Patent FR-2836685-A1 (FR)
Title: Continuous thermal process for cracking polyolefin wastes to produce hydrocarbons
Inventor: DISPONS JEAN
IPC: C10G1/10
Filed: 2002-03-04
Abstract: The initial cracking is done in a closed vessel sealed from atmospheric oxygen using heat from direct contact with very hot chemically inert or unreactive gases heated by burning noncondensable gases from the cracking process itself. A classic feed system such as a screw pushes the wastes continuously into a cage in the vessel which keeps them a small distance from the vessel walls  The very hot gases providing the heat are chemically inert or react very little in the cracking conditions. The solid, liquid and gaseous products from this first cracking phase flow freely to the bottom of the vessel in a sealed chamber which acts as a separator. The gaseous products escape from the separator to be burned in the second stage of cracking; the solid products are retained by a sieve and the liquid products are collected in the bottom of a boiler below the separator together with the superheated steam. These collected liquids undergo a second cracking by boiling heated by one or more gas burners fed by the noncondensable gases from various cracking stages. During a third cracking stage vapours from the boiler pass through a tube bundle reactor heated to between 400 and 600 deg. C where the vapours decompose into fractions whose boiling point is not greater than that of diesel (about 360 deg.C). Superheated steam is injected during the second and third cracking stages to obtain steamcracking conditions. The vapours leaving the reactor are send to a classic rectification column where they are rectified in liquid cuts with different boiling points chosen according to the needs and into gases noncondensable at ambient temperature. These latter are sent to a gasometer to feed the gas burners in the various cracking phases, with the surplus gas forming a primary material rich in alpha-olefins. Steam is injected near the burner flame(s) in the first cracking stage at low pressure superheated to 500 deg.C. The heat lost in the burned gases in the various burners is recovered in a water tube exchanger producing low pressure superheated steam between 400 and 600 deg. C for use in the third cracking stage. The gases and vapours from the first cracking stage and separated from the liquid and solid products in the separator are burned using additional air in the burners heating the boiler and reactor in the second and third stages.  An Independent claim is also included for the use of a particular version of this process where the wastes are compressed by a piston (1) in a tube (2) which is several millimetres thick, with a diameter of between 0.3 and 2m, preferably between 0.3 and 0.5m and a length three or four times its diameter. The surface of the piston is covered with strong points not more than 0.3m long, parallel to the axis of the tube and the first third of the tube is perforated allowing water to flow through. The tube is extended at its end by a mesh cylinder (3) in refractory metal for 2 to 6m, closed at the end by a grille. This mesh part is enclosed in a cylinder (4) of refractory steel several millimetres thick coated externally with a classic insulating layer. The walls of this cylinder are several centimetres from the mesh cylinder except at its end, where the cylinder extends beyond the mesh cylinder by about one diameter to form a cylindrical chamber closed by a flat or dished base with an closable opening for combustion gases (20) and a superheated steam injector (21).