Page 218 - Synthetic Fuels Handbook

204 CHAPTER SEVEN

Refinery gas from different sources varies in suitability as hydrogen plant feed. Catalytic
reformer off-gas, for example, is saturated, very low in sulfur, and often has high-hydrogen
content. The process gases from a coking unit or from a fluid catalytic cracking unit are
much less desirable because of the content of unsaturated constituents. In addition to ole-
fins, these gases contain substantial amounts of sulfur that must be removed before the gas
is used as feedstock. These gases are also generally unsuitable for direct hydrogen recovery,
since the hydrogen content is usually too low. Hydrotreater off-gas lies in the middle of the
range. It is saturated, so it is readily used as hydrogen plant feed. Content of hydrogen and
heavier hydrocarbons depends to a large extent on the upstream pressure. Sulfur removal
will generally be required.
As hydrogen use has become more widespread in refineries, hydrogen production has
moved from the status of a high-tech specialty operation to an integral feature of most refin-
eries. This has been made necessary by the increase in hydrotreating and hydrocracking,
including the treatment of progressively heavier feedstocks (Speight, 2007b). The contin-
ued increase in hydrogen demand over the last several decades is a result of the conversion
of petroleum to match changes in product slate and the supply of heavy, high-sulfur oil, and
in order to make lower-boiling, cleaner, and more salable products. There are also many
reasons other than product quality for using hydrogen in processes adding to the need to
add hydrogen at relevant stages of the refining process, the most important being the avail-
ability of hydrogen.
Hydrogen has historically been produced during catalytic reforming processes as a by-
product of the production of the aromatic compounds used in gasoline and in solvents. As
reforming processes changed from fixed bed to cyclic to continuous regeneration, process
pressures have dropped and hydrogen production per barrel of reformate has tended to
increase. However, hydrogen production as a by-product is not always adequate to the
needs of the refinery and other processes are necessary. Thus, hydrogen production by
steam reforming or by partial oxidation of residua has also been used, particularly where
heavy oil is available. Steam reforming is the dominant method for hydrogen production
and is usually combined with pressure-swing adsorption (PSA) to purify the hydrogen to
greater than 99 percent by volume.
The gasification of residua and coke to produce hydrogen and/or power may become an
attractive option for refiners. The premise that the gasification section of a refinery will be
the garbage can for deasphalter residues, high-sulfur coke, as well as other refinery wastes
is worthy of consideration.
Of the processes that are available for the production of hydrogen, many can be con-
sidered dual processes insofar as they also produce carbon monoxide and, therefore, are
considered as producers of synthesis gas. For example, most of the external hydrogen is
manufactured by steam-methane reforming or by oxidation processes. Other processes
such as ammonia dissociation, steam-methanol interaction, or electrolysis are also avail-
able for hydrogen production, but economic factors and feedstock availability assist in the
choice between processing alternatives.
The processes described in this section are those gasification processes by which hydro-
gen is produced for use in other parts of the refinery.

7.2.1 Chemistry

In steam reforming, low-boiling hydrocarbons such as methane are reacted with steam to
form hydrogen:
CH + H O → 3H + CO ΔH =+97,400 Btu/lb
4 2 2 298 K