Patent Application
20100199717
This invention relates to methods and systems for the separation of mixtures containing carbon dioxide, hydrocarbon, and hydrogen. The invention may be used, for example, for the separation of a syngas to produce hydrogen and nitrogen which may be used in the synthesis of ammonia.
The term “synthesis gas”, also known as “syngas” refers to a gas mixture containing carbon dioxide and/or monoxide and molecular hydrogen generated by the gasification of a carbon-containing fuel to a gaseous product with a heating value. Syngas is produced, for example, by steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal and in some types of waste-to-energy gasification facilities. Syngas is used, for example, as intermediates in creating synthetic natural gas, and for producing ammonia or methanol. Syngas is also used as an intermediate in producing synthetic petroleum for use as a fuel or lubricant.
In the synthesis of ammonia from stoichiometric air (as a source of nitrogen) and hydrocarbons (as a source of hydrogen), the hydrocarbon, such as methane, is made to react with steam at elevated temperatures to generate H2, CO, CO2H2O. This produces a raw syngas containing, in addition to these compounds, residual unreacted hydrocarbon, as well as N2, and other air constituents. The N2 and H2 must then be separated from the other components of the syngas for the generation of ammonia.
There are two main problems in the production of ammonia. One problem relates to the fact that excessive amounts of hydrocarbon, typically methane, remain unreacted in the conversion of the hydrocarbon to H2 and CO, so that the ammonia yield is far from optimal. This so-called hydrocarbon slip can be reduced by using high reforming temperatures. Secondly, CO2 must be removed from syngas to prevent poisoning of the catalyst used in the ammonia conversion, and this CO2 removal requires high capital costs and is also costly in terms of energy consumption.
Significant work has been applied to the development of methods for the removal of carbon dioxide from a syngas. The processes can be separated into four general classes; absorption by physical solvents, absorption by chemical solvents, adsorption by solids, and distillation.
The high relative volatility of methane with respect to carbon dioxide makes cryogenic distillation theoretically very attractive. However, the methane/carbon dioxide distillative separation has a significant disadvantage in that solid carbon dioxide exists in equilibrium with vapor-liquid mixtures of carbon dioxide and methane at particular conditions of temperature, pressure, and composition. Obviously, the formation of solids in a distillation tower has the potential for plugging the tower and its associated equipment. Increasing the operating pressure of the tower will result in warmer operating temperatures and a consequent increase in the solubility of carbon dioxide, thus narrowing the range of conditions at which solid carbon dioxide forms. However, additional increases in pressure will cause the carbon dioxide-methane mixture to reach and surpass its critical conditions. Upon reaching criticality, the vapor and liquid phases of the mixture are indistinguishable from each other and therefore cannot be separated. A single-tower equilibrium separation operating in the vapor-liquid equilibrium region bounded between carbon dioxide freezeout conditions and the carbon dioxide-methane critical pressure line may produce a product methane stream containing 10% or more carbon dioxide.
Various methods have been devised to avoid the conditions at which carbon dioxide freezes and yet obtain an acceptable separation. Two processes which utilize additives to aid in the separation are disclosed in U.S. Pat. No. 4,149,864 to Eakman et al, and U.S. Pat. No. 4,318,723 to Holmes et al.
Eakman et al discloses a process for separating carbon diokide from methane in a single distillation column. If insufficient hydrogen is present in the column feedstream, hydrogen is added to provide a concentration from about 6 to 34 mole percent, preferably from about 20 to about 30 mole percent. The separation is said to take place without the formation of solid carbon dioxide. The tower pressure is preferably held between 1025 and 1070 psia.
Holmes et al adds alkanes having a molecular weight higher than methane, preferably butane, to the tower feed to increase the solubility of carbon dioxide and decrease its freezing temperature line. The additive n-butane is added at an amount from about 5 moles to 30 moles per 100 moles of feed.
U.S. Pat. No. 4,511,382 to Valencia et al discloses separating acid gases, particularly carbon dioxide, from methane by cryogenic distillation in which an effective amount of a light gas, preferably helium, is added to a stream containing methane and carbon dioxide and cryogenically distilling the mixed stream to produce a liquid carbon dioxide stream and an enriched methane stream. The distillation tower or at least a portion thereof may then be operated at a pressure higher than the critical pressure of methane.
A process for the separation of carbon dioxide from a predominantly methane stream is described in U.S. Pat. No. 2,888,807 to Bocquet. The separation requires the use of two distillation columns arranged in series. When the carbon dioxide is present at a concentration below 8 mole percent, the feed is introduced into the first column of the series, and where the carbon dioxide is present at concentration above 8 mole percent, the feed is introduced directly into the second column of the series. The first column is operated at or below the critical temperature of methane such that feed to each column provides a carbon dioxide concentration below which, on cooling at the operating pressure of the column, would produce a solid carbon dioxide phase. Effluents from the top of the second column contain substantially the same concentration of carbon dioxide as the feeds to the first columns. The operating pressure applied to the second column is maintained above a critical pressure defined as that at which the carbon dioxide phase will exist, and above which pressure a solid carbon dioxide phase will not coexist with a vapor.
U.S. Pat. No. 7,090,816 to Malhotra et al discloses a method for the purification of syngas, such as occurs in the manufacture of ammonia, using cryogenic distillation. Refrigeration for the distillation is obtained from waste fluid expansion using a liquid expander to recover mechanical work from the waste fluid. This method reduces pressure loss in the syngas stream and reduces compression and power relative to similar ammonia generating processes.
In one of its aspects, the invention provides a method for the separation of a mixture containing H2, hydrocarbon, and CO2. In accordance with this aspect of the invention, the mixture is introduced into a distillation column having a side stream. Distillation of the mixture using a column having a side stream generates three streams, as follows:
In a preferred embodiment of the method and system of the invention, the mixture further comprises N2. In a most preferred embodiment, the H2 and N2 are present in a molar ratio of 3:1. This may be achieved by mixing the hydrocarbon with a stoichiometric amount of air, as is known in the art. Embodiments in which the H2 and N2 are present in this molar ratio are useful for generating H2 and N2 for use in the manufacture of ammonia. Thus, in another of its aspects, the invention provides a method and system for the production of ammonia. In accordance with this aspect of the invention, a mixture containing H2, N2, hydrocarbon, and CO2 is introduced into a distillation column to produce the three streams described above. The top stream comprising H2 and N2 is then used to generate ammonia by any method known in the art.
The process has several degrees of freedom allowing flexibility in determining the operating parameters of the system, such as methane slip and side stream composition, and can improve the efficiency of the raw syngas generating process.
Thus, in one of its aspects, the present invention provides a method for the separation of a mixture containing H2, hydrocarbon, and CO2, the method comprising introducing the mixture into a distillation column having a side stream to generate:
The hydrocarbon is preferably, although not necessarily, methane. In a preferred embodiment of the method of the invention, the mixture further comprises N2 which is obtained by the method in the top stream. In a most preferred embodiment, the H2 and the N2 are present in the mixture in a molar ratio of 3H2:1N2. Thus, in another of its aspects, the invention provides a method for generating ammonia.
In another of its aspects, the invention provides a system for generating and separating a mixture containing H2, hydrocarbon, and CO2, the system comprising:
The means for generating a mixture containing H2, hydrocarbon and CO2, may be any such system known in the art.
The hydrocarbon of the system is preferably, although not necessarily, methane. In a preferred embodiment of the system of the invention, the mixture further comprises N2 which is obtained by the method in the top stream. In a most preferred embodiment, the H2 and the N2 are present in the mixture in a molar ratio of 3H2:1N2. Thus, in yet another of its aspects, the invention provides a system for generating ammonia.
In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawing, in which:
The distillation column 2 generates a condenser feed stream 14 containing primarily the H2 and N2 in gaseous form. The output stream 14 is introduced into a condenser 18 that generates a liquid reflux 20 that returns to the column 2, preferably to the top tray 22 of the column. The reflux of the top stream to the column is preferably performed using a reflux ratio between 0.001 and 10, and more preferably between 0.5 and 2. It is possible to alter the temperature gradient in the column by varying the reflux rate.
The reflux 20 serves as a cooling source inside the column for the trays 8 above the feed tray 12. In the lower part of the column, where the temperature is higher, only small amounts of liquid nitrogen are present, and most of the cooling for the trays 8 below the feed tray 12 is provided by liquid hydrocarbon and liquid CO2. An overhead product stream 21 containing primarily gaseous H2 and N2 is drawn off from the condenser 18. As explained above, the overhead product stream 21 can be used in the synthesis of ammonia. The distillation column 2 also generates a reboiler feed stream 24 containing primarily hydrocarbon and CO2. The reboiler feed stream 24 is introduced into the reboiler 4. In the reboiler 4, hydrocarbon boils, and may be withdrawn as a vapor side stream 30, while liquid CO2 is withdrawn as a bottom stream 32. The liquid CO2 in the bottom stream is easier to dispose of than gaseous CO2. The reboiler generates a boilup 26 that is returned to the column 2 preferably to the bottom tray 28 of the column 2. The vapor side stream 30 can be recycled and reformed and used to generate new feed stream 3. Recycling of the hydrocarbon increases the utilization of the hydrocarbon, thus increasing the ammonia yield.
The column preferably has a pressure between 5 bar to a critical pressure of the mixture, and more preferably between 7 bar to 55 bar.
In an alternative embodiment, (not shown), a side stream is withdrawn from a tray 8 in the column 2, instead of withdrawing the side stream 30 from the reboiler 4. In another embodiment, the feed stream 3 is introduced directly into the reboiler which is set to conditions under which CO2 is a liquid at its bubble point and is withdrawn.
The method and system of the invention was implemented on the process simulator UniSim Design Version R370Build 13058 of Honeywell.
In this example, the operating parameter values shown in Table 1 were used in the simulation. The thermodynamic package used was the Peng Robinson Sour Vapor package.
Table 2 shows in Column (a) the feed stream 3 to the column 2 that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 2.
Table 3 shows the composition of the processed syngas, or overhead stream 14, as well as its flow rate and the pressure of the stream 14, as determined by the simulation.
Table 4 shows the composition of the side stream 30 that was generated.
Table 5 shows the composition of the bottom stream 32 that was generated.
Table 6 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.
Table 7 shows the energy consumption. Energy costs were calculated assuming a use of a nitrogen refrigerant utility at a cost of 1 million dollar/million kcal/hr/yr. The energy demand and cost were determined by the reflux ratio. When running the column at 15 bar the reflux composition was primarily liquid nitrogen.
Table 8 shows the steady state tray composition profile of the column (molar flows, kgmole/hr).
9.3 × 10−9
3.7 × 10−6
1.3 × 10−3
In this example, the energy consumption of the overall process, 7.15 Gcal/mton ammonia) similar to the energy consumption of existing processes. is the lowest of all the simulations that were performed.
In this example, the operating parameter values shown in Table 9 were used in the simulation. The thermodynamic package used was the Peng Robinson package.
Table 10 shows in Column (a) the feed stream 3 to the column 2 that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 2.
Table 11 shows the composition of the processed syngas, or overhead stream 14, as well as its flow rate and the pressure of the stream 14, as determined by the simulation.
Table 12 shows the composition of the side stream 30 that was generated.
Table 13 shows the composition of the bottom stream 32 that was generated.
Table 14 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.
Table 15 shows the energy consumption. Energy costs were calculated assuming a use of a nitrogen refrigerant utility at a cost of 1 million dollar/million kcal/hr/yr. For the reboiler, refrigerated brine was used as a utility at a cost of $4×104/million kcal/hr/yr. The energy demand and cost were determined by the reflux ratio. When running the column at 45 bar the reflux composition was primarily liquid nitrogen and methane.
Table 16 shows the steady state tray composition profile of the column molar flows (kgmole/hr).
A phase diagram for the five component mixture of this invention is unavailable in the literature. However, from an analysis of the phase diagram of the corresponding binary system (CO2/CH4), and the fact that a high concentration of H2 leads to an increase of the critical pressure and also to a decrease in the freezing pressure of the CO2, it can be concluded that under the conditions (pressure and temperature) of this example the working conditions of the system of the present invention in which CO2 freezing is prevented are broader than those of the binary system.
It is worth noting that for this multi-component mixture, the vapor pressure line of pure CH4 will not limit the separation boundary at low temperatures. Each of the gasses in the column of the multi-component mixture, other than the methane, has a lower critical temperature than methane.
In this example, the input stream was first cooled to a temperature of −100° C. which condenses most of the CO2 in the feed stream. The cooled feed stream was then passed though a flush allowing most of the CO2 to be removed from the other components of the feed stream, before being introduced into the column. The operating parameter values shown in Table 17 were used in the simulation. The thermodynamic package used was the Peng Robinson package.
Table 18 shows in Column (a) the feed stream 3 to the column 2, before passing through the flush, that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 18.
Table 19 shows in Column (a) the feed stream 3 to the column 2, after having passed through the flush, that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 19.
Table 20 shows the composition of the processed syngas, or overhead stream 14, as well as its flow rate and the pressure of the stream 14, as determined by the simulation.
Table 21 shows the composition of the side stream 30 that was generated.
Table 22 shows the composition of the bottom stream 32 that was generated.
Table 23 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.
Table 24 shows the energy consumption. Energy costs were calculated assuming a use of a nitrogen refrigerant utility at a cost of 1 million dollar/million kcal/hr/yr. The energy demand and cost were determined by the reflux ratio. When running the column at 15 bar the reflux composition was primarily liquid nitrogen.
Since the concentration of CO2 in the column is low due the flushing of the CO2, freezing of any CO2 in the column will not occur under the pressure 15 bar. This simulation also corresponds to a system in which the cooled feed stream is fed directly into the reboiler.
In this example, the operating parameter values shown in Table 1 were used in the simulation. The thermodynamic package used was the SRK package.
Table 26 shows in Column (a) the feed stream 3 to the column 2 that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 26.
Table 27 shows the composition of the processed syngas, or overhead stream 14, as well as its flow rate and the pressure of the stream 14, as determined by the simulation.
Table 28 shows the composition of the side stream 30 that was generated.
Table 29 shows the composition of the bottom stream 32 that was generated.
Table 30 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.
Table 31 shows the energy consumption. Energy costs were calculated assuming a use of a nitrogen refrigerant utility at a cost of 1 million dollar/million kcal/hr/yr. The energy demand and cost were determined by the reflux ratio. When running the column at 15 bar the reflux composition was primarily liquid nitrogen.
The ammonia yield of this example (628.4 ton/day) is the greatest of the presented examples. The bottom product of this example (utilizing the SRK package) contains only CO2 and CH4 as opposed to Examples 1 and 2. Thus, in Example 1 where, in addition, there were also small amounts of liquid N2 and liquid Ar, there was a relatively low bottom stream temperature (−108° C.), in comparison to the relatively high bottom stream temperature of Example 4 (−53.44° C.).
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL08/01043 | 7/29/2008 | WO | 00 | 1/29/2010 |
| Number | Date | Country | |
|---|---|---|---|
| 60952575 | Jul 2007 | US |
