Reforming with intermediate reactant injection

Details Reforming with intermediate reactant injection Reforming with intermediate reactant injection

2000-10-12

2003-03-04

Langel, Wayne A. (Department: 1754)

Compositions

Gaseous compositions

Carbon-oxide and hydrogen containing

C423S650000, C423S652000

Reexamination Certificate

active

06527980

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable
BACKGROUND OF THE INVENTION
Carbon monoxide can be produced by reforming hydrocarbon feed with steam and carbon dioxide at high temperatures. The reaction can occur in a steam methane reformer (SMR) which contains catalyst-filled tubes housed in a furnace. The synthesis gas (also called syngas) exiting the reformer contains carbon monoxide along with hydrogen, carbon dioxide, steam and unconverted methane according to the equilibrium established from the following reactions:
CH
4
+ H
2
O ←→ 3H
2
+ CO
Steam Reforming
H
2
O + CO ←→ H
2
+ CO
2
Water Gas Shift
CH
4
+ CO
2
←→ 2H
2
+ 2CO
Carbon Dioxide Reforming
The synthesis gas is cooled and separated into various products, i.e., carbon monoxide or syngas of different hydrogen to carbon monoxide (H
2
/CO) ratios, recycle carbon dioxide, and by-product hydrogen, using separation technologies such as amine absorption, adsorption and/or cryogenic separation.
Carbon monoxide, the principal product of a reformer plant, is used in the manufacture of isocyanates and polycarbonates through phosgene chemistry. Other processes, such as certain oxoalcohol production processes, require a low 1:1 H
2
/CO ratio synthesis gas. In SMR, with a natural gas feed and with full carbon dioxide recycle from the process gas only, the lowest H
2
/CO ratio obtainable is about 3. Thus, hydrogen is typically produced far in excess of what is required. Some hydrogen can be used to fire the reformer; the remainder must be exported at fuel value. The requirement of finding a fuel home for this excess hydrogen introduces a constraint. Also, using a value-added material such as hydrogen to substitute for readily available fuels is not the most economical choice.
As is well known in the industry, injecting carbon dioxide into the reformer hydrocarbon feed very effectively reduces the H
2
/CO ratio towards what is required. Besides recycling carbon dioxide separated from the synthesis gas product, additional carbon dioxide can be obtained from the furnace flue gas or imported into the plant from an outside source. Under conditions of complete CO
2
recycle, the reformer steam to carbon (S/C) ratio is irrelevant in the determination of the H
2
/CO ratio. However, it determines the amount of carbon dioxide recycled from the process gas. The combination of recycled carbon dioxide and imported carbon dioxide yields the reformer carbon dioxide to carbon (CO
2
/C) ratio. For a given H
2
/CO ratio there is an infinite combination of the reformer S/C and CO
2
/C ratios. Table 1 illustrates this for a particular case in which H
2
/CO=0.6, pressure=200 psia (1379 kPa), and temperature=1750° F. (955° C.).
TABLE 1
PRODUCTION OF H
2
/CO (0.6) AT 200 PSIA AND 1750° F.
BOUDOUARD CARBON K-RATIO*
S/C
CO
2
/C
(MINIMUM)
1.00
3.11
0.587
2.00
4.19
0.979
2.31
4.53
1.096
2.60
4.84
1.205
*Boudouard Carbon K-Ratio = K-operating/K-equilibrium
As the S/C is lowered, a lower CO
2
/C ratio is required to produce the specified syngas H
2
/CO ratio. With lower S/C and CO
2
/C, process steam requirement and carbon dioxide recycle are reduced, and methane leakage increases. Producing steam and recycling carbon dioxide are expensive in energy, capital, and power. Hence it is desirable to operate the reformer at the lowest permissible S/C ratio, consistent with a tolerable methane leakage. However, low S/C operation increases the potential of depositing carbon on the reforming catalyst. Carbon deposition deactivates the catalyst resulting in high reactor tubewall temperature and reduced hydrocarbon conversion. It can also block the reformer tubes resulting in high pressure drops inside the reactor. Several carbon formation mechanisms can be involved:
CH
4
←→ C + 2H
2
Methane Cracking
2CO ←→ C + 2CO
2
Boudouard Reaction
H
2
+ CO ←→ C + H
2
O
Reverse of Steam Carbon Reaction
Conventional steam methane reformer conditions are limited to the point of incipient carbon deposition predicted by the carbon forming reaction equilibria. At every temperature between the reformer inlet and outlet, an equilibrated gas composition can be calculated for the given feed stoichiometry. The reaction equilibrium constant for any of the carbon forming reactions can be calculated (K-operating), and compared with the critical value for depositing carbon via that reaction mechanism (K-equilibrium). The K-operating/K-equilibrium ratio should exceed 1 everywhere in the reformer. The minimum value of this ratio as computed for the Boudouard reaction is shown in Table 1. The table shows that in the production of synthesis gas having H
2
/CO ratio of 0.6, the minimum desirable S/C is about 2.31, which gives a narrow operating margin away from conditions where carbon forms. S/C ratios of 1 and 2 are unacceptable because the operating Boudouard K-value is less than the equilibrium value for this reaction and the Boudouard reaction will proceed to the right hand side of the chemical equation as written above, resulting in carbon deposition. The necessity of operating above an S/C of 2.3 implies a CO
2
/C greater than 4.5. The resulting huge carbon dioxide recycle would be accompanied by unacceptably high costs.
It is thus highly desirable to develop a process that produces syngas with a H
2
/CO ratio of 1 or lower and reduces the S/C ratio, yet avoids carbon formation. Such a process would have the following advantages:
reduced costs associated with the recovery and recompression of carbon dioxide;
reduced costs associated with steam generation;
reduced flows, sizes, and duties of all heat exchangers, including the firebox; and
greater energy efficiency in operation of the plant.
U.S. Pat. No. 4,782,096 (Banquy, 1988) and U.S. Pat. No. 4,888,130 (Banquy, 1989) disclose processes for producing a synthesis gas having a H
2
/CO ratio below 2.5. In the processes a hydrocarbon feed is split into two parts. A portion is reformed in a primary reformer at high temperature and pressure. The effluent is combined with the remainder of feed and undergoes a secondary reforming reaction in an adiabatic reactor by reacting with an oxygen rich stream. The overall S/C is very low; e.g., 0.4 in one example. Carbon deposition is reportedly avoided by maintaining the combined feed to the second (autothermal) reformer above a minimum temperature (e.g., 1100° F. or 594° C.), and by designing burners with high mixing efficiency. However, the process of U.S. Pat. No. 4,888,130, as shown in the examples, achieves syngas H
2
/CO ratios greater than 1.5. Also, the process needs expensive oxygen feedstock.
Syngas H
2
/CO ratios of less than 2 can be achieved using autothermal reforming. Carbon formation is avoided by using well mixed burners. With full recycle of carbon dioxide, the minimum syngas H
2
/CO achievable is 1.6. This ratio can be lowered further with imported carbon dioxide. However, autothermal reactors require pure oxygen, and unless inexpensive oxygen is available, they are not economical in comparison with fired reformers. In addition, the oxygen consumption goes up with the amount of carbon dioxide or water recycled or imported into the feed, as necessitated by lower and lower H
2
/CO ratios. Air cannot be used as an oxidant source because the products will be contaminated with nitrogen which is difficult to remove from a hydrogen-carbon monoxide syngas or carbon monoxide product. Also, elaborate safety and shutdown systems are required to handle hot oxygen in the presence of fuel.
U.S. Pat. No. 2,199,475 (Wilcox, 1940) discloses the addition of a controlled amount of carbon dioxide to hydrocarbon gases and steam prior to introduction into the dissociation chamber of a reactor. Oxygen can be added, preferably at an intermediate point in the dissociation chamber. The oxygen is fully consumed in partial combustion of unreformed methane (and hydrogen and carbon monoxide). Temperatures in excess of 2000° F. are thereby reached where the reforming

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