Estimating bitumen viscosity for revamps
Accurate viscosity data for dilbit and its heavy residues define the scope of revamps required to process the feed in crude and vacuum units
TEK SUTIKNO and CARLOS MESTA
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With the growing supply of bitumen based crudes and the reported significant reserves of bitumen, some refineries, especially those in the upper region of North America, have started processing these crudes. Due to high densities (or low API gravities typically less than 10) and other undesirable properties such as the high viscosities of bitumen relative to conventional crudes, one of the common ways for marketing bitumen based crudes is mixing a diluent with the bitumen to become diluted bitumen (dilbit) with a typical API gravity of 20 to 21, which is the typical API gravity range of heavy crudes. However, relative to other typical heavy crudes, the viscosity and content of naphthenic acid, asphaltenes, fine solids and heavy metals are all high or significantly present in dilbits of Canadian origin, in addition to other issues such as phase â€¨separation and crude blend incompatibility, resulting in high fouling rates.
In the crude and vacuum units of an existing refinery intending to process dilbit, the equipment for heat transfer, desalting, and pumping can be inadequate due to the high viscosity of the feed. Additionally, high naphthenic acid contents (or total acid number, TAN) of dilbit increase the corrosion rates of carbon steel equipment operating at high operating temperatures, especially above 400°F (205°C). Product yield distribution from processing dilbit in the crude and vacuum units will significantly change and require higher operating capacities of the downstream units such as the coking unit among others processing heavy fractions. Revamping refineries designed for conventional crudes will typically be necessary to make them suitable for refining dilbit and, as such, the required scope of modification increases as the percentage of dilbit in the crude slate increases.
While the viscosity of conventional heavy crudes can be several orders of magnitude higher than that of the light crudes, bitumen from Canada can typically be a thousand times more viscous than conventional heavy crudes. A maximum viscosity of 350 cSt is typically specified for pipeline delivered dilbit by mixing bitumen with sufficient amounts of diluent. The amount of dilbit required to meet a targeted viscosity at the delivery point will vary, depending mainly on the delivery temperature as well as the particular characteristics of each blending component: bitumen and diluent. When processing 100% dilbit crudes, the heavy fraction or residue of dilbit is essentially all bitumen; therefore, all high boiling point streams in the crude and vacuum units will have high viscosities.
High viscosity dilbit crudes reduce equipment performance and require modification or replacement of equipment. Proper definition of the scope for revamp of the crude and vacuum units necessitates accurate, temperature dependent estimates of the viscosity of the dilbit and its heavy fractions. Depending upon the selected calculation methods and versions, commercial simulation programs do not always provide reasonable estimates for the viscosity of dilbit and its heavy fractions. It is generally worthwhile to verify the dibit viscosity data from the simulation programs against laboratory data or independently estimated values from alternate methods such as data correlations. This article discusses correlations which may be considered for estimating or verifying the viscosities of Canadian dilbit and its heavy fractions.
Crude unit preheat trains
Crude units typically have a raw crude preheat train upstream of the desalter and a desalted crude preheat train downstream of the desalter. These preheat trains recover heat from the product streams of the crude and vacuum towers and minimise the necessary heat input to the crude and vacuum furnaces. The raw crude preheat train also increases the raw crude temperature to meet the inlet temperature requirement of the desalter for optimal performance.
For light and conventional heavy crudes, the desalters generally require 200°F (or lower) to 250°F (93-120°C) inlet temperatures and perform less satisfactorily as crude density and viscosity increase. Desalting dilbits typically requires 280°F (140°C) or higher inlet temperatures to minimise the impacts of high densities and viscosities. Inadequate salt removal will cause severe corrosion, especially at the crude tower overhead. These higher desalter inlet temperatures require modification of the raw crude preheat train to meet the increased preheat duty; however, the overall heat transfer coefficients of exchangers in the preheat trains reduce severely at high viscosities which also result in high pressure drops. High velocities necessary in exchangers to minimise the fouling potentials of dilbit will further increase pressure drop.
Consequently, the required surface areas of the new or modified cold preheat train necessary to process dilbits will increase significantly, and the raw crude charge pump will need to deliver a higher discharge pressure.
Figure 1 shows the sample performance of an exchanger in the raw crude preheat train. As shown, the exchanger originally designed for a light crude of API 37 becomes severely inadequate when preheating dilbit crudes. The overall heat transfer coefficient reduces to about 28% when the crude-side average viscosity of the exchanger increases from about 3 cP average viscosity with an API 37 crude to 200 cP average viscosity of a 100% dilbit crude. The average viscosity on the abscissa of Figure 1 is the viscosity average of raw crude entering and leaving the exchanger. The raw crude side pressure drop of the exchanger increases to about 200% (see Figure 2) when processing 100% dilbit. These reductions in performance become more severe at low dilbit crude feed supply temperatures during winter.
To quantify the reduction in exchanger performance due to the high viscosities of dilbit crudes, accurate, temperature dependent viscosity data of dilbit crudes are essential for properly modifying an existing raw crude preheat train to operate in dilbit services. Overestimating the viscosity will result in unwarranted high capital expenditure, while underestimating will result in poor performances of the preheat train and the desalter. Severe corrosion in the crude tower overhead could result when the desalter does not perform satisfactorily due to feed temperatures lower than the design target. Failure of the preheat train to meet the design duty also leads to higher fired duties of the furnaces. As such, realistic estimates of viscosity data are essential for successful modification of the preheat train and the desalting system.
Dilbit may be blended with other types of crudes, depending upon the crude slates included in the revamp modification scope. 100% dilbit crudes are likely the most viscous, and viscosity data will be necessary. As the diluent content of dilbit will vary to meet the pipeline or other viscosity specifications, accurate and reliable temperature dependent viscosity estimates of the dilbit crudes are needed for defining the scope for modification of the raw crude and desalted crude preheat trains. While a number of correlations or equations for estimating the viscosity of crude blends have been reported, the Cragoe Equation (Equation 1) results in reasonably good estimates of dilbit viscosity from viscosity data of diluent and bitumen:
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