EJE 2: MEDIO AMBIENTE Y PREVENCIÓN DE RIESGOS

5.2.6.1 Natural Crude

Acceptable contamination in a crude oil is difficult to define. Because it is a naturally produced substance, it contains natural materials, some of which are undesirable for a refiner, such as sulfur, nitrogen, carboxylic acids, metals, sand, clay, etc. The levels of these contaminants vary according to the source of the crude oil and its treatment history. A refiner will carry out an assay on a crude oil stream prior to purchasing it, in order to establish its value to that refinery. The value of the crude oil is based on the value and amounts of products that the refiner can make from that crude oil, as well as the cost to the refiner of removing the contaminants that are present in the feed stock. Mixing a different crude oil with the originally assayed crude oil will “contaminate” that oil, and change its value to the refiner, positively or negatively. Typically, a refiner is willing and prepared to accept interfacial levels of contamination, but not levels that are associated with tank bottoms service changes.

5.2.6.2 Synthetic Crude

Synthetic crude oils are significantly different from natural crude oils, both because of their source as well as the fact that they have been processed in an upgrader. Synthetic crude oils (e.g., Alberta based), generally have high aromatics content (considered to be problematic to a refiner) and very low contaminant concentrations (beneficial to refiners). They also usually have no bottoms or residual content because they have been distilled.

Refiners sometimes look at the contamination level in synthetic crude oil based on the residual content. Most synthetic crude oils are “bottomless” when they leave the upgrader site, meaning that they contain no residue. However, during transporta- tion, they can acquire a measurable amount of residue as a result of contamination with other crude oils. This contamination occurs usually from being interfaced with those oils. Sulfur content is also a good indicator of contamination in synthetic crude oils because they usually have very low sulfur contents, of the order of 0.10% by

TAbLE 5-4. industry accepted contamination levels

Contaminant Product % of Contamination in Product

Butane Gasoline * Premium gasoline Regular gasoline 3 Regular gasoline Premium gasoline 1 Jet fuel (kerosene) Regular gasoline 1 Jet fuel (kerosene) Premium gasoline 2 Jet fuel (kerosene) Diesel 2 Any Jet fuel Nil Note: *Depends on butane already added

Pipeline Operation and Batching n 255 weight. Contamination with heavy crude oil will result in a marked increase in the sulfur content of synthetic crude oil due to the high sulfur content of heavy crude oil (2 to 4 wt.%).

It is well understood that contamination by heavy crude oil of lighter grades of crude, such as synthetic, is problematic. It is a common misperception however that contamination of heavy crude oil with a lighter grade of crude oil improves the heavy crude oil. This is unlikely to be correct. The refiner has selected a heavy crude oil for his feedstock based on the properties and distillation yield of that crude oil. Altering that yield and properties may well result in the refiner suffering a loss, perhaps even a serious loss. In one example, a refiner received a heavy crude oil batch, intended for asphalt shingle production, which had been seriously contaminated with a light sweet crude oil. The produced shingles did not meet the requirements for durability, and the asphalt simply melted off the shingles under summer sun.

Contamination can occur from many sources as the crude oil stream moves from its production site to the refiners’ tankage. Common sources include tank farm mani- folds, tank bottoms from tank service changes or common tank usage, leaking valves, interfacial contamination, dead legs, etc. The impact of each of these sources varies according to the volume of the contaminating crude oil. Tank farm manifolds can be a significant contributor, depending on the design and operation of a tank farm. Simple tank farms are easy to analyze, and operating procedures can be developed for minimi- zation of their impacts, including operational sequences and tank service restrictions. Complex tank farms should utilize a manifold specifically designed for minimization of contamination. The use of dead legs should be minimized in the design process, through tight placement of valves, no more than three pipe diameters from a pipe tee. Leaking valves can be identified through careful inventory tracking and control. The largest contributor to contamination as a result of operations (assuming full turbulent flow) is tank bottoms.

Tanks have a minimum working volume (working bottoms). At levels below that minimum working volume, special precautions must be taken for filling and taking suction from that tank, so normally a tank is not operated below its working bottom. When changing service from one crude oil to another, the working bottoms are not considered if the crude oil types are the same, e.g., from one heavy crude oil to another heavy crude oil. However, when planning to change service of a tank between two dissimilar types, the working bottoms must be taken into account. The contamination caused if a tank is simply swapped from one service to another would be unacceptable. There are two alternatives for handling this “incompatible” type of crude oil service change. One is to reduce the volume of bottoms below the working level; by pumping the tank out to minimum suction levels, which could be followed by using vacuum trucks to remove the non-usable volume. The second alternative would be to negotiate with a refiner to accept one or two batches known to be heavily contaminated. Both have disadvantages, the former results in operational restrictions on tank fill rates to restore the working bottoms of the new crude oil in the tank, the latter will probably result in a financial penalty on the contaminated crude oil batch(es).

5.2.6.3 Contamination Level

As a rule of thumb, contamination from interfaces will amount to a few percent of between batches, while contamination from tank bottoms can be as high as 20% to 40% (first batch after service change), see Chapter 8 for details. However, the success- ful batching of differing grades of crude oil depends on maintaining turbulent flow. Reducing the pipeline flow rates such that Reynolds numbers drop below 5000 tends to cause stretching of interfaces, with higher associated contamination costs as a re- sult. The critical parameter in maintaining full segregation is the Reynolds number of the midpoint of the interface between the two batches. Acceptable midpoint Reynolds

number (Re), and the associated contamination, depend on the type of product being transported. Refined products systems typically use a minimum Reynolds number of 20,000, while crude oil lines typically accept lower values of Re, in the range 3000 to 5000. It is generally accepted that Reynolds numbers in the range 2000 to 3500 or so represent transition flow. That is a flow regime that is partially laminar and partially turbulent. Exactly which flow regime is present depends on the nature of the flow history. If the flow rate is increasing and passing from laminar to turbulent, it is more likely that the transition from laminar to turbulent will occur at higher Reynolds num- bers, as long as no external source of turbulence is present. Examples of such an exter- nal source of turbulence might be a pump impellor, a set of short diameter pipe bends, a sudden change in pipe diameter, or any significant pipe roughness. Experiments with water flow have shown that, given optimal flow conditions, laminar flow can be made to continue as high as Re 10,000. Similarly, when decreasing flow rates, turbulent flow tends to drop to transition and laminar flow regions at lower Reynolds numbers. If a pipeline system must operate at marginal flow rates for turbulent flow, it could be ad- vantageous to bring flow rates initially to a rate that corresponds to a Reynolds number of 5000 or so and then reduce the flow rate to the target operating zone.

It is also possible, although rarely done, to operate with heavy crudes in transi- tional or even laminar flow, separated by light crude oils that are in turbulent flow. The midpoint interface must be in turbulent flow for batch segregation to be maintained. This is a sub-optimal operation, and the interface will be slowly shifted as the laminar flow zone captures the heavy tail of the interface. The interface will increase in size from turbulent model predictions. It will also become asymmetric, with a reasonably short interface length from the light crude to the midpoint of the interface (by density), and a longer interface from the midpoint to the heavy crude end of the interface. Little data has been published on this type of operation, but heavy crude has been success- fully batched at Reynolds numbers less than 2000 between light crude batches having Reynolds numbers over 10,000.

An example of this is the line-fill operation of a 2000-km NPS 30 line at 200,000 BBLSD with heavy crude and light synthetic oil. The heavy crude oil batches will flow from the KMP 0 to 300 segment at a Reynolds number of about 840, while the syn- thetic crude would be at a Reynolds number of about 71,000. The Reynolds number of the interface will be about 12,400, which is expected to provide good batch integrity, as described above.