How to calculate vacuum heater residence time? What is the limit to avoid coking?
Answers
|
19/01/2018
|
A:
|
Ashutosh Garg, Furnace Improvements Inc, agarg@heatflux.com
Vacuum heater residence time can be calculated in a heater simulation program which gives the fluid temperature, pressure and vaporization profile for every tube.
There is no direct relationship in coking and residence time. Lower residence time means higher fluid mass, velocity and hence lower coking rates.
|
|
17/01/2018
|
A:
|
Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Calculating residence time would be very difficult due the rapid vaporization that is happening in the last few readiant tubes. Cracking/Coking is a function of the oil being processed, the temperature and the residence time at temperature, so limiting coking by merely setting a maximum residence time may not won't help much.
When cracking/coking is starting to be significant, you will see a rapid increase in non condensibles when you increase furnace outlet temperature. The rate of coking in the furnace can be monitored through the tube skin temperatures. If the furnace operation (feed rate, tower pressure, and temperature) is stable this is quite easy. If operation is variable you may need to normalize the temperatures to account for operational factors on skin temperatures. As cracking/coking becomes worse the slope of the skin temperature vs. time trend line will get steeper.
|
|
17/01/2018
|
A:
|
Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
As you indicated, these are separate questions. To make the calculation, computer models are set up to make calculations on a series of short sections of the tube coil. In these calculations, consideration is given to the changing physical properties of the oil and steam, and the pressure drop in each section. Even the effect of the undesirable cracking is accounted for in the better models. This calculation can also be made by hand, as it was before computers were available to the industry.
As to the target residence time, that depends on the crude source, as some crudes crack easier than others, requiring unique maximum outlet temperatures as well as unique coil residence times.
|