Process for the Preparation of Lower Olefins from Heavy Wax

Abstract

The present invention relates to a process for the preparation of lower olefins from a heavy synthetic oil fraction prepared in a Fischer Topsch process. The process according to the invention comprises mild thermal cracking of the heavy synthetic oil fraction, followed by short residence, high temperature thermal cracking of the product obtained in the first process.

1 . A process for the preparation of lower olefins from a synthetic oil fraction prepared in a Fischer Tropsch process, the process comprising mild thermal cracking of the synthetic oil fraction to form a product, followed by short residence time, high temperature thermal cracking of the product obtained in the mild thermal cracking step, in which process at least 5 wt % of the synthetic oil fraction has a boiling point above 550° C. 
     
     
         2 . The process according to  claim 1 , in which the part of the synthetic oil fraction having a boiling point above 550° C. is at least 10 wt % of the total synthetic oil fraction. 
     
     
         3 . The process according to  claim 1 , in which at least 20 wt % of the synthetic oil fraction has a boiling point above 650° C. 
     
     
         4 . The process according to  claim 1 , in which the Fischer Tropsch process comprises reaction of carbon monoxide and hydrogen over a cobalt or iron based Fischer Tropsch catalyst, at a temperature between 150 and 300° C. and a pressure between 5 and 100 bara. 
     
     
         5 . The process according to  claim 1 , in which the synthetic oil fraction comprises at least 90 wt % paraffins after hydrogenation. 
     
     
         6 . The process according to  claim 1 , in which at least part of any oxygenates and/or olefins in the synthetic oil fraction are removed in a pre-treatment process. 
     
     
         7 . The process according to  claim 1 , in which the initial boiling point of the synthetic oil fraction is at least 250° C. 
     
     
         8 . The process according to  claim 1 , in which the mild thermal cracking process comprises furnace cracking in which the furnace cracking is carried out at a temperature between 500° C. and 700° C. and a residence time up till 6 minutes. 
     
     
         9 . The process according to  claim 8 , in which the conversion of 550° C.+material into 550° C.−material is at least 40%. 
     
     
         10 . The process according to  claim 1 , in which any 550° C.+material present in the product after mild thermal cracking is separated from the reaction product and recycled to the mild thermal cracking. 
     
     
         11 . The process according to  claim 1 , in which the C 2  and C 3  is also sent to the high temperature thermal cracking unit. 
     
     
         12 . The process according to  claim 1 , in which methane produced in the Fischer Tropsch reaction and any unconverted hydrogen and carbon monoxide are used as fuel for mild thermal cracking reaction and/or the high temperature thermal cracking. 
     
     
         13 . The process according to  claim 1 , in which natural gas condensates and/or ethane/propane extracted from natural gas sources are also sent to the high temperature cracking process. 
     
     
         14 . The process according to  claim 1 , in which at least 30 wt % of the synthetic oil fraction has a boiling point above 550° C. 
     
     
         15 . The process according to  claim 1 , in which at least 40 wt % of the synthetic oil fraction has a boiling point above 650° C. 
     
     
         16 . The process according to  claim 1 , in which the synthetic oil fraction comprises at least 50 wt % normal compounds. 
     
     
         17 . The process according to  claim 1 , in which the synthetic oil fraction comprises at least 70 wt % normal compounds. 
     
     
         18 . The process according to  claim 1 , in which the initial boiling point of the synthetic oil fraction is at least 450° C. 
     
     
         19 . The process according to  claim 1 , in which the mild thermal cracking process comprises soaker cracking which is done at a temperature between 400° C. and 500° C. and a residence time between 10 and 60 minutes.