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Apr-2015

Increasing distillate production at least capital cost

Investing in improvements to the atmospheric crude unit can deliver increased distillate yields with short payback times

Joe Musumeci, Steven W Stupin, Brandon Olson and Carlos Wendler
Ascent Engineering
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Article Summary
This is the second of two articles which discuss changes to the crude unit to increase distillate production from each barrel of crude processed. The first article reviewed operational adjustments that require zero capital investment. This second article focuses on atmospheric crude unit improvements that require minimal as well as major capital investment with short payback to further increase distillate yield. Improvements to the vacuum unit, which are not discussed here and will be covered in a future article, can also offer tremendous gains to the refiner’s distillate production.

Historically, most US refineries have been designed and operated to maximise gasoline production as a response to the needs of transportation that have led the consumer market. However, shifts in worldwide fuel consumption patterns have caused a decrease in the demand for gasoline and an increase in the demand for diesel fuel. This trend, discussed in our first article (PTQ Revamps 2014), has led to a current diesel price differential of about $0.23/gal over gasoline and is expected to increase to over $0.70/gal by 2030 if the trends continue. Contributing factors to this changing consumption pattern include increased demand in developing countries, a focus on reducing greenhouse gas emissions that has led to more stringent automotive fuel efficiency standards, and increased blending of renewable fuels. Adjusting to these market conditions can allow refiners to maximise refinery profitability by producing higher yields of the most valuable distillate products.

Opportunities for increasing distillate yield range from operational tweaks that require no capital, as discussed in the first article, to simple modifications that can be potentially implemented without a unit shutdown, to major capital projects. Operational tweaks can be identified by test run most successfully with the help of process simulation, but are often limited by the capabilities of the existing equipment. A refiner who is willing to invest in capital projects may benefit the most from a well thought out design plan which will yield the greatest return on investment and maximum profitability.

A key to implementing any plant design plan is to have accurate simulation and design. A simulation tuned to actual plant data is often useful to benchmark the current operation and can be used to compare alternative concepts for optimisation of the proposed upgrades. The alternative cases should focus on variables which have the greatest impact on distillate yield with effort to minimise capital cost often by maximising the reuse of existing equipment. Refinery and unit product specifications should be reviewed to ensure they are current and their purpose understood. This process may require challenging some of the unit’s current target specifications by identifying unwarranted limits on unit operations and possibly resulting in unnecessary over-
processing and increased operating costs. With these specific areas in mind, concepts can be assessed to narrow down options to those with the greatest financial incentives. Simulation and design experience, such as that provided by a skilled process design firm, is critical if the refiner wants to identify new ideas and the best projects to maximise profitability.

Atmospheric tower modifications to increase distillate production
Maximising the capability of the atmospheric tower is often the primary focus when considering crude unit modifications. Improving atmospheric tower operation usually requires the least amount of capital for a given increase in diesel yield. The following sections focus on atmospheric tower modifications that go beyond simple operational adjustments and will require some capital investment in order to improve distillate recovery.

Stripping steam in the atmospheric tower and side strippers
Increased stripping steam vaporises additional distillate range material from the crude; this requires adequate tower capacity and heat removal capabilities to condense and recover the additional distillate above the flash zone. The first article of this series included a case study demonstrating how, in one refinery, increasing the stripping steam rate (and the tower operating pressure, to offset the increase in percent of flood) significantly increased diesel yield and refinery profits. Other refineries may have similar equipment or hydraulic constraints which limit the ability to increase stripping steam rates as a means to improve distillate product recovery. The extent of the equipment constraints could range from flooding in one or more tower sections or in a side stripper to limited condensing capacity or water dew point limitations in the overhead system. Specific modifications are required in order to tackle specific atmospheric tower system limitations which may exist when trying to maximise stripping steam.

Even though stripping steam rates represent a small portion of the tower’s vapour loadings, flooding in one or more atmospheric tower sections or in a side stripper can be the first limit encountered when increasing stripping steam flow. In this case, a designer may consider installing higher capacity trays or replacing trays with packing. Increasing tray spacing is another option for greater debottlenecking. However, the reduction in theoretical stages that can result from this type of modification should be evaluated to ensure that the fractionation remains acceptable.

A thorough review of all auxiliary equipment is necessary to ensure that the limits of the other equipment have not been exceeded. If increased tray spacing or high capacity trays are insufficient to relieve the tower flooding, more extensive modifications may be required. The tower or side stripper diameter may need to be replaced with a new, larger tower section. The additional capacity that results from these modifications will enable the refiner to take advantage of increased stripping stream rates and subsequent increased distillate recovery.

Another common obstacle that can prevent or limit an increase in stripping steam rate is the inability to remove additional energy from the atmospheric tower. Heat removal can be limited by insufficient capacity in the overhead condensing system and increased pumparound duty. Options for increasing overhead condensing capacity include adding exchanger surface area with new exchanger bundles, reconfiguring the existing exchanger train, and adding a new exchanger or air cooler. A designer can evaluate whether heat is currently being rejected to water or air that can instead be recovered into the crude preheat train. Modifications to the tower pumparound circuit(s) should also be considered. Modifications can include changes to the pumps, piping, control systems, and exchangers with a review of the tower’s pumparound section trays or packing. Lower cost options include new impellers, piping jump overs, and exchanger bundles. Larger investment might include new heat exchangers and air coolers along with a major reconfiguration of the crude preheat train.

An accurate simulation model is paramount for the optimisation of the complex interactions between the tower pumparounds and the preheat train in order to maximise energy recovery and to minimise fractionation losses. If an accurate plant-matched simulation model does not already exist for the full crude unit, consider commissioning an experienced engineering firm to assist in developing this powerful and valuable tool.

Reduce atmospheric tower operating pressure
Like stripping steam, reducing the operating pressure of the atmospheric tower increases distillate recovery by reducing hydrocarbon partial pressure and increasing vapour rate, thereby allowing lighter range material to remain in the vapour phase at a given feed temperature. This helps to ‘lift’ heavier materials like distillate up the tower to increase recovery, but will increase vapour loadings because of the additional volumetric flow at lower density and therefore more volume which must be evaluated.

Lowering the pressure also has similar constraints to tower capacity and heat removal as in the previous stripping steam discussion. Lower pressures result in lower condensing temperatures, which lower the temperature driving force for heat transfer in pumparounds and the overhead condenser. Having an accurate simulation of the system will help identify and quantify effects such as this. Aside from cooling capacity, hydraulic limitations can also bottleneck a tower overhead system. The lower cost solution for fixing hydraulic problems can include installing low pressure drop valves, new exchanger bundles, and adding a parallel piping line. Larger investments might include new exchanger systems, and new nozzles and piping. Often, the source of the overhead capacity limit can also be the tower off-gas compressor which should be evaluated.
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