Hydrocracking Plant Energy Consumption Benchmark
Hydrocracking technology plays a major role in meeting the demand for cleaner burning
fuels, good feed stocks for petrochemical plants and better lubricating oils. Through
Hydrocracking process heavy fuel oil can be converted to transportation fuels and
lubricating oils whose quality will be able to meet tightening environmental market
demand. Up to my knowledge, there are three major licensors to the Hydrocracking
process, Chevron-Lummus Global’s, UOP and Axens processes.
Hydrocracking Plant Energy Consumption Benchmark | Energy Efficiency Guidelines in Refinery
In the Hydrocracking plant’s reaction section heavy aromatic feed stock is converted into lighter products under a wide range of very high pressures and fairly high temperatures in the presence of H2 and a catalyst. When the feed is having very high paraffin, the primary function of H2 is to prevent the formation of the polycyclic aromatic compounds. Hydrogen does also reduce the tar formation and the buildup of coke on the catalyst. Hydrogen also serves, as it does in Hydrotreating plants, as the converter of sulfur and nitrogen compounds present in the feed stock to H2S and ammonia, which can be separated later in the plant’s fractionation section. Hydrocracking process produces large amount of butane that can be used for alkylation process as feed stock.
Hydrocracking is a two stage reaction that combines catalytic cracking and hydrogenation in one process, wherein less valuable heavy oil stocks are cracked in the presence of H2 to produce more valuable products. The chemical reaction is characterized by being a high pressure and temperature reaction. Hydrocracking is normally used to process feed stocks having high polycyclic aromatic hydrocarbons and/or high concentrations of sulfur and nitrogen compounds, since such feed stocks will be poisoning the catalysts in other catalytic cracking or reforming processes. The Hydrocracking process is largely dependent on the nature of the feed stock and the competing rate of reactions of hydrogenation and cracking.
In the two stages Hydrocracking process, the feedstock is preheated and mixed with the recycled hydrogen, in the first stage, and sent to the first-stage- reactor. In this reactor, the catalyst essentially converts sulfur and nitrogen compounds to hydrogen sulfide and ammonia besides a limited hydrocarbon cracking. Upon leaving the first stage the hydrocarbon is cooled, liquefied and sent to high pressure separator. Hydrogen is separated and sent back/recycled to the feedstock. The hydrocarbon liquid is charged to the fractionator where desired products are obtained (gasoline, jet fuel, kerosene and gas oil). The fractionator bottoms are then mixed with hydrogen and sent to the second stage. The second stage reaction is more sever from temperature and pressure point of view. The second stage products are also separated from hydrogen before sending it to the fractionator.
From energy consumption point of view, Hydrocracking unit energy consumption increases as feed becomes significantly lighter, from heavy gas oil to middle distillate, due to the lower heat of cracking. The energy requirement also increases as the feed stock shifts from cracked to virgin and as the product slate changes from light to heavy.
The reaction section includes the reactors, one or two stage separators, recycle gas compressor and recycle gas furnace. The separation section consists of the main fractionator, debutanizer, if any, and side-stream strippers, if any. The Hydrocracking unit energy efficiency criteria are based on product rundown temperatures of 250ºF, to be consistent with the lowest heat sinks available (fresh feed temperature of 250ºF and H2 make up temperature).
Hydrocracking Plant TargetingHydrocracking Heat Integration Example:
Heat integration initiative for the Hydrocracking unit intra-process integration is the most important initiative in enhancing the unit EII to push it to become closer to the best-in-class EII of that unit or in some cases to even try to beat it.
The example below shows how to target for the Hydrocracking heating and cooling utilities energy consumption using heat integration technique at different global minimum approach temperatures among hot and cold streams (ΔT_min). As shown below in the table, the Hydrocracking hot streams to be cooled and the cold streams to be heated which are allowed to get into thermal integration in the Hydrocracking unit is listed with its supply temperatures, target temperatures and stream heat capacity flowrates.
Hydrocraking Problem Data
The Hydrocracking unit heating and cooling utilities quantitative evaluation at decreasing ΔT_min is presented in the table below. The table below shows that for typical Hydrocracking unit using moderate ΔT_min. The Hydrocracking unit’s lowest cooling utility required at ΔT_min = 44ºF, before the problem gets concerted to threshold problem, is about 163 MM BTU/h.
It is instructive to mention here that the Hydrocracking unit is a “SOURCE” unit and hence shall be explored for all possible thermal and mechanical waste energy recovery, such as steam generation for instance. It can also be considered for inter-processes energy integration with other refinery and/or petrochemical site-wide units directly and/or indirectly through buffer streams.
The best achievable reduction in the Hydrocracking cooling utility requirement, for instance, using intra-process heat integration is less than 3 % using ΔT_min= 44ºF, measured from the Hydrocracking energy targets cooling utility at ΔT_min = 50ºF. The Hydrocracking unit at such ΔT_min = 44ºF, becomes a “Threshold” problem where the pinch point vanish and only cooling utility is required.
Typical Hydrocracking Plant HEN Retrofit
The overall purpose of the Hydrocracker Unit is to upgrade a nominal 50,000 BPOD of vacuum gas oil from the crude unit into distillate and naphtha.
The unit has alternate processing objectives, either to produce a maximum yield of middle distillate liquid product in the diesel boiling range, or to produce a maximum yield of heavy naphtha product(minimum 65 Vol. percent).
The unit is composed of a high pressure reactor section and a fractionation section to separate the reactor products. The high pressure reactor section is a two-stage configuration having parallel flow through the two reaction stages, with the unconverted material from the first stage reactor flowing to the second stage reactor. The first stage also has a pre-treatment reactor to remove sulphur, nitrogen, oxygen and metals from the fresh feed.
The fractionation section is comprised firstly of a debutanizer. Debutanised reaction products are then distilled in the main fractionator column, which separates the liquid products as overhead naphtha and side-stream middle distillates. Fractionator bottoms are unconverted oils which are recycled to the second stage reactor.
The unit is designed to process straight run heavy vacuum gasoils from either Arabian light or Arabian heavy crudes, or a blend of light vacuum gasoils combined with coker gasoil from a future plant. The majority of the feedstock is received hot, directly from the upstream units where the feed temperature to the hydocracker is controlled.
Actual feed-rates are 50,000 BPOD for the maximum diesel operating mode, and 47,000 BPOD for the maximum naphtha operating mode. The unit can be turned down to 50% of design feed-rate, and is capable of operating at full capacity with up to 50% of the feed entering cold from the feed storage tank.
Pinch Analysis for HEN Retrofit Example:
The following are the cold streams to be heated and hot streams to be cooled considered in the example
The global ΔTmin used for the system is found by drawing the hot and cold composite curves and moving it until the existing hot utility (240 MMbtu/hr) and cold utility (226MMBtu/hr are obtained. The global ΔTmin currently used is134°F.
According to the industry practice, the current global ΔTmin used in HCU is very high. Using ΔTmin equel to 30°F we get the following results depicted in the table and graph below:
From pinch analysis result, we conclude that total of 119 MMbtu/hr can be saved by modifying the existing HXN. This high amount of waste heat is mainly due to the plant’s split into two parts: reaction part (high pressure) and separation part (low pressure). Enhancing waste heat recovery in each section of the plant and /or better integration between the two sections can save most of the waste heat mention above.
(A) Separation Section Heat Integration Modification
The fractionator (C-320) bottom product stream is coming out at 624°F. It is exchanges heat with kerosene re-boiler (E-238) and Naphtha re-boiler (E-329) reaching 585°F before it get cooled down using steam generator (E-340) to reach to 354°F before going as feed to the 2nd reactor. The diesel pumparound heat up the Naphtha re-boiler bottom stream E-330 and Naphtha feed E-332 to Naphtha column. The diesel product is being cooled from 513°F to 114°F using air cooler. The following diagram and table are the depicting base case of this section:
The energy efficiency optimization opportunity/initiative suggests using the fractionator bottom product to pre-heat the debutanizer re-boiler F-310 via bypassing the E-329 in Phase one and then E-328 in Phase two. Both E-329 and E-328 will be using different hot stream/ utility. Such two phases’ are illustrated in more details below:
11.93 MMBtu/hr can be saved via installing a new heat exchanger between the main fractionator bottom product stream and the De-C4 re-boiler stream and via bypassing E-329. For E-329 heat input, there are two solutions: use M.P.S in re-boiler, or use Diesel product, that is currently using air cooling system.
The following is the initiative description:
A total of 22.9 MMBtu/hr can be saved in the furnace by installing inserts in the NHX, E-229 and E-330. Also, the Diesel pump around stream may be used as hot utility and this will affect E-330. The heat exchanger network that achieved the desired saving can be modified using five different options:
(1) Reduce the duty of E-330 and use M.P.S to fulfill needed load for Naphtha re-boiler.
(2) Reduce the duty of E-332, keep load constant on M.P.S and E-330. Use L.P.S to compensate for the needed load for naphtha column feed temperature.
(3) Use the L.P.S as feed preheater for the naphtha column. The diesel pumparound stream is used only for the Kerosene-reboiler and naphtha reboiler, services.
(4) Use diesel product stream to supply 19.19 MMBtu/hr for Naphtha re-boiler. Let the rest be supplied by the DPA stream shown in the graph below:
(5) Use the diesel product as feed preheater for the naphtha column. The diesel pumparound is used only for the Kerosene-reboiler and naphtha reboiler.
(B) Heat Integration between Separation Section and Reaction
The energy saving initiative presented here is the “final” phase of the energy efficiency optimization initiatives in this HCU example. It thermally integrates the reaction section with separation section. We understand the money cost associated with such link, due to the high pressure heat exchanger units, involved is high. However, we recommend considering such thermal integration between the reaction section and separation section due to its impact on energy cost reduction and its applicability as per recent HCU licensed schemes. The following diagram and table are the depicting base case of this section:
The proposed energy saving initiative uses the 1st reactor product stream to preheat the feed tom main fractionator to reduce the heat required load at the F-320. It also uses the main fractionator bottom before E-318 to heat the 1st reactor feed stream. Adding two new heat exchangers will produce a new heat-load path (E-318-cold utility, NHX1, E-103, E-101, NHX2, F-320-hot utility).
This change will result in rising the temperature in to E-103 (feed side). This rise in inlet temperature will result in a decrease in ΔTmin, E-103 shall be modified (increase the surface area or use tubes insert). Beside, E-101 surface area needs to be also revaluated due to E-101 heat duty reduction.
10 Energy Conservation Guidelines/Compliance Points
– Heat integration shall be applied with global ΔT_min less than or equal to 30ºF and the right use of utility level.
– Process-Utilities system shall exhibit best possible synergy.
– Efficiency of rotating equipment shall be more than 80-85%.
– Efficiency of fired heaters shall be more than 90-92%.
– Distillation columns shall be integrated with the rest of the process or among themselves and columns with long difference in its temperature profile shall be adapting inter-coolers or inter-heaters design features.
– Inlet feed to compressors shall be as cold as possible and to turbines as hot as possible.
– Turbines are used whenever possible and let down valves and let down drums shall be minimized.
– The process products’ temperatures shall not be higher than the feed temperatures, and process temperatures to air coolers shall be lower than 200ºF.
– “Source” Processes shall be integrated with adjacent ones and/or produce heating and/or cooling utilities.
– Heat transfer equipment shall always exhibit high U, as high as possible.