5.0 DIESEL HYDROTREATING UNIT

Transcription

1 5.0 DIESEL HYDROTREATING UNIT 5.1 INTRODUCTION PURPOSE The main purpose of the DHDT unit is to hydrotreat the diesel components from atmospheric and vacuum units of existing and expansion units, Delayed Coker unit and fluidized catalytic cracking unit to produce HSD meeting EURO-III/IV specifications. The product diesel will have minimum cetane number of 53.5 or an improvement in cetane number of 10 points over the feed stream and sulphur content of less than 30 ppmw. The other process objectives of the DHDT unit are the stabilization of the wild naphtha generated in the unit such that it can be routed to refinery gasoline pool & the amine treatment of the sour off gases produced in the unit UNIT CAPACITY The diesel hydro-treating unit capacity is 3.5 MMTPA in two chains from feed pre heat upto reaction sections. The turndown ratio of the unit is 50% of the design capacity. The feed to DHDT unit consists of the following streams obtained while processing Arab Mix crude oil (50% of Arab light crude and 50% Arab heavy crude and Quaiboe (QB) crude oil. Straight Run Light Gas Oil (SRLGO) from Crude Distillation Unit Straight Run Heavy Gas Oil (SRHGO) from Crude Distillation Unit Straight Run Kerosene (SRK) from Crude Distillation Unit Straight Run Vacuum Diesel (SRVD) from Vacuum Distillation Unit Total Cycle Oil (TCO) from FCC Unit (FCCU) Light Coker Gas Oil (LCGO) from Delayed Coker Unit (DCU) Coker Heavy Naphtha (CHN) from Delayed Coker Unit (DCU) LICENSOR The technology is Licensed by Institute Francais Du petrole (IFP), Paris, France.

2 5.1.4 MAIN SECTIONS OF THE UNIT The DHDT Unit consists of the following sections: HP Section o Feed Preheat o Reaction Section o HP Amine Treating Section LP Section o Stripper o LP Amine Treating Section o Naphtha Stabilizer 5.2 FEED DIESEL STREAMS AND THEIR CHARACTERISTICS A) Arab Mix Crude oil: Distillation AM SRLGO AM SRHGO AM SRK AM SRVD TBP CUT, c IBP % % % % % % % FBP Sp. 15 C Sulphur, wt% (Max.) Nitrogen, PPM wt. (Max.) Flash point, C Aromatic content wt. % (28) (36) (20) (32) (Est) Cetane Number (Est) 52 (48) 42 (50) Metal content (Ni + Va) <1.0 <1.0 <1.0 <1.0 ppmw Pour point, C (-) (-)

3 B) Quaiboe Crude Oil : Distillation QB SRK QB SRLGO QB SRHGO QB SRVD TBP CUT, c ASTM D86, c IBP % % % % % % % FBP Sp. 15 C Sulphur, wt% (Max.) Nitrogen, PPM wt (Max.) Flash point, C Aromatic content (16) (18.5) (20) (19) wt% (Est) Cetane Number (Est) (36) (42) (40) (41) Metal content (Ni + <1.0 <1.0 <1.0 <1.0 Va) PPMW Pour point, C <

4 C) Cracked Diesel Feed Streams Distillation Coker Light Coker Heavy Coker Light TCO Naphtha Naphtha Gas Oil TBP CUT, c C ASTM D86, c IBP % % % % % % % FBP Sp. 15 C Sulphur, wt% (Max.) Nitrogen, PPM wt (Max.) Flash point, C - < Bromine Number Aromatic content wt% (Est) Cetane Number Metal content (Ni + Va) PPMW - - <1.0 <1.0 Si content (Peak) PPMW Si content (Avg) PPMW Pour point, C - - (-)12 -

5 The unit is designed to process three different types of feed. The proportion of the various diesel components in the design feed is as follows: FEED STREAMS Feed 1 wt% Feed 2 wt% Feed 3 wt% AM SR LGO AM SR HGO AM SRK AM SRVD QB SR LGO QB SR HGO QB SRK QB SRVD CHN LCGO TCO TOTAL The composite feed characteristics of the three feed cases is as follows: Distillation (ASTM D86, c ) Feed 1 Feed 2 Feed 3 IBP % % % % % FBP Sp. 15 C Sulphur, wt% (Max.) Nitrogen, PPM wt. (Max.) Flash point, C Bromine Number Aromatic content wt% Cetane Number

6 Metal content (Ni + Va) <1 <1 <1 PPMW Si content (Avg) PPMW PROCESS DESCRIPTION FEED PREHEAT Hot diesel stream components from CDU/VDU, DCU, TCO & Cold straight Run streams (SRK, SRLGO, SRHGO & SRVD) from storage tank are received into the feed surge drum. The cold feed from storage and hot TCO from FCCU received into the feed surge drum under level and flow control respectively. The proportion of cold feed in the total feed to the unit is about 20%. The composite feed to the unit is routed to a automatic back wash filter (Make: FILTREX) to arrest particles of size greater than 25 microns before receiving into the feed surge drum V-001. At the upstream of the feed surge drum, pressure control valve maintains the backpressure for internal backwash of the filter elements.the pressure in the feed surge drum is maintained by a split range control valve on fuel gas to surge drum and to flare. The feed from the surge drum is pumped under flow control into two parallel streams by feed pump (Make: FLOWSERVE). Diluted anti-fouling agent in hydrotreated diesel is injected at the upstream of the feed pump. The feed diesel in each train of reaction is mixed with the recycle/make up hydrogen and heated in a welded plate type (packinox) feed / effluent exchanger. It is further heated upto the required temperature in a reaction heater. The reactor inlet temperature is controlled by controlling the fuel oil/fuel gas pressure to the burners. A bypass is provided on the feed side of the feed effluent heat exchanger to maintain the effluent temperature to the hot separator. Each reaction section train consists of reactor feed/effluent exchanger, stripper feed preheater, reactor heater and two reactors. Each train can process 50% of the total feed.

7 4.3.2 REACTION SECTION The HDT reactors (Make:JSW,JAPAN) are down flow type and consist of two reactors. The first reactor consists of three beds, first bed containing HR-945 catalyst which is more active towards hydrogenation of olefins, other two beds contain HR-348 which is more active towards desulphurisation and the second reactor has two beds. In order to limit the temperature rise inside the reactor, cold recycle hydrogen gas quenches are provided under flow control to maintain the catalyst bed temperatures. Presently only HDS reactor will be installed and the second reactor shall be installed at a later date during P-15 implementation. Some of the reactions taking place inside the reactor are given below: RSH+H2 RH+H2S R1SR2+2 H2 R1H+R2H+ H2S R1SSR2+3 H2 R1H+R2H+2H2S R-CH=CH 2 + H2 R-CH2-CH3 R-NH2 + H2 RH + NH3 The effluent from the second reactor is cooled in stripper feed preheater under temperature control of the stripper feed. The reactor effluent from the stripper feed/ effluent exchanger is further cooled against reactor feed and routed to hot HP separator. The feed bypass of the exchanger controls the temperature of the reactor effluents. In the hot HP separator two phases are separated. The major part of the hydro-treated liquid diesel is routed to the hot MP separator through power recovery turbine under flow control. The power recovery turbine partially runs the feed pump The vapor phase of hot HP separator is cooled and partly condensed in the reactor air cooler, and routed to Cold HP Separator. Wash water pumps (Make:PERONI, ITALY) have been provided for injecting wash water (3-5 % of the feed) at the inlet of the Air fin coolers to prevent the deposition of ammonium bi-sulfide on the surface of the air fin coolers. The wash water consists of the recycle water from cold HP separator; stripper and stabilizer reflux drums and Stripped sour water from a hydro-processing sour water stripper. In the cold HP separator three phases are separated. The major part

8 of the hydrotreated liquid diesel is routed to the cold MP separator under flow control. The hydrocarbon liquid from the cold MP separator is routed to the stripper through stripper feed/bottom & stripper feed/reactor effluent exchangers under level control. The reaction section pressure is maintained by the hydrogen make up by the Make up Gas Compressor(Make:MITSHIBUSHI, JAPAN). The spill back controller on the make up gas compressors controls the reaction section pressure. A part of the water from the HP separator is sent to wash water vessel and the balance to the sour water stripper. Vapor phase from cold MP separator and Hot MP separator vessels are routed to the stripper overhead under pressure control HP AMINE ABSORPTION SYSTEM The vapor phase from cold HP separator is sent to HP Amine absorption column through a knock out drum. A bypass is provided on the HP amine absorption column to maintain the hydrogen sulphide concentration in recycle gas between 0.1 to 1.0 mole percent. The amine booster pump (Make:KSB) delivers the Lean amine to the amine absorption column. Rich amine from the column bottom is routed the bottom of the LP amine absorber to flash the lighter hydrocarbons. Hydrogen rich gas free of H2S is routed to suction of recycle gas compressor (Make:BHEL) STRIPPER SECTION The hydrocarbon liquid from the cold MP separator and hot MP separator are routed to stripper under level control after heating in stripper feed bottom exchanger and stripper feed/reactor effluent exchanger. Stripper feed/reactor effluent exchanger bypass controls stripper feed temperature. Superheated MP steam is used as stripping steam to maintain the flash point of the diesel from the stripper. The overhead vapors from the stripper alongwith the vapors from cold and hot MP separators are condensed in stripper air fin coolers and in a trim cooler. The stripped diesel from the stripper bottom is cooled against the stripper feed, BFW to generate LP

9 steam, DM water Preheater and finally water cooler. The cooled diesel product is sent to the diesel storage through a coalescer to separate the fine water droplets. The overhead vapors from stripper are cooled in air fin condenser and in a water cooler. The liquid and gas is collected in stripper reflux drum. A part of the liquid naphtha is sent back to the stripper as reflux and balance to the naphtha stabilizer under level control STABILIZER SECTION The naphtha is preheated against the stabilized naphtha in stabilizer feed bottom heat exchanger. The required heat for stabilizing the naphtha is provided in a natural circulation MP steam reboiler. The stabilized naphtha is cooled in a water cooler after exchanging heat with the feed naphtha and sent to the naphtha tank. The product naphtha can be routed to either naphtha tanks, CRU feed tanks or Motor spirit blending header. The over head vapors are cooled in air condenser and collected in stabilizer reflux drum. The liquid from the reflux drum is sent back to the column as reflux under level control LP AMINE ABSORPTION SYSTEM The gas from the stripper reflux drum and stabilizer reflux drum is sent to the LP amine absorber alongwith the Hydrocracker cold flash drum gas and also the gas from the predesulphurisation section of the Hydrogen generation unit for absorbing the hydrogen sulphide. The sweet fuel gas from the LP amine absorber is routed to the Refinery fuel gas header. The rich Amine from the LP amine absorber column is sent to the Amine regeneration unit. The sweet fuel gas is rich in hydrogen content, hence space provision has been kept for facilities towards the recovery of hydrogen from HCU, DHDT and hydrogen Offgas. The sweet fuel gas from the LP Amine absorber will be compressed to 25.5 Kg/cm2 (g) for routing to PSA unit for recovery of hydrogen in future.

10 4.4 MATERIAL BALANCE The material balance for the unit is as follows: Figs. in KG/Hr. INPUT FEED 1 FEED3 SOR EOR SOR EOR Feed Diesel Hydrogen Hydrogen Rich Gas Ex Hcu Total OUTPUT Product Diesel Stabilised Naphtha Off Gas H 2S/NH 3 In Sour Water & Amine Total

11 4.5 PRODUCT PROPERTIES Product Property Specification Destination Sulphur,PPMW < 150 (one reactor) PREP HSD < 30 (After second Blending Hydrotreated reactor installation) Station Diesel Nitrogen,PPMW <5 Cetane Number 53.5;47 for Feed 3 PR HSD Copper Strip corrosion Not Worse than 1 Blending Station Sp Gravity H 2S content PPMW <1 PREP HGU Feed Sulphur, PPMW <5 Blending Stabilized RON 65 Station Naphtha RVP Kg/cm 2 g Benzene Content, vol % 1.0 PREP HGU Copper Strip Corrosion Not worse than 1 FEED Blending Station PREP Naphtha Blending Station

12 4.6 TYPICAL OPERATING CONDITIONS The typical operating parameters are given below for feed1, SOR conditions: S. NO. PARAMETER FLOW TEMP C PR.KG/ CM 2 G 1 Feed to unit, Mt/hr Recycle Hydrogen flow Mt/hr Make Up Hydrogen Flow, Mt/hr Hydrogen Recycle Ratio Feed preheat/ H-001 inlet Reactor R-001 inlet Hydrogen Quench Flow, Mt/hr Reactor R-001 outlet Reactor R-003 inlet Reactor R nd Bed Outlet HP hot separator Liquid outlet flow from HP separator Vapor outlet flow from HP separator, HP SECTION 14 Wash Water flow at HP air fin cooler Make Up Water Flow Mt/hr HP cold separator Hydrogen Gas flow to RGC, Mt/hr MP SECTION 18 Lean Amine flow to HP Amine Hot MP separator Cold MP separator Hydrocarbon Flow to Stripper,

13 STRIPPER SECTION 22 Stripper Top Stripper Bottom Stripping Steam Flow to Stripper, Stripper Reflux Flow, Mt/hr Stripper Reflux drum Hydrotreated Diesel flow, Mt/hr STABILIZER SECTION 28 Naphtha Flow to Stabilizer, Mt/hr Stabilizer Top Stabilizer Reflux Drum Stabilizer Reflux Flow, Mt/hr Stabilizer outlet MP Steam to Reboiler Mt/hr Stabilized Naphtha R/d Flow Mt/hr LP AMINE 37 Off Gas to LP Amine Absorber, Mt/hr Amine to LP Amine Absorber, Mt/hr Sweet Gas Fuel Gas Header, Mt/hr

14 4.7 SALIENT FEATURES OF THE DHDT 1. Production of diesel meeting Euro-III/IV specifications. 2. Processing of 80% hot feed and 20% cold feed. 3. Improvement in cetane number by 10 points. 4. Reaction section in two parallel trains 5. Compact welded plate heat exchangers in place of conventional tubular heat exchangers for reactor effluent / feed exchangers. Heat duty of Welded Plate Heat exchanger is 54 MMKCAL/HR. Smaller heat duty reaction furnaces due to efficient heat recovery in welded plate heat exchangers. 6. Welded plate heat exchangers for stripper feed/stripper bottoms exchangers in place of conventional tubular heat exchangers. 7. Space provision for cracking catalyst in hydrotreater reactors. 8. Amine treatment of hydrogen rich gas from CDU, VDU, HCU and HGU. 9. Installation of improved liquid distributor in the reactors. 10.Provision for processing MTPA of coker light Naphtha alongwith feed. 11.Routing of make up hydrogen to RGC discharge and thereby reduction in recycle gas compressor capacity. 12.Provision for running the unit at 50% capacity with one chain of the reactors. 13.Provision of Duplex filters at the inlet of welded plate exchangers to prevent entry of foreign material. 14.Provision of recirculation line for pressurization of welded plate exchanger shell before entry of fluid into the plates to prevent accidental damage of the exchanger during start up and shutdown. 15.Shutdown valves have been provided in stripper and stabilizer outlet lines to prevent gas entry into the tanks. 16.Provision for routing total sour water from the unit to the sour water stripper in case of requirement. 17.Amine absorbers are designed for both DEA and MDEA as solvent.

15 18.Naphtha stabilizer is designed for naphtha specification on copper strip corrosion. 19.Power recovery turbine for feed pump to recover 910 KW of power. 20.Hot HP separator, Cold MP separator and Hot MP separators are provided for energy saving 21.Common APH system for both the reaction furnaces. 22.Hydrotreatment of cracked diesel streams like light coker gas oil, coker heavy naphtha and total cycle oil from FCCU. 23.DM water preheating to minimize power consumption in Air fin coolers. 24.Use of service water for cooling of glands, bearings briddle for pumps and compressors. 4.8 ENERGY CONSERVATION MEASURES 1. High efficient Welded Plate Heat Exchangers for Reactor /feed effluent heat exchange. 2. Reduction in firing duty in the furnace due to Welded plate Heat Exchanger. 3. Combustion Air preheating with flue gas to improve efficiency of the furnace. 4. Common FD fan for both the reaction furnaces. 5. Power recovery Turbine has been provided for the feed pump to reduce the electrical load of motor. 6. DM water preheating in hydro-treated diesel stream to recover low level heat. 7. LP steam generation by exchange of heat from the hydro treated diesel stream. 8. Hot HP separator has been provided to reduce the vapor load on HP air fin coolers and also to minimize the energy consumption.

16 4.9 COMPARISON BETWEEN DHDT AND DHDS UNITS DHDT Unit Capacity of Unit is 3.5 MMTPA Product Cetane number Improvement by 10 units Product diesel sulphur content shall be less than 350 ppm, 30 ppm after commissioning of 2 nd reactor. High Pressure in reaction system. Cold separator Pressure shall be 108 kg/cm 2 g. There are hot and cold HP and MP separators in the configuration One Welded Plate heat exchanger in each train of the reaction section Power recovery Turbine for the feed pump Stripper feed/bottom exchanger is welded plate heat exchanger DHDS Unit Capacity is 0.7 MMTPA No appreciable change in Cetane number Product diesel sulphur content shall be less than 500 ppm. Cold separator Pressure is 50 kg/cm 2 g. Only cold HP separator in the configuration Only Shell and Tube Exchangers in the reaction section. No power recovery turbine for feed pump. Stripper feed/bottom exchanger is shell and tube heat exchanger Separate Reflux drums for stripper and stabilizer due to high capacity. Steam is on tube side of the stabilizer reboiler Reflux is provided for stabilizer operation Stabilizer Vapors are routed along with stripper over head vapors. Steam is on shell side of the stabilizer reboiler Reflux is not provided for stabilizer operation

17 4.10 SPECIAL EQUIPMENTS/ NEW TECHNOLOGY PACKINOX EXCHANGER (Make-PACKINOX, FRANCE) The Packinox feed/effluent Heat exchanger for hydrotreaters is a large heat transfer bundle inside a pressure vessel. Heat transfer takes place inside the bundle in true counter-current flow. Four bellows compensates for differential thermal expansion between the hot stainless steel bundle and relatively cool low alloy pressure vessel. Manholes are provided at top and bottom end for approaching bundle for maintenance repair. The heat exchanger can operate at temperatures as high as 550 o C and at pressure same as pressure vessel. Packinox exchangers operates at lower approach temperatures thereby increasing the reaction heater inlet temperature, the design heat duty of the exchanger is MMKcal/hr. The design pressure of the exchanger is kg/cm 2 g on hot side and kg/cm 2 g for cold side. A pressurisation line is provided to ensure that the pressure vessel is always pressurised first before entry of fluid into the bundle. Strainers are provided at inlets of plate exchanger to prevent the entry of foreign material inside the exchangers. The exchanger shall be designed for a differential pressure of 31.2 kg/cm 2 g between hot and cold side. The packinox exchanger being used in stripper feed is suited for medium operating temperature and pressure. In this exchanger there is no pressure vessel and a set of bolted tie roots securely clamp and compress the all welded plate bundle the heat duty of exchanger is 13.6 MMkcal/hr. The exchanger is designed for pressure of 22.1 kg/cm 2 g on hot side and 24.2 kg/cm 2 g on cold side. The design differential pressure between hot and cold sides is 24.2 kg/cm 2 g.