Nitric tail gases

Sulfur Dioxide Processing, Repriuts of 1972—1974 Chem. Eng. Prog, articles, AIChE, New York (1975). Contaius thirteen papers on flue gas desulfurization, two on SO2 control iu pulp and paper, one on sulfuric acid tail gas, one on SO2 from ore roasting, and two on NO from nitric acid. 

Weak Acid. Stainless steels (SS) have exceUent corrosion resistance to weak nitric acid and are the primary materials of constmction for a weak acid process. Low carbon stainless steels are preferred because of their resistance to corrosion at weld points. However, higher grade materials of constmction are required for certain sections of the weak acid process. These are limited to high temperature areas around the gau2e (ca 900°G) and to places in which contact with hot Hquid nitric acid is likely to be experienced (the cooler condenser and tail gas preheater). 

Although this hook deals almost exclusively with modern turho-expanders, the tail gas expander in nitric acid plants enjoys a somewhat special relationship with the compressors that are almost always associated with this turhotrain. 

Figure 4-21. Tail-gas expander for a 945-t/day two-pressure nitric acid plant P = 11,200 kW, n = 5,850 rpm. (Source GHH-Borsig.)...

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. 

Yamagushi, M., Matsushite, K. and Takami, K. (1976) Remove nitrogen oxides (NOx) from nitric acid tail gas, Hyd. Proc. 55(8), 101. 

The threshold limit for nitric acid is only 2 ppm. A few breaths of 200 to 700 ppm may be fatal within 5 to 8 hours. Anyone exposed to nitrogen oxide fumes should be observed for 48 hours. It is obviously essential that a tall, well designed stack be employed to aid in the effective dispersion of tail gas. 

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. 

The design must consider three feed streams and two product streams. The three inlet feed streams are strong reaction gases, weak nitric acid solution and make-up water. Two outlet streams flow from the column. These are a lean reaction gas (tail-gas) stream and red product acid. Absorption of nitrous oxides increases as the temperature is reduced. This effect, together with the exothermic oxidation/ absorption reactions, requires installation of an internal cooling circuit. 

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. 

Catalytic Reaction of Ammonia and Air, 7) Energy Recovery by Steam Generation and/or Gas Re-Heating, 8) Gas Cooling, 9) Dual Pressure Only NOx Compression, 10) Absorption with Nitric Acid Production, and 11) Tail Gas Energy Recovery. 

In an economic comparison of abatement systems, a 1991 EPA study indicates that extended absorption to be the most cost-effective method for NOx removal. Selective Reduction matches its performance only in small-capacity plants of about 200 to 250 tonnes per day. Nonselective abatement systems were indicated to be the least cost-effective method of abatement. The results of any comparison depend on the cost of capital versus variable operating costs. A low capital cost for SCR is offset by the ammonia required to remove the NOx. Higher tail gas NOx concentrations make this method less attractive. The investment for extended absorption is partially recovered by increased yield of nitric acid product104. 

Adsorption Abatement uses acid-resistant molecular sieves to absorb the NOx from the tail gas. The adsorbent is periodically regenerated and the NOx recovered, converted to NOj and recycled to the plant for recovery as nitric acid. The tail gas may also be mixed with a reducing agent (usually natural gas or ammonia) and passed through a catalyst to reduce the NOx to elemental nitrogen. When natural gas is used, conditions must be carefully controlled and facilities installed to control the heat released by natural gas that reacts with both the NOx and the oxygen91. 

The advantages of the SCR system are97 ammonia is readily available in a nitric acid plant a low NOx content can be achieved the increase in tail gas temperature is negligible and no oxygen is consumed. 

The capital cost of an integrated SCR unit for a new 1,000 tonne/day plant is estimated to be 1.5% of the total capital cost of the nitric acid plant. This cost includes the cost of the SCR catalyst but excludes spare parts. The capital cost of an end-of-pipe SCR unit for an existing 1,000 tonne/day plant is estimated to be 3% to 6% of the total capital cost of the nitric acid plant. But this is very dependent on the type of nitric acid process. The SCR will increase operating costs by 1.1% when NOx in the tail gas is reduced from 1,000 ppmv to 200 ppmv. The maintenance cost of the SCR unit is typically 2.5% of the capital cost97. 

The SCR of N20 with hydrocarbons is an exothermic reaction and the adiabatic temperature rise is 30 to 100°C, depending on the concentration of added hydrocarbon. In an existing nitric acid plant, the expander is usually designed to work at a very well-defined temperature, so the heat of the N20 removal reaction has to be removed. A tail gas cooler can be used for this purpose. Direct decomposition of N20 produces only 3 to 5°C of heat so cooling of the gases before they enter the expander is normally not necessary with this process221. 

High chromium (20% to 27%) stainless steel is used in the cooler condenser and the tail gas preheater. Compared to low carbon stainless steel, it has better corrosion resistance to nitric acid at elevated temperatures so the extra cost is justified by the longer life104. 

Zirconium is used in nitric acid service for cooler condensers, tail gas preheaters and reboilers. It rivals tantalum in its corrosion resistance to nitric acid at all concentrations up to the boiling point. Its resistance extends up to 230°C and 65 wt %. However it is susceptible to stress corrosion cracking, which can be prevented by avoiding high, sustained tensile stresses104. 

The main unit operations in nitric acid plants are 97 ammonia evaporation, ammonia filtration, air filtration, air compression, air/ammonia mixing, catalytic reaction of ammonia and air, energy recovery by steam generation and/or gas reheating, gas cooling, dual pressure only— NOx compression, absorption with nitric acid production, and tail gas energy recovery. 

The main environmental factor that affects nitric acid process selection is the concentration of NOx in the tail gas. In the United States, gaseous emissions from newly constructed nitric acid plants are limited to 1.5 kilograms NOx per tonne of nitric acid produced with a maximum opacity of 10 percent. 

Adsorption abatement uses acid-resistant molecular sieves to absorb the NOx from the tail gas. The adsorbant is periodically regenerated and the NOx recovered, converted to N02 and recycled for recovery as nitric acid.91... 

Zeolites have also proven applicable for removal of nitrogen oxides (NO ) from wet nitric acid plant tail gas (59) by the UOP PURASIV N process (54). The removal of NO from flue gases can also be accomplished by adsorption. The Unitaka process utilizes activated carbon with a catalyst for reaction of NO, with ammonia, and activated carbon has been used to convert NO to N02, which is removed by scrubbing (58). Mercury is another pollutant that can be removed and recovered by TSA. Activated carbon impregnated with elemental sulfur is effective for removing Hg vapor from air and other gas streams the Hg can be recovered by ex situ thermal oxidation in a retort (60). The UOP PURASIV Hg process recovers Hg from clilor-alkali plant vent streams using more conventional TSA regeneration (54). Mordenite and clinoptilolite zeolites are used to remove HQ from Q2, clilorinated hydrocarbons, and reformer catalyst gas streams (61). Activated aluminas are also used for such applications, and for the adsorption of fluorine and boron—fluorine compounds from alkylation (qv) processes (50). 

A few classicaV studies on the reactivity of HCs to reduce NOx with catalysts indicated that the use of such reductants for controlling mobile NOx emissions was quite attractive to the automotive industry, thereby the advent of a new type of HC-SCR technology in the mid-1980s. An example may be the treatment process of the tail gas from nitric acid production plant via ammonia oxida-tion. The process includes the usual injection of excessive amounts of HCs over supported noble metals such as Pt, Pd and Rh to eliminate the yellowish stack plume due to 0.1 - 0.5% NOx, mainly NO2, from the nitric acid plant. 

The second process is known as selective catalytic reduction (SCR). SCR may also be used in the treatment of nitric acid process tail gas and similar processes, but has achieved prominence through its application to NO removal from electricity-generating power stations, especially those that are coal fired. In SCR, a range of reductants for the NOx can be used the most common is ammonia. The primary reactions involved arc shown in Eqs. (13) and (14). Oxygen is required for this form of NO control, and levels of 2-3% are typically needed for optimum catalyst performance. 

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