Atmospheric Distillation and Vacuum Distillation Units Thermal Coupling | Energy Efficiency


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Heat Integrated CDU/VDU Processes 

As have been mentioned before the thermal coupling or sometimes called inter-process integration between two processes or among several processes is beneficial from the total energy consumption point of view, especially if properly addressed. In such case, the benchmark for minimum energy consumption of both units, ADU and VDU, will be less than standalone benchmarks of each one of the two units.

Atmospheric Distillation and Vacuum Distillation Units Thermal Coupling

ADU/VDU Problem Data

The figures below show simple and straight forward application of HEN thermal coupling of ADU/VDU. The heat exchangers network (HEN) shown is using both the crude distillation unit product streams, pumparound streams and the vacuum unit products streams to heat up the crude feed to the atmospheric distillation unit furnace.

This practice if properly done should reduce the total atmospheric unit furnace duty and vacuum unit furnace duty besides the total load on water and air cooling systems. Otherwise, such integration is useless and will only complicate the operation of the two units and the energy benefits could be at the expense of operating flexibility.
If we can do the same savings without the thermal coupling via proper in-process modification in each individual process then we should not resort to the thermal coupling, unless the saving is really significant.

Anyway, in the application example presented here we assume and it is true that the thermal coupling in the case of the atmospheric distillation unit and vacuum distillation unit is acceptable by the plant operators and it renders a real value in energy saving.
What we are briefly showing in the application example here that a very simple modification in the HEN design especially just before the atmospheric distillation furnace can bring sensible saving in the furnace duty by about 5% and hence reduction in the GHG emission.

Coupled Crude Atmospheric-Vacuum Units Preheat Train (option I)

Coupled Crude Atmospheric-Vacuum Units Preheat Train (Option II)

Coupled Crude Atmospheric-Vacuum Units Preheat Train (Option II)

ADU/VDU Pinch Analysis Example:

The cold streams to be hated and hot streams to be cooled for plant 15 used in this study are listed in the below table:

It is instructive to note here that the reboiler stream for the Vacuum Column re-boiler was not considered in this pinch study since adding it will be above the pinch and it will just increase the hot utility requirement while keeping pinch point and cold utility constant. Hence, it will continue working on utility.

The global ΔTmin used for the system is found by drawing the hot and cold composite curves and moving it until the existing hot utility (383MMbtu/hr) and cold utility (212MMBtu/hr) are obtained; such ΔTmin is 86°F:

Composite Curves (Existing)

Composite Curves (Existing)

Grand Composite Curve (Existing)

Grand Composite Curve (Existing)

Another drawing was done to find hot and cold utilities requirement upon using ΔTmin equal to 30°F. The flowing table and graph shows the result:

Composite Curve s at ΔTmin =30°F

Composite Curve s at ΔTmin =30°F

Grand Composite Curve at ΔTmin =30°F

Grand Composite Curve at ΔTmin =30°F

Pinch analysis shows that a total of 133 MMbtu/hr can be saved by modifying the exciting HXN. Also, from the grand composite curve, shows there is no possibly to vary the utility and but there is possibility to vary the cold utility.

The Vacuum bottom product stream (C-200) is coming out at 672°F and exchanges heat with crude feed in E-291 then, E-292 to reach 489°F. The product split into two streams: stream (1) return back as Quench to the bottom of the column and stream (2) is cooled down to 355°F using 60 psig steam generator E-293 before it goes to the Visbreaker. The following graph and table are the base case:

The energy efficiency optimization initiative is to use the vacuum bottom product stream to heat up the crude oil stream before going to the furnace F-301. Putting this new heat exchanger on the heat-load path (E-293-cold utility, E-292, E-291, NHX, F-301-hot utility) which allow some load transfer from E-293 to the NHX and hence reducing F-301 load by the same amount of the load.

To implement this initiative, another new heat exchanger unit must be installed in parallel with E-292 to keep the Quench temperature constant by using the Visbreaker feed stream (E-293 stream). This modification will result in a change in the Vacuum bottom stream temperature profile into E-291 and E-292.

This change will result in modification in E-291surface area (increased the area or/ and use tubes insert) and evaluate E-292 (with the NHX in parallel). This idea can be implement at one or two phases:

At the first phase, 25.6 MMBtu/hr can be send by installing two new heat exchangers. The figure blow shows the details of this first stage modification. The red unites with the dotted lines are the new units and the units with outside circle will be the one which need area modification in the future.

Reduce load more on the Furnace F-301 via increasing the waste heat recovery in the new unit using tube inserts. At that stage, E-293 can be cancel and this will not disturb VB operation but it will enhance the operation due to new high temperature of VB Feed. The following initiative diagram shows the modification and the associated load of the heat exchangers:


The Bottom Pumparound stream- BPAS of C-200 is coming out of the crude column at 581°F and exchange heat first with the crude feed in E-145 and second with debutanizer re-boiler E-305 before it generate M.P.S in E-146. The following diagram is the base case:

This energy efficiency optimization initiative use Bottom Pumparound stream to heat up the crude oil before going to the furnace F-301. Adding this new heat exchanger on the heat-load path (E-146-cold utility, E-305, E-145, NHX, F-301-hot utility) can transfer heat from E-146 to the NHX and reduce the F-301 load. This will mandate a modification in both E-145 and E-305 (increase the surface area or use tubes insert). This initiative also can be implemented in one or two phases.

At the first phase, 16.6 MMBtu/hr can be saved in the crude furnace by installing a new heat exchanger between the BPAS and the Furnace feed. This modification will reduce the duty on E-146 and the steam generator can be bypassed. The following are the base case diagram and the data table for the modification:

Moving the E-146 to now location shown in the diagram and using it as 2nd re-boiler for the debutanizer (Using M.P.S), this will result a new energy path (E-146-new reboiler, E-305, E-145, NHX, F-301). This can save extra 10 MMbtu/hr in crude furnace via increasing the load on the NHX (tube insert). The following are initiative diagram and result:

Installing a new heat exchanger between bottom pumparound stream and crude oil to F-301 can save up to 26.6 MMBtu/hr. Hence, this reduction is estimated as $ 2.3 MM/yr. the two phase modification can result in a present value equal to $39.1 MM. the second phase making E-146 as re-boiler will support the final initiative in ADU/VDU.
This initiative is considered as a next phase to the 2nd ADU/VDU initiative. In this initiative, both pump around stream (after 2nd initiative modification) and Sid Cut8 stream will be used. The Side Cut 8- SC8 stream of C-200 is currently coming out at 628°F and exchange heat with crude feed in E-281 and then E-283 to reach 489°F. The following is the base case:

In initiative SC8 stream is used to heat up the crude oil before going to the furnace F-301. Adding this new heat exchanger will save the 45 MMBtu/hr shown in graph. To get the maximum benefit from the modification, the total load for the E-305 will be transferred to the NHX between the SC8 and furnace feed. To apply the initiative, the C4 re-boiler will be changed totally to M.P.S and the E-145 & E-281 need to be modified (increase the surface area or use tubes insert) due to the reduction in the temperature driving force.

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