Managing vanadium from high metals crude oils
Improvements in process and equipment design with regard to crude unit and delayed coker distillation column performance can reduce metals content
Scott W Golden, Process Consulting Services
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Managing vanadium in FCC feed streams originating from atmospheric crude, vacuum crude and the delayed coker is becoming more important as refiners increase the amount of high vanadium crudes imported from Canada, Mexico and Venezuela. Vanadium reduces the hydrotreater run length and lowers FCC conversion due to its impact on catalyst activity. The total amount of vanadium in FCC feed streams can be as high as 5–10ppmw, depending on crude oil, vacuum column cutpoint, unit process design, and distillation unit operation and equipment design. Total gas oil metals may be reduced by 20–50%, or heavy vacuum gas oil (HVGO) product yield can be increased for a given metals target by optimising primary distillation system performance.
A 22.0 °API Bochequero Field (BCF) blend is used to illustrate the potential metals reduction based on optimised primary distillation design and operations. In the following example, 145 000bpd of BCF is processed through a conventional integrated atmospheric/ vacuum unit and the vacuum residue in a delayed coker. The FCC feed consists of atmospheric gas oil (AGO), light vacuum gas oil (LVGO) and HVGO from the crude unit and heavy coker gas oil (HCGO) from the delayed coker (Figure 1).
Gas oil metals source
Vanadium in the distillate products consists of volatile vanadium in the hydrocarbon boiling range and the entrainment of non-distillable residues. Volatile metals vapourise at the operating conditions in the unit, so are always present. The amount of entrainment depends on the vacuum unit transfer line velocity, coker drum line velocity and distillation column performance. Entrainment can be virtually eliminated through prudent transfer line and column internal designs. Volatile vanadium depends on the amount of product yielded and the 95 vol% end point (EP) tail. Process and equipment design has reduced volatile vanadium by up to 30% in HVGO and HCGO products.
Before revamping a vacuum unit or coker main fractionator to reduce product vanadium, the source of the metals needs to be determined. Proper distillation equipment design can eliminate the entrained metals, whereas volatile metals must be fractionated. Ultimately, the value of each incremental barrel of product depends on how much vanadium it contains. With some high metals crude oils, it is not economic to produce the incremental barrel, because the vanadium content is so high it dramatically reduces the hydrotreater run length.
Residues do not vapourise at the operating conditions, so they must be entrained with the rising vapour. The simplest method to estimate the quantity of entrainment is to distill 95% of the sample overhead in a laboratory still. A high temperature simulated distillation (HTSD) is then run on the heaviest 5 vol%. Since the HTSD measures atmospheric equivalent temperatures (AET) up to 1380ºF, any entrainment will show up as high boiling point material that should not be present at the operating temperature and pressure of the unit. The entrained material is often not detected if the whole sample is analysed by HTSD, because it typically represents only 0.5–2% of the total stream.
Table 1 shows BCF 22 °API crude (Figure 2) and atmospheric and vacuum residues vanadium content at typical AGO and HVGO product cutpoints. Since these residues contain extremely high vanadium, even small amounts of entrainment will dramatically increase distillate product metals.
For example, 50bpd of entrained atmospheric residue increases AGO vanadium by more than 40%. Since vacuum residue contains over 800ppm vanadium, entrainment of less than 0.5% into HVGO product cannot be tolerated. While entrainment from the flash zone into the wash section always occurs, the wash section must be capable of removing all of it, otherwise, vanadium can increase to over 20ppmw. When refiners switch from low to high metals crude they are sometimes surprised by the HVGO vanadium content. Many vacuum residues have only 40–80ppmw vanadium compared with 800ppmw in the BCF 22 °API.
Coker unit HCGO contains non-distillable material entrained from the coke drum. As the coke drum and main fractionator’s superficial velocity increases, entrainment goes up. The quantity of entrainment can be estimated by analysing the HCGO product heaviest 5 vol% with the use of the previously mentioned HTSD. Entrained non-volatile material from the coke drum may contain well over 1000ppm vanadium. Entrainment happens during both normal operation and from steaming of the hot coke drum prior to coke cutting.
Volatile material needs to be measured as a function of boiling range so that incremental product yield can be valued. First, the 800ºF+ boiling range material must be fractionated into 25ºF cuts by the ASTM D5236 method or in a continuous flash vapouriser. Metals in the individual cuts are determined by ICP-AES.
Crude oil metals distribution is a function of the source. Many Venezuelan, Mexican and Canadian crude oils have high vanadium content, and the 800°F+ boiling-range hydrocarbon includes volatile vanadium compounds. Some, such as Canadian Lloydminster B, have moderate metals in the whole crude. However, the volatile vanadium in the 800°F+ boiling range is high. Vanadium distribution is crude oil dependent. Therefore, distribution must be known to accurately predict HVGO vanadium content. HCGO product vanadium is a function of the vacuum residue metals, vacuum unit cutpoint and the boiling range of the HCGO product. High vanadium vacuum residues produce HCGO with high vanadium, because the volatile vanadium is also high in the coke drum effluent.
AGO, LVGO, and HVGO vanadium
Combined gas oil vanadium depends on the crude oil metals distribution, processing scheme and distillation system design and operation. The distribution of vanadium by boiling range will set the minimum amount in the combined gas oil product. However, the vacuum unit process design and distillation equipment design will have a significant impact on the HVGO product 95 vol% EP, which materially influences vanadium.
Vanadium distribution is highly non-linear, as is evident from the dramatic increase in vanadium above a 1025°F TBP boiling range shown in Figure 3. Therefore, when operating a vacuum unit at an HVGO cutpoint of 1100°F, there will be significant amounts of vanadium in the HVGO product. Table 2 shows the vanadium distribution in the 950–1150°F boiling range for BCF 22 °API. Relatively small changes in the TBP cutpoint increase the vanadium content due to the previously mentioned (highly) non-linear distribution.
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