AU606813B2 - Distribution of liquid in packed column - Google Patents

Description

S& F Ref: 106244 COMMONWEALTH OF AUSTRALIA PATIENTS ACT 1952 fl COMPLETE SPECIFICATIOOV 6 8 1

(ORIGINAL)

FOR OFFICE USE: Class M-'Ih.Y1. LISA ts Int Class ,flo OC, 4' 4' 4'4o~ 4' 4 4'4'G 4' 44 0~4

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Complete Specification Lodged: Accepted: Published: Priority: Related Art: Name and Address 4* 4 4 4' C 4'

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4, 4' 1 4 of Applicant: Norton Company I New Bond Street Worcester Massachusetts 01606-2698 UNITED STATES OF AMERICA Address for Service: Spruson Ferguson, Patent Attorneys Level 33 St Martins Towe r, 31 Market Street Sydney, New South Wales, 2000, Aubtralia Complete Specification for the invention entitled: Distribution of Liquid in Packed Column The following statement is a full description of this invention, including the best method of performing it known to me/us 5845/3

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ABSTRACT

A process for absorbing SO 2 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.

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0 00 o o O 0 0000 o o 6 0 000 000 o a e 00 0 0 oa 0 0i -l1 This invention relates to a method of distributing a liquid to the top of a packed column in liquid-gas contact apparatus, such as 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 4dioxide 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 !i sodium bisulfite. The process includes a separate regeneration facility to convert the bisulfite back to i 20 sulfite and recover sulfur dioxide gas which is compressed and bottled or converted sulfuric acid or 1 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 i the absorber, in the regenerable Wellman-Lord process, non-regenerable flue gas desulfurization processes 2 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.

According to a broad form of the present invention there is provided a method for distributing an aqueous liquid to the top of a packed column in a process for contacting a liquid with a gas, said liquid being distributed from drip fingers having a hydrophylic surface such that the a 0 surface is entirely wet by the liquid.

0 DESCRIPTION OF THE DRANINGS 0o"15 Figure 1 is a schematic of an adsorption column 10, indicating the oL0. 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 ind outlets indicated.

Figure 2 is an exploded broken view of a portion of a preferred 20 liquid distributor having a supply through 20, a spacing element 21, and a 0000 0 flow guide 22. Attached to flow guide 22 are drip firgers or rods 23 0 o 4 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.

0025 Figure 4 is an exploded schematic side view of a similar arrangement 0oOo° to Figure 2, but with the element 21 replaced by spacers Figure 5 is an enlarged cross-sectional view of a portion of element ooa00' 21.

DESCRIPTION OF THE INVENTION In accordance with the invention of our Australian Application 69037/87, by employing liquid distribution at extremely low liquid flow rates and high efficiency random dumped packing, the present /i 8 -t i' -B v' KXW:1104y i, i- -3invention 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 0.05 to 1.0 gallons per minute per square foot, or more preferably from 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 U.S. Patent 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 Cc with trap trays, in the Wellmun-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 Le disposed of in an environmentally safe manner.

In order to distribute liquid onto the packing so as to achieve the low flow rate one generally uses a drip type distributor which provides -4at least 3 and preferably at least 6 to 9 feed points uniformly spaced per square foot. A suitable liquid distributor originally intended for non-polar liquids which can be adapted for this purpose is shown in U.S. Patent 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 hydrophylic coatings, as later described below. Other types of liquid distributors may be employed, and, in the larger diameter towers a spray type distributor, suitably designed for low liquid rates may be preferable, because it is less costly. Any other type of distributor suitable to deliver the required low liquid flow may be employed.

0 S 15 If the distributor is less efficient, the length of cs the packed bed must be increased. Thus less efficient r distribution methods may be employed, with a penalty 000 paid in the higher costs and pressure drop inherent in 0000 the deeper bed.

In accordance with the present invention, when a metal (stainless steel) trough type or drip type distributor is used, the surfaces of the 0 C distributor, outside of the feed troughs which convey 0 00 the liquid, are coated with a hydrophylic microporous coating such that the surface is entirely wet by liquid. This coating 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 -1 L cl L as shown n Figures 7 through 10 of U.S. Patent 4,511,519 was employed, having a diameter of 3-1/2 inches and an axial height of 1-1/4 inches. The packing depth was 7.5 feet. A liquid distributor such as described in U.S. Patent 3,937,769 was employed, having 9 drip fingers per square foot. The operative surfaces of the distributor were coated with a resin latex drag resistant hydrophylic coating as described in U.S. Patent 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 3/16 inches apart, in an 8 mil steel sheet, to help distribute the liquid. The tower was inches in diameter. The liquid composition was composed as follows: Weight R, Na 2

SO

3 17.4 Na 2

S

2 0 5 2.9 Na 2

SO

4

H

2 0 73.7 The test results are shown in Table I.

STable II shows the pressure drop and the calculated mass transfer coefficient for each run.

.1The highest mass transfer coefficients were obtained 1 for the runs in which the gas rate was such that the Stower was operating in the loading zone. That is, the gas rate was sufficiently high to cause increased S 30 liquid hold-up in the tower resulting in increased i pressure drop and gas liquid contacting because of the added space taken up by the liquid.

Table I Liquid Gas Gas SO 2 Parts/Million Run Rate Rate No. GPM/Ft 2 Lbs/hr/ft 2 Inlet Outlet 1 0.145 1813.3 717.2 160 2 0.142 3374.6 711.7 220 -6- Table I (Continued) 3 0.157 3526.0 944.5 510 4 0.312 3631.7 1402.1 400 0.157 1964.1 1065.7 600 6 0.324 3481.4 676.0 Table II Run Pressure Drop Mass Transfer Coefficient KGa No. Inches of H 2 0/ft LB Moles/hr-ft 3

-ATM

1 0.105 13.4 2 0.388 22.1 3 0.429 15.5 4 0.429 28.2 0.113 6.8 6 0.461 39.4 15 The use of a single packed bed greatly re- «00 duces the capital cost of the absorber compared to Spresent 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 i oaQ .also by eliminating the presently required liquid recycle pumps. The packed bed, to achieve these energy savings, preferably uses a high efficiency packing i i r which can achieve in a minimum length of bed the l 25 necessary mass transfer contacting efficiency, and has sufficiently low pressure drop to make the energy S savings possible.

Partial capital cost and energy saving li 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 I 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 rS',e operation but still have some pressure drop reduction. The energy savings here comes from the elimination of some of the trap trays.

i _i F- X 9 P -7- 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 tc, 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 S02 content because of low SO 2 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 o to eliminate potential salt precipitation problems 9, caused by dry areas.

Thus the single bed, with no pump around, employing low liquid rate may be used to absorb SO 2 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 *:emoval where required for coal burning power plants.

In normal operation typically a 10 to stoichiometric excess of sorbent sulfite solution will be fed to the tower, based on the concentration of the i sulfite and the liquid and gas feed rates. In some cases, however, it may be more desireable to fully 0 rc-act the sorbent liquid for efficient regeneration, oo than to remove all of the SO 2 from the gas. In such 00 II 0 cases the liquid feed would be somewhat deficient, on a stoichiometric basis.

Claims (4)

1. A method for distributing an aqueous liquid to the top of a i packed column in a process for contacting a liquid with a gas, said liquid being distributed from drip fingers having a hydrophylic surface such that the surface is entirely wet by the liquid.

2. A method according to claim 1, in which the drip fingers are constructed of stainless steel having an organic hydrophylic coated thereon.

3. A method according to claim 1 or 2 wherein said contacted gas is SO 2

4. A method for distributing an aqueous liquid to the top of a packed column according to claim 1, substantially as hereinbefore described with reference to any one of Runs 1-6. i I DATED this TWELFTH day of NOVEMBER 1990 Norton Company Patent Attorneys for the Applicant i SPRUSON FERGUSON Si i KXW:1104y