Gas separation: processes – Solid sorption – Organic gas or liquid particle sorbed
Reexamination Certificate
2001-07-23
2003-02-11
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Organic gas or liquid particle sorbed
C095S050000, C095S096000, C095S145000, C585S818000, C585S820000
Reexamination Certificate
active
06517611
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to use of adsorbents in purification of relatively impure olefins such as are typically produced by thermal cracking of suitable hydrocarbon feedstocks. More particularly, this invention concerns purification by passing an olefinic stream, containing alkanes, small amounts of acetylenic impurities, carbon oxides and/or other organic components, which are typically impurities in cracked gas oil, in contact with an adsorbent comprising a crystalline titanium silicate under conditions suitable for adsorption of olefins and/or alkynes.
Generally, this invention is directed to separating useful alkenes (olefins) and/or alkynes from alkanes (paraffins) of the same carbon content and is more specifically directed to separating ethylene or propylene from mixed streams of ethane/ethylene or propane/propylene, respectively, using CTS titanium silicate adsorbents.
BACKGROUND OF THE INVENTION
As is well-known, olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. The simplest member of the series, ethylene, is the largest volume organic chemical produced today. Importantly, olefins including ethylene, propylene and smaller amounts of butadiene, are converted to a multitude of intermediate and end products on a large scale, mainly polymeric materials.
Commercial production of olefins is almost exclusively accomplished by pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Thermal cracking feedstocks include streams of ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Because of the very high temperatures employed, commercial olefin processes invariably coproduce significant amounts of acetylene. Required separation of the acetylene from the primary olefin can considerably increase the plant cost.
In a typical ethylene plant, the cracking represents about 25% of the cost of the unit, while the compression, heating, dehydration, recovery and refrigeration sections represent the remaining percentage of the total. This endothermic process is carried out in large pyrolysis furnaces with the expenditure of large quantities of heat, which is provided in part by burning the methane produced in the cracking process. After cracking, the reactor effluent is put through a series of separation steps involving cryogenic separation of products such as ethylene and propylene. The total energy requirements for the process are thus very large, and ways to reduce it are of substantial commercial interest. In addition, it is of significant interest to reduce the amounts of methane and heavy fuel oils produced in the cracking processor and utilize them for other than for their fuel value.
Hydrocarbon cracking is carried out using a feed, which is ethane, propane, or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Ethane, propane, liquid naphthas, or mixtures thereof are preferred feed to a hydrocarbon cracking unit. Hydrocarbon cracking is generally carried out thermally in the presence of a dilution steam in large cracking furnaces which are heated, at least in part, by burning methane and other waste gases from the olefins process resulting in large amounts of NO, pollutants. The hydrocarbon cracking process is very endothermic and requires large quantities of heat per pound of product. However, newer methods of processing hydrocarbons utilize, at least to some extent, catalytic processes, which are better able to be tuned to produce a particular product slate. The amount of steam used per pound of feed in the thermal process depends to some extent on the feed used and the product slate desired. Typically, steam pressures are in the range of about 30 lbs. per sq. in. to about 80 lbs. per sq. in. (psi), and amounts of steam used are in the range of about 0.2 lbs. of steam per pound of feed to 0.7 lbs. of steam per pound of feed. The temperature, pressure, and space velocity ranges used in thermal hydrocarbon cracking processes depend to some extent upon the feed used and the product slate desired, which are well-known and may be appreciated by one skilled in the art. The type of furnace used in the thermal cracking process is also well-known.
Several methods are known for separation of an organic gas containing unsaturated linkages from gaseous mixtures. These include, for instance, cryogenic distillation, liquid absorption, membrane separation and the so-called “pressure swing adsorption” in which adsorption occurs at a higher pressure than the pressure at which the adsorbent is regenerated. Cryogenic distillation and liquid absorption are common techniques for separation of carbon monoxide and alkenes from gaseous mixtures containing molecules of similar size, e.g. nitrogen or methane. However, both techniques have disadvantages such as high capital cost and high operating expenses. For example, liquid absorption techniques suffer from solvent loss and need a complex solvent make-up and recovery system.
Olefin-paraffin separations represent a class of most important and also most costly separations in the chemical and petrochemical industry. Cryogenic distillation has been used for over 60 years for these separations. They remain to be the most energy-intensive distillations because of the close relative volatilities. For example, ethane-ethylene separation (c
2
splitter) is carried out at about −25° C. and 320 lbs. per sq. in. gage pressure (psig) in a column containing over 100 trays, and propane-propylene separation is performed by an equally energy-intensive distillation at about −30° C. and 30 psig. The energy costs in olefin/paraffin separations are enormous. Recent revamps of ethylene plants have involved replacing distillation trays in the towers and heat exchange tubing in condensers and reboilers to reduce energy costs. New methods of process control and manipulation of feed point, product draw, de-ethanizer processing have all been used to control energy usages in an ethylene plant. Obviously, new methods of olefin/paraffin separation, which are less energy intensive as the present distillations, would be welcomed and could replace or at least augment the present C
2
splitter distillation processes.
Listed below are the mole weight and atmosphere boiling points for the light products from thermal cracking and some common compounds potentially found in an olefins unit. Included are some compounds, which have similar boiling temperatures to cracked products and may be present in feedstocks or produced in trace amounts during thermal cracking.
Normal
Mole
Boiling
Compound
Weight
Point, ° C.
Hydrogen
2.016
−252.8
Nitrogen
28.013
−195.8
Carbon Monoxide
28.010
−191.5
Oxygen
31.999
−183.0
Methane
16.043
−161.5
Ethylene
28.054
−103.8
Ethane
30.070
−88.7
Phosphine
33.970
−87.4
Acetylene*
26.038
−84.0
Carbon Dioxide*
44.010
−78.5
Radon
222.00
−61.8
Hydrogen Sulfide
34.080
−60.4
Arsine
77.910
−55.0
Carbonyl Sulfide
60.070
−50.3
Propylene
42.081
−47.8
Propane
44.097
−42.1
Propadiene (PD)
40.065
−34.5
Cyclo-Propane
42.081
−32.8
Methyl Acetylene
40.065
−23.2
Water
18.015
100.0
*Sublimation temperature
Recently, the trend in the hydrocarbon processing industry is to reduce commercially acceptable levels of impurities in major olefin product streams, i.e., ethylene, propylene, and hydrogen. Need for purity improvements are directly related to increasing use of higher activity catalysts for production of polyethylene and polyproypropylene, and, to a limited, extent other olefin derivatives.
It is known that acetylene can be selectively hydrogenated and thereby removed from such product streams by passing the product stream over an acetylene hydrogenation catalyst in the presence of molecular hydrogen, H
2
. However, these hydrogenation processes typically result in the deposition of carbon
Bell Valerie A.
Kuznicki Steven M.
Engelhard Corporation
Lindenfeldar Russell G.
Spitzer Robert H.
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