Methods for producing gasoline boiling range hydrocarbons from methanol or other lower aliphatic oxygenates (referred to hereinafter as an MTG process) are generally known, as exemplified in U.S. Pat. Nos. 3,998,899 (Daviduk et al), 3,931,349 (Kuo) and 4,044,061 (Chang et al). incorporated herein by reference. In an MTG process, methanol is quantitatively converted to hydrocarbons and water. The hydrocarbons are primarily gasoline boiling range materials. Gasoline as such term is used in the instant specification and as such term is commonly used in the petroleum industry is useful as a motor fuel for internal combustion engines. More specifically, gasoline is hydrocarbon in nature, being composed of various aliphatic and aromatic hydrocarbons having a full boiling range of up to about to 430.degree. F., depending upon the exact blend used and the time of year. Although gasoline is predominantly hydrocarbon in nature, various additives which are not necessarily exclusively hydrocarbon are often included. Additives of this type are usually present in very small proportions, e.g., less than 1% by volume of the total gasoline. It is also not uncommon for various gasolines to be formulated with non-hydrocarbon components, particularly alcohols and/or ethers as significant, although not major constituents thereof. Such alcohols, ethers and the like have burning qualities in internal combustion engines which are similar to those of hydrocarbons in the gasoline boiling range. For purposes of this application, the term "gasoline" is used to mean a mixture of hydrocarbons boiling in the aforementioned gasoline boiling range and is not intended to mean the above-referred to additives and/or non-hydrocarbon constituents.
In the basic MTG process, methanol or other C.sub.1 to C.sub.4 aliphatic oxygenates contained in the feedstock are essentially dehydrated, with gasoline and water given off as the primary products. The intermediate-size zeolite catalysts suitable for use in the MTG process, such as ZSM-5, are selectively penetrated by molecules of intermediate size and are thus capable of converting lower aliphatic oxygenates such as methanol, into high octane gasoline. Gasoline selectivity is considered to be extremely high in the MTG process using this type of intermediate pore size catalyst, described in further detail below, because the sizes of the channels are just wide enough to produce hydrocarbons boiling in the gasoline range. That is, the reaction product terminates at about a carbon number of 10.
The conversion of methanol to gasoline boiling range hydrocarbons is accompanied by the formation of substantial amounts of water byproduct, which contains trace levels of oxygenated compounds (about 0.1-0.2 wt %). This water prduct is sent to a waste water treatment plant to reduce the concentration of oxygenates to acceptable limits.
U.S. Pat. No. 2,847,368 discloses a process for the extraction of hydrocarbons from an aqueous medium to obtain a purified water stream. The extraction is conducted in a column and at a temperature of about 100.degree. C. Further, U.S. Pat. No. 3,998,899 (Daviduk et al) refers to an embodiment in an MTG process wherein the separator drum shows a temperature of about 100.degree. F. (about 38.degree. C.) to provide a rough separation of a cooled hydrocarbon-containing effluent into a water phase, a hydrocarbon phase and a gaseous phase. U.S. Pat. No. 3,931,349 (Kuo et al) also discloses prior art separator conditions in an MTG process wherein heat exchange arrangements may contribute to reducing the reactor temperature effluent to about 100.degree. F. (about 38.degree. C.) before entry into a low pressure separator maintained at a temperature of about 100.degree. F. as well. Gasoline boiling components thus separated and recovered may further be separated in a high pressure separator operated at an unspecified temperature.
In practical operation of MTG and MTO process, however, the product separator temperature fluctuates depending upon the cooling water temperature which varies day to night, week to week, etc., depending on the air temperature. In other words, in normal practice the product separator temperature is not controlled at all.
Methods for converting methanol, dimethylether (DME) and other lower aliphatic oxygenates, such as alcohols or corresponding ethers, to olefins using medium pore size zeolite catalysts (referred to hereinafter as an MTO, methanol-to-olefins, process) are also generally known, as exemplified in U.S. Pat. No. 4,543,435 (Gould et al), incorporated herein by reference. In an MTO continuous process, olefinic hydrocarbon products are produced by the catalytic conversion of the oxygenate feedstock to an intermediate lower olefinic stream. This primary phase catalytic reaction also results in the formation of a byproduct waste water effluent stream upon phase separation from the hydrocarbon phase containing a major amount of C.sub.2 -C.sub.4 olefins. Thereafter, the thus-produced olefins can be oligomerized to produce distillate and gasoline, with recovery of an ethene-rich gaseous phase for recycle to the primary catalytic stage, if desired. For example, the olefinic feedstock can be converted to C.sub.5.sup.+ gasoline, diesel fuel, etc. Representative conversion processes for the olefinic feedstock produced in the primary MTO process include the Mobil Olefins to Gasoline/Distillate (MOGD) method referred to by Gould et al, as well as a method for converting C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components into higher hydrocarbons over crystalline zeolites having controlled acidity. The Gould et al patent refers to specific patents directed to these various olefin-to-gasoline methods.
However, in byproduct water streams produced upon operation of MTG and MTO processes, the amount of oxygenate components, contained therein is undesirably high, and in practice, waste water treatment plants must devote valuable resources to reducing the oxygenate content and COD to environmentally acceptable limits.