Q&A

(2011) Question 73: What is your best practice for coke drum velocity to minimize coke carryover to the fractionator?

Coke drum velocity limit depends on the capability of the coker main fractionator and downstream equipment to handle entrained coke.

(2011) Question 74: Please discuss the pros and cons of the various coke drum level technologies.

The process conditions are highly fouling, high temperature, changing pressure and composition, and high velocity hydraulic decoking every cycle.

(2011) Question 75: What are the pros and cons of driving coke VCM (volatile combustible matter) to a low level? What are the lowest green coke VCM you have consistently achieved?

Volatile combustible material (VCM) is an important parameter of Petcoke. VCM is basically unconverted pitch in the coke. The metal and sulphur are controlled by the type of crudes processed, but VCM content of coke is mainly in the control of DCU operators.

(2017) Question 43: What is your experience with intermittent mud washing of single-stage and two-stage desalting? What are the advantages of continuous versus intermittent mud washing?

The primary purpose of a mud wash is to suspend and remove solids that have accumulated in the bottom of the desalter. Some desalting operations use the mud wash to remove solids (asphaltenes and oil-wetted inorganic solids including iron) that have accumulated in the interface of the desalter. The desalter level may be lowered to accomplish this goal in some of these operations.

(2017) Question 44: How do you monitor exchanger fouling? How do you use that information to justify additional work scope during unplanned shutdowns?

Exchanger monitoring software or spreadsheets that pull in refinery process data can be used to monitor fouling trends. Exchanger trains should be evaluated for fouling on a routine basis to provide historical references and trends to compare pre- and post-cleanings.

(2017) Question 46: What is your design service life of atmospheric tower overhead heat exchangers? How does that compare to actual service life? What do you do to better manage corrosion and improve reliability of these heat exchangers?

Heat exchangers are designed for heat recovery and only rarely for corrosion control. The designer’s tool for reliability is to use upgraded materials of construction. Over the years, those on our team have seen even Hastelloy C276 and titanium being used.

(2017) Question 47: What correlations do you use to predict cracked gas make in a vacuum tower from atmospheric tower bottoms based on feedstock properties and heater outlet temperature?

In the past, vacuum towers’ flash zones have operating at or below atmospheric tower flash zone temperatures. These units relied on the vacuum to increase heavy gasoil yields from the atmospheric resid without running the risk of thermal cracking associated with higher temperatures.

(2017) Question 48: What are your design and operating guidelines for vacuum tower bottoms versus crude heat exchangers such as minimum velocities, fouling rates, and pressure drop, and which process fluid should be on the tube side versus the shell side?

KPE believes the typical design for a vacuum bottom versus crude exchanger is for the crude to be on the tube side and the vacuum bottoms to be on the shell side. Most facilities I see have this setup as weli.

(2017) Question 49: How do you use today’s Advanced Process Control (APC) technology to achieve crude unit operation that supports short-and long-term planning goals? How do you achieve alignment between the crude unit APC and those of downstream units?

Without proper resources and attention, Advanced Process Control (APC) technology may experience inconsistencies as applied to crude units. Refineries where crude slate varies frequently may struggle capturing the value that APC provides for steady controls and optimization. Mode-based APC may be applicable, but irregularities in crude compositions may still provide hurdles.

(2016) Question 24: How do you manage reactor maldistribution once identified?

Reactor maldistribution is a phenomenon that is typically caused by inadequately designed or installed reactor internals, improperly/unevenly loaded catalyst bed, catalyst migration from an upstream bed, or process fouling. The main problems generated by flow maldistribution are the overuse of part of a catalyst inventory and the formation of hot spots, which can create a process safety risk, as well as limit the performance of the cycle.

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