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Apr-2015

Seven rules of sedimentation in hydrocracking

Guidelines to mitigate sedimentation fouling in heavy sour crude processing together with case studies illustrating how the improvements are captured.

SCOTT SAYLES, ROBERT OHMES and RICK MANNER
KBC Advanced Technologies
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Article Summary
Conversion of heavy oils is still a major economic consideration for refiners around the world. Recent discoveries of light oil in North America have reduced the emphasis on heavy oil conversion processes but the balance of crudes being processed still contains a significant heavy crude fraction. One of the processes that is fundamental to upgrading heavy sour crude is the ebullated bed and slurry hydrocracker. One area of operational challenge is the formation of a solid phase that incurs fouling of equipment. The formation of the solid phase is commonly referred to as sedimentation.

Sedimentation is difficult to define and so is consensus on theory, composition and the behaviour that causes its formation.1,2,3 Occurrence is verified by unit fouling, resulting in:
• Short runs for equipment clean out
• Loss of exchange performance
•  Ebullation difficulties
• Limitations to unit performance.

Testing methods are solvent extraction based and are a reasonable way to monitor sedimentation but are not useful to predict its occurrence. The following ‘rules’ use typical methods to determine performance and provide guidance. The rules provide a basis to reduce sedimentation and improve unit performance.

Unit description

The ebullated bed and slurry hydrocracking processes utilise elevated temperatures and pressures to convert vacuum residua to lighter products. The ebullated bed unit uses a fluidised bed of catalyst with the ability to replace the catalyst on-line, while the slurry reactor has a continuous catalyst with the feed as a slurry through the reactor.6 The reactor configuration for an ebullated bed unit is shown in Figure 1.

An ebullated bed design is shown in Figure 2 with locations for potential improvements indicated.8 These changes will be discussed, as well as how the improvements are captured using case studies.

Evaluation methodology
Understanding of the potential for sedimentation in a given unit configuration requires knowledge of the reaction chemistry, process design and operating goals. Hydrocracking residua chemistry is a function of thermal conversion.4 The catalyst’s role is to saturate the thermally cracked product. The reaction mechanism then is: first crack, then saturate. This leaves the process subject to a requirement to achieve close contact between the catalyst, oil and hydrogen. The evaluation of sedimentation is based upon experience and an understanding of the thermal cracking functionality as the determining factor in overall conversion.

Rule 1: reactor temperature
Increased temperature increases sedimentation. Options are available to achieve higher conversion without increased sedimentation and are affected by keeping reactor temperatures low.2 For example, recycling the vacuum residua allows higher conversion at lower reactor temperatures and lower sedimentation.

Supporting observations are:
• Sedimentation increases as reactor temperature increases for constant space velocity
• Decreased reactor space velocity at the same reactor temperature decreases sedimentation
• Thermal kinetics controls the hydrocarbon conversion, not hydrocracking; this leads to production of unstabilised heavy streams that contribute to sedimentation4
• Catalyst functionality is for stabilising the cracked stream, not cracking the feed
• Some catalysts have improved asphaltene conversion and so allow higher temperature operation for constant sedimentation.
Changes to the reactor design lower temperature at constant conversion, reducing sediment:
• At the top of the reactor (see Figure 1), the cup or pan separates gas from the liquid in the reactor. Poor performance of the separator leads to higher levels of gas in the reactor section, increasing sediment by requiring higher reactor temperatures for the same conversion. Modifications lower gas hold-up, decreasing the temperature for a given conversion.
• At the bottom of the reactor (see Figure 1), the plenum mixes gas and liquid before distribution into the catalyst bed. Modifications to the distribution of gas and liquid allow better contacting in the first 10% of the catalyst bed, permitting lower gas rates for the same level of sulphur and nitrogen removal
• The initial gas and liquid are combined external to the reactor in the mix tee. Modifications increase the oil/hydrogen mixing, reducing maldistribution at the inlet to the reactor and benefiting the plenum performance
• All these design changes reduce sedimentation at constant conversion by reducing reactor temperature.
Rule 2: incompatibility
Hydrocracked products are incompatible with each other and form sediments. Mixing light hydrocracked products with heavy results in more sedimentation:

Light + Heavy = Sediment   
or
L + H = S
For example:
(0%*1000°F-) + (100%* 1000°F+) less sediment

(50%*1000°F-) + (50%* 1000°F+) more sediment

Applying the rule indicates a light oil to quench the vacuum tower bottoms potentially creates sediment or poor vacuum tower fractionation may increase sedimentation.

HPS hydroclones
Removing light entrained gases from the high pressure separator (HPS) bottoms liquid reduces sediment. Rapid separation of the gas from the liquid can be accomplished using hydroclones in the hot separator. An example of a hydroclone from EGS Systems, Inc is shown in Figure 3.
Separation efficiency is high and the light ends are removed from the heavy product, reducing sedimentation.

Rule 3: asphaltene conversion
High asphaltene conversion directionally reduces sedimentation, but not always. Sometimes asphaltene conversion removes the sedimentation component from the mix. Other times the partial conversion of asphaltene leaves it prone to sedimentation.

Catalyst effect
Some catalyst are reported to 
be effective in reducing asphaltenes.3 In general, lower asphaltene levels seem to relate to lower sediment and have the following effects:
• Lower sediment allows higher reactor temperatures and conversion to the sediment limit (Rule 1)
• Other catalysts have reduced asphaltenes without changing sedimentation.
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