Chemistry of inorganic compounds – Nitrogen or compound thereof – Ammonia or ammonium hydroxide
Reexamination Certificate
2002-11-13
2004-11-02
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Nitrogen or compound thereof
Ammonia or ammonium hydroxide
C422S148000
Reexamination Certificate
active
06811762
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to method and apparatus for the production of a product by catalytic reaction of a pressurized synthesis gas. For example, one embodiment of the present invention relates to the production of ammonia by catalytic reaction of pressurized synthesis gas comprising hydrogen and nitrogen. More specifically, the present invention relates to an improved method for purification of make-up synthesis gas, i.e., synthesis gas which is added to the catalytic reactor to replace reacted synthesis gas.
RELATED ART
U.S. Pat. No. 3,350,170 issued Oct. 31, 1967 to J. A. Finneran et al, discloses a process for carrying out cyclic synthesis reactions at elevated pressures and is particularly concerned with improvements in the method of compressing the fresh and recycle synthesis gases in such process. This patent well illustrates the type of synthesis process with which the present invention is concerned. As shown in FIG. 1 of U.S. Pat. No. 3,350,170, fresh synthesis gas 10 is introduced into a centrifugal compressor together with gas 42 recycled from a converter 38 in which hydrogen and nitrogen are catalytically converted to ammonia. The recycle gas exiting from converter 38 thus contains product ammonia as well as unreacted hydrogen and nitrogen. The recycle gas is reintroduced via line 24 into the compressor. The compressed outlet gas 26 thus comprises a mixture of the recycle gas plus the fresh (make-up) gas introduced via line 10. The product ammonia is separated in separation vessel 31 and the ammonia-depleted compressed synthesis gas travels to the converter 38 via lines 33, 34 and 35. Line 46 is used to separate a purged gas from the synthesis loop in order to prevent build-up of impurities in the synthesis loop defined by lines 42, 24, 26 and 33.
In conventional ammonia synthesis processes, removal of H
2
O from make-up synthesis gas is accomplished by mixing make-up gas containing about 160 ppm H
2
O with recycle gas at the compressor recycle wheel inlet. The gas discharged from the compressor is then cooled and chilled with H
2
O being absorbed in the condensing NH
3
. The NH
3
and absorbed H
2
O are separated from the gas in a separator. The converter is fed with gas from the separator, which separated gas is substantially H
2
O-free or at least has only a very small residual H
2
O content. The separated gas may contain, for example, about 1.9% NH
3
. There are several disadvantages with this system. The refrigeration power required is higher because of the dilution of converter effluent with make-up gas that lowers the NH
3
concentration and the dewpoint. This transfers load from the higher to the lower temperature chillers, which require more power per ton of refrigeration. Also, product NH
3
is compressed in the recycle wheel, adding to the power demand imposed on the compressor. A significant improvement in reduced energy requirements can be realized for this system, as shown in U.S. Pat. No. 1,815,243, by incorporating a dehydrator.
A 1989 paper by H. Bendix and L. Lenz of VEB Agrochemie Piesteritz, the former German Democratic Republic (East Germany), was presented at a meeting of the American Institute of Chemical Engineers. The paper is entitled
Results and Experiences on Revamping of Large
-
Scale Ammonia Single
-
Line Plants
and discloses the addition, via a Venturi tube, of liquid ammonia to the synthesis gas discharged from the third stage of the synthesis gas compressor. The stated purpose is to dry the synthesis gas.
A paper by M. Badano and F. Zardi was presented at the 28 Feb.-2 Mar. 1999 Nitrogen '99 meeting in Caracas, Venezuela sponsored by British Sulphur Publishing. The paper is entitled
Casale Group Experience in Revamping Ammonia, Methanol and Urea Complexes
and discloses scrubbing with liquid ammonia, ammonia synthesis gas between the second and third stages of the synthesis gas compressor.
Another prior art expedient is shown in U.S. Pat. No. 1,830,167 and Canadian Patent 257,043. This method involves scrubbing the combined make-up and recycle gas stream with liquid NH
3
prior to preheating the stream and sending it to the converter. Normally, there is no need to scrub the recycle stream since there are no impurities in it. A drawback of the scheme of these patents is that it distributes impurities through the entire gas stream. It is then more difficult to effect complete impurity removal because the impurities are diluted by being dispersed throughout the entire gas stream. In order to treat the combined stream, the scrubbing apparatus must be much larger and more costly than would be required for scrubbing the makeup gas stream alone, since it is treating a gas volumetric flow which is 4-5 times greater than the make-up gas stream alone. Accordingly, the scheme of U.S. Pat. No. 1,815,243 and Canadian Patent 257,043 adds to the scrubbing load by combining the recycle and make-up streams prior to scrubbing.
Other prior art expedients include the use of molecular sieves to remove H
2
O from make-up gas by adsorption. The concept of dehydrating make-up gas permits the stream with the highest NH
3
content, the effluent from the converter, to feed the chilling system. This saves considerable refrigeration power and can allow a significant capacity increase in plants that are limited by the size of the refrigeration compressor. The power savings is accomplished because of the elevated dew point that results in some condensation with cooling water and a transfer of load from the low to the high temperature chillers which need less power per ton of refrigeration. Removal of H
2
O by molecular sieves also enables omitting the purge gas chiller that uses the coldest NH
3
refrigeration.
The H
2
O-free (and NH
3
-free) make-up gas is then mixed with recycle gas, compressed in the recycle wheel and fed to the converter. This system has one advantage over competing technologies, which is that the converter feed has a low NH
3
content, about 1.4%. However, this advantage is offset by other factors such as the heat required for regeneration of the molecular sieves, the operating complexity because of the requirement for numerous switching valves needed for the cyclic operation to adsorb and desorb H
2
O from the molecular sieves, higher maintenance costs and the high capital cost of the molecular sieve vessels, heat exchangers, filters, piping and valves. The energy saving is estimated to be about 0.53 MM Btu/ST (where ST means short ton or 2000 pounds), compared to a standard secondary flash design.
Another prior art concept is shown in U.S. Pat. No. 3,349,569. This patent discloses installation of an NH
3
scrubber at the inlet of the synthesis gas compressor, to use liquid NH
3
to absorb H
2
O from make-up synthesis gas. This allows make-up gas to be mixed with the recycle gas and to be fed directly to the ammonia converter. The converter effluent then goes directly to a cooling/chilling system of the type described above in connection with the use of molecular sieves. A substantial chilling effect takes place because of the heat required to vaporize NH
3
, which comes from chilling the make-up gas. The essentially H
2
O-free make-up gas, which contains about 4.9% NH
3
, is then mixed with recycle gas as described above in connection with the use of molecular sieves.
There are several disadvantages with this system. Over-chilling of make-up gas due to excessive NH
3
evaporation resulting from low pressure results in a scrubber overhead and compressor inlet temperature (−27° F.) which is below the minimum (−20° F.) for standard materials of construction. More expensive low-temperature materials of construction are needed for the scrubber, and the compressor will have to be re-rated (if possible). A re-rating of the compressor can sometimes be done if its original materials of construction were satisfactory for more severe operating conditions. Otherwise, an upgrade of the compressor low pressure case may be required and this is costly. Another disadvantage of this method is that NH
3
Langel Wayne A.
Libert Victor E.
Libert & Associates
Shannahan Cornelius E.
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