|
Abstract
|
Methods
and systems are provided for converting methane in a feed stream to
acetylene. The method includes removing at least a portion of heavy metals
from a hydrocarbon stream. The hydrocarbon stream is introduced into a
supersonic reactor and pyrolyzed to convert at least a portion of the methane
to acetylene. The reactor effluent stream may be treated to convert acetylene
to another hydrocarbon process. The method according to certain aspects
includes controlling the level of heavy metals in the hydrocarbon stream.
BACKGROUND
OF THE INVENTION A process is disclosed for removing contaminants from a
process stream and converting methane in the process stream to acetylene
using a supersonic flow reactor. More particularly, a process is provided for
removal of trace and greater amounts of heavy metals or compounds containing
heavy metals. This process can be used in conjunction with other contaminant
removal processes including mercury removal, water and carbon dioxide
removal, oxygenates, and removal of sulfur containing compounds containing
these impurities from the process stream. Light olefin materials, including
ethylene and propylene, represent a large portion of the worldwide demand in
the petrochemical industry. Light olefins are used in the production of
numerous chemical products via polymerization, oligomerization, alkylation
and other well-known chemical reactions. Producing large quantities of light
olefin material in an economical manner, therefore, is a focus in the
petrochemical industry. These light olefins are essential building blocks for
the modern petrochemical and chemical industries. The main source for these
materials in present day refining is the steam cracking of petroleum feeds.
The cracking of hydrocarbons brought about by heating a feedstock material in
a furnace has long been used to produce useful products, including for
example, olefin products. For example, ethylene, which is among the more
important products in the chemical industry, can be produced by the pyrolysis
of feedstocks ranging from light paraffins, such as ethane and propane, to
heavier fractions such as naphtha. Typically, the lighter feedstocks produce
higher ethylene yields (50-55% for ethane compared to 25-30% for naphtha);
however, the cost of the feedstock is more likely to determine which is used.
Historically, naphtha cracking has provided the largest source of ethylene,
followed by ethane and propane pyrolysis, cracking, or dehydrogenation. Due to
the large demand for ethylene and other light olefinic materials, however,
the cost of these traditional feeds has steadily increased. Energy
consumption is another cost factor impacting the pyrolytic production of
chemical products from various feedstocks. Over the past several decades,
there have been significant improvements in the efficiency of the pyrolysis
process that have reduced the costs of production. In a typical or
conventional pyrolysis plant, a feedstock passes through a plurality of heat
exchanger tubes where it is heated externally to a pyrolysis temperature by
the combustion products of fuel oil or natural gas and air. One of the more
important steps taken to minimize production costs has been the reduction of
the residence time for a feedstock in the heat exchanger tubes of a pyrolysis
furnace. Reduction of the residence time increases the yield of the desired
product while reducing the production of heavier by-products that tend to
foul the pyrolysis tube walls. However, there is little room left to improve
the residence times or overall energy consumption in traditional pyrolysis
processes. More recent attempts to decrease light olefin production costs
include utilizing alternative processes and/or feed streams. In one approach,
hydrocarbon oxygenates and more specifically methanol or dimethylether (DME)
are used as an alternative feedstock for producing light olefin products.
Oxygenates can be produced from available materials such as coal, natural
gas, recycled plastics, various carbon waste streams from industry and
various products and by-products from the agricultural industry. Making
methanol and other oxygenates from these types of raw materials is well
established and typically includes one or more generally known processes such
as the manufacture of synthesis gas using a nickel or cobalt catalyst in a
steam reforming step followed by a methanol synthesis step at relatively high
pressure using a copper-based catalyst. Once the oxygenates are formed, the
process includes catalytically converting the oxygenates, such as methanol,
into the desired light olefin products in an oxygenate to olefin (OTO)
process. Techniques for converting oxygenates, such as methanol to light
olefins (MTO), are described in U.S. Pat. No. 4,387,263, which discloses a
process that utilizes a catalytic conversion zone containing a zeolitic type
catalyst. U.S. Pat. No. 4,587,373 discloses using a zeolitic catalyst like
ZSM-5 for purposes of making light olefins. U.S. Pat. No. 5,095,163; U.S.
Pat. No. 5,126,308 and U.S. Pat. No. 5,191,141 on the other hand, disclose an
MTO conversion technology utilizing a non-zeolitic molecular sieve catalytic
material, such as a metal aluminophosphate (ELAPO) molecular sieve. OTO and
MTO processes, while useful, utilize an indirect process for forming a
desired hydrocarbon product by first converting a feed to an oxygenate and
subsequently converting the oxygenate to the hydrocarbon product. This
indirect route of production is often associated with energy and cost
penalties, often reducing the advantage gained by using a less expensive feed
material. Recently, attempts have been made to use pyrolysis to convert
natural gas to ethylene. U.S. Pat. No. 7,183,451 discloses heating natural
gas to a temperature at which a fraction is converted to hydrogen and a
hydrocarbon product such as acetylene or ethylene. The product stream is then
quenched to stop further reaction and subsequently reacted in the presence of
a catalyst to form liquids to be transported. The liquids ultimately produced
include naphtha, gasoline, or diesel. While this method may be effective for
converting a portion of natural gas to acetylene or ethylene, it is estimated
that this approach will provide only about a 40% yield of acetylene from a
methane feed stream. While it has been identified that higher temperatures in
conjunction with short residence times can increase the yield, technical
limitations prevent further improvement to this process in this regard. While
the foregoing traditional pyrolysis systems provide solutions for converting
ethane and propane into other useful hydrocarbon products, they have proven
either ineffective or uneconomical for converting methane into these other
products, such as, for example ethylene. While MTO technology is promising, these
processes can be expensive due to the indirect approach of forming the
desired product. Due to continued increases in the price of feeds for
traditional processes, such as ethane and naphtha, and the abundant supply
and corresponding low cost of natural gas and other methane sources
available, for example the more recent accessibility of shale gas, it is
desirable to provide commercially feasible and cost effective ways to use
methane as a feed for producing ethylene and other useful hydrocarbons. In the
process of the present invention, it has been found important to minimize the
concentration of water as well as carbon monoxide and carbon dioxide to avoid
the occurrence of a water shift reaction which may result in undesired
products being produced as well as reduce the quantity of the desired
acetylene. Other contaminants should be removed for environmental, production
or other reasons including the repeatability of the process. Since variations
in the hydrocarbon stream being processed in accordance with this invention
may result in product variations, it is highly desired to have consistency in
the hydrocarbon stream even when it is provided from different sources.
Natural gas wells from different regions will produce natural gas of
differing compositions with anywhere from a few percent carbon dioxide up to
a majority of the volume being carbon dioxide and the contaminant removal
system will need to be designed to deal with such different compositions. It
has been found that a variety of hydrocarbon containing heavy metals or
compounds containing heavy metals need to be removed from hydrocarbon
streams. These include arsenic, antimony, tin, lead, vanadium, nickel, iron
and tungsten. These metals can interfere with the functioning of catalysts in
downstream processes and equipment
|
