Q&A

(2016) Question 6: What is your experience with having a vent depropanizer off-gas unit in order to manage tower pressure, and what might be the cause of and solution to the problem?

The typical and probably most obvious driver for the need to vent from a depropanizer column is the presence of non-condensable gases. Most commonly, this is ethane and ethylene that come in with the feed.

(2016) Question 7: How is propane content in the refrigeration loop optimized against the compressor capacity to minimize contractor temperature? Do you have a good process model to predict the optimum propane content?

There is an optimum propane concentration in the refrigerant that will allow the alkylation unit to operate at a maximum alkylate throughput when the unit is up against limitations of both the compressor and the reaction zone heat removal capability. That optimum concentration of propane will vary from unit to unit and can be different seasonally. It can also be different from night to day or if exchangers are fouled, etc.

(2016) Question 60: When is your return on investment (ROI)adequate to justify installing a desalter to treat purchased FCC feeds? What other factors besides FCC catalyst impact and unit corrosion are used to determine ROI? How do these desalters differ, mechanically and operationally, from a conventional crude oil desalter?

This is a three-part question. The first question is: When is the return on investment (ROI)adequate to justify installing a desalter to treat purchased FCC feeds? The capital and operating costs of an FCC feed desalter are supported by improvements in performance, operating costs, and reliability.

(2016) Question 61: How many inside/outside operators staff your FCC plant? What other processes are included in their scope of responsibility?

[United Refining Company (URC)] Our FCC unit has five operators per shift. Typically, two people are inside running the DCS (distributed control system) and board-mounted controls while the other three are doing rounds in the unit.

(2016) Question 62: We have run a full-burn FCCU for many years. We are considering processing more resid and operating in a partial CO combustion mode. What is a carbon runaway, and how can it be addressed?

A carbon runaway or “snowball” occurs in an FCC that is operating in partial CO combustion mode and very high CO levels. In the runaway situation, the carbon on regenerated catalyst (CRC) rises to levels such that additional coke that is laid down on the catalyst cannot be burned from the catalyst in the regenerator, causing the CRC to continue rising like a snowball that gets bigger and bigger as it rolls down a hill.

(2016) Question 63: What are your current methods used for regenerator cyclone temperature control? Do you use water sprays or steam injection?

In the early days of FCC, the normal bed and dilute-phase operating temperatures –being less than 1100°F –did not support CO combustion. These units operated with high levels of carbon on catalyst and about 10 vol% CO in the flue gas with almost no excess oxygen in the flue gas.

(2016) Question 64: What are your typical operating guidelines to prevent compressor surge episodes? How closely do your FCCU operators approach the actual surge line of a compressor before adjusting operation?

[United Refining Company (URC)] The unit engineer/compressor manufacturer should be able to provide guidelines to prevent surge episodes. These guidelines may include safe windows of operation for air rates, discharge pressures, and discharge temperatures.

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