Steam reforming

Details Steam reforming Steam reforming

2001-02-13

2003-02-25

Langel, Wayne A. (Department: 1754)

Chemistry: fischer-tropsch processes; or purification or recover

With preliminary reaction to form hydrogen or a carbon oxide

Water utilized in the preliminary reaction

C252S373000

Reexamination Certificate

active

06525104

ABSTRACT:

This invention relates to steam reforming and in particular to the catalytic steam reforming of hydrocarbons. Steam reforming is widely practised and is used to produce hydrogen streams and synthesis gas for a number of processes such as ammonia, methanol and Fischer-Tropsch process for the synthesis of carbon-containing compounds such as higher hydrocarbons. In a steam reforming process, a desulphurised hydrocarbon feedstock, e.g. natural gas or naphtha, is mixed with steam and passed at elevated temperature and pressure over a suitable catalyst, often nickel on a suitable refractory support, e.g. alumina or calcium aluminate cement. The composition of the product gas depends on, inter alia, the proportions of the feedstock components, the pressure and temperature. The product normally contains methane, hydrogen, carbon oxides, steam and any gas, such as nitrogen, that is present in the feed and which is inert under the conditions employed. For applications such as methanol synthesis it is generally desirable that the molar proportions of hydrogen and carbon oxides are such that the value of ([H
2
]−[CO])/([CO
2
]+[CO]), where [H
2
], [CO
2
] and [CO] are the molar proportions of hydrogen, carbon dioxide and carbon monoxide respectively in the product, is about equal to 2. For applications such as Fischer-Tropsch synthesis, it is desired that the ratio of [H
2
]/[CO] is about 2 and the amount of carbon dioxide present is small. For other applications it may be desirable to have a [H
2
]/[CO] ratio below 2.
It has been proposed in U.S. Pat. No. 4,999,133 to make a synthesis gas having a low CO
2
content and a [H
2
]/[CO] ratio below 2.5 from feedstocks such as natural gas by a steam reforming process wherein in a first, primary reforming, stage part of the feedstock is reformed with steam in a fired furnace, the remainder of the feedstock is added to the primary reformed product of the first stage and the resultant mixture subjected to a stage of secondary reforming wherein the feed is subjected to partial combustion by the addition of oxygen and then passed over a secondary reforming catalyst. In an example there is described the production of a gas having a carbon dioxide content of 2.6% (on a dry basis) and a [H
2
]/[CO] ratio of 2.18 by reforming at a pressure of about 49 atms. abs. with a secondary reformer exit temperature of 1149° C. 83% of the feedstock bypasses the first reforming stage and over 0.51 moles of oxygen are added per gram atom of hydrocarbon carbon fed to the reforming stages. Not only is a large proportion of oxygen required, but also the high outlet temperature presents metallurgical problems and the need to introduce the feedstock that has bypassed the first reforming stage into the hot feed to the secondary reforming stage presents engineering difficulties.
It has been proposed in U.S. Pat. No. 4,910,228 to make synthesis gas having the stoichiometric composition for methanol synthesis but having a [H
2
]/[CO] ratio of about 2.9 by a primary/secondary reforming process using a heat exchange reformer for the primary reforming stage with the heat required for the primary reforming being supplied by the sensible heat in the secondary reformed product.
We have realised that by the use of a heat exchange reformer, the use of such large amounts of oxygen and consequent very high temperatures can be avoided and a product having the desired composition can be achieved if a major proportion, if not all, the carbon dioxide is separated from the secondary reformed product, before or after subsequent processing, and recycled to the feed. With a light feedstock, such as natural gas, a [H
2
]/[CO] ratio of 2 or below can be achieved even with recycle of substantially all of the carbon dioxide. This enables more synthesis gas to be made from a given amount of feedstock. On the other hand, with alternative processes such as autothermal reforming or partial oxidation, when using a light feedstock in order to obtain a [H
2
]/[CO] ratio of 2 some carbon dioxide has to be exported.
It has been proposed in GB 2 179 366 to produce a gas stream having a [H
2
]/[CO] value of about 2 using such a heat exchange reformer with recycle of carbon dioxide separated from the product. In the process of that reference part of the feedstock is fed to the primary, i.e. heat exchange, reformer while the remainder is fed to the directly to the secondary reformer. In the examples, the process was operated under conditions such that the product reformed gas (before carbon dioxide separation) had a relatively high carbon dioxide content, ranging from about 27% to about 42% by volume on a dry basis. The amount of carbon dioxide that was separated and recycled was thus large, and exceeded the amount of feedstock on a molar basis. The separation and recycle of such a large amount of carbon dioxide would render such a process uneconomic.
We have realised that such a process can be operated to produce a reformed gas having a significantly lower carbon dioxide content, thus obviating the need for separation and recycle of such large amounts of carbon dioxide.
Accordingly the present invention provides a process for the production of a gas for use in the synthesis of carbon-containing compounds comprising the steps of:
a) subjecting a gaseous mixture containing at least one hydrocarbon and 0.6 to 2 moles of steam per gram atom of hydrocarbon carbon in said mixture to catalytic primary reforming at an elevated temperature in a heat exchange reformer by passing said mixture through tubes containing a steam reforming catalyst heated by a hot gas stream flowing past the exterior of said tubes, thereby forming a primary reformed gas stream;
b) without the addition of further hydrocarbon, subjecting the primary reformed gas stream to secondary reforming wherein it is subjected to partial combustion with a gas containing free oxygen to form a hot partially combusted gas stream which is then passed through a bed of a secondary reforming catalyst, thereby forming a secondary reformed gas stream;
c) passing said secondary reformed gas stream past the exterior of the tubes of the heat exchange reformer as said hot gas stream, thereby heating the gas inside said tubes and cooling said secondary reformed gas stream;
d) further cooling said secondary reformed gas stream, condensing steam therefrom, and separating out the condensed water to give a de-watered secondary reformed gas stream, the reforming conditions being selected to give a de-watered secondary reformed gas stream having a carbon dioxide content below 20% by volume;
e) recovering carbon dioxide from said de-watered secondary reformed gas stream, before or after use of the latter for synthesis of the carbon-containing compounds, and
f) adding the recovered carbon dioxide to the gaseous mixture before the latter is fed to the heat exchange reformer, the amount of recycled carbon dioxide being 0.2 to 0.6 moles per gram atom of hydrocarbon carbon in the gaseous mixture.
In the present invention, the primary reforming is effected using a heat exchange reformer. In one type of heat exchange reformer, the catalyst is disposed in tubes extending between a pair of tube sheets through a heat exchange zone. Reactants are fed to a zone above the upper tube sheet and pass through the tubes and into a zone beneath the lower tube sheet. The heating medium is passed through the zone between the two tube sheets. Heat exchange reformers of this type are described in GB 1 578 270 and WO 97/05 947.
Another type of heat exchange reformer that may be used is a double-tube heat exchange reformer as described in aforesaid U.S. Pat. No. 4,910,228 wherein the reformer tubes each comprise an outer tube having a closed end and an inner tube disposed concentrically within the outer tube and communicating with the annular space between the

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