H2S PRODUCTION FROM A MOLTEN METAL REACTOR
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of prior co-pending application USSN
08/421 ,102, filed April 13, 1995, (attorney docket 6391NUS), now US 5,577,346
granted November 26, 1996.
The application is somewhat related to USSN 08/163,468, filed December 7,
1993, (attorney docket 6431MUS).
CROSS REFERENCE TO RELATED PATENTS
This application is related to US 5,435,814, issued July 25, 1995, teaching a
two-zone molten metal decomposition apparatus and process.
BACKGROUND OF THE INVENTION
I. FIELD OF THE INVENTION:
This invention relates to gasification of sulfur and hydrocarbon containing
streams in molten metal.
II. DESCRIPTION OF THE PRIOR ART:
Molten metal, especially molten iron, baths are well known and widely used as
gasifiers. The high temperatures in such baths rapidly decompose, by theπΗal action,
a variety of solid, liquid and gaseous feeds into hydrogen and/or carbon oxides. Such
processes are well known, e.g., U.S. Patents 4,574,714 and 4,602,574 to Bach teach
a molten iron gasifier. Another, and preferred, molten metal reactor is disclosed in US
5,435,814, MOLTEN METAL DECOMPOSITION APPARATUS, Chailes B. Miller
and Donald P. Malone.
One of the problems of molten metal processing is that the feeds to such
processes are rarely pure materials. If the feed were a pure hydrocarbon, such as
methane with no significant amount of chlorides, trash metals, sulfur or other
impurities, design and operation of the molten metal reactor is simple. Consider the
case of methane conversion. The reactor need only be designed to thermally convert
the CH4 into hydrogen (which is thermally stable and rapidly released as pure
hydrogen gas) and carbon (which rapidly dissolves in the molten iron). There are no
feed impurities and no slag forms.
Once the refiner has to depart from such ideal fuels as methane, design of the
reactor becomes complicated. Molten metal reactors are best at converting difficult
streams not otherwise amenable to processing - resids, ground up tires, old pesticides
and the like. Such materials present many challenges to the engineer charged with
converting them to useful products (or at least making the offending material go
away), but for now the focus is on one pervasive impurity - sulfur. The problem of
sulfiir in the feed is pervasive in refinery processing, coal combustion and molten
metal processing. It is instructive to review how each of these processes has dealt
with feed sulfur.
Crude oil invariably contains sulfiir. Sulfiir is so pervasive in crude oil that its
presence in greater or lessor amounts makes crude oil sour or sweet. Refiners have
evolved efficient ways to convert sulfiir in feed into solid sulfiir product. Sulfur is a
valuable product in its elemental foπn. In refineries, the crude is generally
catalytically hydrotreated to convert sulfur compounds to H2S which is eventually
converted in a Claus unit to elemental sulfur. The processing is expensive, both in
teπns of operating and capital expense required to hydrotreat feeds, but essential.
In coal processing, sulfur is generally dealt with by stack gas scrubbing or by
burning the coal in a bed of ground up limestone or dolomite. It is possible to burn
coal in California if a Circulating Fluidized Bed (CFB) coal combustor is used.
Relatively small amounts of coal are added to a much larger circulating inventory of
crushed alkaline material. The sulfiir components in the coal are oxidized to fonri
sulfur oxides, which then react with tons of circulating, high temperature, ground
dolomite.
In molten metal processing (and to some extent in steel making), sulfur is
oxidized during processing to fonri sulfiir oxides. The produced sulfiir oxides then
react with alkaline material added to the bath to foπn a slag layer. One example of
this approach is a vitreous layer used above a molten metal bath, as taught in US
5,354,940. A typical vitreous layer was five inches of 40% calcium oxide, 40%
silicone dioxide and 20% aluminum oxide.
Although it has been known for years that it is possible to release some H2S
from a molten iron bath no one has made any productive use of this finding. In
UK 1 , 187,782, Nixon taught that a molten metal conversion process converting
methane to hydrogen would also refine the iron bath:
"The hydrogen produced in the cracking zone has a refining effect on the metal contained in the molten metal bath in contact with it. For instance, the sulphur content of the molten iron tends to be reduced as a result of the reaction:
FeS + H2 = Fe + H2S."
The EXAMPLE in the Nixon patent showed conversion of methane to high
purity hydrogen (H2 volume % purity was 99.86 and 99.70). The sulfur content of the
hydrogen gas was not reported, but presumably trace amounts of H2S were present,
at least during the early stages of the process as sulfiir present in the iron was removed
by the refining effect of the hydrogen production. In this example, the sulfiir was in
the metal and no sulfiir in the feed.
Some use has been made of molten metal baths to absorb H2S. though the bath
in question was not a molten iron bath and operated at a lower temperature than
molten iron baths.
I did not like the conventional approach to dealing with feed sulphur. Now,
refiners add large amounts of alkaline material to foπn an alkaline slag layer which has
to be tliick enough to react with sulphur compounds as they fonn or extract dissolved
sulfur from the metal. The added alkaline material consumes significant amounts of
energy when dumped into the reactor to foπn a slag layer. This slag layer in turn
creates a removal problem and eventually a disposal problem.
I wanted to be able to deal with sulfur containing feeds without unnaturally
altering the heat balance of the reactor by adding large amounts of alkaline material
to deal with the feed sulfiir. I discovered a way convert much, and potentially all, of
the feed sulfur to H2S. Such material, while highly toxic, is easily handled in any
modem refinery using conventional amine scrubbing, Claus conversion and the like
techniques. H2S is a dangerous material, but refiners have been efficiently converting
it to elemental sulfiir for over 50 years.
The solution was surprisingly simple. Change the operating conditions in the
molten metal reactor so that strong reducing environment was created. Rather than
do this adding hydrogen at ruinous expense, do it cheaply by letting the carbon level
build up in the reactor. High dissolved carbon levels in a molten iron bath could be
used to create conditions where most, or all, of the feed sulfiir could be converted to
H2S.
BRIEF SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for producing H2S from
a sulfur containing feed in a molten metal bath comprising dissolving at least 3 wt.%
carbon in a molten metal bath comprising at least 50 wt.% Fe; charging a sulfur,
hydrogen and carbon containing feed to such bath and theπnally decomposing said
feed to produce H2S and carbon, emitting the H2S as a vapor product from or above
said molten metal bath and dissolving at least a portion of said carbon in said molten
metal bath; and at least periodically adding oxygen or an oxygen containing gas to
said molten metal bath to oxidize at least a portion of said dissolved carbon from said
bath.
DETAILED DESCRIPTION DESCRIPTION OF THE PREFERRED EMBODIMENTS
FEED MATERIALS:
Any sulfur containing feed may be used, ranging from noπnally gaseous
materials, such as sulfiir containing natural gas streams to noπnally liquid and even
solid materials. The invention is especially useful at converting heavy distillate,
vacuum and other resids, solvent deasphalted pitch (SDA), aromatic extracts, FCC
slurry oil and the like into useful products without hydrotreating. In addition, the
invention can also efficiently process many sulfur containing solids such as trash,
garbage, tires, coal, virtually any other sulfur-containing material.
If the sulfur containing feed does not contain enough hydrogen, then some
hydrocarbon rich material can be added with the sulfiir containing feed to provide
sufficient hydrogen to theπnally decompose the feed to a reduced foπn or sulfiir rather
than an oxidized form of sulfiir.
PRODUCTS:
H2S is the prefeπed product. Preferably most, and ideally essentially all, of the
feed sulfur is converted to ELS.
Other products of molten metal processing include CO, C02, H2, perhaps some
methane and even some soot. In my process, a little soot can be a good thing in that
it ensures that the molten metal bath is saturated with carbon. The production of solid
soot is not especially beneficial, and will require some sort of filter or bag house
means to remove soot. Many units will have or can readily install such equipment
anyway. For these units with a baghouse or electrostatic precipitator or the like
filtering means to remove entrained solids, the presence of soot incurs no additional
capital expense and provides an easy way to ensure a reducing "atmosphere" is
always present in the molten metal
CONTROLS:
Conventional analog or digital controls may be used, measuring temperature,
preferably with optical or infrared pyrometer or protected theπnocouple; carbon by
spectrometers; level by nuclear radiation and admitting feed, CH3, C02, H20 to
maintain temperature, which must be high enough (e.g., at least 1150°C (2101°F) to
maintain the particular metal carbon composition liquid and dissolved carbon level and
H2 production within preset limits. Temperature of the molten metal is preferably
1 150° to 1600°C (2102° to 2912°F), more preferably 1250° to 1500°C (2282° to
2732°F) during feed to the reactor or crucible and usually preferably 50° to 150°C
(122° to 302°F) higher during the oxidation cycle within the single-chamber reactors
or crucibles.
REACTOR DESIGN
The process does not require any special reactor design. Any molten metal
reactor apparatus can be used. Many people will prefer to use reactors similar to
those used in steel manufacturing, e. g. the design shown 4,574,714 or in US
5,322,547 (but preferably without the slag layer).
I prefer to use a pressurized system, such as that shown in US 5,435,814.
The hardware, per se, foπns no part of the present invention.
PROCESS VARIABLES
Minimum dissolved carbon whenever the bed sees sulfiir containing feed should
be at least 2.0 wt.%, but preferably is higher.
Control of dissolved carbon level and other conditions in a molten metal reactor
allows a majority of the sulfur content of the feed to be converted to H2S, rather than
sulfur oxides or required sulfur removal as slag. High dissolved carbon levels,
preferably in excess of 3 wt. % in a molten metal iron bath, and limited oxygen
addition, allow molten metal refiners to process sulfur containing feeds without
addition of lime or other similar alkaline materials, peπnitting slag foπnation to be
reduced or eliminated.
MODIFICATIONS
Reference to documents made in the specification is intended to incorporate
such patents or literature.
What is claimed is: