Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
2001-06-04
2004-07-06
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
C073S195000
Reexamination Certificate
active
06758101
ABSTRACT:
FIELD OF INVENTION
This invention relates to methods and apparatus for improving process efficiency of the steam reforming of hydrocarbons (SRH) method of hydrogen gas production. More particularly, the process improvements use a Coriolis flowmeter to control the steam to carbon ratio in SRH hydrogen gas production.
PROBLEM
Hydrogen is an increasingly valuable commodity having many uses, such as coolant in electrical equipment, fuel for space exploration, and in chemical manufacturing of commercially important products, especially ammonia, methanol, oxo alcohols and hydroformed gasoline. Hydrogen demand is increasing due to regulatory requirements that spur the development of better performing and cleaner fuels.
The primary method of producing hydrogen in commercial quantities is steam reforming of hydrocarbons (SRH). The process may be performed on hydrocarbon gasses or low-octane petroleum fractions under process conditions that typically involve high heat and pressure. Where the reformation process is performed without a catalyst, it is generally known in the art as thermoforming. SRH is most efficient when a catalyst, such as nickel, molybdenum or platinum, facilitates the reaction. A low sulfur hydrocarbon feedstock is needed to avoid poisoning the catalyst. SRH is well known in the art and is described in a variety of publications, such as R. N. Shreve,
Shreve's Chemical Process Industries
, McGraw-Hill, Inc., pp. 106-109 (1984); and D. M. Considine,
Chemical and Process Technology Encyclopedia
, McGraw-Hill, Inc., pp. 592-596 (1974), which are hereby incorporated by reference to the same extent as though their content is fully repeated herein.
Hydrogen gas production by the SRH method involves reacting a hydrocarbon feedstock with steam. In general, hydrocarbon feedstocks contain a variety of hydrocarbons, and the reaction chemistry proceeds according to ideal stoichiometric equations for each type of hydrocarbon. A variety of different reactions occur, depending upon the feedstock. The most important reactions can be generally categorized as:
A. Dehydrogenation of cyclohexanes to yield aromatic hydrocarbons;
B. Dehydrogenation of certain paraffins to yield aromatics;
C. Isomerization including the conversion of straight-chain to branched chain carbon structures, such as octane to isooctane;
D. Reformation of methane in natural gas to produce carbon dioxide and hydrogen; and
E. Reformation of naptha to yield synthetic natural gas.
A preferred manner of generating large quantities of hydrogen is to use a natural gas feedstock that contains a large portion of methane. The reaction proceeds as shown in Equation 1:
CH
4
+H
2
O=>CO+3H
2
, (1)
where H
2
O is preferably present as steam.
This class of reaction similarly operates on other gas fractions in the feedstock for complete decomposition, in an ideal sense, of the hydrocarbon into carbon dioxide and hydrogen. For example, a further reaction including propane (C
3
H
8
) in the hydrocarbon feedstock proceeds according to Equation (2):
C
3
H
8
+3H
2
O=>3CO+7H
2
, (2)
where H
2
O is preferably present as steam.
More generally, this overall class of reaction proceeds as shown in Equation (3):
C
n
H
m
+nH
2
O=>nCO+(
m/
2
+n)H
2
(3)
The foregoing equations predict the reaction of lighter hydrocarbons, especially, methane, butane and propane, as well as some liquids, such as naptha. Heavier ends tend to react differently and, while some proceed according to the above Equation (3), other such reactions as isomerization occur, also with resultant hydrogen production.
The equations demonstrate a concept that different amounts of steam are required to complete the reaction, depending upon the feedstock composition. For example, one mole of methane requires one mole of steam in Equation (1), whereas one mole of propane consumes three moles of steam in Equation (2). In commercial manufacturing facilities that operate upon feedstocks of various compositions, these differences impose a potentially significant materials balance problem. The process may be further complicated by using oxygen as a reagent, which results in lower steam consumption.
A wide range of feedstocks can be utilized as the hydrocarbon feedstock in a SRH hydrogen production unit. Hydrocarbons such as natural gas, methane, propane, butane and naphtha can be used as the hydrocarbon feedstock, either alone or in combination. Economics and the availability of particular hydrocarbon feedstocks may dictate the use of different hydrocarbon feedstocks from one period to another.
A particular problem arises in petroleum refineries because the SRH feedstock composition constantly changes. The hydrocarbon feedstock for a hydrogen production unit can come from several sources within the refinery, and these sources contribute different hydrocarbons. One particular example of a combined-source hydrocarbon includes the refinery fuel gas system. Numerous process results contribute to the fuel gas system by adding different hydrocarbons, which may be directed in a combined stream to the hydrogen gas production unit. If one of the contributing refinery fuel gas system processes is shut down or changes in terms of output volume, the composition of the fuel gas system output changes. The changes in feedstock composition require corresponding changes in the SRH process conditions, such as heat, pressure and flow rate, in order to optimize process efficiencies and minimize environmental pollution.
It is presently a problem to accurately measure the fractional composition of a hydrocarbon feedstock in a hydrogen production unit. It is a further problem to control the ratio of steam to carbon in a hydrogen production unit based upon the composition of the feedstock. These measurement and control problems reduce SRH process efficiencies while increasing associated environmental pollution problems.
A traditional approach to measuring the hydrocarbon feedstock in a hydrogen gas production unit involves measuring the hydrocarbon feedstock with a volumetric flowmeter. While somewhat useful in addressing the reaction balance problem arising from combined feedstocks, however, volumetric meters are incapable of providing a full measurement solution. When the composition of the hydrocarbon feedstock changes or a substitute hydrocarbon feedstock is used, the amount of carbon contributed to the hydrogen gas production unit can change dramatically within a given unit of volume, as can the required amount of steam to complete the reaction. For example, under identical conditions of temperature and pressure, the amount of steam required to react with a volume of propane is approximately three times greater than the amount of steam required to react with the same volume of methane. The problem is exacerbated by real gas behavior where the lighter gasses tend to have higher compressibility factors.
Another approach to measuring the hydrocarbon feedstock is to determine the composition of the hydrocarbon feedstock with a gas chromatograph. However a gas chromatograph cannot provide real-time composition data for a hydrocarbon feedstock.
It is known to use Coriolis mass flowmeters to measure mass flow and other information with respect to materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al., of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flowmeters typically comprise a flowmeter electronics portion and a flowmeter sensor portion. Flowmeter sensors have one or more flowtubes of a straight or curved configuration. Each flowtube configuration has a set of natural vibration modes, which may be of a simple bending, torsional, radial or coupled type. Each flowtube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes of the vibrating, material filled systems are defined in part by the combined mass of the flowtubes and the material within the flowtubes. Material flows into the flowmeter se
Duft Setter Ollila & Bornsen LLC
Micro Motion Inc.
Patel Harshad
LandOfFree
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