4.0 HYDROGEN GENERATION UNIT (PREP) 4.1 INTRODUCTION

Transcription

1 4.0 HYDROGEN GENERATION UNIT (PREP) 4.1 INTRODUCTION PURPOSE The objective of the unit is to produce hydrogen by steam reforming of full range naphtha i.e. C O C SR naphtha & coker naphtha to meet the hydrogen requirements for the reactions in Hydrocracker unit of 1.7 MMTPA capacity and a Diesel hydrotreating Unit of 3.5 MMTPA capacity UNIT CAPACITY The hydrogen section has a capacity of 1,40,000 MTPA comprising of two parallel trains, each train is designed to produce 70,000 MMTPA of Hydrogen The unit is designed based on the full range straight run naphtha C 5-140cut obtained during processing of high sulphur crude oil in AVU. The unit is also designed for a mixture of SR Naphtha and Coker Light Naphtha obtained from DCU. The turn down ratio of the unit is 30% for each section LICENSOR The technology for hydrogen reformer has been obtained from M/s Haldor Topsoe A/S, Denmark and for PSA System from UOP, Belgium MAIN SECTIONS OF THE UNIT The Hydrogen Plant is divided into the following Sections. Pre-Desulphurisation section Hydrogen Section The hydrogen section consists of: Second stage hydrogenation Pre-reforming Reforming CO-Conversion PSA purification system

2 4.2 FEED STOCK PROPERTIES The typical characteristics of SR and Coker LT. Naphtha are given below: S.NO PARAMETER SR NAPHTHA COKER LIGHT NAPHTHA 1 TBP Cut range C C - 2 ASTM distillation, c IBP 43-5% Vol % Vol % Vol % Vol % Vol % Vol % Vol EP Specific 15 0 C PONA Analysis,% Vol. Paraffin s Olefins Naphthenes Aromatics Cyclic olefins Acyclic olefins C/H ratio Sulphur Total/mercaptan, 600/ / Chloride,PPMW 4-8 LHV, Kcal/kg Water, PPMW Metal Content,(ppbw Max) Arsenic 20 Lead 20 Nickel 20 Vanadium 20 Copper 20 Iron 20 Total 80 Nitrogen, ppmw (Max) Silicon, ppmw (Max) 5 Potential Gum gm/m * Provision for processing OHCU light naphtha in HGU feed has also been kept.

3 4.3 PROCESS DESCRIPTION OF THE UNIT PRE-DESULPHURIZATION SECTION The Coker Light Naphtha is received from the Delayed Coker Unit into Coker Naphtha feed surge vessel. Coker Naphtha is pumped by the coker Naphtha feed pump and mixed with the recycle and make up hydrogen. The mixed stream is preheated against reactor effluents in a heat exchanger and further preheated against Saturated HP steam and sent to the Si-Adsorber Reactor. In the Si-Adsorber Reactor diolefins are converted into paraffin s and Si is adsorbed on the catalyst. SR Naphtha is received into the SR Naphtha feed surge vessel from the off-site Hydrogen feed storage Tanks. SR Naphtha is pumped by the SR naphtha feed pump and joins with the pre-treated Coker Naphtha at the outlet of Si- Adsorber reactor. The combined feed is further preheated against 1 st hydrogenation reactor effluent and further preheated in a fired heater and sent to the 1 st Hydrogenator. In the 1 st Hydrogenator balance olefins are saturated and sulphur compounds are converted to hydrogen sulphide. The reactor effluents are cooled against feed streams and condensed in Air fin coolers, water cooler and collected in the cold separator. Boiler feed water is injected at the inlet of air fin coolers/feed effluent exchanger for washing the dry ammonium bisulphate salts. In the cold separator three streams are separated. They are unreacted hydrogen, liquid hydrocarbons and water. The unreacted hydrogen is routed to suction of recycle gas compressor (Make: BPCL) which is recycled back & mixed with the incoming feed. A part of recycle gas is purged to maintain the hydrogen sulphide concentration between % in recycle gas. The liquid hydrocarbons are sent to the stripper after heating against the stripper bottoms in a heat exchanger. The vapours from the stripper are condensed in water cooler and sent back as reflux. The required heat is supplied to the stripper bottom material in a steam reboiler. The Naphtha from stripper bottoms contain about 5 ppm sulphur and is cooled against stripper feed and sent to the Feed surge drum of the Hydrogen section. When Hydrogen section is under circulation the sweet Naphtha can be sent to the Sweet Naphtha

4 Storage tank in the Unit after cooling in a water cooler. Provision for routing naphtha to HGU feed & Naphtha Blending Station has also been kept HYDROGEN SECTION A. HYDROGENATION The naphtha feedstock containing upto 5ppm of S compounds must be desulphurised because catalysts in reforming section are extremely sensitive to S compounds. The hydrogenation catalyst (TK-250) is a Cobalt Molybdenum catalyst. Sulphur present in the form of mercaptans, disulphides, sulphides and thio-sulphides and carbonyl-sulphur in the naphtha feed is converted to H2S. Some of typical reactions are shown below. RSH+H2 R1SR2+2 H2 R1SSR2+3 H2 C2H5 SH+ H2 CH3S C2 H5+2 H2 C2 H5SS C2H5+3 H2 RH+H2S R1H+R2H+ H2S R1H+R2H+2 H2S C2H6+ H2S C2H6+CH4+ H2S 2C2H6+2 H2S The Sweet Naphtha from the Pre-Desulphurisation section is received in to the feed surge drum. The feed Naphtha is pumped by the feed pump and is mixed with the recycle hydrogen. The mixed stream is preheated in an exchanger against Saturated HP steam and is further preheated upto the reaction temperature against super heated HP steam and sent to the 2 nd hydrogenator. In the 2 nd hydrogenator balance sulphur compounds are converted into hydrogen sulphide and chlorides into hydrogen chloride. The products from the reactor are sent to the ZnO adsorbers, where hydrogen chloride is adsorbed by the Chloride guard and the hydrogen sulphide by the ZnO catalyst. The two sulphur adsorbers are identical where the second reactor acts as a guard vessel. Each of the reactors is loaded with HTZ-5 sulphur adsorption catalyst. The sulphur in naphtha stream from the ZnO Adsorbers is less than 0.05PPM, which is mixed with the steam and is preheated in the Pre-reformer preheat

5 coil in the waste heat section of the reformer furnace and sent to the Prereformer. B. ADIABATIC PRE-REFORMING In the pre-reformer all the higher hydrocarbons are converted to hydrogen, CO, CO 2 & Methane. This is an adiabatic process. The pre-reformer is charged with RKNGR, (HTAS proprietary Catalyst). C. TUBULAR REFORMING The tubular reformer is constructed as a duplex furnace, each cell containing a single row of 120 centrifugally cast tubes made of high alloy Cr-Ni steel and filled with reformer catalyst R-67-7H. The tubes in each cell are heated by 180 burners arranged in 6 horizontal rows on each side of the furnace. In the reformer furnace, balance hydrocarbons and methane are converted into hydrogen, carbon monoxide and carbon dioxide. The flue gas flow is upward with outlet at the top of the chamber whereas the process gas flow downwards inside the tubes and is collected in cold collector at the bottom of the furnace. The flue gas outlet temp. is about C. PSA Off gas from PSA unit and vaporised naphtha are used as fuel in the reformer. The flue gas heat from the reformer furnace is used for: - Preheating of reformer feed. - Preheating of pre-reformer feed. - Superheating of Steam. - Heating of combustion air. - Generation of saturated HP steam. - Preheating of combustion air. The process gas leaving the tubular reformer will pass through a waste heat boiler where saturated HP steam is produced. The process gas is cooled in BFW pre-heater up to the reaction temperature of the medium temp shift converter. The steam reforming of hydrocarbons can be described by the following reactions.

6 C nh m + nh 2O CH 4+H 2O nco + (n+m/2) H 2 +Heat CO 2+H 2+ Heat D. CO-CONVERSION i) MEDIUM TEMPERATURE SHIFT CONVERSION The reformed gas contains about 20 % CO and CO is reacted with steam to produce CO2 and hydrogen in presence of MT shift catalyst. In the shift converters CO is converted into hydrogen and thereby increases the production of hydrogen. The medium temperature shift converter is filled with LK-811 catalyst. Lower temperatures and presence of more steam favour a low equilibrium content of CO. The process gas is cooled against BFW in a BFW preheater upto the LT shift converter inlet temperature. Shift reaction is an exothermic reaction: CO + H2O CO2 + H2 + Heat ii) LOW TEMPERATURE SHIFT CONVERSION Further conversion of CO to CO 2 & hydrogen to an optimum level is achieved in LT shift reactor. The LT shift reactor is filled with LK-823 catalyst. The inlet temperature of both LT & MT shift converter are the same whereas the outlet and bed temperatures are higher in MT shift converter. The process gas from the LT Shift converter is cooled against BFW in a BFW pre heater and sent to a separator where condensate is recovered and sent to the deaerator. The process gas is further cooled in DM water Preheater, Air fin cooler, water cooler and sent to the second condensate separator.the recovered condensate from the second condensate separator is sent to the Deaerator. The Process gas is sent to the PSA system for purification to produce 99.99% pure hydrogen.

7 E. PSA SYSTEM The PSA system of the hydrogen unit consists of 12 bed adsorbers, controlled by redundant PLC for each chain of hydrogen unit. The feed to PSA system is process gas from Shift converter at the down stream of the steam reformer. The PSA system produces: i) High purity hydrogen Product (99.9% volume) ii) PSA off gas for use as fuel in steam reformer. The PSA system can operate at any feed flow rate from 30% to 100% of the design flow rate without loss in recovery and purity. When the feed flow rate is reduced below 30% of the design flow rate the recovery will decrease however the product quality shall be maintained. The temperature of the feed gas to PSA must be maintained at design temperature or below. High feed gas temperature will result in too high moisture content in the feed gas which lowers adsorbent efficiency. 4.4 MATERIAL BALANCE Material Balance for Pre-Desulphurisation Section Input Kg./Hr. %wt. Coker Naphtha Feed Sr Naphtha Feed Recycle Hydrogen Total Output Desulphurised Naphtha H2S Rich Stream Fuel To Furnace Total

8 Material Balance for each chain of Hydrogen Unit Input: KG/HR %wt Sweet Naphtha As Feed Process Steam Fuel Naphtha Total Output Product Hydrogen Process Condensate PSA Off Gas/ Naphtha To Reformer Fuel Total PRODUCT PROPERTIES PROPERTY SPECIFICATIONS Destination Hydrogen Purity, Mole % CO+CO 2, PPMV Max Chlorine + Chlorides, PPMV Nitrogen, PPMV Max H 2O, PPMV Max CH4, Mol % BALANCE DHDT/HCU/REFINERY H2 HEADER

9 4.6 TYPICAL OPERATING CONDITIONS The typical operating conditions of the Pre-desulphurisation section are as follows: S.NO PARAMETER FLOW MT/HR TEMP. C PRESSURE Kg/cm2g 1 Coker Naphtha feed Make up hydrogen flow, Nm 3 /hr Recycle hydrogen flow Nm 3 /hr Si-Adsorber Reactor inlet Si-Adsorber reactor outlet SR Naphtha flow rate Ist hydrogenator Reactor inlet, Nm 3 /hr 8 Ist hydrogenator Reactor outlet, Nm 3 /hr 9 Naphtha separator H 2S stripper feed, Nm 3 /hr H 2S stripper Top, Nm 3 /hr H 2S stripper Bottom Steam flow to reboiler Sweet Naphtha flow rate to Reformer Trains

10 4.6.2 TYPICAL OPERATING CONDITIONS OF THE REFORMER SECTION (ONE CHAIN) The typical operating conditions of the hydrogen section are as follows: S.NO PARAMETER FLOW TEMP. C PRESSURE Kg/cm2g 1 Naphtha feed MT/HR Recycle hydrogen flow Nm 3 /hr 3 IInd hydrogenator Reactor inlet 4 Chlorine/S-Adsorber outlet Process steam flow rate Steam to Carbon ratio Fuel Naphtha flow rate PSA off Gas Fuel Flow rate Nm 3 /hr 9 Pre-Reformer inlet, Nm 3 /hr Pre-Reformer outlet, Nm3/hr Reformer inlet Nm3/hr Reformer outlet MT shift Reactor inlet Nm3/hr MT shift Reactor outlet Nm3/hr 14 LT shift Reactor inlet LT shift Reactor outlet PSA inlet Product hydrogen Flow rate Nm 3 /hr 18 HP Export steam flow rate LP Export steam flow rate

11 4.7 SALIENT FEATURES OF HYDROGEN UNIT 1. Full range naphtha feed (C5-140 cut) instead of light naphtha(c5-90) 2. Designed to process feed Naphtha with sulphur content up to 600 ppm for SR Naphtha and 5900 ppm in case of coker Naphtha. 3. Two stage hydrogenation to reduce zinc oxide catalyst consumption 4. Silica adsorber and diolefin saturation reactor provided for utilizing Coker light naphtha. 5. Liquid naphtha quench in hydrotreater reactors. 6. Provision for cleaning of tube bundle of the Naphtha super-heater. 7. Provision of chlorine guard in S-adsorption reactors. 8. Steam turbines for FD and ID fans and HP BFW pump. 9. Elimination of steam ejector for recirculation of pre-reformed gas. 10. Vortex flow meter for vapor naphtha and steam flow measurements 11. Load temperature management scheme for optimizing the hydrogen unit operation. 12. Reformer operation at low steam to carbon ratio of 1.85 is also possible. 13. Filters provided in the hydrogen product to prevent carryover of PSA adsorbent particle to compressors in HDS section of HGU. 14. Centralized condensate receiving and pumping system. 4.8 ENERGY CONSERVATION MEASURES 1. Hot feed from Pre-Desulphurisation section to hydrogen section. 2. Pre heating of combustion air upto 550 o C. 3. Preheating of DM water to recover low-grade heat from the process gas % Burners in the 6 th row of the furnace to optimize the fuel consumption. 5. Flashing of condensate to recover LP steam. 6. Low NOX burners

12 4.9 COMPARISON BETWEEN EXISTING AND EXPANSION HYDROGEN UNIT Expansion HGU SR Naphtha feed is C o C TBP cut Feed Sulphur is max 600 ppmw (SR Naphtha) & 5900 ppmw in Coker naphtha. Coker Naphtha can be processed. Hydrogen Production capacity is MTPA in two trains Pre-Desulphurisation section takes care of sulfur in feed. Two stage Hydrogenation FD/ID fans are HP to MLP back Pressure Turbine driven. One BFW pump is HP-MLP Back Pressure Turbine driven Chlorine Guard is provided in S- Adsorber. PSA is designed for only Reformed Gas Hydrogenation Catalyst is TK-250 Zinc Oxide catalyst is HTZ-3 Si Adsorber is provided Existing HGU SR Naphtha feed is C 5-90 o C TBP cut Feed Sulphur is max 200 ppmw Coker Naphtha can not be processed. Hydrogen Production capacity is MTPA in single train. No Pre-Desulphurisation section. Single stage Hydrogenation FD/ID fans are variable speed motor driven. Both BFW pumps are motor driven Chlorine Guard is being provided. PSA is designed for Reformed as well as CRU/OHCU hydrogen Rich gases. Hydrogenation as Catalyst is TK-550 Zinc Oxide catalyst is HTZ-5 Not required as the feed is SR Naphtha only. Filter with 5 micron screens is provided in Product hydrogen to prevent PSA adsorbent dust to compressor. Filter were installed later on.