Hydrocarbons can catalytically be converted to synthesis gas, i.e. mixtures of hydrogen (H2) and carbon monoxide (CO), by using steam. As is explained in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword “Gas Production”, this so-called steam reforming is the most frequently used method for the production of synthesis gas, which subsequently can be converted to further important basic chemicals such as methanol or ammonia.
Steam reforming of natural gas proceeds strongly endothermally. It is therefore carried out in a reformer furnace in which numerous catalyst-containing reformer tubes are arranged in parallel, in which the steam reforming reaction takes place. The reformer tubes mostly are fired by means of burners, which are mounted on the upper side or underside or on the side walls in the interior space of the reformer furnace and directly fire the space between the reformer tubes.
After preheating by heat exchangers or fired heaters the hydrocarbon-steam mixture enters into the reformer tubes after final heating and is converted there at the reforming catalyst to obtain carbon monoxide and hydrogen. The composition of the product gas is determined by the reaction equilibrium; beside carbon monoxide and hydrogen, the product gas therefore also contains carbon dioxide, non-converted methane and steam.
For energy optimization and/or in feedstocks with higher hydrocarbons, a so-called prereformer after the preheater can be used for pre-cracking the feedstock. In a further heater, the pre-cracked feedstock then is heated to the desired inlet temperature into the main reformer, for example the steam reformer. Conventional prereforming can be defined as steam reforming process at limited temperatures (distinctly below 700° C.). It leads to a gaseous intermediate product whose main constituents are methane and steam. The intermediate product contains no or only small amounts of higher hydrocarbons. This intermediate product normally is treated further in a steam reformer referred to as main reformer.
As is explained in Ullmann's Encyclopedia of Industrial Chemistry, loc. cit., nickel-based catalysts normally are used for steam reforming. The same are sensitive to catalyst poisons such as sulfur, arsenic, copper, vanadium, lead and chlorine or halogens in general. Sulfur in particular distinctly lowers the catalyst activity and can virtually be found in all feedstocks which are suitable as feed for the steam reformation. Earlier desulfurizing systems were operated with impregnated activated carbon as adsorbent and at ambient temperature. The efficiency of this adsorption method, however, differs in dependence on the specific sulfur compounds in the feed gas stream. For this reason, desulfurizing systems now are preferred in which the removal of sulfur compounds is effected on zinc oxide as sorbent at temperatures of 350 to 400° C. These desulfurizing systems based on zinc oxide are very reliable in the absorption of hydrogen sulfide and, with limitations, sulfur compounds such as carbonyl sulfide and mercaptans. The removal of cyclic organic sulfur compounds, such as e.g. thiophenes, on the other hand normally requires a hydrogenation on cobalt-molybdenum or nickel-molybdenum catalysts with hydrogen or a hydrogen-containing gas at temperatures of typically 350 to 380° C. Reference here is also made to hydrogenating desulfurization or hydrodesulfurization (HDS). The cyclic sulfur compounds are hydrogenated to hydrogen sulfide which then is bound in a downstream fixed zinc oxide bed. This hydrogenation at the same time can be used to hydrogenate unsaturated hydrocarbons, for example olefins or diolefins, in the feed gas. Because the temperature range for this hydrogenation usually is limited to 250 to 400° C., the content of unsaturated compounds in the feed gas must be limited, as their hydrogenation proceeds strongly exothermally.
For steam reforming the use of methane-containing natural gas as feedstock or so-called feed is dominant; but, depending on local availability, there are also used other hydrocarbons, such as naphtha, liquefied gas or refinery gases. The U.S. patent specification U.S. Pat. No. 3,477,832 for example describes a process for producing a synthesis gas by catalytic steam reforming of naphtha and similar hydrocarbons. Naphtha in the sense of this application is understood to be hydrocarbons with a mean carbon number of seven, which contain straight and branched hydrocarbons, a certain amount of aromatic and olefinic hydrocarbons, and various impurities, for example sulfur components. In order to transfer this substance mixture liquid at ambient conditions into a feed stream for steam reforming, it is evaporated and heated, wherein the temperature ideally should lie between 260 and 316° C., but by no means should exceed 343° C., as otherwise a decomposition of components contained in the feed will occur by forming undesired carbon deposits in the steam reforming plant and in upstream plant components.
The International Patent Application WO 2011/016981 A2 discloses a process for producing a mixed feed stream for a steam reforming plant, wherein refinery gases, for example FCC waste gas or coker waste gas, serve as a basis, to which gases a natural gas stream is admixed. Beside olefins, both refinery gases also contain diolefins such as 1,3-butadiene. It is taught that the olefin content and the content of such streams in organic sulfur compounds requires a pretreatment, in order to avoid the deposition of carbon on the reforming catalyst or the poisoning of the catalyst with sulfur compounds which otherwise might lead to a deactivation of such catalyst.
The U.S. patent specification U.S. Pat. No. 7,037,485 B1 also teaches the formation of a mixed feed for steam reforming, comprising a natural gas stream and a refinery gas stream. The two feed streams initially are combined and mixed and subsequently heated by indirect heat exchange against the hot product stream from the CO conversion plant downstream of the steam reforming plant. Subsequently, the heated mixed stream is treated in a reactor which contains a catalyst active both for the hydrogenation of the olefins and the sulfur compounds and for the partial autothermal reformation (ATR). Steam used as temperature moderator and optionally oxygen is added to the reactor when the reactor is to be operated in the partial ATR mode, wherein pre-reforming of the higher hydrocarbons to methane and synthesis gas constituents already is effected. What is disadvantageous is the logistics needed for supplying steam and oxygen. Too much addition of steam also can lower the temperature in the reactor too much and thus negatively influence pre-reforming. Furthermore, the downstream sulfur separation then is impaired as well.
U.S. Pat. No. 7,037,485 B1 furthermore teaches two embodiments of the process, in which the natural gas stream and the refinery gas stream are pretreated completely separately and are combined only before introduction into the steam reforming unit. It is very disadvantageous that all pretreatment devices accordingly must be present twice.