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Temporary Cooling Equipment Can Resolve Temporary Condensation Problems in Vacuum Distillation Units

By Massimo Capra at Aggreko. November 9, 2016

Introduction

Vacuum distillation plays a critical role in improving yields for petroleum refiners by producing high-value petroleum products out of the heavier oils left over from atmospheric distillation.  A vacuum distillation unit (VDU), or tower, consisting of vacuum distillation columns, operates under a vacuum or at a pressure significantly lower than atmospheric.  When the VDU is performing optimally, the low pressure reduces the boiling point of the heavy oil charge stock such that lighter products can vaporize without the risk of cracking or decomposing the feed.  However, when the VDU is not operating at optimal conditions, the vacuum pressure might rise, causing both yields and profitability to suffer.

Many of the causes of elevated VDU top pressure—warmer summer temperatures, temporary changes in operating conditions to capitalize on opportunity feedstocks, etc.—are short-term disturbances.  VDUs suffering from these temporary cooling challenges could therefore stand to benefit, in the form of more valuable distillate yields, from equipment that can help control the process operation temperature.

Temporary cooling solutions should be customized to an operator’s specific process conditions and needs.  After a thorough analysis of each VDU, including the duty to be performed, desired performance target, available cooling water temperature and flow rate, and the fouling tendency of the condensers, an optimal solution can be determined.  Equipment selection can be further optimized to comply with electrical power availability, lay-out restrictions, and plant accessibility to allow for proper emergency response.

Following a joint hazard and operability analysis, a pre-engineered package comprising different elements — chillers, heat exchangers, cooling towers — can be deployed in as little as a few days or weeks.  The rapid speed at which solutions are engineered, procured, commissioned and de-commissioned makes the financial returns from temporary cooling solutions attractive when temperature problems are temporary, or when the refinery wants to exploit specific, short-term market opportunities.

The VDU can be designed for a number of operational modes, from wet to dry, depending on the amount of steam injected into the unit.  Comprehensive rental equipment solutions such as those provided by Aggreko Process Services (APS) have been successfully deployed in various VDU operating modes, as the two case studies below demonstrate.

Case Study #1 - Dry VDU

In a dry operational mode, no steam is injected into the VDU and the unit can operate at pressures as low as 5 to 15 mm Hg abs.  In this mode, two ejectors are typically installed in series to directly remove non-condensable gas from the VDU overhead.

A refiner operating in this mode saw their top vacuum column pressure unexpectedly increase from 20 mmHg to 61 mmHg.  An investigation pointed to heavy fouling of the first and second condensers as the cause of the pressure increase, thanks to a higher than normal pressure drop on the tube side.

The higher pressure drop reduced cooling water flow rate, as indicated by the lower ΔT of the cooling water.  This in turn overloaded the second ejector stage, which resulted in a higher suction pressure.  Poor performance of the first-stage ejector also contributed to a substantial increase in tower operating pressure.

By chilling the cooling water to each condenser from 34°C to 31°C with a temporary solution consisting of chillers and various ancillaries, the refiner was able to recapture a higher HVGO yield of 1,850 BPSD.  The temporary cooling equipment, which was deployed in only eight working days, remained operational at the refiner’s site for 93 days.

Case Study #2 - Wet VDU

In a wet operating mode, a considerable amount of steam is injected into the VDU to act as stripping steam, or into the fired heater tubes to increase fluid velocity and prevent tube coking.  The majority of the steam injected in either of these locations is condensed and collected at the top of the column.

In this mode, the column operating top pressure is in the range 50 - 65 mmHg abs, which is relatively mild in comparison with other operating modes.  In fact, operating at this pressure makes condensation possible with plant cooling water.  The system used to produce the vacuum typically consists of two or three ejector stages.  In this arrangement, the condenser upstream of the first-stage ejector serves to remove as much hydrocarbon vapors and steam as possible.  The better the condensation, the easier it will be to maintain the desired vacuum pressure and distillates yield.

A VDU, operating in wet mode at a capacity of 25,955 BPSD, was simulated to estimate the theoretical distillates yield improvement that could be obtained by applying a temporary cooling solution at the pre-condenser.  It was theorized that condenser inlet cooling water temperatures obtained by this temporary equipment installation would improve condensation.

In the base case, it was assumed that a lower-than-designed cooling water flow rate caused the condenser to foul, resulting in a top condenser temperature of 54°C and a tower top pressure of 100 mmHg abs.  Assuming a constant feed rate, the top column pressure and temperature progressively decreased as the solution was applied.  By chilling the condensing water, the amount of heat removed by the top and bottom pump increased. This led to an increase in LVGO and HVGO quantity at approximately the same quality level.  The net effect was an increase in higher valued distillate material at the expense of lower value vacuum tower bottoms.

At the higher vacuum pressure of 100 mmHg abs, the total distillates yield (LVGO + HVGO) was 40.6%.  It was assumed that the temporary cooling solution would allow the vacuum tower pressure to decrease to 75 mmHg abs.  At this lower pressure, the distillates yield would increase to 47.6%.  At current tower feed rates, this would result in an increase of 1,794 BPSD in HVGO.  Assuming a $10/bbl marginal value difference between HVGO and tower bottoms material, the net increase in distillate production would yield approximately $1,600,000 during three months of summertime operation.

Conclusion

Engineered temporary cooling solutions, such as those provided by Aggreko, can add great value to operations profitability when seasonal and fouling conditions prevent the VDU from reaching the optimal condensation pressure and temperature, or when the refiner wants to capitalize on short-term opportunities.


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