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Performance of waxy crudes as FCC feeds

Advantages and challenges: determining the optimal blending ratios with waxy crude and VGO combinations

Erick Gamas, Cliff Avery, Gerbrand Mesu and David De Villiers
Albemarle Corporation
Alan Yahev, Newfield Exploration Company
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Article Summary
Waxy crudes from formations in the US state of Utah were considered intractable just five years ago, but the relatively higher prices of conventional crudes make the economics of these Utah crudes’ transportation by truck or rail car acceptable.

In this area Newfield Exploration has a long-term commitment to supply approximately 38 000 b/d of the waxy crude to refineries in the area. Newfield is the largest oil producer in Utah with current production of 39 000 barrels of oil equivalent per day (boe/d). Total waxy crude production from this region is 65000 b/d, with Newfield’s share comprising 45%. Deposits of 700 million boe have been reported in the region, which seems to indicate a reliable and long-term supply of the crude.

At the onset, the transportation logistics to refinery processing centres were complex and there was a steep learning curve for the midstream infrastructure and refiners processing these waxy crudes, such as fouling at all locations of the transportation network, plugging of pipes and rail car unloading. Nonetheless, there are compelling reasons for refiners wanting to process these waxy crudes.

The high paraffinic content of these waxy crudes makes them suitable for certain lubes plants. For those refining facilities with coker units, the waxy crudes’ low metals content also makes them suitable for anode grade coke feedstocks. The crudes’ low content of light ends allows for blending with conventional FCC feeds, allowing for complete bypassing of the crude distillation unit (CDU). The low sulphur and low metals (Ni and V) contents results in lower sulphur FCC fuels and longer catalyst life.

Crude feed sources and their fractions (vol%) are shown in Figure 1 for conventional gas oils, tight oils, waxy crudes and others. Small amounts of resid and some distillate fractions are present in the conventional gas oils (KVGO and PVGO). Tight oils present significant fractions of light ends that require processing through the crude unit. In comparison, the waxy crudes (yellow wax, black wax and Uintah crude) contain less than 10% naphtha and can therefore be blended with FCC/RFCC feeds. 

Advantages and challenges
Waxy crude prices are lower than conventional crudes and additional energy savings are incurred by bypassing the CDU for direct cracking in the FCC. The low Ni and V in these waxy crudes results in relatively low rates of FCC catalyst deactivation and easier cracking. This process advantage is why these feeds can be potentially used for blending with much heavier cheap feeds. Processing waxy crudes through the FCC produces higher volume gains and higher percentages of high value added products, such as higher LCO cetane.

Although waxy crudes’ characteristics are such that they provide refiners with the flexibility to bypass certain process steps, such as circumventing the CDU for direct upgrading in the FCC unit, these crudes are not without their challenges, such as lower RON in FCC gasoline. Beginning with a lack of compatibility when blending with other streams, their high wax fractions lead to asphaltene precipitation and low carbon residue for delta coke. FCC engineers must be cautious of gas production constraints and occlusion of external FCC catalyst pore structure from the crudes’ high Fe content, resulting in loss of catalyst activity. Careful selection of high accessibility catalyst technology is key for activity and selectivity control.

From lab to commercial operations
Laboratory testing programs are an accurate and cost effective tool when process similarities at the commercial scale are considered. Information obtained from lab testing provides directional information to infer commercial performance, such as determining the yield structure response in an FCC reactor from patterns of feed and catalyst contacting. Supporting process models yield estimates for individual operations. In addition, use of E-cat in lab testing for feed screening provides better representation of commercial conversion than lab deactivated catalysts.
In designing a testing program to assess the potential of waxy crudes as FCC feeds, a proprietary Albemarle Fluid Simulation Test (FST) lab reactor was used for waxy crude cracking tests. In these instances, processing through the CDU was bypassed. Catalytic cracking FST tests of neat waxy crudes and blends with VGO were carried out using a high accessibility E-cat as shown in Table 1.

Feed properties vary from conventional VGOs to neat waxy crudes. For example, black wax has high Concarbon of 3.5 as compared to 0.3 for PVGO. High sodium feeds such as 70/15/15 Uintah/YW/BW (2542 ppm) require an FCC feed desalter. Very high Fe requires high accessibility and Fe tolerant catalyst.

Cracking yields
While lower RON can be expected when processing FCC feeds with higher paraffin content, there is nonetheless a consistent increase in conversion and gasoline yields. In addition, the overcracking conversion limit can be expanded by using feeds with higher paraffin content.
There is a consistent loss of LCO with conversion through the FCC unit and a corresponding LPG increase. With higher feed paraffinicity comes the potential for higher cetane LCO, but with lower yields. With these combinations of feeds (YW, BW, Uintah-Wax, PVGO, KVGO, and so on), an economic LCO/LPG optimum has been identified with an apparent trend underlying the yield patterns. 

The underlying yield patterns indicated in Figure 2 reveal an optimal blending ratio to balance LCO/LPG. This apparent trend shows conversion of LCO to LPG without loss of gasoline. In this case, LCO cetane and gasoline RON, in addition to yields, are optimisation indexes.

Higher yields of LPG C3 and C4 fractions shown in Figure 3 follow a trend consistent with increased feed paraffinicity, where the impact of catalyst combined with feed effect are revealed at highest severity. The figure shows a 5% C4s delta with 1.5 higher CTO at 932°F with neat waxes. The same underlying trend with feed quality is observed in propylene yields, where propylene yields show a strong response to feed quality (for example, +5.3 wt% from PVGO) and cracking severity (+1.88 wt% for 63°F higher temperature). 

A linear decrease in C3 and C4 olefinicity is observed as feed improves. For example, a 3.0% reduction in C3 olefinicity and a 22% reduction in C4 olefinicity are observed with lowest values corresponding to yellow wax.

Paraffinic bottoms can be converted to extinction at high severity. It can be demonstrated that feed synergy is observed with coke yields of waxy crudes and VGO blends, where coke yield of blends noted in the previous figures is similar to VGO coke. For example, coke yield from PVGO is higher than coke yield from PVGO-Wax blend, with CCR at a higher magnitude than the PVGO (by a factor of five [5X]).

Feed CCR from waxy crudes is not reflected in FST coke yields, where an inverse relationship is actually observed. FST coke yields response to CTO is linear, where a change in slope and trend is observed with blends revealing a possible synergy in behaviours when combining feeds (see Figure 4). Feed CCR is not reflected on delta coke due to high paraffinicity of feed bottoms in waxy crude mix. For example, Uintah-Wax mix behaves like a conventional VGO despite having a higher paraffinicity than KVGO.

Conclusions and recommendations
Results from lab cracking tests with a high accessibility E-cat support business opportunities for commercial operations using waxy crudes, partially replacing VGO/resid fractions. Use of black wax requires an FCC feed desalter as practised by major refiners buying VGO.

An optimal blending ratio was identified, driven by LCO/LPG as well as cetane/RON economics for individual refiners. Waxy crudes in VGO impart higher cetane numbers to LCO products, whereas lower gasoline RON requires a balanced management approach. Identifying potential synergies of feeds in wax/VGO blends is a key benefit for refiners. In any event, refiners should maintain close communication with the FCC catalyst technology supplier to optimise the catalyst formulation and particle accessibility when evaluating opportunities to process wax/VGO feeds.
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