Tail gas emissions

Gas Treatment and Sulfur Recovery SO, NO. and HjS from vent and tail gas emissions. 

Normal operation should be well within the environmental regulation limits set by the EPA. Liquid waste is virtually non-existent and can be sent to the normal sewerage drains. Any acid spills should be diluted. Tail-gas emissions are thought to be less than 1000 ppm of nitrogen oxides (about half the current EPA limit). Should tail-gas emissions exceed this figure, then a catalytic combustor would be necessary to reduce nitrogen oxide levels to below 400 ppm. 

The product specification requires 11 670 kg/h of 60% (wt.) nitric acid solution, excluding dissolved nitrogen oxides. This specification must be achieved while restricting the level of tail-gas emissions to below 1000 ppm of nitrogen oxides. This acid product requires approximately 2340 kg/h of deionized make-up water. 

There are several performance parameters required from this unit, including the production of 60% (wt.) nitric acid product (on a dissolved gas-free basis). This product must be obtained while ensuring the tail-gas emissions are kept below 1000 ppm. 

NONE No Flow 1. Pump failure. Deficient quality product and high NOx tail gas emission levels. a) Install LOW LEVEL ALARM on LIC at the base of the absorption column. 

PART OF High NOx Composition I 1. Improved yield from reactor. Higher tail-gas emission levels possible. k) Manually increase make-up water flowrate. 

MORE OF More Flow 5. Increased gas feed at inlet 6. Decreased NOx absorption. Transfer line subject to higher pressures. As for 5. Tail-gas emission levels up. Covered by b) and c). Covered by b) and c). d) Look to altering make-up water feed rate in response. ... 

More Temperature 6. Higher inlet temperatures. Less dissolved NOx in acid but higher NOx tail-gas emissions. Covered in Tables 9.6, 9.7 and 9.8... 

Tail gas emissions are controlled by improving the S02 conversion efficiency and by scrubbing the tail gas. In a double absorption process plant, a five-bed converter has 0.3 percent unconverted S02, as compared with 0.5 percent for a four-bed converter. A Lurgi Peracidox scrubber may be used to remove up to 90 percent of the residual S02 in the tail gas from a double absorption plant. Hydrogen peroxide or electrolytically produced peroxymonosulfuric acid is used to convert the S02 to H2S04 in the Lurgi scrubber. 

The absorber tail gas contains about 20 mol % hydrogen and has a higher heating value of ca 2420 kj/m (65 Btu/SCF). With increased fuel costs and increased attention to the environment, tail gas is burned for the twofold purpose of generating steam and eliminating organic and carbon monoxide emissions. 

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

Low-temperature activity promotion is an issue in mobile (diesel) applications, but may not be a critical issue in several stationary applications, apart from those where the temperature of the emissions to be treated is below 200°C (for example, when a retrofitting SCR process must be located downstream from secondary exchangers, or in the tail gas of expanders in a nitric acid plant). In the latter cases, a plasmacatalytic process [91] could be interesting. In the other cases, the use of NTP together with the SCR catalyst is not economically viable. However, the synergetic combination of plasma and catalysts has been shown to significantly promote the conversion of hazardous chemicals such as dioxins [92], Although this field has not yet been explored, it may be considered as a new plasmacatalytic SCR process for the combined elimination of NO, CO and dioxins in the emissions from incinerators. 

Hydrosulfreen A process for removing sulfur compounds from the tail gas from the Claus process. It combines the Sulfreen process with an upstream hydrolysis/oxidation stage, which improves efficiency and optimizes the emission control. Developed jointly by Lurgi and Societe National Elf Aquitaine, and installed in 1990 in the Mazovian Refining and Petrochemical Works, near Warsaw, Poland. See also Oxysulfreen. 

Liquefaction of chlorine is always incomplete, because the non-condensable impurities carry chlorine at its vapour pressure as they leave the liquefaction process. This exit gas, or tail gas, is handled in any of several different ways. It is of course an intolerable plant emission, and the contained chlorine must at least be destroyed before the gas is released to the atmosphere. There is also a powerful economic incentive for recovering much of the chlorine in some usable form - Silver s estimate of the value of the chlorine in the tail gas produced in the United States alone in 1981 was 50 million [3]. 

One of the main drivers for the development of new sulphuric acid catalysts over the last decades has been the desire to reduce S02 emissions from sulphuric acid plants without costly tail gas cleaning or an additional interbed... 

The background for the development of VK69 was a need for reduction of S02 emissions from double-absorption plants by installing a more active catalyst at low temperature downstream from the intermediate absorption tower. Clearly, the catalytic solution should be more competitive than the alternatives, e.g. tail gas scrubbing or triple-absorption layout, in terms of capital and operating costs. In the following, the required technical performance of the catalyst with respect to S02 oxidation activity, mechanical strength and pressure drop is discussed, and input from the literature and from practical experience in the field is presented. Reviews of the extensive literature published on sulphuric acid catalysts can be found in [2-5],... 

Process Alternatives. Process alternatives for sulfur recovery are shown schematically in Figure 2. The choice of either elemental sulfur or sulfuric acid will depend on economics and markets related to each plant location. Elemental sulfur may be produced by gas-phase oxidation (the Claus process) or liquid-phase oxidation (e.g., the Stretford process). Stretford units were described in Section 1 and are well discussed in the literature (1, 2> 5) Claus sulfur recovery efficiency is usually less than required by current air emission standards. Therefore, some form of tail-gas treating is required. Sulfuric acid may be produced by the well-known contact process (6). This process is licensed by a number of firms, each of which has its own... 

Tail gas cleanup is required because a well-designed Claus plant with three catalytic stages and fresh catalyst will recover only 95-97% of its feed sulfur (8), which is not generally sufficient to meet current emission standards. In addition, feed impurities and catalyst aging will reduce overall recovery in some plants to about 92% just before catalyst changeout. Therefore, tail-gas cleanup is required. Tail-gas treating processes are generally classified as follows ... 

Tail Gas Cleanup Process Efficiency - Required process efficiency depends on applicable emission regulations. Low-efficiency processes result in up to 99.0-99.5% overall sulfur recovery when combined with the Claus plant and include the Sulfreen, SNPA/Haldor-Topsoe, CBA, IFP, and Beavon Mark II processes. High-efficiency tail-gas treating processes can achieve overall sulfur recoveries of 99.8% and above under ideal conditions. These include the Beavon Mark I, SCOT, Trencor, and Wellman-Lord processes. 

In the eady 1970s, air pollution requirements led to the adoption of the double contact or double absorption process, which provides overall conversions of better than 99.7%. The double absorption process employs the principle of intermediate removal of the reaction product, ie, S03, to obtain favorable equilibria and kinetics in later stages of the reaction. A few single absorption plants are still being built in some areas of the wodd, or where special circumstances exist, but most industtialized nations have emission standards that cannot be achieved without utilizing double absorption or tail-gas scrubbers. A discussion of sulfuric acid plant air emissions, control measures, and emissions calculations can be found in Reference 98. 

Tail gas scmbbers are sometimes used on single absorption plants to meet S02 emission requirements, most frequently as an add-on to an existing plant, rather than on a new plant. Ammonia (qv) scrubbing is most popular, but to achieve good economics the ammonia value must be recovered as a usable product, typically ammonium sulfate for fertilizer use. A number of other tail gas scrubbing processes are available, including use of hydrogen peroxide, sodium hydroxide, lime and soda ash. Other tail gas processes include active carbon for wet oxidation of S02, molecular sieve adsorbents (see MoLECULARSIEVEs), and the absorption and subsequent release of S02 from a sodium bisulfite solution. 

The primary problem with the systems that have been developed to utilize lower feed sulfur concentrations is the reduced efficiency of sulfur recovery. This effect is not so apparent in lost sulfur revenues as it is in the increased size and cost of the downstream tail-gas treating unit required to minimize the sulfur emissions from the overall sulfur-treatment section. 

The sulfur plant of the energy refinery, as contrasted to other sulfur plant applications, will have a high concentration of carbon dioxide. This CO2 will have a significant impact, both chemically and thermally, on the sulfur plant operation and downstream tail-gas treating. Energy demands and/or sulfur emissions may be adversely affected, relative to known operations. 

Beavon, D.K., Hass, R.H. and Muke, B., "High Recovery, Lower Emission Promised for Claus-Plant Tail Gas", Oil Gas Journal, (1979) March 12. 

Furnace temperatures have also been shown to be important in controlling the formation of COS. While COS has little effect on the downstream Claus catalyst efficiency its presence in the gas stream leads to higher loading of the reductive tail gas clean-up processes (e.g. SCOT, BSR, see environment) or to higher SO2 emissions in the stack gas. The recent developments regarding the control of its formation in the front end furnace are thus a significant contribution to the improvement of environmental quality control. 

Although the Claus catalytic conversion is a highly efficient process as presently employed in sulfur recovery plants the continuing efforts to reduce sulfur emissions to atmosphere demand that the last possible ounce of efficiency be squeezed from the process. Whether further small but critical improvements in the already high sulfur recovery efficiency can be achieved by more fine tuning of the converters and their catalyst charge remains to be seen. What cannot be accomplished in the catalytic converters will be achieved in the tail gas desulfurization processes. 

Reductive Tail Gas Treatments. It was largely as a result of the effort to achieve better than 99% recovery that the reductive tail gas desulfurization processes (46) were developed in the 1970 s. The two main methods are the Beavon Sulfur Removal (BSR) (47) and the Shell Claus Off-Gas Treatment (SCOT) (48) processes. Both of these processes are now widely used as tail gas desulfurization units on sulfur recovery plants and can readily achieve point source emission levels below 250 ppm and below 100 ppm if necessary to meet regulatory standards. 

Off-gas from the coke burner ( -Gas) contains nitrogen, sulphur dioxide (SO2), hydrogen sulphide (H2S), carbon monoxide (CO), carbon dioxide (CO2), water vapour and other trace contaminants. The -Gas is directed to the CO Boiler for incineration where sulphur compounds are converted to SO2. The Boiler flue gas is passed through electrostatic precipitators for particulate control and then emission to atmosphere. The CO Boiler also serves as the Sulphur Plant tail gas incinerator. Maximum sulphur emissions are 146 tonnes/day or 10.6% of sulphur contained in bitumen feed to the cokers. 

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