of CO2 and/or off-gas A′ that comprises H2 and CO, can, together with the first synthesis gas substream S′, be run into the pressure-swing adsorption 51 or can be mixed with the off-gas A from the pressure-swing adsorption 51 and, therewith, be used together as fuel.
On account of the separation according to the invention of CO2, said CO2, after the separation, is advantageously present at a high pressure of preferably at least 20 bar, and so correspondingly energy can be saved for the otherwise necessary compression of the CO2 for the purpose of urea synthesis. This is principally due to the fact that regeneration is performed during the temperature-swing adsorption by means of heating the adsorbent, and so in comparison the pressure drop occurring during regeneration in the pressure-swing adsorption is avoidable.
In addition, in the presence of the CO2 purification 53 by catalytic oxidation, CO2 arriving from the temperature-swing adsorption advantageously need not be cooled, since it must have a correspondingly elevated temperature for the catalytic oxidation.
The use of an appropriately designed catalytic oxidation can balance out the fluctuations in composition formed during the desorption and thus ensure a CO2 quality as uniform as possible. The control can be adapted, in such a manner, for example, that the oxygen requirement of the catalytic oxidation is taken into account and thus an oxygen concentration in the CO2 as constant as possible is always maintained, for example below 0.7% by volume, in particular below 0.6% by volume, or in particular <0.35% by volume. This control possibility is advantageous for the stability and energy efficiency of the subsequent urea plant. For control of the O2 content in the CO2, the desorption of the combustible components can be calculated in advance on account of the heating. Then, the amount of air can be set accordingly. There is also the possibility, e.g., of additionally measuring and controlling the O2 content in the CO2.
As a result, the invention permits the integration of known technologies such as, e.g., POX, ASU (cryogenic air separation), pressure-swing adsorption and temperature-swing adsorption, into one plant concept or method concept which can provide sufficient CO2 for urea synthesis, and so complete reaction of the ammonia that is manufactured is possible, wherein the required CO2 is provided at a high pressure level, and so a high-cost additional compression can he avoided.
| Number | Date | Country | Kind |
|---|---|---|---|
| DE 102015015524.5 | Dec 2015 | DE | national |
