General Tips for Energy Saving in Crude Distillation | Energy Conservation
Let us start first with the distillation column and then we move to the feed pre-heat train.
In crude tower, as with the case for any other distillation column, the general design principles established for many years recommend the use of more trays to reduce the required reflux, hence the load on both the reboiler and the condenser. Tower pressure drop can be minimized to avoid high flash zone temperatures and/or bottoms stripping steam rates. Besides, more stripping trays can be used to minimize stripping steam requirements.
Naturally, these items will need more capital but a life cycle economic analysis taking into consideration the escalation in energy process may render legitimacy to such solutions. In case of an existing crude tower, it may be possible to use more efficient trays or modify existing ones. These techniques are applicable to both atmospheric crude tower and vacuum tower, too.
Another concept that has been established and found attractive is operating the vacuum tower at low pressure. Low pressure operation in vacuum tower is attained by placing the vacuum jets directly on the tower overhead upstream of the main condensers. Since this method of operation can reduce the tower overhead pressure from approximately 60 mm Hg to about 25 mm Hg, only small amounts of steam are required in the tower to obtain the required hydrocarbon vapor pressure. Since the jets consume less steam than is backed out of the tower, there is a net savings in steam consumption. The application of vacuum pump instead of using steam ejectors might be attractive if the plant is using cogeneration and produces its own power and exporting the surplus as is the case in many new refineries.
The tips discussed above are addressing the situation where it is desired to save energy consumption though equipment changes. If all the products are exactly on specification, no quality giveaway, there is tinny way to reduce energy consumption through non-design modifications. However, if the products that are being produced are better than necessary, the unit performance can be optimized to save energy utility.
Operations optimization and stable operations programs mean adjusting product yields, pumparounds duties, stripping steam and feed temperature such that all desired product yields and specifications are attained but not exceeded in face of disturbances and crude switches, if any.
In the crude atmospheric distillation tower sometimes it is desirable to minimize over flash if the lower side-stream back end fractionation is not critical. Since there are few trays in the wash section/ flash zone excess over flash is ineffective for back end fractionation. For an atmospheric tower going from 3%LV over flash to 5%, a 12°F reduction in ASTM temperature is achieved. An additional 4% over flash only results in an additional 8°F approximately.
Another possible area for potential energy saving is the side stream steam stripping. The incremental effectiveness of steam decreases as the steam rate increase. The degree of improvement in separation with steam flow rate depends on the product cut width (boiling range), and where the product is withdrawn from the tower, the narrower the cut width the less the improvement. Also, for typical crude units, stripping steam appears to have its most significant effect in the intermediate and lower parts of the tower.
It is important to note here that the “elephant” in saving potential in crude distillation unit is attained via the proper integration between the feed and side products and the pumparounds and the wider integration between the atmospheric unit and the vacuum unit as well as the integration among these two units and the rest of the refinery units directly or indirectly.
These facts are always easy said than done due to the complexity of the oil refinery.
The crude unit is in itself is very complex and there are many interactions involved.
For instance the graph shown below represents the top part of an atmospheric distillation unit. There are three products here, the overhead product and the two side-streams stripped products. Assume that the yields of the three products are fixed. Also, assume that the distillate product has some back end distillation specification.
This specification can only be met by a unique top reflux rate. This reflux rate is achieved by letting more or less heat pass through the pumparound. Remember that stripping only improves the front end of the stream. Stripping has little effect on the back end and quality of an adjacent higher product stream. Also, remember that the adjustment of the duties only affects the fractionation above the pumparound that is changed, not below it.
As a direct consequence of the top reflux rate and the fixed side stream #1 rate, the reflux rate to the section between the side stream #1 and the side stream #2 is also fixed (mass balance). However, this reflux rate may not be suitable to effect the desired fractionation to meet a specification on the back end of the first side stream.
Hence, it would be necessary to let more heat ride through the pumparound to generate more top reflux and as a sequence more side stream #1 reflux. Now, the distillate product would be better than required. If a decrease in then side stream #1 yield was allowed, it would be possible to meet the side stream #1 specification without increasing the top reflux flow rate. It should be noted that increasing the stripping steam rate to the side stream #2, stripper would not significantly improves the side stream #1 back end.