Utilization of decomposition gases in atmospheric-vacuum tubestill units

Abstract

Negative sides of process-flow diagrams for utilization of acidic decomposition gases from vacuum towers of production atmospheric-vacuum tubestill units in the radiation chambers of active furnaces at oil refineries (OR) are examined. New flow diagrams for utilization of decomposition gases from vacuum towers are proposed: adsorption of hydrogen sulfide by amine solutions; and combustion in a firebox using a special lining. It is pointed out that utilization of decomposition from vacuum towers in a firebox with subsequent recuperation of the heat of exiting exhaust gases and warming of a thermal agent is an economically effective technical solution recommended for implementation at OR.Utilization of acidic decomposition gases from vacuum towers in the radiation chambers of active furnaces in atmospheric tubestill units is practiced at many domestic oil refineries (OR). Let us examine the entire spectrum of negative consequences of adoption of such a technical solution, and possible alternate schemes for resolution of this problem.Decomposition gases are characterized by a high content of hydrogen sulfide H 2 S (10-45%, depending on the physicochemical composition of the feedstock and operating temperature regime of the furnace), and low excess pressure (p = 0.015-0.030 MPa) and temperature T = 25-50°C.Adsorption of hydrogen sulfide by diesel-fuel cuts in a mixing condenser with and without additives of surfaceactive substances (SAS) is carried out, for example, to lower the H 2 S content in decomposition gases at OR. The hydrogen sulfide-enriched diesel cuts are then pumped either into an electric desalting unit (to lower the viscosity of the crude), or into the feedstock of the hydrotreating unit; this is more optimal, since there is no mixing condenser in the flow diagrams of vacuum-creating equipment at many plants.As a rule, the circuit for utilization of acidic gases is as follows: separator -plain-or alloy-steel piping -fire barrier -separate inlet to radiation furnace, or inlet through fuel-oil line of combined burner. The degree of H 2 S conversion for this circuit does not exceed 60-70%, and the hydrogen sulfide is converted to SO 2 (SO 3 ) in the burning process. During oxidation of the H 2 S, a small amount of SO 2 (normally from 1 to 5%) is oxidized to SO 3 . The degree of conversion will depend on such factors as the temperature profile throughout the volume of the flue gases, the amount of sulfur dioxide in the flue gases, and the effect of aluminum as a component part of the refractory, etc.The SO 3 that has formed at temperatures ranging from 230 to 340°C enters into reaction with the water contained in the flue gases, forming sulfuric acid, which results in a number of serious problems: sulfuric acid contributes to a shift in the dew point to as high as 200°C, and condensation of the sulfuric acid may result in rapid surface corrosion of the metalwork of the furnace, the tubes of the soils in the convection chamber, and the recuperators extracting the heat of the exiting flue gases [1].The low d...