The reactor comprises: a lower reactor region (2) for accommodating a
sulfur melt (3); non-pressure-bearing first caverns (4) for temporary
accommodation of a
product gas mixture that is formed by
exothermic reaction and comprises
hydrogen sulfide,
sulfur and
hydrogen; non-pressure bearing second caverns (8) arranged above the first caverns and for temporary
accommodation of the
product gas mixture formed in the first caverns and for formation of further
hydrogen sulfide by
exothermic reaction of
sulfur and hydrogen to form a
product gas mixture; and a gas collecting region (6). The reactor comprises: a lower reactor region (2) for accommodating a sulfur melt (3); non-pressure-bearing first caverns (4) for temporary
accommodation of a product gas mixture that is formed by
exothermic reaction and comprises
hydrogen sulfide, sulfur and hydrogen, and a supply device for controlled supply of pressurized
gaseous hydrogen per first cavern; non-pressure bearing second caverns (8) arranged above the first caverns and suitable for temporary accommodation of the product gas mixture formed in the first caverns and for formation of further
hydrogen sulfide by exothermic reaction of sulfur and hydrogen to form a product gas mixture; and a gas collecting region (6) for accommodating the product gas mixture at elevated temperature and elevated pressure relative to standard conditions. One of the second caverns comprises a supply device suitable for controlled supply of pressurized
gaseous hydrogen. The reactor further comprises: non-pressure-bearing third caverns arranged above the second caverns; non-pressure-bearing installed devices suitable for continuous transfer of a total amount of product gas mixture formed in the lower reactor region to the gas collecting region, where a catalyst is present in the installed devices suitable for reacting sulfur and hydrogen still present in the product gas mixture to form
hydrogen sulfide; an inner wall that allows continuous circulation of the sulfur melt according to an
airlift pump principle in the course of operation of the reactor with involvement of a space between an outer reactor wall and the inner wall; a
reflux condenser suitable for condensation of the sulfur present in the product gas mixture; and an input line suitable for transport of the product gas mixture from the gas collecting region to the
reflux condenser and a return line suitable for return of the condensed sulfur to the reactor. One of the second or third caverns has a greater volume than each of the first caverns, and / or one of the second or third caverns has lower heat removal for construction reasons than each of the first caverns. The installed devices are arranged such that, the installed devices are present in
thermal contact with the sulfur melt after sufficient filling of the lower reactor region with the sulfur melt. The catalyst is cooled by transfer of heat to the sulfur melt if the installed devices contain the catalyst. An independent claim is included for a process for preparing hydrogen
sulfide by exothermic reaction of sulfur with hydrogen at elevated temperature and elevated pressure relative to standard conditions to form a product gas mixture comprising hydrogen sulfide and sulfur.