Off-gas streams

Propylene has many commercial and potential uses. The actual utilisation of a particular propylene supply depends not only on the relative economics of the petrochemicals and the value of propylene in various uses, but also on the location of the supply and the form in which the propylene is available. Eor example, economics dictate that recovery of high purity propylene for polymerisation from a smaH-volume, dilute off-gas stream is not feasible, whereas polymer-grade propylene is routinely recovered from large refineries and olefins steam crackers. A synthetic fuels project located in the western United States might use propylene as fuel rather than recover it for petrochemical use a plant on the Gulf Coast would recover it (see Euels, synthetic). 

The off gas stream consists of all inlet nitrogen plus excess oxygen. 

In all three mechanisms, the reactions are performed at about 800°C and the oxygen-containing by-products (H20, and C02) are removed in the off-gas stream. 

Compounds A and B are available in the off-gas stream from an absorption column at concentrations of 20 moles/m3 each. 14 m3/sec of this fluid is to be processed in a long isothermal tubular reactor. If the reactor may be assumed to approximate a plug flow reactor, what volume of pipe is required to obtain 80% conversion of species A ... 

The first report of the cyclodimerization of fluorinated olefins was provided by Lewis and Naylor,3 working at E.I. DuPont de Nemours Co., in 1947. While studying the pyrolysis of polytetrafluoroethylene (PTFE), the compound octa-fluorocyclobutane was isolated from the pyrolysis off-gas stream. The researchers identified the product and speculated that it was formed by the cyclodimerization... 

The off-gas stream from hydrotreating is usually very rich in hydrogen sulfide and light fuel gas. This gas is usually sent to a sour gas treatment and sulfur recovery unit along with other refinery sour gases. 

Sulfur is removed from a number of refinery process off-gas streams (sour gas) to meet the sulfur oxide emissions limits of the Clean Air Act and to recover salable elemental sulfur. Process off-gas streams, or sour gas, from the coker, catalytic cracking unit, hydrotreating units, and hydroprocessing units can contain high concentrations of hydrogen sulfide mixed with light refinery fuel gases. 

Influent streams with contaminant concentrations greater than 0.01% generally cannot be treated by air stripping. Even at lower influent concentrations, air strippers may not be able to meet cleanup goals. Typically, some air pollution control technology is required to capture or destroy contaminants in the off-gas stream or the treated effluent. 

The in situ nature of this treatment also minimizes potential exposure to humans and the environment. Ex situ options like excavation require repeated worker handhng of the contaminated soil and increased opportunity for volatilization of contaminants (leading to off-site contamination). The off-gas stream generated as part of the SPSH process can be treated using conventional off-gas treatment technologies such as catalytic oxidation, thermal oxidation, condensation, and granular activated carbon (GAC). 

Dowex Optipore is a polymeric adsorbent used to treat chlorinated volatile organic compound (CVOC) contaminated off-gas streams from remediation processes such as air stripping of groundwater and soil vapor extraction. According to the vendor Dowex Optipore adsorbent has the following advantages over activated carbon ... 

The system is mounted on two mobile trailers. The first trailer contains a rotary kiln in which soil is heated to 300 to 600°F. The off-gas is ducted from the rotary kiln to a cyclone separator where relatively coarse matter is removed and a heat exchanger cools the gas to about 400°F. The cooled off-gas goes to the baghouse, which filters relatively fine particles from the off-gas stream. The final unit is an afterburner or fume incinerator that heats the contaminated air to approximately 1800°F to destroy the contaminants. 

The vendor states that its chemical oxidation (ChemOx) technology is very cost effective because no off-gas stream is formed, can be retrofitted in existing facilities, and can treat large volumes of wastewater (D17707V, p. 2). 

Most metals cannot be treated by ATDU technology. Soils may require pretreatment if they contain oversized materials or greater than 30% moisture. The off-gas stream may require additional treatment by activated carbon filtration or thermal oxidization to remove light hydrocarbons. 

To accomplish the thermal desorption, contaminated media are heated, generally between 300 and 1000°F, thus driving off the water, volatile contaminants, and some semivolatile contaminants from the contaminated media and into the off-gas stream. The removed contaminants are then treated by thermal oxidation in an afterburner, condensed in a single- or multiple-stage condenser, or captured by carbon adsorption beds. 

Currently, large volumes of off-gas hydrogen are either flared, burned as fuel, or routed to secondary operations within the chemical and petroleum refining industries. Such off-gas streams represent a potentially significant source of hydrogen that, if economically recoverable, could partially satisfy the near term (1988-1990) hydrogen supply problems in the U,.S. 

Assess the potential of a thermal treatment of the off-gas, assuming that the reaction takes place in a plug-flow reactor. Determine the optimum reactor conditions for destroying N2O, while optimizing the yield of NO. The off-gas stream can be assumed to have the following composition (volume-based) 30% N20,0.7% NO, 300 ppm CO, 3% H20,4% O2, balance N2. 

Pollution Control. Zeolite adsorbents can effectively remove pollutants such as S02, H2S, and NO from industrial off-gas streams at near ambient temperature (54-57). Since water vapor usually exists along with these acidic compounds, an acid-stable or acid-resistant zeolite adsorbent is necessary for a long service life. Union Carbide announced three new processes for pollution control recently. They are the PuraSiv-Hg process for mercury vapor removal, the PuraSiv-N process for NO removal from nitric acid plant off-gas, and the PuraSiv-S process for S02 removal from... 

Jubin, R. T. A Literature Survey of Methods to Remove iodine from Off-Gas Streams Using Solid Sorbents. 0RNL/TM-6607, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 1979. 

A few ammonia plants have been located where a hydrogen off-gas stream is available from a nearby methanol or ethylene operation (e.g., Canadian plants at Kitimat, BC and Joffre, Alberta). Gas consumption at such operations range from 25 million to 27 million BTU per tonne of ammonia, depending on specific circumstances. Perhaps more important, the capital cost of such a plant is only about 50% of the cost of a conventional plant of similar capacity because only the synthesis portion of the ammonia plant is required. However, by-product carbon dioxide is not produced and downstream urea production is therefore not possible56. 

Next, characteristic properties of components are listed to select appropriate separation method (Table 3.7). Because the trace components belong to different chemical families, we eliminate gas-phase catalytic oxidation or hydrogenation. More specific chemical-based techniques remain. A first one is reversible chemical absorption. As solvents we may enumerate liquid redox systems (chelated iron), caustic washing solutions, amines or special formulations, as Selexol . Since H2S and C02 both have an add character, we may expect that a certain amount of C02 will pass in the off-gas stream. Dry chemical treatment could also be used, as reaction of H2S with iron-sponge or impregnated wood chips. 

A further requirement placed on the catalyst is a low pressure loss. This applies particularly if the conversion in a single pass through the reactor is low, so that a large amount of gas has to be recirculated. It is of prime importance for off-gas purification, in which large off-gas streams must be treated with minimal additional cost. 

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