Lube vacuum column revamp

The basis of a lube vacuum column revamp and its results are discussed. The primary goals were to increase lube product fractionation, minimise HVGO product

contaminants and improve vacuum bottoms specifications for asphalt production

The lube column at Marathon Petroleum Companys (MPC) Cattlesburg, Kentucky, USA, refinery was revamped in 2006 (Figure 1) to improve lube product fractionation, reduce the heavy vacuum gas oil (HVGO) products micro-carbon residue (MCR) and metals, and improve the vacuum bottoms specifications for the production of asphalt. Prior to the revamp, the HVGO product was black and cylinder stock yield was excessive. Cylinder stock was fed to another vacuum unit so that the lube column bottom product could meet the asphalt product specifications, as well as recover a portion of the HVGO boiling range material in the cylinder stock. However, reprocessing this stream consumed some of the other units capacity and increased its heater firing. All the projects justification benefits were met following startup. Moreover, a lower column operating pressure and improved stripping efficiency led to a 4.0 Mbpd higher crude charge rate due to the lower heaters cracked gas production freeing up some compressor capacity to process more crude.

Process flow schemeThe previously mentioned Figure 1 shows the simplified process flow scheme for the atmospheric and vacuum column prior to the revamp. Off-gas from the atmospheric crude and lube vacuum columns was handled with a common compressor. Atmospheric column heavy gas oil (HGO) product was routed to the vacuum column to recover some of the lube-quality material. The vacuum column produced light vacuum gas oil (LVGO), side stream (SS) #1, side stream (SS) #2, HVGO, cylinder stock and asphalt products. It was necessary to yield cylinder stock to meet the asphalt specifications on the bottom product. LVGO and HVGO were routed to cat feed hydrotreating. SS#1 and SS#2 were lube-quality base stocks targeted for further processing.

The vacuum unit is a wet design with coil and residue stripping steam, plus a precondenser prior to the first-stage ejector. The top column operating pressure on a wet column is set by the ejector system load, which changes with the seasonal cooling water temperature. During winter, the first-stage ejector load is lower due to the reduced cooling water temperature; hence, the columns top pressure is as low as 35 mmHg. This pressure increases to 55 mmHg in the summer when the cooling water temperature is higher. The vacuum column internals consisted of three packed beds and 12 trays to remove heat and fractionate the feed into five side-

cut products (Figure 2). The tray pressure drop was approximately 3.5 mmHg per tray, with the packing contributing only a small amount, which resulted in a flash zone pressure of 95105 mmHg, depending on the ambient temperature. The HVGO product was black from entrained vacuum residue. Furthermore, the trays were prone to damage and leaks, resulting in poor fractionation, excessive cylinder stock yield and difficulty in meeting asphalt specifications.

Typical unit charge was a blend of Middle Eastern crudes and was limited by off-gas compressor capacity. During the summer, the atmospheric columns

Kevin Basham Marathon Petroleum Company LLCEdward Hartman Process Consulting Services Inc

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Vacuum bottoms

Preflashcrude

LVGO

HGOproduct

Off-gas

Crude off-gasEjector off-gas

HVGOCylinder stock

Strippingsteam

SS #1

Sour water

Oil

SS #2

Steam

Ejectorsystem

Fuelgas

Atmosphericcrude

Lube vacuum

Figure 1 Lube vacuum unit

overhead receiver temperature increased, producing more off-gas. At the same time, the vacuum ejectors off-gas rate was higher because the heaters outlet temperature was at maximum. The crude rate had to be reduced once the off-gas compressor reached its maximum capacity.

In 2005, a study was completed to determine the cost/benefit of revamping the unit. Major economic incentives were improving the lube products fractionation, minimising the HVGO product MCR and metals contaminants, producing on-specification asphalt and improving internals reliability.

Lube products fractionationFractionation between lube products depends on reflux and the number of theoretical stages. Increasing either one improves fractionation. This requires high-efficiency mass-transfer internals and minimum column flash zone pressure. Prior to the revamp, the column pressure drop was high, stripping efficiency was low, and fractionation between lube cuts and HVGO was poor. Before the mid-1980s, most lube vacuum columns were designed with bubblecap trays and occasionally valve trays. Yet, trays have the disadvantages of high pressure

drop and inherently poor efficiency in lube column service. In 1984, the first large-diameter lube vacuum column was revamped from bubblecap trays to structured packing, with several others modified since then. Some were successful, while others were not due to poor-quality liquid distributors.

Trays have inherently low efficiency in lube columns because the liquid rate is low and the tray weir length is large (because of large diameters). Conversely, structured packings inherent efficiency is good at a low liquid rate, assuming a high-quality liquid distributor is used. A structured packing beds efficiency is largely controlled by the liquid distributors performance. Since column diameters are large and liquid rates are generally low (~1 gpm/ft2 of column area), distributing the liquid uniformly is a challenge.

MPCs lube vacuum column flash zone pressure was high because it had 12 trays above the flash zone. Trays produce a high pressure drop per theoretical stage because each tray generates about 3.5 mmHg pressure drop and three trays are needed to achieve a theoretical stage. Thus, each theoretical stage creates approximately 10 mmHg pressure drop. Conversely, structured packing produces only 1.5 mmHg per theoretical stage.

Minimising HVGO product contaminantsHVGO product contaminants consist of volatile MCR and metals in the product boiling range as well as entrained vacuum residue containing high amounts of MCR and metals. There will always be some volatile contaminants present, irrespective of the column design, but the amount of volatile contaminants depends on the efficiency of the wash zone and residue stripping. Surprisingly, many vacuum columns produce black HVGO product because of poorly designed column flash zone, wash zone and stripping section internals. In this case study, because the unit processed low metal crudes and operated at low cutpoints, eliminating entrainment would reduce the HVGO products metals and MCR to low levels.

Vacuum residue entrainment in the flash zone depends on transfer line critical-flow expansions, the flash zone vapour horn and wash section internals. Poorly designed transfer lines with high pressure drop critical-flow expansions at the column inlet nozzle generate fine mists that are difficult to remove. Improperly designed flash zone vapour horns do not separate the feed stream vapour and liquid or properly distribute the vapour to the wash section. Poor vapour distribution results in high localised vapour velocity in parts of the

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HVGO

Cylinder stock

Vacuum bottoms

SS #1

Ejector

HGO

Feed

Strippingsteam

LVGOproduct

SS #2

CW

Figure 2 Lube column before revamp

Vacuum bottoms

Very lowstrip-out

Higher heateroutlettemperature

Strippingsteam

Shedtrays

S-1S-2

S-3S-4

Figure 3 Residue stripping section before revamp

wash section, causing the HVGO product to be black from entrainment. Yet, entrainment can be nearly eliminated through prudent column internal designs, as long as the column wash section capacity factor does not exceed 0.40.45 ft/sec.

Residue strippingThe efficiency of the vacuum column stripping section influences the HVGO yield, asphalt quality and HVGO products metals. Yet, it is often overlooked as an important design variable. Maximising stripping efficiency will raise the HVGO products TBP cutpoint by 2060F, reduce metals in the HVGO product by 50% or more and allow for the production of higher-valued asphalt grades from the same crude. Greater efficiency also reduces the vacuum heater outlet temperature by up to 30F in some instances. Moreover, residue stripping vapourises a lower boiling range hydrocarbon than the heater produces for the same amount of vapourisation. Therefore, it reduces the HVGO products MCR and nickel and vanadium content.

Residue stripping uses steam to reduce the oil partial pressure on the trays. Even a well-designed tray has only 25% efficiency, with a typical tray achieving just 510% efficiency. Since the stripping trays vapourise lower boiling range material than the heater, the metals content is lower than the same volume of material produced in the heater. In addition, improved efficiency vapourises more flash zone liquid, allowing the heater outlet temperature to be reduced. Since this unit produces asphalt, it is the asphalt specifications that determine the HVGO products cutpoint rather than a specific target. Another benefit is that the asphalt product viscosity is easier to meet because the light material that must be removed to meet the viscosity specifications is stripped out. Before the revamp, the stripping section was designed with four baffle trays, so it had very little efficiency, which resulted in a low strip-out and made it difficult to meet the asphalt specifications (Figure 3).

Revamp process flow schemeThe revamp flow scheme is shown in Figure 4. All the trays above the flash zone were replaced with packing, and other minor modifications were made to the existing packed beds, plus a much more efficient stripping section was installed. New packed column internals reduced the flash zone pressure by about 30 mmHg. A lower flash zone pressure allowed for a lower heater outlet temperature, which reduced cracked gas production (Figure 5). Improved stripping efficiency further reduced the

heater outlet temperature, lowering cracked gas production again.

The crude/vacuum unit capacity is constrained by the off-gas compressors capacity. By reducing the amount of ejector off-gas, more atmospheric column overhead receiver off-gas can be handled, allowing for an increased crude charge rate (Figure 6).

Column designPrior to the revamp, the column was designed with a combination of grid, random packing, valve and bubble cap trays, and baffle trays. The new internals would: Reduce the column pressure drop Provide the required fractionation for lube oil production

Vacuum bottoms

Preflashcrude

LVGO

HGOproduct

Off-gas

Crude off-gasEjector off-gas

HVGO

Cylinder stock

Strippingsteam

SS #1

Sour water

Oil

SS #2

Steam

Ejectorsystem

Fuelgas

Atmosphericcrude

Lube vacuum

Figure 4 Revamp process flow scheme

Figure 5 Flash zone temperatures and pressures

Vacuum bottoms

Flash zoneconditions

Increasedvacuumdistillate

Lowerheater

crackedgas

730 710

105 75

Feed

Temperature, FPressure, mmHg

Before revamp After revamp

Strippingsteam

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Improve HVGO quality by eliminating vacuum residue entrainment Maximise stripping section efficiency Provide adequate wash oil to keep the wash bed wetted and prevent coking while minimising cylinder stock yield Improve internals reliability. The existing 12 valve and bubblecap trays were replaced with structured packing beds and associated internals. A new vapour horn was installed to minimise entrainment and improve vapour distribution to the wash section. The four existing baffle trays used for

stripping were replaced with five sieve trays.

The fractionation section used structured packing and high-quality narrow trough liquid distributors to maximise efficiency within the available height. The wash section was installed in an internal shroud to allow for maximum lube product fractionation. Existing tray support rings were reused to reduce the installation time. The new internals have improved mechanical design to maintain integrity during normal operating conditions and upsets (Figure 7).

Stripping section efficiency was dramatically improved by changing from baffle to sieve trays, increasing the number of trays from four to five, designing each tray with an optimised hole area for higher tray efficiency and installing a new collector to feed the top tray (Figure 8).

Side-strippersThe side-strippers tray design was modified to increase tray efficiency, allowing for more strip-out of the product distillation front-end. The changes were low cost, with the tray active panels replaced with an optimised open area and a combination of light and heavy valves.

Revamp resultsThe primary goals were to improve lube product fractionation, minimise HVGO product contaminants, meet vacuum bottoms properties for asphalt production and improve internals mechanical reliability. All objectives were met. The HVGO product MCR and metals have been reduced. The wax colour from SS #2 is now clear because colour bodies from residue entrainment have been eliminated. In addition, the crude charge rate has been increased by 4.0 Mbpd as a result of reducing the vacuum heaters cracked gas production.

The authors would like to thank all who participated in the successful completion of this revamp.

Kevin Basham is a technical services co-ordinator at the Marathon Petroleum Company LLC Cattlesburg, Kentucky, USA, refinery. He has 15 years experience in the optimisation and troubleshooting of crude distillation units. Edward Hartman is a process engineer with Process Consulting Services Inc in Houston, Texas, USA. His primary responsibilities include process design packages (PDPs) and equipment design for refinery units.

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Off-gas totreating

Crude off-gas(higher rate)

Vacuum off-gas(reduced rate)

Off-gascompressor

Figure 6 Off-gas compressor

HVGOWash oil

Cylinder stock

Vacuum bottoms

SS #1

Ejector

HGO

Feed

Strippingsteam

LVGOproduct

SS #2

CW

Figure 7 Lube column after revamp

Vacuum bottoms

Higherstrip-out

Reducedheater outlettemperature

Strippingsteam

S-2S-1

S-3S-4S-5

Figure 8 Stripping section after revamp