A process for the preparation of sulfuric acid in a contact sulfuric acid manufacturing facility comprising an interpass absorber wherein said facility is retrofitted to be operated in accordance with a process that recovers the heat of absorption of SO 3 in useful form at a temperature of at least 150°C, the process comprising: burning a source of sulfur in a gas comprising excess oxygen to produce a sulfur oxide-bearing gas stream comprising a combustion gas comprising sulfur dioxide and oxygen; contacting the sulfur oxide-bearing gas stream comprising said combustion gas with a catalyst for conversion of sulfur dioxide to sulfur trioxide, thereby converting the sulfur oxide-bearing gas stream to a conversion gas containing SO 3; contacting the conversion gas in a primary absorption zone with a primary absorption liquid comprising sulfuric acid, thereby absorbing sulfur trioxide and/or transferring sulfuric acid from the conversion gas to the primary absorption liquid; circulating said absorption liquid between said primary absorption zone and an indirect heat exchanger in which heat generated by reaction of sulfur trioxide and water, condensation of sulfuric acid, and/or absorption of sulfur trioxide into the primary absorption liquid is transferred to a heat transfer fluid, thereby heating the heat transfer fluid to at least 150°C; and contacting the gas stream exiting the primary absorption zone with a secondary absorption liquid comprising sulfuric acid in a secondary absorption zone, residual SO 3 contained in the gas stream entering said secondary absorption zone being recovered as sulfuric acid in the secondary absorption liquid, wherein said secondary absorption zone is comprised by an interpass absorber existing in said facility prior to said retrofit, and the mass flow ratio of said secondary absorption liquid to gas in said secondary absorption zone is between 1.0 and 7.0 or between 14 and 18. burning a source of sulfur in a gas comprising excess oxygen to produce a sulfur oxide-bearing gas stream comprising a combustion gas comprising sulfur dioxide and oxygen; contacting the sulfur oxide-bearing gas stream comprising said combustion gas with a catalyst for conversion of sulfur dioxide to sulfur trioxide, thereby converting the sulfur oxide-bearing gas stream to a conversion gas containing SO 3; contacting the conversion gas in a primary absorption zone with a primary absorption liquid comprising sulfuric acid, thereby absorbing sulfur trioxide and/or transferring sulfuric acid from the conversion gas to the primary absorption liquid; circulating said absorption liquid between said primary absorption zone and an indirect heat exchanger in which heat generated by reaction of sulfur trioxide and water, condensation of sulfuric acid, and/or absorption of sulfur trioxide into the primary absorption liquid is transferred to a heat transfer fluid, thereby heating the heat transfer fluid to at least 150°C; and contacting the gas stream exiting the primary absorption zone with a secondary absorption liquid comprising sulfuric acid in a secondary absorption zone, residual SO 3 contained in the gas stream entering said secondary absorption zone being recovered as sulfuric acid in the secondary absorption liquid, wherein said secondary absorption zone is comprised by an interpass absorber existing in said facility prior to said retrofit, and the mass flow ratio of said secondary absorption liquid to gas in said secondary absorption zone is between 1.0 and 7.0 or between 14 and 18.