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Jan-2011

Clean diesel project

A process review, including the improvement of atmospheric and vacuum distillation columns, for the reformulation of the diesel pool

Luigi Valagussa
Koch-Glitsch Italia
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Article Summary
The reformulation of diesel oil fuel was introduced by the European Union in 2009. The aim of the Fuel Directive is to reduce pollution from vehicle emissions. One feature of this directive, discussed in this article, is the specification that the maximum distillation final point of the diesel oil is set at 360°C for 95% of distilled volume measured according to the ASTM D86 analysis (D86T95). Additionally, the Fuel Directive requires a reduction of sulphur content to a maximum 10 ppm. Both the specifications affect refinery operations. More specifically, catalytic processes such 
as hydrocracking (HDC) and hydrotreating (HDT) are used to improve the diesel yield and to limit the sulphur content respectively. A more extensive analysis, which looks at the atmospheric distillation unit (CDU) and vacuum distillation unit (VDU), may improve total diesel production and minimise the impact on existing units of the refinery. This analysis has the benefit of lower capital cost for the project; furthermore, the improvements can no longer be postponed in view of the market regulation of diesel oil.

Study approach
The scope of this work was to analyse and compare different ways to achieve the required diesel distillation end point from the CDU/VDU as specified by the shorter diesel oil cut (D86T95=360°C), while minimising the impact on the HDC and HDT downstream units and optimising the total diesel production yield.

A refinery requested a licensor to conduct a feasibility study and to draw up a cost estimate for the revamp of the refinery’s HDT unit to achieve the specified clean diesel oil requirements.

No consideration was given and no modification was proposed to produce in-specification, straight-run diesel from the existing CDU. Therefore, the design basis for the HDT revamp was a diesel stream with D86T95=360°C at the HDT battery limit. The HDT does not significantly change the diesel distillation endpoint. The HDT feasibility study investigated whether this diesel stream property can be achieved by mixing the present straight-run diesel from the CDU with an additional light stream to reach the maximum distillation point. The evaluation showed that if a D86T95=325°C blending stream is mixed with the present straight-run diesel stream at D86T95=382°C, the light blending should be about 1.7 times (by volume) the total atmospheric diesel produced. This solution does not look very attractive because it requires an increase in treatment capacity, which can affect the revamp of the existing HDT unit substantially and the size of a new unit as well. It also omits any consideration of the availability of a light blending stream and the total cost.

Two main points to consider from this example are:
• The clean diesel project must take the overall refinery process scheme and production capacity into account
• Alternatives are available to improve clean diesel production and to limit the impact on the new/existing equipment and facilities.

Present scheme
The main features of the existing refinery (see Figure 1) are listed below. The refinery is presently producing non-clean diesel oil:
• The crude oil-handling capacity is 125 000 b/d. To increase the handling capacity, the atmospheric column was revamped some years ago by Koch-Glitsch with Superfrac trays and Flexipac HC structured packing; it thus contains the most advanced column internals technology
• Light diesel oil (LDO) and heavy diesel oil (HDO) are produced in the atmospheric column. These streams are mixed together and fed to the HDT unit. The analysis of this straight-run diesel oil is D86T95=382°C, and this does not change significantly after HDT treatment. The present HDT unit is not able to reduce the sulphur content to the required level for clean diesel oil
• The atmospheric residue (AR) is fed to the vacuum column, which is equipped with three beds of grid packing (old style). One bed at the top of the column for top pumparound service produces a light vacuum gas oil (LVGO), a second bed at the middle of the column for mid pumparound service produces a heavy vacuum gas oil (HVGO), and a third bed at the bottom of the column is a wash bed. The two gas oils are mixed together and fed to the HDC unit
• The diesel stream from the HDC unit is mixed with the HDT straight-run diesel and a final diesel stream with D86T95=375°C (value from refinery), which is above the distillation endpoint requirement for clean diesel oil, is produced.

The refinery has to consider two main modifications in order to comply with the clean diesel specification: first, the revamp of the HDT unit (to reduce sulphur content), which will be approached and solved by a catalyst licensor; and, second, the distillation point specification of the diesel stream.

Alternative scheme to produce clean diesel oil
The first step was to evaluate whether the CDU and VDU columns can reach the target through enhanced performance. It is clear from Figure 1 that the deep cut of HDO greatly affects the diesel quality. In addition, no sensible benefit is achieved by modifying the internals of the existing 
atmospheric column to produce in-specification, straight-run diesel oil and by minimising the loss of HDO yield. The main reasons are:
• There is a low margin for improving the HDO final distillation points because the present column internals are already designed according to the most advanced and efficient technology
• Any attempt to maximise the straight-run diesel oil, includingdifferent splitting of LDO/HDO, affects existing equipment and operations (for instance, pumparound and heat recovery). The total diesel oil yield must be reduced and the heavy ends that are not recoverable into the HDO stream must be left in the atmospheric 
residue fed to the vacuum column. An increase in atmospheric residue could affect the existing vacuum heater performance, which is already limiting the present operations.

On the other hand, a total revamp of the vacuum column internals (from the old grid packing to new structured packing beds) can produce the LVGO stream in specification for clean diesel oil and therefore be suitable for blending with the atmospheric straight-run diesel oil. This avoids additional processing of this stream through HDC. The vacuum column revamp calls for four new structured packing beds, instead of the three existing beds, with the additional bed located below the top pumparound bed for fractionation service. This is necessary to produce the in-specification LVGO product.
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