Gas waste

The preheated gases react exothermically over the first-stage catalyst with the peak temperature ia the range of 330—430°C, depending on conditions and catalyst selectivity. The conversion of propylene to waste gas (carbon dioxide and carbon monoxide) is more exothermic than its conversion to acroleia. At the end of the catalyst bed the temperature of the mixture drops toward that of the molten salt coolant. 

Waste Gas Streams. Several methods of decomposing phosgene in waste gas streams are used. The outlet gas from the phosgene decomposition equipment is continuously monitored for residual phosgene content to ensure complete decomposition. 

Decomposition by Caustic Scrubbing. The waste gas stream is led through packed towers where a sodium hydroxide solution is introduced at the top of the towers. Venturi scmbbers can also be used. Makeup sodium hydroxide is added under pH control (32). 

Decomposition with Moist Activated Carbon. The waste gas stream is passed through packed activated carbon towers where water is fed at the top of the towers. The water is normally recycled. If the hydrochloric acid concentration in the recycled water exceeds 10%, the decomposition efficiency is greatly reduced. Thus, a sufficient supply of fresh water must be assured and a hydrochloric acid stream continuously taken out (33). 

Combustion. The waste gas stream is burnt to convert phosgene to carbon dioxide and HCl. An advantage of this method is that all components of the waste gas, such as CO and solvent, are burnt (34). 

The waste gas remaining after removal of ammonia and recovery of hydrogen cyanide contains enough hydrogen and carbon monoxide that it is flammable and has enough heat value to make it a valuable fuel. It is usually used to displace other fuel ia boilers. 

After removal of the unreacted ammonia and recovery of hydrogen cyanide, the waste gas is essentially all hydrogen suitable for other chemical use. The advantages of the BMA process are the high ammonia and natural gas yields and the usehil hydrogen waste gas, but the high investment and maintenance for the converter is a decided disadvantage. 

The fluohmic process is a third process for manufacturing hydrogen cyanide, which is being appHed in Spain and AustraUa. This process involves the reaction of ammonia with a hydrocarbon, usually propane or butane, in a fluidized bed of coke particles. The endothermic heat of reaction is suppHed electrically through electrodes immersed in the fluid bed. Yields from propane and ammonia are reportedly above 85% and the waste gas is essentially hydrogen, but the costs for electricity are high. Thus this process is appHcable only when there is an inexpensive source of power. 

There are situations where thermal oxidation may be preferred over catalytic oxidation for exhaust streams that contain significant amounts of catalyst poisons and/or fouling agents, thermal oxidation may be the only technically feasible control where extremely high VOC destmction efficiencies of difficult to control VOC species are required, thermal oxidation may attain higher performance and for relatively rich VOC waste gas streams, ie, having >20 25% lower explosive limit (LEL), the gas stream s explosive properties and the potential for catalyst overheating may require the addition of dilution air to the waste gas stream (12). 

Pretreatment of the raw waste gas to remove particulates, adjust temperature and humidify to saturation. 

SOURCE Data compiled from Ottengraf, S. P. P, Biological Systems for Waste Gas Elimination, 1987, Table 3. 

Based upon the above-mentioned species interactions, pilot-scale testing is generally recommended to accurately size a biofilter bed for a multicomponent waste gas stream. 

Substantial energy can be recovered using low-grade waste heat, process gas, or waste gas pressure letdown. 

The eyeles in these power reeovery applieations are relatively simple. Figures 1-2 and 1-3 are typieal examples. The eyele eonfigura-tions involve the removal of solids or liquids ahead of the expander, and often the ineoming stream is heated so its temperature will not reaeh its frost point at the diseharge. This addition of heat also inereases the amount of available power. Some examples of this applieation are expansion of waste gas, waste produets of eombustion in oxidation proeesses, waste earbon dioxide, and expansion of high-pressure synthesis gas streams. 

Gas waste stream disposal operating costs can be significant. Incineration with or without a catalyst usually requires additional fuel. A heat balance provides the amount. If sulfur must be removed before releasing the waste gas, all operating costs associated with its removal and recovery must be included. 

The converse of Case 3 is a desire to make an inert mixture (such as a waste gas stream) flammable. Figure 4 shows a similar flammability envelope for all concentrations of interest. 

Pollutant Loading-. Waste gas pollutant loadings can range from 20 to 4,500 grams per standard cubic meter (g/sm ) (9 to 1,970 grains per standard cubic foot (gr/sef)). Multiple-tray settling chambers can only handle inlet dust concentrations of less than approximately 2.3 g/sm (1.0 gr/sef) (Mycock, 1995 Parsons, 1999 Steinbach, 1999 Josephs, 1999). 

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