| American Fuel & Petrochemical Manufacturers

Question 77: What operating procedures are do you use to mitigate/prevent coking issues in the vacuum tower from loss of wash oil pumps?

Wash oil loss can result in coking in either the wash oil delivery system or in the wash bed, or both. The best method is to prevent coking is to keep the wash oil in service. Auto-start of standby wash oil pumps, alternate wash oil supply sources (AGO), and putting pumps on critical service power supply are used.

Question 76: What is considered industry Best Practices with respect to the control of thermal cracking in vacuum tower bottoms? If quench is not available, what other parameters do you monitor and control? How do you establish the target control points?

Thermal cracking of liquid in the vacuum tower bottoms depends upon time and temperature. The higher the temperature, the higher the cracking rate. The longer the residence time, the more cracking. The key to reduced cracking in the vacuum tower bottoms is to keep the bottoms temperature down and the residence time low.

Question 75: Do you draw liquid hydrocarbon from the overhead system in the vacuum tower after the pre-condenser? What is the operating temperature of the tower overhead? What issues do you observe?

At vacuum tower top temperatures over 135-150°F some material heavy enough to condense after the pre-condenser goes overhead. In units with extremely paraffinic feeds the hydrocarbon can form wax in the pre-condenser, leading to reduced heat transfer and higher vacuum tower pressures.

Question 74: What are the main causes of vacuum jet ejector poor performance? What methods have you used to mitigate these issues?

Diagnosing poor vacuum jet ejector performance can be tricky.

Question 73: There is increasing economic incentive to operate crude units to yield maximum middle distillate volume. What targets are you using to quantify the degree of separation between naphtha and kerosene and between diesel and gas oil? What amount of distillation curve overlap do you consider good practice? What guidance are you given to achieve the target separation?

Middle distillates are maximized within the constraints of D86, flash point and density specifications. There is some optimizations play between the pump around heat recovery, which is specific to a unit, and maximum distillate recovery.

Question 72: What discrepancies do you see between simulation predictions and actual crude and vacuum operational data?

What discrepancies do you see between simulation predictions and actual crude and vacuum operational data?

Question 71: What is your best practice for responding to a ruptured heater tube in the crude unit?

What is your best practice for responding to a ruptured heater tube in the crude unit?

Question 70: Please discuss the merits and detriments of using low-base strength or high-base strength neutralizers for corrosion control in atmospheric column overhead.

We do not recommend the use of low base strength neutralizers that have a pkb lower than Ammonia for the obvious reason that these are not able to compete with Ammonia in the chloride salt formation. We prefer higher base strength neutralizers but that is only part of the selection criteria, other important criteria are vapor/liquid partitioning, chloride salt properties, oil/water distribution in OVHD and desalter, volatility, toxicity and cost/performance.

Question 69: What is your experience with using thermal scans or other methods to monitor tube wall temperature furnaces?

This is a common practice in furnaces that have severe operation (VBU, Coker, etc.). All VBU’s have multiple fixed TMT measurements on all coils. Thermal imaging is used in furnaces that have specific TMT issues, it does require expertise and specialized thermal imaging cameras in order to get useful data. Thermal imaging will not work on tubes that have external fouling.

Question 68: What is your experience with toxicity issues at the Water Treatment Plant as related to naphthenic acid content in desalter brine water? What strategies are you employing employed to mitigate this issue?

The top causes of fouling with tight oil are asphaltene, destabilization on blending with asphaltene bearing crudes and also the potential for thermal production of foulant material.