US20100199717A1

US20100199717A1 - Method and system for the separation of a mixture containing carbon dioxide, hydrocarbon and hydrogen

Info

Publication number: US20100199717A1
Application number: US12/671,207
Authority: US
Inventors: Baruchi Barry Baruch Kimchi
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-07-29
Filing date: 2008-07-29
Publication date: 2010-08-12
Also published as: WO2009016625A3; WO2009016625A2

Abstract

The invention provides a method and system for the separation of a mixture containing H2, hydrocarbon, and C02. The generated mixture (3) is introduced into a distillation column (2) having a side stream to generate a top stream comprising H2 (21), a middle (volatility) stream comprising hydrocarbon (30); and a bottom stream comprising C02 (32). In a preferred embodiment, the mixture further comprises N2 which is obtained in the top stream (21). In a most preferred embodiment, the H2 and the N2 are present in a molar ratio of 3H2:1N2. The generated H2 and N2 may be used for the synthesis of ammonia. Thus, the invention also proves a method and system for the generation of ammonia.

Description

FIELD OF THE INVENTION

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.

BACKGROUND OF THE INVENTION

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 N₂and H₂must 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 H₂and CO, so that the ammonia yield is far from optimal. This so-called hydrocarbon slip can be reduced by using high reforming temperatures. Secondly, CO₂must be removed from syngas to prevent poisoning of the catalyst used in the ammonia conversion, and this CO₂removal 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.

SUMMARY OF THE INVENTION

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:

- (i) a top stream comprising H₂;
- (ii) a middle stream comprising hydrocarbon; and
- (iii) a bottom stream comprising CO₂.

In a preferred embodiment of the method and system of the invention, the mixture further comprises N₂. In a most preferred embodiment, the H₂and N₂are 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 H₂and N₂are present in this molar ratio are useful for generating H₂and N₂for 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 CO₂is introduced into a distillation column to produce the three streams described above. The top stream comprising H₂and N₂is 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:

- (i) a top stream comprising H₂;
- (ii) a middle (volatility) stream comprising hydrocarbon; and
- (iii) a bottom stream comprising CO₂.

The hydrocarbon is preferably, although not necessarily, methane. In a preferred embodiment of the method of the invention, the mixture further comprises N₂which is obtained by the method in the top stream. In a most preferred embodiment, the H₂and the N₂are 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:

- (a) means for generating a mixture containing H₂, hydrocarbon, and CO₂; and
- (b) a distillation column having a side stream configured to receive the mixture and to generate from the mixture:
  - (i) a top stream comprising H₂;
  - (ii) a middle stream comprising hydrocarbon; and
  - (iii) a bottom stream comprising CO₂.

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 N₂which is obtained by the method in the top stream. In a most preferred embodiment, the H₂and the N₂are 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.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

FIG. 1 shows a system for separating a mixture containing H₂, hydrocarbon, and CO₂, in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a system 1 for separating a feed mixture 3 containing H₂, N₂, hydrocarbon and CO₂, in accordance with one embodiment of the invention. The system 1 comprises a distillation column 2, a reboiler 4 and a condenser 18. The distillation column 2 comprises one or more trays 8, as is known in the art for distillation columns. The feed mixture 3 may be a gas, or liquid/gas mixture. The feed mixture 3 is introduced into the distillation column 2 at an inlet 10 and is deposited into a feed tray 12 of the column 2. After the mixture reaches equilibrium in the feed tray 12, the H₂and N₂, flow up the column while the CO₂is liquefied and flows to the bottom by gravity. The working pressure inside the column should be above about 5 bar in order to assure that the CO₂can exist in the liquid phase in the column. Part of the hydrocarbon entering the column is liquefied, mixed with the CO₂, and then flows down towards the bottom of the column. Gaseous hydrocarbon first rises in the column and liquefies by the liquid nitrogen, and then flows down and re-evaporates. In every tray there is a liquid/vapor equilibrium whose composition is determined by the tray's temperature.
The distillation column 2 generates a condenser feed stream 14 containing primarily the H₂and N₂in 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 H₂and N₂is 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 CO₂is withdrawn as a bottom stream 32. The liquid CO₂in 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 CO₂is a liquid at its bubble point and is withdrawn.

EXAMPLES

The method and system of the invention was implemented on the process simulator UniSim Design Version R370Build 13058 of Honeywell.

Example 1

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 1

Syn loop pressure	15 bar
primary reforming temperature	700° C.
Number of trays	12 plus condenser and reboiler
inlet temperature	−55° C. (feed stream in gaseous state)
Location of feed inlet	Tray 7 from the bottom
Reflux ratio	1.9
Side stream draw stage	Reboiler
Side stream flow rate	300 kgmole/hour

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 2

	Flow rate		partial pressure
Feed Stream	[kgmol/hr]	molar fraction - y	[atm]
(a)	(b)	(c)	(d)

CO₂	654	0.164	2.455
H₂	2206	0.552	8.280
CH₄	368	0.092	1.381
N₂	751	0.188	2.819
Ar	17.59	0.004	0.066
Total	3996.59	1.000	15

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 3

Syngas (overhead	Flow rate		partial pressure
stream):	[kgmol/hr]	molar fraction - y	[atm]
Species	(b)	(c)	(d)

CO₂	0	0	0
H₂	2207	0.748	11.22
CH₄	0.0033	1 × 10⁻⁶	1.5 × 10⁻⁵
N₂	735.6	0.249	3.735
Ar	6.131	0.004	0.06
Total	2948.75	1	15

Table 4 shows the composition of the side stream 30 that was generated.

TABLE 4

	Flow rate		partial pressure
	[kgmol/hr]	molar fraction - y	[atm]
Side Stream:	(b)	(c)	(d)

CO₂	5.4	0.018	0.27
H₂	0	0	0
CH₄	270.59	0.905	13.575
N₂	14.03	0.046	0.69
Ar	8.77	0.029	0.435
Total	298.8	1.000	15

Table 5 shows the composition of the bottom stream 32 that was generated.

TABLE 5

	Flow rate		partial pressure
Bottom stream	[kgmol/hr]	molar fraction - x	[atm]
(a)	(b)	(c)	(d)

CO₂	648.6	0.865	12.975
H₂	0	0	0
CH₄	97.41	0.13	1.95
N₂	1.372	0.00183	0.0275
Ar	2.682	0.035	0.525
Total	750.064	1.000	15

Table 6 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.

TABLE 6

ammonia product yield	593.6	ton/day

purity of the ammonia yield

0.997

Ammonia temperature	−28°	C.
Ammonia pressure	15	bar

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 7

Condenser duty	7.08 × 10⁶	Energy cost of	$7.08
	kcal/hr	Condenser	million/year
Reboiler duty	1.06610×⁶	Energy cost of	$1.0661
	kcal/hr	Reboiler	million/year

Table 8 shows the steady state tray composition profile of the column (molar flows, kgmole/hr).

TABLE 8

	CO₂		CH₄
Stage	(Vap)	H₂(Vap)	(Vap)	N₂(Vap)	Ar (Vap)	Temperature

Condenser	1.01 × 10⁻²⁶	2208	1.55 × 10⁻⁴	735.8	5.526	−185.5
12	6.21 × 10⁻²²	2484	1.17 × 10⁻²	5984	84.91	−169.9
11	3.58 × 10⁻¹⁹	2368	6.8 × 10⁻²	7123	154.7	−168.6
10	1.45 × 10⁻¹⁶	2347	0.3185	7146	232	−168.4
9	5.76 × 10⁻¹⁴	2343	1.461	7016	337.8	−168.2
8	2.3 × 10⁻¹¹	2339	6.671	6809	484.5	−167.8
7	9.3 × 10⁻⁹	2333	30.32	6486	681.7	−167.2
6	3.7 × 10⁻⁶	2321	135.6	5896	918.2	−165.3
5	1.3 × 10⁻³	2290	568.8	4577	1066	−158.5
4	0.2306	2241	1796	2274	747.8	−143.4
3	8.181	2229	2965	1125	281.8	−138.1
2	0.3702	1.156	40.59	13.89	4.67	−123.7
1	2.649	0.1531	206.2	26.98	17.31	−117.5
Reboiler	6.481	3.18 × 10⁻³	287.4	8.145	11.93	−108.3

			CH₄
Stage	CO₂(Liq)	H₂(Liq)	(Liq)	N₂(Liq)	Ar (Liq)	Temperature

Condenser	6.2 × 10⁻²²	276.4	1.15 × 10⁻²	5248	79.38	−185.5
12	3.6 × 10⁻¹⁹	160	6.78 × 10⁻²	6387	149.2	−169.9
11	1.45 × 10⁻¹⁶	139.2	0.318	6411	226.4	−168.6
10	5.75 × 10⁻¹⁴	134.8	1.46	6280	332.2	−168.4
9	2.3 × 10⁻¹¹	131.1	6.67	6073	479	−168.2
8	9.27 × 10⁻⁹	125.3	30.32	5750	676.2	−167.8
7	3.72 × 10⁻⁶	113.1	135.6	5160	912.7	−167.2
6	1.33 × 10⁻³	82.19	568.8	3841	1061	−165.3
5	0.2306	33.09	1796	1539	742.3	−158.5
4	8.181	21.44	2965	388.9	276.2	−143.4
3	655.3	1.59	412.3	22.42	19.54	−138.1
2	657.6	0.1562	578	35.5	32.19	−123.7
1	661.4	6.25 × 10⁻³	659.1	16.67	26.8	−117.5
Reboiler	648.7	1.99 × 10⁻⁵	97	0.7374	3.48	−108.3

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.

Example 2

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 9

Syn loop pressure	45 bar
primary reforming temperature	800° C.
Number of trays	10 plus condenser and reboiler
Location of feed inlet	Tray 6 from the bottom
inlet temperature	−55° C. (feed stream in gaseous state)
Reflux ratio	1.3
Side stream draw stage	Tray	4 from the bottom
Side stream flow rate	290 kgmole/hour

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 10

	Flow rate		partial pressure
Feed Stream	[kgmol/hr]	molar fraction - y	[atm]
(a)	(b)	(c)	(d)

CO₂	716	0.176	7.92
H₂	2266	0.556	25
CH₄	317	0.078	3.51
N₂	763	0.187	8.415
Ar	9.5	0.0023	0.103
Total	4071.5	1	45

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 11

Syngas (overhead	Flow rate		partial pressure
stream):	[kgmol/hr]	molar fraction - y	[atm]
Species	(b)	(c)	(d)

CO₂	9.683 × 10⁻⁵	3.11 × 10⁻⁸	1.39 × 10⁻⁶
H₂	2266	0.729	32.805
CH₄	77	0.025	1.125
N₂	755.2	0.243	10.935
Ar	9.088	0.0029	0.1305
Total	3107	1	45

Table 12 shows the composition of the side stream 30 that was generated.

TABLE 12

	Flow rate		partial pressure
	[kgmol/hr]	molar fraction - y	[atm]
Side Stream:	(b)	(c)	(d)

CO₂	53.727	0.185	8.325
H₂	0	0	0
CH₄	236.94	0.815	36.675
N ₂	8	0.0275	1.2375
Ar	0	0	0
Total	290.667	1	45

Table 13 shows the composition of the bottom stream 32 that was generated.

TABLE 13

	Flow rate		partial pressure
Bottom stream	[kgmol/hr]	molar fraction - x	[atm]
(a)	(b)	(c)	(d)

CO₂	662.7	0.995	44.775
H₂	0	0	0
CH₄	3.077	4.62 × 10⁻³	0.2
N₂	0.0065	9.76 × 10⁻⁶	4.4 × 10⁻⁴
Ar	0.003	4.5 × 10⁻⁵	2.02 × 10⁻⁴
Total	665.75	1	45

Table 14 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.

TABLE 14

ammonia product yield	617.8	ton/day

purity of the ammonia yield

0.977

Ammonia temperature	−28°	C.
Ammonia Pressure	15	bar

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 15

Condenser duty	6.582 × 10⁶	Energy cost of	$6.58
	kcal/hr	Condenser	million/year
Reboiler duty	2.872 × 10⁶	Energy cost of	$0.13
	kcal/hr	Reboiler r	million/year

Table 16 shows the steady state tray composition profile of the column molar flows (kgmole/hr).

TABLE 16

	CO₂		CH₄
Stage	(Vap)	H₂(Vap)	(Vap)	N₂(Vap)	Ar (Vap)	Temperature

Condenser	2.48 × 10⁻⁵	1938	43.7	646.3	7.661	−166.1
10	5.17 × 10⁻²	2196	1390	2685	57.69	−126.2
9	0.7223	2083	2645	1772	47.57	−114.5
8	4.416	2071	3256	1203	31.33	−110.3
7	20.66	2072	3464	995.6	21.79	−108.2
6	88.52	2062	3311	910.9	16.67	−104
5	2.862	4.929	41.23	8.343	0.2172	−76.69
4	37.77	4.351	388.4	33.9	1.481	−55.54
3	100.2	1.562	962.4	40.67	2.652	−16.26
2	113.7	0.2723	1048	21.95	2.071	0.9492
1	122.2	4.15 × 10⁻²	986.6	10.2	1.385	6.963
Reboiler	166.3	2.7 × 10⁻³	433.5	2.231	0.4723	9.122

			CH₄
Stage	CO₂(Liq)	H₂(Liq)	(Liq)	N₂(Liq)	Ar (Liq)	Temperature

Condenser	5.17 × 10⁻²	257.4	1346	2038	50.03	−166.1
10	0.722	145.1	2601	1126	39.91	−126.2
9	4.416	132.5	3212	556.6	23.67	−114.5
8	20.66	134	3420	349.3	14.13	−110.3
7	88.52	123.9	3267	264.6	9	−108.2
6	596.1	8.019	450.2	32.75	1.392	−104
5	631	7.441	797.4	58.31	2.656	−76.69
4	666.6	1.562	1096	41	2.775	−55.54
3	680.1	0.2724	1181	22.3	2.194	−16.26
2	688.6	4.16 × 10⁻²	1120	10.54	1.508	0.9492
1	732.8	2.83 × 10⁻³	566.6	2.564	0.5954	6.963
Reboiler	566.4	1.23 × 10⁻⁴	133.1	0.3323	0.1231	9.122

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 H₂leads 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 CO₂freezing 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 CH₄will 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.

Example 3

In this example, the input stream was first cooled to a temperature of −100° C. which condenses most of the CO₂in the feed stream. The cooled feed stream was then passed though a flush allowing most of the CO₂to 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 17

Syn loop pressure	15 bar
primary reforming temperature	700° C.
Number of trays	12 plus condenser and reboiler
Location of feed inlet	Tray 7 from the bottom
inlet temperature	−100° C. (feed stream in gaseous state)
Reflux ratio	0.5
Side stream draw stage	reboiler
Side stream flow rate	410 kgmole/hour

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 18

	Flow rate		partial pressure
Feed Stream	[kgmol/hr]	molar fraction - y	[atm]
(a)	(b)	(c)	(d)

CO₂	711.2	0.169	2.535
H₂	2285	0.542	8.13
CH₄	447.7	0.106	1.59
N₂	761	0.18	2.7
Ar	9	0.002	0.003
Total	4213	1	15

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 19

	Flow rate		partial pressure
Feed Stream	[kgmol/hr]	molar fraction - y	[atm]
(a)	(b)	(c)	(d)

CO₂	121.3	0.033	0.495
H₂	2285	0.63	9.45
CH₄	447.7	0.1235	1.8525
N₂	761	0.21	3.15
Ar	9	2.48 × 10⁻³	0.0372
Total	3624	1	15

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 20

Syngas (overhead	Flow rate		partial pressure
stream):	[kgmol/hr]	molar fraction - y	[atm]
Species	(b)	(c)	(d)

CO₂	0	0	0
H₂	2286	0.735	11.025
CH₄	57.9	0.0186	0.28
N₂	761.9	0.245	3.675
Ar	9	2.89 × 10⁻⁶	4.3 × 10⁻⁵
Total	3107	1	15

Table 21 shows the composition of the side stream 30 that was generated.

TABLE 21

	Flow rate		partial pressure
	[kgmol/hr]	molar fraction - y	[atm]
Side Stream:	(b)	(c)	(d)

CO ₂	30	0.0731	1.1
H₂	0	0	0
CH₄	380	0.926	13.89
N₂	0	0	0
Ar	0	0	0
Total	410	1	15

Table 22 shows the composition of the bottom stream 32 that was generated.

TABLE 22

	Flow rate		partial pressure
Bottom stream	[kgmol/hr]	molar fraction - x	[atm]
(a)	(b)	(c)	(d)

CO₂	92.21	0.9	13.5
H₂	0	0	0
CH₄	9.764	0.095	1.43
N₂	0	0	0
Ar	0	0	0
Total from bottom	101.947	1	15
stream
CO₂from flush	589.9	1	15
Total (Flush + Column)	691.874	0.986	15

Table 23 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.

TABLE 23

Ammonia product yield	619	ton/day

purity of the ammonia yield

0.988

Ammonia temperature	−28°	C.
Ammonia pressure	15	bar

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.

TABLE 24

Condenser duty	3.466 × 10⁶	Energy cost of	$3.466
	kcal/hr	Condenser	million/year
Reboiler duty	1.466 × 10⁶	Energy cost of	$1.466
	kcal/hr	Reboiler r	million/year

Since the concentration of CO₂in the column is low due the flushing of the CO2, freezing of any CO₂in 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.

Example 4

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 25

Syn loop pressure	15 bar
primary reforming temperature	750° C.
Number of trays	10 plus condenser and reboiler
Location of feed inlet	Tray 6 from the bottom
inlet temperature	−55° C. (feed stream in gaseous state)
Reflux ratio	1.2
Side stream draw stage	reboiler
Side stream flow rate	199 kgmole/hour

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 26

	Flow rate		partial pressure
Feed Stream	[kgmol/hr]	molar fraction - y	[atm]
(a)	(b)	(c)	(d)

CO₂	769	0.189	2.835
H₂	2330	0.573	8.595
CH₄	177	0.0435	0.65
N₂	776	0.191	2.865
Ar	9.36	0.0023	0.0345
Total	4061	1	15

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 27

Syngas (overhead	Flow rate		partial pressure
stream):	[kgmol/hr]	molar fraction - y	[atm]
Species	(b)	(c)	(d)

CO₂	1.9 × 10⁻⁹	6 × 10⁻¹³	~0
H₂	2330	0.74	11.1
CH₄	29.84	9.48 × 10⁻³	0.142
N₂	776	0.246	3.69
Ar	9.3	2.95 × 10⁻³	0.04425
Total	3146	1	15

Table 28 shows the composition of the side stream 30 that was generated.

TABLE 28

	Flow rate		partial pressure
	[kgmol/hr]	molar fraction - y	[atm]
Side Stream:	(b)	(c)	(d)

CO₂	83	0.417	6.25
H₂	0	0	0
CH₄	116	0.583	8.75
N₂	0	0	0
Ar	0	0	0
Total	199	1	15

Table 29 shows the composition of the bottom stream 32 that was generated.

TABLE 29

	Flow rate		partial pressure
Bottom stream	[kgmol/hr]	molar fraction - x	[atm]
(a)	(b)	(c)	(d)

CO₂	685.4	0.956	14.35
H₂	0	0	0
CH₄	31.33	0.044	0.65
N₂	0	0	0
Ar	0	0	0
Total	716.7	1	15

Table 30 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.

TABLE 30

Ammonia product yield	628.4	ton/day

purity of the ammonia yield

0.992

Ammonia temperature	−28°	C.
Ammonia pressure	15	bar

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.

TABLE 31

Condenser duty	7.718 × 10⁶	Energy cost of	$7.718
	kcal/hr	Condenser	million/year
Reboiler duty	1.883 × 10⁶	Energy cost of	$1.883
	kcal/hr	Reboiler r	million/year

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 CO₂and CH₄as opposed to Examples 1 and 2. Thus, in Example 1 where, in addition, there were also small amounts of liquid N₂and 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.).

Claims

1. 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:

(i) a top stream comprising H₂;

(ii) a middle (volatility) stream comprising hydrocarbon; and

(iii) a bottom stream comprising CO₂.

2. The method according to claim 1 wherein the mixture further comprises N₂which is obtained by the method in the top stream.

3. The method according to claim 2 wherein the H₂and the N₂are present in a molar ratio of 3:1.

4. The method according to claim 1 or 3 wherein the hydrocarbon comprises methane.

5. The method according to any one of claims 1 to 4 wherein freezing of the CO₂is substantially prevented.

6. The method according to any one of the previous claims wherein the middle stream is obtained as a gas from a reboiler of the column.

7. The method according to any one of claims 1 to 6 wherein the middle stream is obtained from a tray of the column.

8. The method according to any one of the previous claims where in the mixture is introduced into a reboiler of the column.

9. The method according to any one of claims 1 to 7 where in the mixture is introduced into a tray of the column.

10. The method according to any one of the previous claims wherein reflux of the top stream to the column is performed using a reflux ratio between 0.001 and 10.

11. The method according to claim 10 wherein reflux of the top stream to the columns is performed using a reflux ratio between 0.05 and 2.

12. The method according to any one of the previous claims wherein the column has a pressure between 5 bar to a critical pressure of the mixture.

13. The method according to claim 12 wherein the column has a pressure between 7 bar to 55 bar.

14. The method according to any one of the previous claims further comprising cooling the mixture prior to introducing the mixture to the column.

15. The method according to claim 14 further comprising introducing the cooled mixture into a flush to remove CO₂prior to introducing the stream into the column.

16. The method according to claim 15 wherein the cooled mixture is introduced into a reboiler of the column.

17. A system for generating and separating a mixture containing H₂, hydrocarbon, and CO₂, the system comprising:

(a) means for generating a mixture containing H₂, hydrocarbon, and CO₂; and

(b) a distillation column having a side stream configured to receive the mixture and to generate from the mixture:

(i) a top stream comprising H₂;

(ii) a middle stream comprising hydrocarbon; and

(iii) a bottom stream comprising CO₂.

18. The system according to claim 17 wherein the means for generating the mixture further generates N₂obtained in the top stream.

19. The system according to claim 18 wherein the means for generating the mixture generates a mixture containing H₂and N₂in a ratio of 3:1.

20. The system according to claim 17 or 19 wherein the hydrocarbon comprises methane.

21. The system according to claims 17 to 20 wherein freezing of the CO₂is substantially prevented.

22. The system according to any one of claims 17 to 21 wherein the middle stream is obtained as a gas from a reboiler of the column.

23. The system according to any one of claims 17 to 21 wherein the middle stream is obtained from a tray of the column.

24. The system according to any one of claims 17 to 23 where in the mixture is introduced into a reboiler of the column.

25. The system according to any one of claims 17 to 23 wherein the mixture is introduced into a tray of the column.

26. The system according to any one of claims 17 to 25 wherein reflux of the top stream to the column is performed using a reflux ratio between 0.001 and 10.

27. The system according to claim 26 wherein reflux of the top stream to the columns is performed using a reflux ratio between 0.05 and 2.

28. The system according to any one of claims 17 to 27 wherein the column has a pressure between 5 bar to a critical pressure of the mixture.

29. The system according to claim 26 wherein the column has a pressure between 7 bar to 55 bar.

30. The system according to any one of claims 17 to 29 wherein the mixture is cooled prior to introducing the mixture to the column.

31. The system according to claim 30 wherein the cooled mixture is introduced into a flush to remove CO₂prior to introducing the mixture into the column.

32. The system according to claim 30 wherein the cooled stream is introduced into a reboiler of the column.

33. The system according to claim 18 or 19 further comprising means for generating ammonia from the top stream.

34. The method according to claim 2 or 3 further comprising generating ammonia from the top stream.

35. The method according to claim 34 wherein the hydrocarbon is methane.

36. The system according to claim 33 wherein the hydrocarbon is methane.