Upgrading olefins to make gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. In U.S. Pat. No. 4,227,992 Garwood and Lee disclose the operating conditions for the Mobil Olefin to Gasoline/Distillate (MOGD) process for selective conversion of C.sub.3.sup.+ olefins to mainly aliphatic hydrocarbons. In a related manner, U.S. Pat. Nos. 4,150,062 and 4,211,640 (Garwood et al) discloses a process for converting olefins to gasoline components. Typically, the process recycles gas or liquid hydrocarbons from a high-temperature, high-pressure separator downstream of the catalyst bed back into the reaction zone where additional olefins are converted to gasoline and distillate products. If the reaction of the olefins in converting them to distillate and gasoline is allowed to progress in the catalyst stream without any measures taken to prevent the accumulation of heat, the reaction becomes so exothermically accelerated as to result in high temperatures and the production of undesired products.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization/polymerization using a medium pore shape selective acid crystalline zeolite, such as ZSM-5 type catalyst, process conditions can be varied to favor the formation of either gasoline or distillate range products. At moderate temperature and relatively high pressure, the conversion conditions favor aliphatic distillate range product having an initial point of at least 165.degree. C. (330.degree. F.). Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions do not convert a major fraction of ethylene. One source of olefinic feedstocks of interest for conversion to heavier fuel products is the intermediate olefin-rich light oil obtained from Fischer-Tropsch conversion of synthesis gas.
It is a main object of this invention to provide a continuous system for upgrading lower and intermediate olefins, such as Synthol light oil, to a valuable heavy distillate fuel product. Control apparatus and processes for operating a production plant are provided in a unique system for monitoring process conditions and effecting changes in recycle flow ratio and reactor temperature.
An improved continuous catalytic conversion system has been devised for upgrading olefinic feedstock in contact with a bed of oligomerization catalyst at elevated temperature and pressure in an enclosed reaction system to produce an effluent stream comprising a mixture of heavy hydrocarbons and lighter hydrocarbons including means for flashing and phase separation of a liquid product stream rich in heavy hydrocarbons and a vapor stream rich in lighter hydrocarbons. A recycle control system is provided, including condenser means for condensing a major amount of said flashed vapor stream rich in lighter hydrocarbons, a surge drum for receiving the condensed stream, fluid handling means operatively connected between the surge drum and reactor for repressurizing and combining a predetermined amount of recycled hydrocarbons with olefinic feedstock, and valve means for withdrawing a vapor slipstream between said flashing means and the condenser means to divert flashed vapors. Measurement and process control functions are provided by means for detecting liquid level of condensed hydrocarbons in said surge drum and generating a signal representative of the liquid level; and control means responsive to the level signal and operatively connected with the slipstream diverting valve means, whereby vapor slipstream flow is increased with increased surge drum liquid level above a predetermined level.
It is a further object of this invention to provide a novel technique for controlling and varying the composition and weight ratio of the gasoline and distillate produced. It allows a weight ratio change in product yields without changing the feed composition or rate. Also, if distillate is to be used for diesel fuel, a distillate of increased specific gravity may be produced to meet diesel fuel specifications, and to enhance its lubricating properties.
These and other objects and features of the invention will be understood from the following detailed description and drawings.