Converter tower

SO2 is converted to SO3 in the so-called converter tower filled with 4 packed beds of V2O5 catalyst on a silica carrier. The reaction is highly exothermic and intermediate cooling of the process gas flow between the various beds with indirect air coolers is required. 

Notwithstanding the low process air dewpoint, some sulphuric acid/oleum mist condenses in the coolers following the converter tower at temperatures of about 45-50 C. This highly reactive mist can affect the quality of the subsequent sulphonation reaction and therefore a Wgh-efficiency demister is installed before the actual sulphonation step. 

S02/air leaving the burner will contain traces of "ash" and "dust" stemming from the refractory material lining the sulphur burner. Therefore the cooled S02/air is passed through a hot gas filter before entering the converter tower. 

The converter tower should be made of stainless steel 316. The catalyst mass on each of the 4 beds is "sandwiched" between two layers of quartz the top layer ensures proper gas distribution to the underlying catalyst bed and the underlayer prevents catalyst pellets falling through the stainless steel supporting grid. There is a manhole above each bed, used for catalyst inspection and catalyst replacement. Ideally these manholes should be accessible by a permanent scaffolding construction with a ladder and a small platform to each bed position. Thermocouples are located after entry and at the exit of each of the converter beds. The operation of the converter is controlled by the entry temperature of each bed. It should be verified that each thermocouple is correctly positioned and in good condition. 

At plant shut-downs the catalyst should be purged with dry process air for half an hour to remove the SO3 absorbed by the catalyst itself. After the cleaning with air, care must be taken that moist air does not enter the converter tower. Therefore all valves must be closed to prevent ambient air coming into contact with the catalyst. The catalyst is hygroscopic especially when it has been used for the SO2 SO3 conversion. Catalyst handling and sieving instruction are found in Appendix 3. 

The converter tower catalyst has to be preheated before plant start-up. A gas or oil-fired preheater (Ballestra) supplies the hot combustion gas which heats the dried process air, using the SO2 cooler as a preheater. A temperature of 4(X)°C in the catalyst tower is attained after about 3 hours. 

MM and Mazzoni prefer an electric preheater in a closed circuit. Dried process air is circulated through the converter tower, hot gas filter and electrical elements. Electrical heating offers the possibility of variable heat input. For example, during weekend stoppage (not recommended ) the electrical preheater can be set to maintain a high temperature in die converter tower and hot gas filter. This results in a quick start-up, which contributes to long catalyst life and consistendy high conversion. Moreover, a rapid start-up will increase the actual effective production time. 

SO2 concentrations in exhaust gas leaving the sulphonation system will be, during steady-state operation, of the order of 2 000 mg/m assuming 98% conversion in the SO2-SO3 converter tower. However, during plant start-up, concentrations of the order of 50 000 mg/m may occur (see 5.9.2.). The level should be reduced to less than 5 mg/m SO. in the exhaust gas leaving,the caustic soda scrubber under steady-state conditions and about 15 mg/m during initial start-up PQnditions>... 

There should also be prepared a mechanical/instrumental catalogue, containing specific drawings and/or data about plant items like compressors, chiller unit, dosing pumps, transfer pumps, main equipment items like sulphur burner, converter tower, drying equipment, reactors, mixers, etc. etc., instruments and control system. 

Make a sketch of the converter tower (see figure 10), taking into account that the catalyst beds have to be inspected annually and that sometimes the catalyst has to be removed for sieving or partial or complete replacement. Input data Appendix 3 Safety data and handling of Monsanto Vanadium Pentoxide catalyst. 

The utihty iadustry utilizes fans typically from 6.7—10 m diameter ia banks of 8 to 12 fans ia wet cooling towers. These towers cool the water used to condense the steam from the turbiaes. Many towers may be needed ia large plants requiring as many as 50 to 60 fans 12 m in diameter. These fans typically utilize velocity recovery stacks to recoup some of the velocity pressure losses and convert it to useful static pressure work. 

Separate ketdes and backwash towers are frequendy used to convert ion-exchange resins from one ionic form to another prior to packaging, and to cleanse the resin of chemicals used in the functionalization reactions. Excess water is removed from the resin prior to packaging by a vacuum drain. Both straight line filters and towers or columns are used for this purpose. 

Gas contact is typically carried out in absorption towers over which the alkaline solutions are recirculated. Strict control over the conditions of absorption are required to efficiendy capture the NO and convert it predominantly to sodium nitrite according to the following reaction, thereby minimizing the formation of by-product sodium nitrate. Excessive amounts of nitrate can impede the separation of pure sodium nitrite from the process. 

After the SO converter has stabilized, the 6—7% SO gas stream can be further diluted with dry air, I, to provide the SO reaction gas at a prescribed concentration, ca 4 vol % for LAB sulfonation and ca 2.5% for alcohol ethoxylate sulfation. The molten sulfur is accurately measured and controlled by mass flow meters. The organic feedstock is also accurately controlled by mass flow meters and a variable speed-driven gear pump. The high velocity SO reaction gas and organic feedstock are introduced into the top of the sulfonation reactor,, in cocurrent downward flow where the reaction product and gas are separated in a cyclone separator, K, then pumped to a cooler, L, and circulated back into a quench cooling reservoir at the base of the reactor, unique to Chemithon concentric reactor systems. The gas stream from the cyclone separator, M, is sent to an electrostatic precipitator (ESP), N, which removes entrained acidic organics, and then sent to the packed tower, H, where SO2 and any SO traces are adsorbed in a dilute NaOH solution and finally vented, O. Even a 99% conversion of SO2 to SO contributes ca 500 ppm SO2 to the effluent gas. 

Burning Pyrites. The burning of pyrite is considerably more difficult to control than the burning of sulfur, although many of the difficulties have been overcome ia mechanical pyrite burners. The pyrite is burned on multiple trays which are subject to mechanical raking. The theoretical maximum SO2 content is 16.2 wt %, and levels of 10—14 wt % are generally attained. As much as 13 wt % of the sulfur content of the pyrite can be converted to sulfur trioxide ia these burners. In most appHcations, the separation of dust is necessary when sulfur dioxide is made from pyrite. Several methods can be employed for this, but for many purposes the use of water-spray towers is the most satisfactory. The latter method also removes some of the sulfur... 

Ma.nufa.cture. In a typical process, a solution of sodium carbonate is allowed to percolate downward through a series of absorption towers through which sulfur dioxide is passed countercurrently. The solution leaving the towers is chiefly sodium bisulfite of typically 27 wt % combined sulfur dioxide content. The solution is then mn into a stirred vessel where aqueous sodium carbonate or sodium hydroxide is added to the point where the bisulfite is fully converted to sulfite. The solution may be filtered if necessary to attain the required product grade. A pure grade of anhydrous sodium sulfite can then be crystallized above 40°C because the solubiUty decreases with increasing temperature. 

Gas leaving the converter is normally cooled to 180—250°C using boiler feedwater in an "economizer." This increases overall plant energy recovery and improves SO absorption by lowering the process gas temperature entering the absorption tower. The process gas is not cooled to a lower temperature to avoid the possibiUty of corrosion from condensing sulfuric acid originating from trace water in the gas stream. In some cases, a gas cooler is used instead of an economizer. 

Constmction of new power plants in the coal region of the western United States presents serious problems in states whose laws dictate zero effluent. In these plants, cooling-tower water withdrawn from rivers cannot be returned to them. In these situations, cooling-tower effluent is purified by distillation (vapor-compression plants have predominated) and by a combination of distillation and membrane technology. The converted water then is used as boiler feedwater the plant blowdown (effluent) is evaporated from open-air lined pools, and pool sediment is periodically buried back in the coal mine with the flue ashes. 

The second law of thermodynamics focuses on the quaUty, or value, of energy. The measure of quaUty is the fraction of a given quantity of energy that can be converted to work. What is valued in energy purchased is the abiUty to do work. Electricity, for example, can be totally converted to work, whereas only a small fraction of the heat rejected to a cooling tower can make this transition. As a result, electricity is a much more valuable and more costly commodity. 

The overhead of the depropanizer is sent to the propylene fractionator. The methylacetylene (MA) and propadiene (PD) are usually hydrogenated before entering the tower. An MAPD converter is similar to an acetylene converter, but operates at a lower temperature and in the Hquid phase. Due to recent advances in catalysis, the hydrogenation is performed at low temperatures (50—90°C) in trickle bed reactors (69). Ordy rarely are methylacetylene and propadiene recovered. 

Ethylene capacity = 450, 000 t/yr tower material is carbon steel. To convert kPa to atm, divide by 101.3. 

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