Category : Featured Plants Manufacturer : Shell Oil Company Plant Type : Hydrotreater Capacity : 31,150 bbls/day
Technology Provider: Shell
Capacity: 31,150 bbls/day
Operating Pressure: 150 bar
Hydrocracker Reactor Section Unit 7500 converts a feed of Flashed Distillate, Waxy Distillate and Deasphalted Oil (DAO) in a Hydrogen rich atmosphere into mainly Naphtha, Kerosene and Gas Oil products which are sent to Fractionation section U7600.
The first reactor of the 1st stage, R7501, adsorbs heavy metals and provides the hydrodenitrogenation, hydrodeoxygenation and hydrodesulphurisation reactions of the feed. Initial cracking of the feedstock into products takes place in the second reactor R7502. Unconverted feed is recycled to the second stage reactors, R7503 & 4, where further cracking takes place, achieving approximately 96% conversion. The design recycle ratio was 150% (combined feed ratio of 1.5), but a recycle ratio of 100% is now targeted. A 4% bleed to fuel oil blending is normally run. The reactors R7501, 2, 3 & 4 are constructed of Chrome Molybdenum with an internal weld overlay of stainless steel to a thickness of 7.5mm. The cycle length of the unit is typically 4 years, based on deactivation of both the pre-treat and hydrocracking catalyst.
The Hydrogen and H2S rich gases are removed from the reactor effluent in two parallel high pressure (HP) and low pressure (LP) separator systems prior to being sent to U7600 for fractionation.
Feed System - First stage charge pump P7501 is a Mitsubishi 7 stage centrifugal pump. The pump is driven by an HP to MP back pressure steam turbine taking suction from V7501 at 6 bar and discharges into the circuit at approx. 150 bar. Second stage charge pump is also a Mitsubishi 7 stage centrifugal pump. The pump is driven by an HP -> MP back pressure steam turbine taking suction from V7506 at 6 bar and discharging into the reactor circuit at approx. 150 bar.
Reactor Systems - The first reactor R7501 consists of 4 catalyst beds. The first bed is loaded with a layered mixture of inert high voidage material, low activity demetallisation catalyst and high activity demetallisation catalyst. The primary function of the inerts is to act as a “graded bed”, designed to combat high top bed pressure drops. The demetallisation catalyst is designed to remove metals such as nickel, sodium and vanadium. The second bed is again a mixture, this time of demetallisation and pre-treatment catalyst. The pre-treatment catalyst is also loaded into the remainder of R7501 and is where the conversion of Nitrogen to Ammonia, Sulphur to H2S and Oxygen to water occur. The second reactor R7502 is loaded with a mixture of 1st stage cracking catalysts, Nickel/Tungsten on a composite Zeolite/amorphous base. R7502 holds the largest volume of catalyst, having the largest sized beds.
Feed/Effluent Exchangers - First stage liquid feed gains preheat in E7501B/A before entering the first stage reactors R7501 and R7502. Second stage liquid feed gains preheat in E7510, E7508 and E7506B/A before entering the second stage reactors R7503 & R7504.
Separator System - The unit is designed with a four separator system, which enables most of the reactor products to separate out in V7502 and sent to the fractionator via the hot LP separator V7503.
Wash Oil System - Wash Oil is used to enhance hydrogen purity of the recycle gas, thus increasing the H2 partial pressure in the reactors. It does this by re-contacting with the hydrocarbon vapors in the vapor stream from V7502 prior to entering the E7512s. P7502, a HP -> MP steam turbine with drive assisted by the power recovery turbine PRT7502, pumps the Wash Oil to the injection point upstream of E7512 at a rate of 3300t/d.
Wash Water System - Wash water of 1200t/d is injected upstream of the effluent air cooler to avoid both the possibility of ammonium salt fouling and the generation of very concentrated ammonium salt solutions when the water dew point is reached. The most important feature of the wash water system is to provide adequate mixing (scrubbing) between the effluent and liquid wash water.
Gas Circuit - K7502 is a circulating compressor, moving approximately 2600 t/d of Hydrogen to R7501/7502/7503/7504 and all the reactor quenches. The aim is to maintain H2 partial pressure, for catalyst protection and provide a surplus of H2 to the reactions occurring.
Fresh make up gas is provided to the hydrocracker by K7501A/B/C a 3 x 50% compression system, which takes hydrogen rich gas and compresses it over 4 stages from 18barg to 150barg. The design intake of make-up gas is 133 t/d in terms of pure Hydrogen but today up to 170 t/d can be achieved.
Hydrocracker Fractionation Section - The hydrocracker Fractionation section separates the cracked material from received from the hydrocracking reactors / High pressure circuit. The separated product streams are: Refinery Gas, LPG, Tops, Naphtha, Light Kero, Medium Kero, Heavy Kero, Gas Oil and Recycle Oil (MVC Bottoms). The MVC Bottoms are recycled to the second stage reactors of U7500 for further conversion. The Hydrocracker Fractionation Section can process a maximum hydraulic limit of 9600 t/d of “total” intake, and can operate satisfactorily at less than 50% of design throughput.
Main Fractionation Column - The unit is fed by two streams from the Hydrocracker Reactor Section Unit 7500, Light Fractionator Feed and Heavy Fractionator Feed.
Heavy Fractionator Feed (HFF) is fed from V7503 via heat exchangers E7502, E7504, E7507 and E7601A/B/C prior to entering C7601. The HFF enters C7601 at tray 11.
Light Fractionator Feed (LFF) is fed from V7505 and enters C7601 at tray 24.
C7601 produces the following products: Full range naphtha (which is further processed via the Gas Tail equipment), Light Kero, Medium Kero, and Fractionator bottoms (which are sent for further processing in the Mild Vacuum column).
C7601 is equipped with 48 calming section valve trays, 1 partial draw-off tray, 49(top circulating reflux) and 3 total draw-off trays, 1(F7601), 25(V7602, C7603 Medium Kero), 37(C7602, V7601 Light Kero).
C7601 is equipped with 3 reflux systems, a pumped top circulating reflux, a Van de Marel light kero reflux and a Van de Marel medium kero reflux.
C7601 is provided with a fired reboiler, F7601.
Overhead Naphtha Recontacting System - Overhead vapors from C7601 are partially condensed in E7621A-D and accumulated in V7611. The non-condensed vapors in V7611 are sent via ADIP 4 (U7830) for H2S removal, compressed via K7601 and recontacted with condensed liquid ex V7611 at higher pressure. The recontacted cooled vapor/liquid is passed to V7612 via air cooler E7622. The recontacting system reduces the loss of heavy hydrocarbons into refinery gas, increasing the overall naphtha yield.
Naphtha Processing “Gas Tail” System – The “Gas Tail” processing system is designed to process up to 1200t/d of Naptha minus product. The “Gas Tail” is feed from the Naphtha recontacting drum V7612, and consists of a Debutaniser C7606 designed to remove LPGs and Refinery Gas from the Naphtha/Tops, and a Naphtha Splitter C7607 designed to remove the Light Naphtha Tops from the Heavy Naphtha. C7606 overhead system has the flexibility to produce either liquid LPGs (liquid mode) or Gas (Gas mode).
Mild Vacuum Column - The mild vacuum column C7605 consists of a rectifying section only with feed entering the column below tray 1. Feed to C7605 is heated in F7602.
An ejector system is provided, and controls the column pressure typically at 120-140mbara.
C7605 was originally designed with 21 trays, but trays 2 through 7 have been replaced with Mellaplus 425Y structured packing to improve separation efficiency between Gasoil and MVC Bottoms.
Tray 21 is a total draw off tray for the Heavy Kerosene product. The Heavy Kerosene is also used to provide three separate reflux streams to C7605; The Cold Front Reflux and Top Reflux are both returned to C7605 above tray 1, whilst the Hot Heavy Kerosene reflux is returned to column above tray 20.
Gasoil is removed from the column via total draw off tray 12.
MVC lower circulating reflux (LCR) is drawn from tray 8. The LCR provides a number of heating duties within U7600 and is also critical in maintaining the recycle cut point between Gas Oiland MVC bottoms. The LCR can be returned to C7605 in two locations; above tray 11 and above the packed section (for packed bed wetting).
MVC Bottoms Circulation Systems - Recycle Oil Level in C7605 is pumped by P7608A/B, providing heat transfer through E7601's. Flow then passes to HPNA chambers, V7691A/B, and returns via P7691. P7691 provides additional discharge flow pressure to MVC Bottoms flow to compensate for the pressure drop across the HPNA chambers. Flow now passes to HP steam generator E7653A/B, normally all the MVC bottoms flow through E7653, however exchanger bypass 76HC043 is available for additional heat supply to the downstream exchangers, or to retain temperature for 2nd stage feed ex V7606 (EOR). MVC bottoms ex E7607 then splits two ways and passes through E7651 on one stream, for MP steam generation, and through E7625 on the other, for C7606 reboiler duty.
Waste Heat Boilers - Hydrocracker Steam Generation Unit 7650 is designed to utilize waste heat from Unit 7500 and 7600 flue gases and also MVC bottoms and lower reflux strams, for the production of Superheated High Pressure and Medium Pressure steam. In addition the flue gases also pre-heats the Circulating Boiler Feed Water from V7651. The design capacity is based on end-of-run operation of the Hydrocracker at full throughput. The High Pressure Superheater has a maximum steam outlet temperature limitation of 450°C. The Medium and High Pressure Superheated steam produced is fed into the HP and MP steam mains.
HPNA Chambers – V7691A/B HPNA chambers are a lead/lag system loaded with activated carbon to reduce the level of HPNA’s present in the 2nd stage feed, reducing both fouling of the second stage feed/effluent exchangers and also reducing fouling and deactivation rate of the second stage catalyst.
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