Jan-2011

Maximisation of VGO through deep-cut distillation for refinery margins

Appropriate choice of crude and/or crude oil blends as feedstock and a vacuum tower revamp enable higher production of vacuum gas oil

Rajeev Kumar, Chithra V, Shalini Gupta, Sonal Maheshwari, Peddy V C Rao and N V Choudary.
Bharat Petroleum Corporation Ltd, India

Article Summary

Over the years, improvements to distillation towers have delivered better separation and higher yields.^1,2 Vacuum towers have been improved by the introduction of packing designs that deliver better mass transfer and separation. Packings have the advantage of lower pressure drops compared with trays, reducing, for instance, flash zone pressures and the overall column pressure drop.^3,4 Thus, more efficient vacuum tower operation meets the current demand for processing heavy crude oils and delivering high heavy vacuum gas oil (HVGO) yields.

Traditional vacuum towers deliver a HVGO/residue cutpoint of approximately 520–565°C.⁵ Deep-cut operation starts when cutpoints are increased by 50°C (see Figure 1). In this way, an increase in product recovery by up to 2–5 wt% is possible. However, vacuum column operations present an arduous environment for efficient fractionation, which makes upgrading product from bottoms to distillate more difficult. Shifting of delta initial boiling residues to vacuum gas oil (VGO) is the real challenge for a deep-cut VGO maximisation process to meet the design feedstock criterion of the secondary units.^6,7

In the current refining climate, processing opportunity crude oils in atmospheric units⁸ and vacuum towers in deep-cut mode operation can provide economic and operational benefits to the refiner. To obtain deep-cut yields, a revamp of the vacuum unit is required, and a proper design and engineering study is essential. This should include crude type, exchanger, furnace, tower, ejector system and so on, to optimise capacity, low-pressure capability and overall revamp cost. However, a simplified analysis can benefit the project’s initial feasibility study. Accurate feed characterisation and effective process simulation are key factors in evaluating deep-cut options.

A technical evaluation of deep-cut maximisation has been included in the present work. The evaluation includes a study on: selection of the crude basket; true boiling point distillation for deep cuts at each 10°C cut for analyses and endpoints; feed characterisation to meet the criterion for secondary units; and an economic analysis. Thus, increasing the demand for feed enhancement in HVGO yields can provide a large economic incentive per barrel, reducing residue production and so improving the gross refining margin.

Challenges in processing deep-cut VGO
VGO from the vacuum tower is a feedstock to secondary units —HCU/HDS and FCC units — for maximising distillates production. The endpoints of VGO processed in these units are generally 520–565°C and vacuum residues (VR) are used for producing low-value products such as fuel oils, low-sulphur heavy stock and bitumen. Extending the endpoints of VGO up to 600°C can be further utilised for conversion to high-value distillates in the secondary processing units. The diversion of deep-cut VGO (520–600°C) from VR means up to 2–5 wt% more of the crude feedstock is available to the secondary units. This can result in more distillates and fewer low-value products.

To obtain deep-cut VGO, vacuum tower operations are quite severe, which causes changes in VGO composition. In such conditions, the primary concerns are production of increased levels of Conradson carbon residues (CCR), larger quantities of high molecular weight polycyclic aromatics and asphaltenes in VGO, which could lead to higher coke make in the secondary units, and metal contaminants that could poison the catalysts. All secondary units are vulnerable to these factors, which could lead to a heavy penalty through catalyst poisoning and effect on run length. The real challenge for a deep-cut VGO maximisation process is to meet the design feedstock criteria of secondary units for parameters such as CCR, C7 insoluble asphaltenes, nitrogen and metals. Not only has distillation technology improved, but conversion units are also better equipped to handle heavier gas oil feeds. A methodology has been designed to debottleneck these units by deep-cut characterisation for each 10°C cut range. This will help to restrict the maximum cutpoint for HVGO and increase conventional yields.

Selection of crude oil feedstock
A crude oil basket was selected from various parts of the world. For the most part, high and medium °API crude oils were selected for the study. Feedstock characterisation for critical properties was carried out by standard ASTM methods and the results are shown in Table 1.

TBP distillations for deep cuts
True boiling point (TBP) distillation (atmospheric) was carried out in a batch set up of 15 theoretical stages, in accordance with ASTM D2892, up to 360°C cuts. The residue was transferred to a pot still for vacuum operation at 0.5–0.1 mm Hg, in accordance with ASTM D5236. The composite (atmospheric and vacuum) distillations were carried out up to AET 600°C cuts. The yield profile is shown in Figure 2.

Deep-cut characterisations and secondary units
VGO drawn from the vacuum column is used as feed to secondary processing units. The extent of deep cut and its endpoints from the vacuum column is determined by CCR, metals content (V, Ni) and C7 insoluble asphaltenes in the VGO. Generally, secondary units are designed for a feedstock with Ni <1 ppm, V <1 ppm, and C7 insoluble asphaltenes at 500 ppm (maximum). However, the composite metal content (Ni+V) can be up to 3 ppm (maximum), with a penalty in run length.5 In view of this, crude characterisation studies are necessary to determine the cutpoint at which VGO can be extracted for study.

Thus, the metals content for VGO cutpoints up to 600°C (that is for every 10°C rise from 550 to 600°C), along with CCR and C7 insoluble asphaltene content data, are required to revamp the existing vacuum column to decide the endpoint for VGO cut. To understand the deep cuts and endpoints for processing in the secondary units, small cuts beyond 550°C have been made, such as 550–560°C, 560–570°C, 570–580°C, 580–590°C and 590–600°C. These cuts were characterised for their physico-chemical properties to meet the criteria as feedstock for secondary processing (see Table 2). The primary concerns here are high levels of CCR and asphaltenes in VGO, which could cause higher coke make in the secondary units, while higher metals content could poison the catalysts. These 
critical parameters were characterised for varying VGO endpoints for various crude oils (see Figures 2–6).