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11/06/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
“At a constant percent overflash, by adjusting the pumparound and overhead condenser duties up and down the column, the cut in question can be changed at the expense of one or more cuts up and down the column.”
Sorry. I contradicted myself to an extent with that statement. At a constant % overflash, the internal reflux in the lower section is set, and, at a constant stripping steam rate, the degree of fractionation is set (overlap of distillation curves). Then, if you increase furnace outlet temperature, or reduce pressure if there is such an opportunity, you are either increasing the % overflash or shifting vacuum gas oil to the lower side draw of the atmospheric column. And, as I said, if the destinations are the same, the only advantage would be to unload the vacuum column somewhat. If you don’t change the overflash at the higher temperature, the overall utilities are about the same because that incremental material does not need to be “lifted” in the vacuum column.
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11/06/2010
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S Banik, Centre for High Technology, sbdr@rediffmail.com
The answers posted give a good direction to the solution of the problem stated. We faced similar situation in a crude distillation unit which had valve trays in the wash and bottom stripping sections.With increase in stripping steam extra gas oil could be lifted but it was getting coloured. Subsequently we replaced valve trays in both the sections with structured packing. In the process we increased no of theoretical trays in the wash and stripping sections. We could lift additional 2-3% extra gas oil on crude basis with same operating conditions. We could also lift additional gas oil by increasing stripping steam as entrainment from flash zone could be controlled.
In the present case it is important to have process details and the hardware details also for suggesting suitable solution.
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09/06/2010
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keith bowers, B and B Consulting, kebowers47@gmail.com
Mr Ragsdale's answer covers the entire subject quite thoroughly. He importantly notes it may be uneconomic to 'sharpen' the cuts on the crude unit if these streams are later processed by hydrotreating units having product fractionation sections. The incremental operating cost in those downstream units will be far less than trying to get the last bit of fractionation possible on the crude tower.
Refinery optimization is not a simple business. One must understand the physical constraints, the marginal operating costs, the marginal product (or intermediate stream) values and the linkages between them. This is NOT a trivial exercise and tweaking the crude unit without considering the impact on total products value and costs of alternative processing to achieve similar results will almost always result in a 'solution' that is far from optimal and may even have NEGATIVE 'benefits.' This is where a good PIMS LP model with recursion of process stream properties, and using process unit yield/cost models in the Excel Data Tables can uncover otherwise 'hidden' opportunities.
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09/06/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
This is a good question because many engineers ignore the big picture. I assume one or both of the following: Either the lowest side draw and the vacuum gas oil do not have the same destination, or you simply need to reduce the load on the vacuum unit. If neither of those is true, there is no need to “sharpen” the cut between them. If sharpening the cut is advantageous, the big picture needs to be considered. In a modern refinery, most streams from the crude unit are hydrotreated, and the fractionation downstream of those reactor sections provide another opportunity to sharpen the cuts between ultimate products, with negligible affects on unit capacities and utilities consumptions. On that basis, it is difficult to justify any sharpening of cuts at the crude unit, either before it is designed or after it is built.
After taking all of that into account, and it is desired to sharpen the cut in question, the keys are the stripping steam rate as you mentioned and the internal reflux rate in the lower section of the atmospheric column. That rate is called “overflash”, usually set at 5% of crude rate for design. More than that is considered wasted utilities, and less than 5% is difficult to control to. For a given cut point between the atmospheric and vacuum columns, the overflash is set by the atmospheric furnace outlet temperature. I have seen columns operating with 25% overflash, but it was uneconomical to do so. At a constant percent overflash, by adjusting the pumparound and overhead condenser duties up and down the column, the cut in question can be changed at the expense of one or more cuts up and down the column. A case study using simulation software can demonstrate the process described and compare the economics. One of the variables associated with the distribution of overhead and “side duties” is the tray loading in each fractionation section relative to its tray design capacity. When designing a new unit, that translates to optimum column diameter. Good luck!
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09/06/2010
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The yield of reduced crude is basically dictated by the conditions in the bottom of the fractionator. The bottom of the tower is basically a flash, not a traditional distillation. Stripping steam will only remove some incremental light ends.
To make a significant shift in yields you need to change the flash conditions by changing the temperature, pressure or both in the bottom of the fractionator. To lower the reduced crude yield you would need to either decrease tower pressure or increase furnace outlet temperature or a combination of the two. Typically there are few options to lower tower pressure much without spending some capital. Your most likely option is to increase furnace outlet temperature.
When you do either of these steps however you will be taking more heavy material up the column as well, so the end point on the cut above reduced crude will increase. If you can't tolerate the increased end point on that stream you are probably pretty much at your minimum reduced crude yield for that feed slate.
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09/06/2010
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Vishnu Ram A S, RIL, vishnu.selvaraj@ril.com
First of all the recovery of the crude column is reflected in the overflash of the column. Check for the overflash volume if there is facility.
- Usually this problem can be rectified with pressure drop across the column. Check whether your column is running with minimum required pump around flows and internal reflux flows. You can reduce the excess flows which create the pressure drop by decreasing the return temp. to the column of pump around.
- Column top pressure can be reduced if your condenser is having margin.
- Check for the color and distillation of the Heavy at. Gas oil and increase FBP components without affecting the quality.
- Sometimes if you only increase the stripping steam without altering any changes above the flash zone, you will land up in increase of RCO further as the flash zone pressure increases in this case. First create margin in the pressure drop of the column as suggested above and then increase the steam rate.
- Reply for further clarification if req. But with proper specifications, pl.
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