Spray dryer-scrubber

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

Many different typas of spray device have been developed and no generalized correlations are available for predicting performance. They can be categorized into two basic types (I) preformed spray, which includes countercurrent, cocurrent, and crosscurrent spray lowers, spray dryers, cyclonic spray devices, and injector venturis and (2) gas atomized spray, which consists primarily of venturi scrubbers and related designs. 

Tihe aqueous carbonate process (ACP) has been under development at Atomics International for the last 4% yr. The program aims to establish a technology which eliminates or minimizes the major problems encountered in operating most other sulfur dioxide removal processes. That technology includes the use of sodium carbonate as the scrubbant in the modified spray dryer and the complete regeneration of the sulfur dioxide removal products to recover elemental sulfur and produce sodium carbonate for reuse in the spray dryer-scrubber. 

One of the primary reasons that the spray dryer-scrubber is able to achieve excellent sulfur dioxide removal with such low liquid-to-gas ratios is the small size of the droplets produced by the high speed centrifugal atomizer. This type of atomizer also has an easily controlled turndown capability which is a desirable feature that has been demonstrated in the pilot tests. As gas flow decreases, the amount of sodium carbonate solution can be decreased in direct proportion without interfering with sulfur dioxide removal efliciency. The atomizer actually produces finer droplets at the lower liquid flow rates. This appears to compensate for any gas-liquid mixing problems that could impair performance. 

The proper operation of a spray dryer-scrubber also requires that a dry product be formed and subsequently removed from the gas stream. Pilot tests have shown that the product salts will be dry and collectable if the gas temperature at the dryer outlet is maintained about 20 °F above its dewpoint. This also tends to minimize plume formation. The cyclone collectors used in the pilot tests removed 89-99% of the product. Although this was excellent performance by mechanical collectors, particulate emission standards will require either replacement of the cyclones or additional collection devices in series with the cyclones. The system design presently favored involves using cyclones to remove the bulk of the product and adding a small electrostatic precipitator for final particulate removal. The sodium salts produced in the spray dryer-scrubber... 

SDS Spray-dryer scrubber (used interchangeably with SDA). 

Semidry Scrubbers The advantage of semidry scrubbers is in that they remove contaminants by way of a solid waste that is easier to dispose of (less expensive). Initially, the scrubbing medium is wet (such as a lime or soda ash slurry). Then a spray dryer is used to atomize the slurry into the gas which evaporates the water in the droplets. As this takes place, the acid in the gas neutralizes the alkali material and forms a fine white solid. Most of the white solids are removed at the bottom of the scrubber while some are carried into the gas stream and have to be removed by a filter or electrostatic precipitator (discussed later). Although semidry systems cost 5-15% more than wet systems, when combined with a fabric filter, they can achieve 90-95% efficiencies. Dry scrubbers are sometimes used in a very similar fashion, but without the help of gas-liquid-solid mass transfer, these systems use much higher amounts of the solid alkali materials. 

Wet and dry scrubbers Dry scrubbers are generally preferred today. Scrubbers can be used for the removal of both particulate matter and toxic sulfur gases (dry scrubber or spray dryer desulfurizers). 

The drying air is cooled in a scrubber/condenser where the water, evaporated in the spray dryer, is removed. The air is subsequently recycled to the direct gas-fired air heater. A vent will exhaust a volume of drying air equal to the volume of combustion products from the gas burner. However, most of the air is recycled in the system. 

Estimate the water requirement of a spray dryer (dry SO2 scrubber) at a coal-fired incineration facility that treats 150,000 Ib/h of a flue gas at 2180°F. Assume an approach temperature to the adiabatic saturation temperature (AST) of 40°E The AST can be assumed to be 180°R... 

List the advantages the spray dryer has over traditional wet scrubbers. 

Among the inherent advantages that the spray dryer enjoys over wet scrubbers are... 

With the current emphasis on environmental protection, many spray dryers are equipped with additional means to collect even the finest fraction. This collection is often destructive to the powder. Equipment in use are wet scrubbers, bag or other kinds of filters, and in a few cases incinerators. 

Spray-dryers are simpler and more compact than conventional wet scrubbers and have a lower capital and operating cost. Also, they do not produce large quantities of wastewater, and the spent absorbent is dry, thereby eliminating the need for thickening and filtration of the sludge. However, if the same dust precipitator is used for both the fly-ash and fhe spray-dryer product, the mixture of fly-ash and spent absorbent that they produce is unmarketable, and must be disposed of. Also, they require more expensive absorbents than conventional wet scrubbers. They are most suitable for retrofitting small plants that burn medium-sulfur coals, where capital costs and space restrictions are more of a consideration. ... 

In a single-stage spray dryer the process is finished when most of the moisture in the pore space has dried. The agglomerates accumulate in the lower part of the spray-drying chamber and are removed by the suction of a fan driving a dust-collection system. The agglomerates are collected in a cyclone while dust is collected in a wet scrubber. Material-laden scrubber water may be recirculated and mixed with the liquid feed. 

The first pilot scrubber tests were conducted using simulated flue gas to establish the feasibility of sulfur dioxide s reacting with sodium carbonate solutions and slurries in a spray dryer. Subsequent tests were conducted at the Mohave generating station, where a 5-ft diameter modified spray dryer was used to test sulfur dioxide removal from a side stream of flue gas from this coal-fired power plant (Figure 4). The spray dryer had been in operation for over 20 yr in various drying applications prior to modification to a sulfur dioxide scrubber. It was used in over 100 tests at Mohave without a single operational problem. 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. Miscellaneous gas-absorption equipment could include acid gas scrubbers that are commonly classified as either wet or dry. In wet scrubber systems, the absorption tower uses a lime-based sorbent liquor that reacts with the acid gases to form a wet/solid by-product. Dry scrubbers can be grouped into three catagories (1) spray dryers (2) circulating spray dryers and (3) dry injection. Each of these systems yields a dry product that can be captured with a fabric filter baghouse downstream and... 

FIGURE 9.37 Example of a closed-cycle layout (1) spray dryer (2) cyclone (3) feed tank (4) exhanst fan (5) wet scrubber (6) scrubber cooling system (7) fan (8) heater. (From Strumillo, C., Podstawy Teorii i Techniki Suszenia, 2nd edn., Wydawnic-two Naukowo-Techniczne, Warsaw, Poland, 1983.)... 

FIGURE 33.10 Spray dryer installation (1, drying chamber 2, atomizer 3, air dispenser 4, air heater 5, feed pump 6, main cyclone collector 7, wet scrubber 8, pneumatic conveying system 9, conveying cyclone). (Courtesy of A/S Niro Atomizer, Soeborg, Denmark.)... 

---