Apr-2012

Optimise hydrocracker operations for maximum distillates

Estimation of an accurate operating window for a hydrocracker enables refiners to minimise yields of light ends and maximise diesel production

PRASHANT PARIHAR, RAVI K VOOLAPALLI, RAJEEV KUMAR, SRINIVASULU KAALVA, BISWANATH SAHA and P S VISWANATHAN
Bharat Petroleum Corporation Ltd

Article Summary

The typical reaction mechanism during hydrocracking consists of the primary cracking of heavier feedstocks such as vacuum gas oil (VGO) and the secondary cracking of middle distillates (see Figure 1). The range of products obtained are gases, naphtha, kerosene, diesel and unconverted oil. Understanding the scheme and controlling the mechanism is helpful in optimising product yields. Refiners always target the maximisation of high-value products such as diesel and the minimisation of low-demand products such as light ends, including naphtha and kerosene. In view of this, the design and operation of a hydrocracking unit has been studied for maximising the diesel fraction through optimisation of process parameters.

Fixed-bed downflow reactors are widely used for hydrocracking feedstocks such as VGO and heavy gas oils from the coker. The process employs high pressures (85-200 bar), high temperatures (350-420°C) and multiple catalysts in the presence of hydrogen to obtain high-quality products from poorer quality feedstock, thus adapting to current refining trends and the need to process opportunity crude oils to obtain high margins.^1-3 

Hydrocracking offers wide flexibility in terms of feed and product quality and yields. The most common configuration is a two-stage process with or without recycle of bottom product (see Figure 2). This configuration allows the use of specific catalysts for each stage and offers flexibility. The severity of each stage can also be better controlled to yield products at the desired levels. Understanding the conversion, yields and product qualities obtained from competing conversion processes enables optimisation of a hydrocracking unit to achieve a target yield pattern.4-6 Thus, optimisation of the process parameters to minimise lower value products such as naphtha and kerosene has become essential to the refiner. The present study focuses mainly on three different approaches: optimum conversion in stages I and II; recycle feed cut point control; and manipulation of catalyst average temperature (CAT) for maximising the diesel yield. The estimation of an operating window to obtain the right choice of distillate yield pattern will help to meet the requirement for high-value products. 

Methodology for maximising diesel and minimising light ends
In hydrocracking, a balance exists between middle distillates (diesel and kerosene) and light ends (naphtha and gas). The net reaction can be represented as follows:
                                      
Feed + H2 →  H2S + NH3 + LPG + FG + Naphtha + Kerosene + Diesel + UCO

Selectivity and yield of a particular distillate fraction can be improved through the manipulation of operating conditions in the reactors and fractionator. In order to maximise diesel yield, three approaches have been explored for analysing the effect on the distillate yields from the hydrocracker (see Figure 3). In a two-stage hydrocracker, optimum severity in stages I and II, the quality of recycle feed and the extent of secondary cracking of diesel will determine the maximum diesel yield and minimal production of light ends. The methodologies for maximisation of the diesel yield are discussed below.

Optimum conversion in stages I and II 
The first stage of a hydrocracking unit is mainly a hydrotreating stage with partial conversion of VGO, while hydrotreated feed is selectively cracked into diesel in stage II. Conditions in stage I are more severe than in stage II. Typically, high-temperature and high-pressure conditions lead to more light ends. Optimum stage I conversion helps to achieve a more balanced load for diesel-selective hydrocracking in stage II. Severity in stages I and II can be altered by changing the temperature and pressure conditions. In order to determine the kinetics of the hydrocracking process, laboratory experiments were conducted under isothermal conditions using a commercial hydrocracking catalyst and feeds to mimic two-stage hydrocracker configurations. The effects of varying process variables on hydrocracking performance were studied. The resulting data have been used for understanding and interpreting the effect of process variables on stages I and II performance with respect to product yields. The experimental data generated have been regressed and interpolated for predicting the effects of temperature and pressure on hydrocracker yields.

Increase recycle feed cut point (fractionator control)  
Severe operating conditions in a VGO hydrocracker result in cracking of the diesel fraction to light ends. If secondary cracking of diesel is reduced, maximisation of the diesel yield at the expense of light ends is possible in the hydrocracker while operating at target conversion levels. The purpose of the fractionator is to remove hydrocracked products from unconverted oil so that only unconverted oil undergoes hydrotreating/cracking in the next reactor/stage. Ideally, hydrocracked products should not see catalyst again. In a two-stage recycle configuration, the separation efficiency of the main fractionator is key to achieving this objective. The presence of a distillate fraction in the recycle feed to the second-stage reactor is reflected in the recycle cut point temperature. A recycle cut point lower than the design will promote secondary cracking of middle distillates to naphtha and gas, thereby increasing light ends yields at the expense of middle distillates. The presence of distillates in the recycle feed will also enhance vapourisation of the feed, resulting in dry spots and improper utilisation of active catalyst.

Minimise secondary cracking of middle distillates in stage II
Hydrocracking involves series parallel reactions. The heavier fractions undergo cracking and hydrogenation, which results in the formation of distillates and light ends. In a conventional approach, as catalyst loses its activity, the inlet temperature of feed to the reactor is increased to attain a higher CAT in order to operate the hydrocracker at constant conversion. The reaction’s exothermic behaviour is simultaneously controlled by inter-bed recycle gas quench flows. The proposed approach focuses on manipulation of the CAT of the second-stage reaction section of the hydrocracker unit in such a way that the diesel fraction yield is maximised selectively. These diesel maximisation  approaches are discussed below.

Results and discussion Optimum conversion in stages I and II
In a two-stage hydrocracker unit, conversion in stages I and II holds the key for controlling product yield pattern and quality. Conversion in turn is controlled by varying CAT and/or reactor pressure. Effects of CAT and pressure on the yield pattern of a hydrocracker are discussed below.