Table 842 Disposal Or Recovery Of Sulfidic Spent Caustics

Methoda

Description

Operating Information or Conditions

Beneficial Characteristics

Possible Adverse Characteristics

Sale of Sulfidic Causticsb

Burning

Deep Well Disposal Continuous Regeneration

Flue Gas Neutrali-

zationd

Mineral Acid Neu-tralizatione

Oxidation

Ocean Dumping

Sulfidic caustics are sold to a commercial plant which recovers sodium sulfide, sodium hydrosulfide and disulfides.

A light hydrocarbon stream is in continuous contact with caustic in an absorption tower. The spent caustic is regenerated by contact with steam in another tower.

Contact with flue gas liberates H2S and mercaptans with simultaneous formation of a sodium salt solution. Only small amounts of cresylic acids and thiophenols are present.

Neutralization and steam stripping liberate H2S and mer-captans with simultaneous formation of a sodium salt solution. Only small amounts of cresylic acids and thio-phenols are present.

Sulfidic caustics are continuously in contact with air and steam in a packed column. Sulfides are oxidized to thio-sulfate and mercaptans are oxidized to disulfides.

Concentrated caustic (25°-30°Be') should be used in the refinery to minimize shipping costs for spent caustics. Sulfidic caustics should be segregated from phenolic caustics.

Typical operating conditions in the regenerator are 215°-240°F and 1-10 psig. Mer-captan off gases released during regeneration are burned.

Operating conditions are the same as for phenolic caustics. However, the quantity of H2S and mercaptans in the overhead gas is much greater. Sulfur recovery is necessary in most cases.

Operating conditions are the same as for phenolic caustics. However, the quantity of H2S and mercaptans in the overhead gas is much greater. Sulfur recovery is necessary in most cases.

Typical operating conditions are 165°-225°F and 60-85 psig. Disulfides phase separate and are decanted. The thiosulfate solution is then released to the sewer system.

Sulfidic caustics may not return a profit for some refineries; however, sales should pay for the transportation charges and eliminate the need for inplant disposal facilities.

Process is simple and relatively inexpensive.

Same as for phenolic caustics, except cresylic acids and thiophenols are present in small quantities only.

Resulting sodium salt solution may be less contaminated than with flue gas neutralization. Cresylic acids and thio-phenols are present in small quantities only.

The process is simple and relatively inexpensive. The immediate oxygen demand of the sulfides is satisfied and the ultimate oxygen demand is reduced from 2 to 1 lb of oxygen per lb of sulfide.

Shipping costs may be prohibitive for some locations. Caustic treating facilities within the refinery may need to be revised before switching to concentrated virgin caustic.

Process is limited to absorption of mercaptans. Any sulfides absorbed will not be removed during regeneration. Consequently, caustic eventually becomes spent and poses a disposal problem.

Same as for phenolic caustics, except SO2 emissions are more of a problem.

Same as for phenolic caustics except SO2 emissions are more of a problem.

Drainage of the thiosulfate solution to the sewer greatly increases the dissolved solids content of the refinery effluent waters. Although the ultimate oxygen demand is reduced at least 50%, the remaining COD is still very high.

aMethods are listed in descending order of overall ability to recover or dispose of sulfidic caustics without creating additional pollution problems.

bThe Merichem Company purchases sulfidic caustics.

cSame as for phenolic caustics. Refer to Table 8.4.1.

dSome NaHCO3 is formed simultaneously with Na2CO3.

eH2SO4 and HCl can be utilized.

• lower acid content of spent acid

• improvement of subsequent processes due to solubility of compounds formed during pickling

• cleaner rinsing

• better drying

• improved overall quality of steel produced

Hydrochloric acid pickling is not without problems. The acid is more expensive than sulfuric acid, and more corrosive. When converting from sulfuric acid to hydrochloric acid operation, storage, pickle tanks, and fume hoods must be replaced.

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