hydrocracking steratgies

A hydrocracking strategy 

for a competitive market

A description of a novel hydrocracking technology that offers refiners a cost effective way of complying with European 2005 diesel specifications, taking into account new configurations necessary to meet capacity and quality targets.
The refining industry is faced with many challenges that may significantly affect refinery processing schemes over the next five years and beyond. First, the recommendations of the European Auto-Oil I and II programmes have resulted in unprecedented gasoline and automotive diesel fuel quality requirements (Table 1). 

In 2005, the maximum automotive diesel sulphur level will be reduced to 50ppm. Sulphur levels  as low as 10ppm will be the rule in some countries. At the same time, the specified cetane  number, polyaromatics, specific gravity and 95 percent distillation point will also undergo changes. These will directly affect the refinery diesel pool formulation. The probable reduction of 10°C to 20°C of the diesel 95 per cent distillation point will impact on the refinery diesel output.

For example, a 10°C reduction in diesel cut-point will diminish the diesel pool volume by about 5  per cent. Second, as a result of increasing road transport in Europe and the success of diesel automobile engines, particularly in France, the projected demand for diesel fuel shows an  increase of at least 18 per cent over the next 10 years, and even more in some scenarios, while the market share for naphtha is expected to decline slightly (Table 2).

  Jet fuel demand is also climbing rapidly (+40 per cent) due to the expansion of air transport. Demand for domestic fuel  oil continues its longterm decline. European refineries must therefore adapt their product slate significantly, considering that dieselproduction capacity is already bottle-necked while the  previously projected diesel end-point specifications will result in a 5 to 10 per cent loss of current production. These long-term trends will compel refiners to modify their processing configurations in order to increase higher-quality middle distillate production. In the past, the prevailing market  forecasts and economics led refiners to invest in FCC complexes to satisfy the more immediate gasoline needs, which is why most European refinery conversion  schemes were limited to FCCs and visbreakers. To produce high yields of high-quality middle  distillates from vacuum gasoil (VGO), refiners will have to invest in hydrocrackers to meet future market demand. Although they are not likely to shut down FCC operations on the sole premise  that FCC units do not produce high yields of high quality middle distillates, they will have to implement some reduction in FCC throughputs. A realistic case study will include an FCC unit and a hydrocracker.

Hydrogen management is another key point to consider when comparing technical solutions in addressing the difficult problem of diesel quantity and quality. The optimum overall scheme will  be one that selectively adds hydrogen into the diesel pool.

Case studies

The European refinery  case studies detailed in this article processes 10 megatons/year of a North Sea crude. The refinery’s bottom-of-the-barrel units include:

** An FCC plant
** A vacuum residue visbreaking unit
** A diesel HDT plant, sized for year

2000 specifications (not shown in thefigures that follow).The residue from the visbreaker is used for production of 40cSt fuel oil. The effects of adding the following hydrocracking technologies were analysed:
** Mild hydrocracking
** High pressure hydrocracking
** IFP hydroconversion technology(Hytail).

The study draws upon information gathered from over 40 industrial units. The results, primary  product distribution, diesel and domestic fuel oil pool constitution, and main product characteristics prior to hydrotreatment, are given in Tables 3 to 5.

Base case

Refinery’s year  2000 configuration (Figure 1)

The refinery produces slightly more naphtha and less diesel than  European demand. With a cetane number of 49 and a sulphur content of 2400ppm, the entire diesel pool must be sent to a deep HDS plant where the specifications of 51 cetane and  350ppm sulphur are easily obtained with the North Sea crude oil.  The domestic fuel oil (DFO)  characteristics are close to the specifications. To obtain the minimum cetane number required for this stream, one third of the domestic fuel oil will be hydrotreated. The deep diesel HDS flow is therefore 2.82 MTPA for the refinery's crude oil throughput of 10 MTPA.

Case 1

After 2005 (no change in configuration) The impact of year 2005 specifications on refinery  performance was studied. Primarily due to the reduction of diesel oil ASTM D-86 95 per cent  point from 360°C to 340°C, diesel fuel production is reduced from 30 per cent of the total naphtha-plus-middle-distillates obtained in year 2000, to 27.9 per cent in year 2005.  Considering the market share of 34 per cent given in Table 2 for the same year, the gap  between refinery production and market need for 2005 will have more than doubled compared  with that for the year 2000.  By 2005, with the same refinery configuration, the FCC throughput  will have been increased by 12 per cent, which, of course, does not fit well with the need to  increase both diesel pool quantity and quality (Figure 2).

Because of the increased light cycle oil (LCO) production, the cetane in the pool is also deeply affected.  Diesel oil cetane number before hydrotreating is only 47.9, which represents a gap of 4 to 6 points compared with the  expected 2005 specifications. With this type of feedstock, such an increase in cetane number requires extremely severe operating conditions that cannot be achieved in the existing HDT  plant, even after revamping. In case a minimum cetane number is required for the domestic fuel  oil, the entire DFO oil pool must also be deeply hydrotreated. In these conditions, the total  middle distillates hydrotreating throughput is increased by 24 per cent, and a new high-pressure  HDT must be installed. The average naphtha pool sulphur level will be 270ppm, which implies adding high-severity, post-treatment units to attain the required sulphur levels of 10–50ppm. This  level of severity will be accompanied by some octane loss, which means that, in some  cases, the octane balance could well become tight and will require more investment.

Case 2 

Mild hydrocracker Based on preliminary studies, the addi- tion of a high-pressure hydrocracking unit was not an economic solution due to the high investment requirement andhigh hydrogen consumption. In the alternative case, a mild hydrocracker (designed for 30 per cent conversion and a residue maximum sulphur content of 0.05 wt%) is added to the refining scheme upstream  of the FCC unit (Figure 3). 

The advantages of this configuration on FCC operation are well  known:
** Reduced FCC coke, slurry and LCO yields in favour of LPG and naphtha yields
** Increased LCO cetane number
** FCC naphtha sulphur levels reduced to 15ppm
** Reduction in FCC SOx emissions.

Adding a mild hydrocracker significantly increases the diesel-to-naphtha ratio and the refinery product slate almost matches market requirements.  The results from the refinery simulation in this configuration show that most of the LCO produced by the  FCC can be used to control fuel viscosity. Under these conditions, the cetane numbers in the  diesel and DFO pools are significantly increased (Table 5). 

With a cetane number of 39 and a sulphur content of 600ppm, the DFO meets specifications without further hydrotreatment, even in  countries where a cetane number of 40 is specified. In this case, a small amount of cetane  booster will be added to the pool.  The diesel fuel, with a cetane number of 48.2 before HDT, will  still require severe hydrotreating to attain a minimum of 52. The necessary cetane gain of 3.8  points cannot be accomplished in the existing unit without revamping. Depending on the case,  the cetane gain can be achieved in a new high-pressure HDT plant (preferably), or in the most favourable cases, in the existing plant after extensive revamping.  Total HDT throughput is 9 per  cent lower compared to the current situation. A sulphur level as low as 15ppm in the FCC  naphtha will result in a diesel pool well below the most stringent foreseeable specification of  10ppm, achieved with virtually no octane loss. Case 3 New technology Hytail is a new IFP     hydrocracking technology specifically developed to solve European refining problems (Figure4).

It is optimised for cracking heavy atmospheric gasoils or light vacuum gasoils that will be in  excess in refineries in 2005 after reducing diesel 95 per cent point specification. Because it  operates under much milder operating conditions than a conventional hydrocracker, and the  process flow scheme is much simpler, this unit investment cost is remarkably low, and the  economics are attractive. The key feature of this process is that it operates at pressure levels  and requires capital investment comparable to those of a mild hydrocracker, but still offers the  high conversion levels and the high diesel quality of a high pressure hydrocracker. If we plot  diesel quality (ie, polyaromatics in this example) as a function of hydrogen partial pressure for  the same conversion level of 80 per cent, a zeolite catalyst can be used, enabling the unit to  operate at a 25-bar lower hydrogen partial pressure, for the same level of quality (Figure 5).  

This is explained by the zeolite’s greater activity, which requires a much lower operating  temperature, and which in turn favours more efficient hydrogenation. Zeolite catalyst systems appear as the best choice in Europe where the emphasis is on maximum quality diesel fuel.
Moreover, selecting a light feedstock (LVGO or HGO) instead of the usual hydrocracking feed  (HVGO) makes possible an additional 20- to 25-bar reduction in hydrogen partial pressure.  The result is that, in the Hytail process, which basically cracks light feedstocks on a zeolite catalyst,  the operating pressure can be about 50 bar lower than in conventional plants. The process offers the same product yield slate and quality as the HP hydrocracker, but with a much lower hydrogen partial pressure resulting in a lower investment cost (Table 6).

Another characteristic of this low investment process is that its flowscheme is simpler than that  of an HP hydrocracker. It is similar to a diesel HDT plant with an additional column, the diesel–residue splitter). The combination of these features results in attractive economics (Table 7).

Hytail investment cost is only about 400 per cent of the investment of an HP consumed hydrocracking unit, while H2 in the unit is only 28 per cent of the HP hydrocracker. The last case  in this study concerns the addition of a Hytail plant having previously installed a mild  hydrocracker. Feedstock to the Hytail plant is the lighter third of the total vacuum gas oil (TBP  cut: 350–410°C), while feedstock to the mild hydrocracking unit is the 410565°C cut. Taking into account the conversion in the mild hydrocracker, the FCC throughput is reduced to 55 per
cent of design. To maintain a minimum FCC feed rate at 70 per cent of design, an extra 1.2  MTPA of atmospheric residue was imported. Owing to the high quality of diesel fuel from the Hytail unit, the diesel pool and domestic fuel oil pool cetane num- bers are increased to 50.7 and 40.7,respectively, before the HDT step. The domestic fuel oil meets specifications. With  such a high quality level, the diesel fuel can be reasonably upgraded to the sulphur specification  by the revamped diesel HDT plant. Taking into account that Hytail diesel is sent directly to the  diesel pool, the new HDT unit’s throughput is only 77 per cent of its original design.

Economics 

Table 8 provides the results of an economic comparison between a mild hydrocracking unit, a  mild hydrocracker plus a Hytail unit, and an FCC unit plusan upstream high pressure  hydrocracker that replaces the Hytail and mild hydro- cracking units. 

The high pressure  hydrocracker conversion level is adjusted in order to obtain a refinery material balance and product quality pattern equivalent to those obtained with a mild hydrocracker plus a Hytail unit.  Because both Hytail and mild hydrocracking units operate at much lower pressures, the  investment for these two units is lower than the investment for a high-pressure hydrocracker of the same capacity. The hydrogen consumption is much higher in the HP hydrocracking case because this unit operates at higher pressure and hydrogenates the residue that is sent to the FCC to a high degree. In the mild hydrocracking-plus-Hytail scheme, hydrogen is selectively added to the diesel fuel with as little hydrogen as possible going to the unconverted residue being sent to the FCC.

Conclusion 

Hydroconversion, whether or not combined with some degree of reduction in the FCC throughput, is the key option for European refineries when their objec-
tive is to meet diesel 2005 specifications and production requirements. While diesel sulphur specifications are often achievable by revamping existing diesel HDT units, the required cetane level (acharacteristic that is not easily improved) appears as the refinery bottleneck. Mild hydrocracking is a cost effective option that will solve part of the problem. It also leads to very  low sulphur content in the FCC naphtha, enabling the most stringent sulphur specification
in the naphtha pool to be met.  Hytail is a new hydrocracking tech- nology that fits well with the European constraints. When the mild hydrocracker is combined with a Hytail unit, the refinery’s diesel-to-naphtha ratio increases to meet the projected European yield slate, while the total  production of naphtha, jet, diesel and domestic fuel oil is increased by 14 per cent. In that case, the diesel oil pool properties attain values that enable them to meet the 2005 target, using the existing diesel HDT units, after a moderate revamp. The comparison between the combined Hytail-plus-mild-hydrocracking units and a stand-alone high pressure hydrocracking unit, operating at partial conversion and offering the same refinery material balance and product quality,  shows that Hytail-plus-mild-hydrocracking is the more effective solution in terms of  capital investment and hydrogen management.

P Marion  D Duée  E Benazzi IFP