Acid heat recovery

Smith, R. M., Sheputis, J., Kim, U. B., and Chin, Y. B., Sulfuric Acid Heat Recovery System (HRS) Operations at Namhae Chemical Corporation, Korea, paper read at Sulphur 88, Vienna, Austria, Nov. 6-9, 1988. 

Fig. 24.7. Schematic of acid heat to steam energy recovery system, after Puricelli et al., 1998. It is for intermediate H2S04 making, Fig. 9.6. Note (i) the double packed bed H2S04 making tower and (ii) boiler. Industrial acid heat recovery H2S04 making towers are 25m high and 10 m diameter. They produce 2000 to 4000 tonnes of H2S04 per day. For photographs see Sulfur, 2004.--------- large flows. small flows.

Corrosion rates for materials in contact with acid or gas usually increase with increasing temperarnre. A notable exception is stainless steels used for acid heat recovery systems (Chapter 24) which exhibit low corrosion rates in a narrow window of high temperature and acid concentration (Fig. 30.1 Viergutz, 2009). 

Figure 30.1 Operating windows for MECS acid heat recovery systems (HRS) indicating low corrosion rates (window A-B-C) at high temperatures ( 180 °C) and high acid concentrations (>99 mass% H2SO4 Viergutz, 2009). Low corrosion rates also occur at lower temperatures ( 100 °C) and lower acid concentrations ( 98 mass% H2SO4) as shown in window D-E-F. 310M stainless steel is commonly used for HRS. Copyright 2013 MECS, Inc. All rights reserved. Used by permission of MECS, Inc.

Cost estimates are for a 3 1 double contact type acid plant constructed of modem materials (mostly stainless steel) with a steam turbine generator for producing electrical energy. The acid plant is constructed on a new site with little or no existing infrastructure. Sulfur is purchased and received at site in solid form. Hot acid heat recovery systems such as MECS s HRS and Outotec s HEROS are not included. 

Considerable hot gas heat is recovered as electrical energy, shown in the table as electricity credit. Hot acid heat recovery is not included. Sulfur [Mice, /tonne (inass% S in H2SO4/100) = l75/tOTine 0.33. ... 

The plant treats 140,000 Nm /h of feed gas with an SO2 concentration of 8 volume%. Hot acid heat recovery is not included. No heat recovery to steam equipment is used. All excess heat is rejected to the atmosphere as warm air. 

In a 500 ml. three-necked flask, fitted with a reflux condenser and mechanical stirrer, place 121 g. (126-5 ml.) of dimethylaniline, 45 g. of 40 per cent, formaldehyde solution and 0 -5 g. of sulphanilic acid. Heat the mixture under reflux with vigorous stirring for 8 hours. No visible change in the reaction mixture occurs. After 8 hours, remove a test portion of the pale yellow emulsion with a pipette or dropper and allow it to cool. The oil should solidify completely and upon boiling it should not smell appreciably of dimethylaniline if this is not the case, heat for a longer period. When the reaction is complete, steam distil (Fig. II, 41, i) the mixture until no more formaldehyde and dimethylaniline passes over only a few drops of dimethylaniline should distil. As soon as the distillate is free from dimethylaniline, pour the residue into excess of cold water when the base immediately solidifies. Decant the water and wash the crystalline solid thoroughly with water to remove the residual formaldehyde. Finally melt the solid under water and allow it to solidify. A hard yellowish-white crystalline cake of crude base, m,p. 80-90°, is obtained in almost quantitative yield. RecrystaUise from 250 ml. of alcohol the recovery of pure pp -tetramethyldiaminodiphenylmethane, m.p. 89-90°, is about 90 per cent. 

Owing to the cycHc nature of the TBRC operation, waste heat recovery from the off-gases is not practical and the SO2 content of the gas varies with the converter cycle. In order to supply a relatively uniform flow and strength SO2 gas to a sulfuric acid plant, a system has been installed at RonnskAr whereby the SO2 from fluctuating smelter gases is partially absorbed in water. During smelter gas intermption, SO2 is stripped with air and the concentrated gas deflvered to the acid plant. 

Commercial condensed phosphoric acids are mixtures of linear polyphosphoric acids made by the thermal process either direcdy or as a by-product of heat recovery. Wet-process acid may also be concentrated to - 70% P2O5 by evaporation. Liaear phosphoric acids are strongly hygroscopic and undergo viscosity changes and hydrolysis to less complex forms when exposed to moist air. Upon dissolution ia excess water, hydrolytic degradation to phosphoric acid occurs the hydrolysis rate is highly temperature-dependent. At 25°C, the half-life for the formation of phosphoric acid from the condensed forms is several days, whereas at 100°C the half-life is a matter of minutes. 

A further enhancement to the HRS process whereby the exhaust from a gas fired turbine is used to superheat steam from the HRS process is also possible (129). The superheated steam is then fed through a turbogenerator to produce additional electricity. This increases the efficiency of heat recovery of the turbine exhaust gas. With this arrangement, electric power generation of over 13.6 kW for 1 t/d (15 kW/STPD) is possible. Good general discussions on the sources of heat and the energy balance within a sulfuric acid plant are available (130,131). 

In recent years alkylations have been accompHshed with acidic zeoHte catalysts, most nobably ZSM-5. A ZSM-5 ethylbenzene process was commercialized joiatiy by Mobil Co. and Badger America ia 1976 (24). The vapor-phase reaction occurs at temperatures above 370°C over a fixed bed of catalyst at 1.4—2.8 MPa (200—400 psi) with high ethylene space velocities. A typical molar ethylene to benzene ratio is about 1—1.2. The conversion to ethylbenzene is quantitative. The principal advantages of zeoHte-based routes are easy recovery of products, elimination of corrosive or environmentally unacceptable by-products, high product yields and selectivities, and high process heat recovery (25,26). 

Fig. 1. Schematic of nitric acid from ammonia showing integration of reactor heat recovery, power recovery from tailgas, and air compression (3).

A more obvious energy loss is the heat to the stack flue gases. The sensible heat losses can be minimized by reduced total air flow, ie, low excess air operation. Flue gas losses are also minimized by lowering the discharge temperature via increased heat recovery in economizers, air preheaters, etc. When fuels containing sulfur are burned, the final exit flue gas temperature is usually not permitted to go below about 100°C because of severe problems relating to sulfuric acid corrosion. Special economizers having Teflon-coated tubes permit lower temperatures but are not commonly used. 

In sulfuric acid production involving heat recovery and recovery of waste sulfuric acid, acids of various concentrations at high temperatures can be dealt with. Corrosion damage has been observed, for example, in sulfuric acid coolers, which seriously impairs the availability of such installations. The use of anodic protection can prevent such damage. 

Anodic protection allows the use of materials under unfavorable conditions if they are also passivatable in sulfuric acid. CrNi steels [material Nos. 1.4541 (AISI 321) and 1.4571 (AISI 316 Ti)] can be used in handling sulfuric acid of 93 to 99% at temperatures up to 160°C. This enables a temperature of 120 to 160°C to be reached, which is very suitable for heat recovery. 

When cooling combustion flue gas for heat recovery and efficiency gain, the temperature must not be allowed to drop below the sulfur trioxide dew point. Below the SO3 dew point, very corrosive sulfuric acid forms. The graph in Figure 1 allows determination of the acid dew point us shown in Example 1. 

These three approaches to reject heat and exhaust fuel recovery with power generation apply primarily to the higher temperature, solid oxide (1800 F) and molten carbonate (1200 F), fuel cell systems operating on CH4 fuel. The lower operating temperatures of the phosphoric acid (400 F) and polymer electrolyte (175 F) fuel cells severely limit the effectiveness of thermal cycle based power generation as a practical means of heat recovery. 

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