METHOD FOR REMOVING SO2 FROM GAS STREAMS

Abstract: A process for absorbing SO₂ from a gas in a packed column employs a low aqueous sulfite feed rate of 0.05 to 1 gallon per minute per square foot of tower cross section, or preferably 0.1 to 0.3 gallons per minute per square foot. Lower capital costs and lower operating costs result because of less tower equipment, lower pressure drop, reduced oxidation to sulfate, and elimination of pump around as compared to the conventional processes. Also disclosed is an improved liquid distributor.

Description: This invention relates to an improvement in liquid-gas contact apparatus and process where sulfur dioxide is absorbed into or reacted with the liquid. A particular application is in the absorber of the Wellman-Lord process for the removal of sulfur dioxide from exhaust gases by aqueous sulfite solution, particularly from combustion power plant flue gases, although it is also applicable to exhaust streams from smelters, sulfuric acid plants, or any other sulfur dioxide containing gas streams. As an example of a process requiring an aqueous liquid-gas contact apparatus, the Wellman-Lord process of sulfur dioxide removal from stack gases employs an aqueous alkali sulfite solution (usually sodium sulfite) which chemically combines with the sulfur dioxide gas in an absorption tower by forming sodium bisulfite. The process includes a separate regeneration facility to convert the bisulfite back to sulfite and recover sulfur dioxide gas which is compressed and bottled or converted into sulfuric acid or to elemental sulfur. In the case of flue gases from coal fired plants, a separate unit for removal of fly ash and chlorides is included. Because of large capital costs and high energy requirements to overcome the pressure drop in the absorber, in the regenerable Wellman-Lord process, non-regenerable flue gas desulfurization processes have been the more popular choice in the past, in spite of the major problem of disposal of solid waste produced by such processes. The present invention is an improvement in absorption apparatus and its operation and in particular the process of the invention relates to the absorber of the Wellman-Lord process whereby capital costs and parasitic power requirements are reduced so as to make the regenerable process more economic and thus avoid the solid waste problem of the non-regenerable processes. DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of an adsorption column 10, indicating the position of a liquid distributor 11, an optional packing retainer 12, packing support 13, and conventional gas distributor and liquid collection, if necessary, 14, with the liquid and gas inlets and outlets indicated. Figure 2 is an exploded broken view of a portion of a preferred liquid distributor having a supply trough 20, a spacing element 21, and a flow guide 22. Attached to flow guide 22 are drip fingers or rods 23 having ends adjacent packing (shown schematically) at 24. Figure 3 is a schematic rib view, assembled, of the parts of a preferred distributor shown in Figure 2. Figure 4 is an exploded schematic side view of a similar arrangement to Figure 2, but with the element 21 replaced by spacers 25. Figure 5 is an enlarged cross-sectional view of a portion of element 21. DESCRIPTION OF THE INVENTION By employing liquid distribution at extremely low liquid flow rates and high efficiency random dumped packing, the present invention can achieve the absorption step in a single relatively short packed column of low pressure drop, and without the requirement of recycle of the absorbate solution prior to regeneration. The flow rate for liquid fed to the top of the tower is from 0.000034 to 0.00068 m³/sec./m² (0.05 to 1.0 gallons per minute per square foot), or more preferably from 0.000136 to 0.000204 m³/sec./m² (0.2 to 0.3 gallons per minute per square foot). Only fresh absorbant solution is employed, without recycle. With the use of such low liquid flow rates, the tower packing is chosen so that its operation is compatable with the low liquid rate. One suitable type is that in which the operation is primarily one of combination and separation of droplets, rather than by a spreading of the liquid on large wetted areas of the packing. A suitable packing of this type is a high void volume packing shown in US-A-4 511 519 to Hsia, the disclosure of which is incorporated herein by reference. The packing has a large number of drip points, a relatively low surface area, but relatively long total length of non-aligned interacting edges. Because of the low liquid rate of the present invention, with one half to one tenth of the liquid retention time of the conventional practice using trayed towers or a plurality of packed towers with trap trays, in the Wellman-Lord process, the problem of oxidation of sulfite to sulfate is reduced by an amount of 50% to as much as 90%. This is important in that the sulfate reduces the absorption efficiency of the liquid and thus must be removed in the regeneration process, adding capital and operational expenses to the process. In addition, the sulfate salts are a solid waste which must be disposed of in an environmentally safe manner. In order to distribute liquid onto the packing so as to take advantage of the low flow rate a drip type distributor which provides at least 32.29 (3) and preferably at least 64.58 (6) to 96.87 (9) feed points uniformly spaced per m² (square foot) may be employed. A suitable liquid distributor originally intended for non-polar liquids which can be adapted for this purpose is shown in US-A-4 264 538. While this type of distributor as disclosed in the patent was designed for organic liquids, it can be adapted for use in the present invention by use of hydrophilic coatings, as later described below. Usually a metal (e.g. stainless steel) trough type distributor is used. The surfaces of the distributor, outside of the feed troughs which convey the liquid, should be coated with a hydrophylic microporous coating which causes a film of liquid to be formed on such surfaces as the liquid flows on them by gravity. The drip fingers, which are coated, should also all extend to be close to or contacting a surface of the packing (or packing retainer, if used). The distance should not be substantially greater than the diameter of the liquid droplets being fed, so as to avoid entrainment of the liquid by the countercurrent flowing gas. Such an arrangement produces maximum efficiency and can avoid the need for a demister at the exit of the absorber. In the following examples a packing element as shown in Figures 7 through 10 of US-A-4 511 519 was employed, having a diameter of 88.9 mm. (3-1/2 inches) and an axial height of 31.75 mm, (1-1/4 inches). The packing depth was 2286 mm. (7.5 feet). A liquid distributor such as described in US-A-3 937 769 was employed, having 96.87 (9) drip fingers per m² (square foot). The operative surfaces of the distributor were coated with a resin latex drag resistant hydrophylic coating as described in US-A-4 467 070, sold by Hydromer, Inc., Whitehouse, N.J. The distributor also included a dimpled, perforated plate as shown at 21 in Figures 2, 3, and 5. The dimples were alternately facing in opposite directions. The dimples 50 of Fig. 5 in this case were 4.7625 mm (3/16 inches) apart, in an 0.2032 mm. (8 mil) steel sheet, to help distribute the liquid. The tower was 762 mm, (30 inches) in diameter. The liquid composition was composed as follows: Weight % Na₂SO₃ 17.4 Na₂S₂O₅ 2.9 Na₂SO₄ 6.0 H₂O 73.7 The test results are shown in Table I. Table II shows the pressure drop and the calculated mass transfer coefficient for each run. The highest mass transfer coefficients were obtained for the runs in which the gas rate was such that the tower was operating in the loading zone. That is, the gas rate was sufficiently high to cause increased liquid hold-up in the tower resulting in increased pressure drop and gas liquid contacting because of the added space taken up by the liquid. Run No. Liquid Rate m³/m² (GPM/Ft²) Gas Rate kg/s.m² (Lbs/hr/ft²) Gas SO₂ Parts/Million Inlet Outlet 1 0.0000985 (0.145) 2.4592 (1813.3) 717.2 160 2 0.0000964 (0.142) 4.5767 (3374.6) 711.7 220 3 0.0001066 (0.157) 4.7821 (3526.0) 944.5 510 4 0.0002119 (0.312) 4.9254 (3631.7) 1402.1 400 5 0.0001066 (0.157) 2.6638 (1964.1) 1065.7 600 6 0.0002200 (0.324) 4.7216 (3481.4) 676.0 80 Run No. Pressure Drop kPa/m (Inches of H₂O/ft) Mass Transfer Coefficient K G a kg Moles/sec./m³ kPa (LB Moles/hr-ft³-ATM) 1 0.0858 (0.105) 0.000588 (13.4) 2 0.3171 (0.388) 0.000970 (22.1) 3 0.3506 (0.429) 0.000681 (15.5) 4 0.3506 (0.429) 0.001238 (28.2) 5 0.0923 (0.113) 0.000299 (6.8) 6 0.3767 (0.461) 0.001730 (39.4) The use of a single packed bed greatly reduces the capital cost of the absorber compared to present design practice used in Wellman-Lord plants. The single packed bed also greatly reduces the operating cost, primarily in the area of the blower horsepower needed to feed the gas through the absorber but also by eliminating the presently required liquid recycle pumps. The packed bed, to achieve these energy savings, preferably uses a high efficiency packing which can achieve in a minimum length of bed the necessary mass transfer contacting efficiency, and has sufficiently low pressure drop to make the energy savings possible. Partial capital cost and energy saving benefits can be achieved by using less than optimum arrangements of this process invention. That is, rather than replacing all of the pump-a-round sections in prior absorption processes with a single packed bed, it is possible to combine some of the pump-around sections into two or three sections. These sections can still employ liquid recycle to achieve the high liquid rate operation but still have some pressure drop reduction. The energy savings here comes from the elimination of some of the trap trays. The elimination of the trap trays also saves considerable capital; the trap trays are very expensive because of their stainless steel construction. One further partial simplified configuration is possible. This configuration uses two beds. The top section would be a packed section using the once through regenerated absorption solution at low liquid rate. Thus, making an off gas with the lowest possible SO₂ content because of low SO₂ vapor pressure in the regenerated solution. The lower bed would be recirculated for the benefit of the higher liquid rates for easier liquid distribution, and would tend to eliminate potential salt precipitation problems caused by dry areas. Thus the single bed, with no pump around, employing low liquid rate may be used to absorb SO₂ from a gas stream directly from its source, or, as the final stage, from any intermediate pollution control apparatus including fly ash removal and chloride removal where required for coal burning power plants. In normal operation typically a 10 to 20% stoichiometric excess of sorbent sulfite solution will be fed to the tower, based on the concentration of the sulfite and the liquid and gas feed rates. In some cases, however, it may be more desireable to fully react the sorbent liquid for efficient regeneration, than to remove all of the SO₂ from the gas. In such cases the liquid feed would be somewhat deficient, on a stoichiometric basis.

Claims: A method of removing SO₂ from a gas stream which comprises passing said gas stream to the bottom of at least one liquid-gas contact bed in a tower and feeding aqueous sulfite solution to the top of the bed, and withdrawing gas depleted in SO₂ from the top of the bed and bisulfite solution from the bottom of the bed, characterized by the fact that the SO₂ depletion is essentially completed in a single pass of the aqueous sulfite solution through the bed using an aqueous sulfite flow rate through the bed of from 0.000034 to 0.00068 m³/sec./m² and a trough type distributor having a hydrophilic surface for supplying a low liquid flow rate of the solution to the top of the bed, said bed containing random dumped packing for carrying out said liquid-gas contact. A method according to claim 1, in which the flow rate is 0.000136 to 0.000204 m³/sec./m². A method according to claim 1 or 2, in which the total sulfite content of the entering liquid per unit of time is at least equal to the sulfur dioxide content of the entering gas, per unit of time. A method according to any one of the preceding claims, in which the aqueous sulfite is an alkali sulfite. A method according to any one of the preceding claims, in which the liquid is distributed from drip fingers having said hydrophilic surface. A method according to claim 5, in which the drip fingers are constructed of stainless steel having an organic hydrophylic coated thereon.

Inventors: RUKOVENA, FRANK
Applicants: NORTON COMPANY
Application year: 1987
Main classification: B01D53
IPC codes: B01D53