Process for production of needle coke from coal tar pitch and petroleum streams

Abstract

The present invention relates to a process for needle coke production by co-processing coal tar pitch (CTP) with petroleum streams. The process utilizes a ‘multi-residence time mild thermal cracking’ of coal tar pitch with petroleum streams (CLO, PFO) followed by solvent-assisted selective separation of undesirable quinoline insoluble (QI) and solid catalyst fines from the mixed pitch and delayed coking thereafter to produce needle coke. The solvent employed for feedstock purification is a mixture prepared from an aromatic solvent which is a lighter fraction formed by mild and severe thermal cracking of mixed feedstock, an aliphatic solvent and a polar extraction solvent.

We claim: 
     
         1 . A process for preparing a needle coke, the process comprising:
 subjecting a feed stream to a multi-residence time mild thermal cracking in presence of a petroleum stream to obtain a mixed pitch, wherein the feed stream comprises coal tar pitch (CTP), and wherein the petroleum stream is selected from the group consisting of clarified oil (CLO), pyrolysis fuel oil (PFO) and a combination thereof;   purifying the mixed pitch through a solvent-assisted selective separation, to remove an undesirable quinoline insoluble (QI) and solid catalyst fines to obtain a purified mixed pitch, wherein the mixed pitch has a boiling point above 350° C.; and   subjecting the purified mixed pitch to delayed coking to produce the needle coke and a lighter hydrocarbon component.   
     
     
         2 . The process as claimed in  claim 1 , wherein the multi-residence time mild thermal cracking comprises:
 heating the feed stream in a mild cracking furnace;   sending the heated feed stream to a bottom of a mild cracking reactor;   introducing the petroleum stream from a middle section of the mild cracking rector to carry out the multi-residence time mild thermal cracking of the heated feed stream in presence of the petroleum stream to produce a plurality of reactor effluents,   wherein the reactor effluents comprise mixed product vapors of CTP, CLO, and PFO;   sending the reactor effluents from a top section of the mild thermal cracking reactor to a fractionator for fractionating and to produce components comprising a plurality of off-gases, a light oil, a middle oil and the mixed pitch;   routing the plurality of off-gases to a gas concentration section for recovery; and   sending the light oil to a solvent tank for use as an aromatic solvent and middle oil to a hydrotreating unit for recovery.   
     
     
         3 . The process as claimed in  claim 1 , wherein purifying the mixed pitch through a solvent-assisted selective separation comprises:
 treating the mixed pitch with a solvent in a solvent treatment vessel;   settling a heavy pitch-solvent mixture comprising impurities in a gravity settler; withdrawing the heavy pitch-solvent mixture from a bottom of the gravity settler;   withdrawing a purified pitch-solvent mixture from a top of the gravity settler;   heating the heavy pitch-solvent mixture in a first heater and then routing to a first solvent recovery column;   heating the purified pitch-solvent mixture in a second heater and then routing to a second solvent recovery column for solvent recovery;   withdrawing the purified mixed pitch from a bottom of the second solvent recovery column, wherein the solvent to the mixed pitch has a weight ratio in a range of 0.5:1 to 10:1, and wherein the solvent treatment vessel is operated at a temperature in a range of 60° C. to 350° C. and at a pressure in a range of 1 to 80 Kg/cm 2  (g).   
     
     
         4 . The process as claimed in  claim 2 , wherein the mild cracking furnace operates at an outlet temperature of coil in a range of 390° C. to 475° C., wherein the mild cracking reactor operates at a temperature in a range of 380° C. to 475° C., and is maintained at a pressure between 5 to 15 Kg/cm 2  (g), wherein the petroleum stream and the feed stream in the mild cracking rector have a hydrocarbon residence time between 1 to 30 mins, and wherein the fractionator is operated at a bottom temperature between 260° C. to 390° C. 
     
     
         5 . The process as claimed in  claim 2 , wherein the mild cracking furnace operates at an outlet temperature of coil in a range of 430° C. to 460° C., wherein the mild cracking reactor operates at a temperature in a range of 420° C. to 450° C., and is maintained at a pressure between 7 to 12 Kg/cm 2  (g), wherein the petroleum stream and the feed stream in the mild cracking rector have a hydrocarbon residence time between 5 to 20 mins. 
     
     
         6 . The process as claimed in  claim 2 , wherein the plurality of off-gases comprises fuel gas, LPG and naphtha, wherein the light oil has a boiling point in a range from 140 to 270° C., and the middle oil has a boiling point in a range from 270° C. to 350° C. 
     
     
         7 . The process as claimed in  claim 1 , wherein the coal tar pitch has a Conradson Carbon Residue (CCR) in a range of 10 to 47 wt %, and has a quinoline insoluble (QI) content in a range of 1 to 16 wt %; wherein the petroleum stream comprises CLO in a range of 1.5 to 19 wt %, and PFO in a range of 1 to 15 wt %; and the CLO has a basic sediment and water (BS&W) content in a range of 0.05 to 2.5 wt %. 
     
     
         8 . The process as claimed in  claim 1 , wherein the needle coke has a coefficient of thermal expansion (CTE) in a range of 1.33×10 −6 /° C. to 1.21×10 −6 /° C. 
     
     
         9 . The process as claimed in  claim 3 , wherein the solvent treatment vessel operates at a temperature in a range of 70° C. to 200° C. and at a pressure in a range of 1 to 50 Kg/cm 2  (g). 
     
     
         10 . The process as claimed in  claim 3 , wherein the solvent comprises an aromatic solvent, an aliphatic solvent, a polar extraction solvent with carbon numbers ranging from 3 to 15, wherein the aromatic solvent is selected from the group consisting of light oil, coker kero, coal tar oil, anthracene oil, naphthalene oil, benzene, xylene, creosote oil, quinoline, toluene, and a combination thereof, wherein the light oil is obtained by mild thermal cracking, wherein the coker kero is obtained by delayed coking of aromatic feedstocks, wherein the aromatic feedstocks comprise coal tar pitch, CLO, PFO, and wherein the coal tar oil is obtained by distilling coal tar, wherein the aliphatic solvent is selected from the group consisting of cyclohexane, kerosene, n-pentane, n-hexane, n-heptane, diesel oil, naphtha and a combination thereof; and wherein the polar extraction solvent is n-methyl-2-pyrrolidone (NMP). 
     
     
         11 . The process as claimed in  claim 10 , wherein the solvent comprises 10 to 80 wt % of the aromatic solvent, 10 to 80 wt % of the aliphatic solvent, and 10 to 70 wt % of the polar extraction solvent. 
     
     
         12 . The process as claimed in  claim 3 , wherein the delayed coking of the purified mixed pitch comprises:
 i. mixing the purified mixed pitch withdrawn from the bottom of the second solvent recovery column with a hydrocarbon stream comprising hydrotreated vacuum gas oil (HDT VGO) to obtain a mixture, wherein the hydrotreated vacuum gas oil (HDT VGO) is in a range of <0.1 to 10 wt %;   ii. routing the mixture to a mixed fractionator (MF) column, wherein the mixture mixes with an internal recycle stream and is drawn out from a bottom of the mixed fractionator column as a secondary feedstock, wherein a bottom of the mixed fractionator column operates at a temperature in a range of 260° C. to 390° C.;   iii. sending the secondary feedstock withdrawn from the bottom of the mixed fractionator column to a coker furnace at a predefined hydrocarbon feed rate for heating at a temperature predefined for severe thermal cracking;   iv. additionally introducing a dilution steam or a boiler feed water stream to the coker furnace to obtain a hot feedstock comprising a heavy oil which prevent coking in furnace tubes and create additional turbulent flow;   V. routing the hot feedstock from the coker furnace into a plurality of coke drums in the feeding step of the coking cycle, wherein the heavy oil is cracked and converted to the needle coke and the lighter hydrocarbon component;   vi. sending the lighter hydrocarbon component to the mixed fractionator column for separation to obtain an overhead vapor containing the plurality of gaseous products from a top section of the mixed fractionator column; and the plurality of liquid products containing the coker kero, the light coker gas oil, the heavy coker gas oil and the coker fuel oil from the mixed fractionator column with different cut ranges of 140 to 270° C., 270° C. to 350° C., 350° C. to 480° C. and 480° C.+ respectively;   vii. routing the overhead vapor containing the plurality of gaseous products to a gas concentration section for recovery; and   viii. routing the coker kero stream to the solvent tank for use as an aromatic solvent.   
     
     
         13 . The process as claimed in  claim 12 , wherein the coker furnace operates at an outlet temperature of coil between 450° C. to 530° C., wherein the dilution steam or the boiler feed water stream is injected in the coker furnace in a range of 0.5 to 3 wt % of the hydrocarbon feed. 
     
     
         14 . The process as claimed in  claim 12 , wherein the coker furnace operates at an outlet temperature of coil between 490° C. to 515° C. 
     
     
         15 . The process as claimed in  claim 12 , wherein the plurality of coke drums operates at a temperature in a range of 440° C. to 520° C., and is maintained at a pressure in a range of 1 to 10 Kg/cm 2  (g), and has a feeding cycle time in a range of 12 to 56 hrs. 
     
     
         16 . The process as claimed in  claim 12 , wherein the plurality of coke drums operates at a temperature in a range of 470° C. to 500° C. and is maintained at a pressure in a range of 2.5 to 7 Kg/cm 2  (g). 
     
     
         17 . The process as claimed in  claim 12 , further comprising removing the needle coke deposited in the plurality of coke drums in a maintenance cycle by using a high-pressure water jet. 
     
     
         18 . The process as claimed in  claim 12 , wherein the lighter hydrocarbon components comprise off-gases comprising fuel gas, LPG, naphtha, gas-oils, and a combination thereof. 
     
     
         19 . The process as claimed in  claim 12 , wherein the plurality of coke drums operates at a temperature in a range of 470° C. to 500° C. and at a pressure in a range of 2.5 to 7 Kg/cm 2  (g). 
     
     
         20 . The process as claimed in  claim 1 , wherein the needle coke derived from the purified mixed pitch has a crystallinity of 77.8% to 80.3%.