Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks comprising ethane or ethane/propane mixtures, known as gas cracking, or propane, butane, naphtha, NGL (natural gas liquids), condensates, kero, gas oil and hydrowax, known as naphtha cracking. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into, for instance, methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process.
In an OTO process, an oxygenate such as methanol or dimethylether is provided to a reaction zone of a reactor comprising a suitable conversion catalyst and is converted to ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the oxygenate such as methanol is converted to higher hydrocarbons including C4+ olefins, paraffins and carbonaceous deposits on the catalyst. The catalyst is regenerated to remove a portion of the carbonaceous deposits by methods known in the art, for example heating the catalyst with an oxygen-containing gas such as air or oxygen.
The effluent from the reactor, comprising the olefins, any unreacted oxygenates such as methanol and dimethylether and other reaction products such as water, is separated from the bulk of the catalyst, usually by one or more cyclonic separation devices. The remaining effluent, may then be treated in a number of steps to provide separate component streams, including the desired olefin streams and by-product streams. Even after separation of the bulk of the catalyst, some solids, such as catalyst fines, will remain in the reaction effluent stream.
In order to increase the ethylene and propylene yield of the process, a separated stream containing C4+ olefins may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene.
Following reaction in the OTO reactor, the reaction effluent stream must be cooled before being treated to provide separate component streams. Conventionally, the reaction effluent stream is cooled to around 140 to 350° C. using one or more heat exchangers, often one or more transfer line exchangers (TLEs), before being contacted with a cooled aqueous stream in a quench tower. A quench tower comprises at least one set of internals such as packing and/or trays.
In usual operation, the gaseous stream to be quenched is fed into the quench tower below the internals and one or more cooled aqueous streams is fed into the quench tower above the internals. Thus, the gaseous stream travels upwards through the quench tower and is brought into contact with the one or more cooled aqueous streams travelling downwards through the tower (counter-currently to the gaseous stream). The cooled gaseous stream is removed from the top of the quench tower. An liquid stream containing condensed materials is removed at the bottom of the tower.
U.S. Pat. No. 6,870,072 describes such a quench tower process for recovering heat from the reaction effluent stream in an OTO process. In U.S. Pat. No. 6,870,072, the reaction effluent stream is quenched by contacting it with a quench medium, typically in a quench device, specifically a quench tower. The water cools the reactor effluent stream and removes solids. The water containing the solids is cooled and re-used as quench medium in the quench tower.
As well as aqueous material, liquid hydrocarbons and oxygenates will be present in the quench tower liquid stream. Solid material, such as catalyst fines may also be present. Separation of these different materials is carried out either in the bottom of the quench tower or in a separate settler. Solids are removed as a slurry. Liquid hydrocarbons are separated as a waste stream or for use as fuel. Aqueous material is separated and can be recycled, with cooling, to the quench tower or a different part of the process. A separated oxygenate containing liquid stream will usually be subjected to further separation in order to provide oxygenates for re-use as the oxygenate co-feed in the OTO reactor.
In known processes, separation of the aqueous material, solids and hydrocarbons is carried out by gravity separation using level control with a vertical weir. However, the presence of oxygenates in the quench tower liquid stream can cause difficult separation and foam formation, particularly in regions of turbulent flow.
It would be desirable to provide an improved process for the separation of the component materials in the quench tower liquid stream.