July 2021

Process Optimization

Advances in light ends processing units using DWCs

For those who have been in the refining industry for many years, it is obvious that significant changes are occurring.

Martin, G. R., Sulzer GTC Technology

For those who have been in the refining industry for many years, it is obvious that significant changes are occurring. Refining is not going away, but refineries built for long-term existence will not be built like those of the past. Forces and needs are dictating changes in the refining business climate.

Dividing wall columns (DWCs) are a useful tool for helping meet these demands. Using DWCs in the design of complex light ends processing units can provide a lower-CAPEX design over that of a conventional gas plant. DWC technology has already proved its value through numerous applications. However, it must be used in the appropriate application to yield a successful outcome. This article addresses the use of DWCs to fractionate light hydrocarbon streams. While the applications shown are common for the refining industry, this technology also can be used in other types of processing plants where light hydrocarbon streams are processed.

DWCs reduce the amount of materials of construction and, in many applications, reduce the energy required to produce end products. In other applications DWCs may only reduce the materials of construction. The reduction in energy consumption has a continuous benefit, with DWCs often saving 10%–30% over conventional distillation configurations. This varies depending on the process, and comparisons should always be evaluated to justify the design selection.

In the past, CAPEX savings in materials of construction were the primary and often only interest, but changing business dynamics are forcing businesses to consider even nonprofit issues, such as carbon footprint. With the use of DWCs it is common for the materials of construction of the major processing equipment to be reduced by 30%. Not only is CAPEX reduced, but this reduction in materials of construction also results in energy savings and less impact on the environment.

Pressure on corporations to reduce fossil fuel consumption, reduce carbon emissions and make less of an impact on the environment have made these considerations a real factor in business decisions. In simple terms, this is the concept of doing more with less. DWCs not only give processing plants a lower-CAPEX means of providing the distillation tools to meet a plant’s processing objectives, but they also provide improvement on the environmental impact. These advancements make sense for the business world—i.e., they allow changes that improve business economics while simultaneously enabling operators to do more with less, thereby reducing their impact on the environment.

As previously noted, it is common to reduce construction materials of the major processing equipment by 30% when using a DWC design over a conventional one. If the volume of metal and other construction materials needed to build a plant is reduced, then the mining, shipping and processing of the metal ore could be reduced, which would reduce the shipping of this metal to fabrication shops, the fabrication of finished products and the shipping of the products, all of which could potentially lead to a savings of 30% in environmental impact. If all businesses could similarly reduce their environmental impact in building new distillation columns, buildings and other business-related assets, then this would have a significant environmental impact.

Light ends processing

Processing of light ends is commonly required in many different processing units. Common applications include:

  • Saturated gas plants (SGP)
  • FCC gas concentration plants
  • Coker gas plants
  • NGL gas plants
  • Flare gas recovery.

DWCs can be used in these applications, as well as others, to lower CAPEX and OPEX and to reduce the impact on the environment.

Saturated gas plants

A simplified process flow diagram (PFD) of a common conventional SGP configuration is shown in FIG. 1. Numerous deviations have been used in the flow scheme for SGPs over the years. Designs include complex configurations with refrigeration units; more moderate designs (such as that shown in FIG. 1); lower-cost, simple designs with low LPG recovery; or no SGP where the FCC gas concentration unit processes the saturated gas streams.

FIG. 1. Conventional SGP simplified PFD.

Within the design of SGPs such as that shown in FIG. 1, many variations have been used. Debutanizers have been installed upstream of the deethanizer, the primary absorber has been separated from the deethanizer, sponge absorbers have been omitted, intercoolers have been used and not used on the primary absorber, refrigeration systems to supply cold absorption oil have been included, and other deviations have resulted in variations of this basic flow scheme.

Regardless of which conventional PFD has been implemented, the conventional SGP design requires numerous columns to provide quality molecular management and component recoveries. In FIG. 1, the seven product streams (offgas, propane, isobutane, normal butane, isopentane, normal pentane and heavy naphtha) require seven to eight columns to fractionate the crude unit naphtha and lighter compounds into the streams shown in FIG. 1 for sale or further processing. One technologya used in SGPs significantly reduces CAPEX.In three vessels, the DWC technology can produce the same product streams as shown in FIG. 1.

FIG. 2 is one configuration of the proprietary technologya for SGPs. The sponge absorber, primary absorber, deethanizer and depropanizer are combined into a single vessel; the debutanizer and deisobutanizer are combined into a second vessel; and the depentanizer and deisopentanizer are combined into a third vessel. The conventional process flow configuration with seven vessels has been combined into three by utilizing DWC technology. In addition, the required number of condensers, reboilers and pumps is less. Fewer pieces of equipment are needed to construct, operate and maintain. This is the future of all technically advanced SGP designs.

FIG. 2. SGP utilizing proprietary LPGb and GWCc technologies.

The depentanizer and deisopentanizer have been combined into a single vessel producing isopentane (iC5), normal pentane (nC5) and heavy naphtha streams. This design works well when producing a high-purity nC5 stream feeding a C5 isomerization unit. If the isomerization unit is for C5/C6 isomerization, then a different configuration is used for the DWC. In this case, a four-cut DWC that produces iC5, nC5/C6, benzene and benzene precursor concentrate, and heavy naphtha streams is the optimum design configuration. High-purity streams are obtained with optimal control of benzene and benzene precursors.

Similarities exist in the design of DWCs to conventional FCCU, coking unit and SGP designs. The proper DWC design takes advantage of these similarities, which have been proven in numerous units operating for years. Using a stabilized naphtha stream as a source of lean oil enables improvement in propane recovery. It is more common to use debutanized naphtha as the lean oil, but naphtha that has been stabilized more thoroughly offers an even better lean oil for propane recovery. The composition of this lean oil stream, along with other operating conditions, determines the vapor/liquid equilibrium at the top of the absorber.

Using lean oil that has been only debutanized will produce enough gasoline-range components in the primary absorber offgas so that a sponge absorber is often used downstream to recover these components. However, when utilizing lean oil that has been more deeply stabilized, it is sometimes possible to eliminate the sponge absorber from the process. In this case, the gasoline components in the offgas are sufficiently low to allow for this design.

The gas compressor discharge cooler and receiver provide multiple functions. The heat of compression must be removed by this exchanger. The separation of the vapor and liquid phases occurs in the receiver before the two phases are fed to the primary absorber and the deethanizer. If fed as a single stream to a single column, the phases are more difficult to separate and, if not separated, could result in premature column flooding. Combining the deethanizer offgas and the primary absorber bottoms with the gas compressor section discharge provides another stage of absorption to improve propane recovery. The high-pressure receiver also provides the settling time necessary to remove water from the system that otherwise can lead to water entrapment issues. These and other similarities are utilized to provide an effective and reliable design.

The DWC design utilizes two advanced separation units.b,c These units are based on well-proven designs that optimize plant CAPEX and maximize product purity to achieve maximum recovery. In comparing FIGS. 1 and 2, seven conventional vessels are combined into three vessels, using DWC technology to provide significant savings in CAPEX and plot space. Not only are the number of vessels reduced, but also the number of foundations is reduced by having fewer columns. The amount of vessel metal is reduced significantly, as is foundation load and material cost. A significant reduction in column insulation, ladders, platforms, etc., is achieved. The number of condenser systems (exchangers, drum, pumps, controls) is reduced from five to two, and the number of reboiler systems is reduced from six to five.

The DWC SGP design can easily operate over the typical range of SGPs and is a lower-CAPEX solution. The simplistic design configuration helps achieve equal or better product recoveries and purities, as compared to any conventional system, but at lower CAPEX. This SGP design can be used without the addition of a high-cost refrigeration system to provide propane recovery of greater than 97%. Molecular management and light ends recovery will become more important than ever to enable shifts to petrochemical feedstock and higher-octane gasoline production. DWCs offer lower-CAPEX means of attaining these demands.

DWC internals design

Some in the industry have expressed reluctance to use DWC technology. However, the introduction of packing internals has been a topic of interest over the past century and, as with DWCs, there was a reluctance to use packing at that time. Great success has been experienced in using packing instead of trays. At the same time, some significant failures have been experienced by those who did not possess adequate design knowledge. DWC internals are no different. Hundreds of applications using DWC designs with success already exist. However, to create this success requires detailed knowledge in designing the process and the column internals.

While a DWC tray can be similar to a conventional tray, it also has some distinctive features that allow proper tray operation. FIG. 3 shows a shop setup of two-pass trays for a DWC that has since been installed. To some degree, the tray is no different than a standard tray design, but here each tray is segmented on each side of the dividing wall.

FIG. 3. Shop setup of trays for a DWC.

The basic tray operation is essentially no different than a conventional design. However, there are many enhancements in the design, especially in the transition areas. The proper splitting of vapor and liquid to the two sides of the wall, and how to achieve this, are also important. In addition, the shape of the tray leads to changes in operating parameters, such as weir loadings, which must be taken into consideration. While a DWC tray has similarities to a conventional tray, it also has differences that must be taken into account for proper tray operation.

For an equivalent selection of column internals, the design of a DWC tray can provide the same reliability and fouling resistance as a conventional tray design; there is no difference in a DWC from a conventional design. Both require someone who understands the process and how to properly design the column internals and incorporate reliability and fouling resistance into the design as dictated by each process. The selection of the type of column internal is as important with a DWC as it is with a conventional column. If structured packing does not work in a conventional gas plant depropanizer, then it should not be expected to be adequate for use in a DWC that is being used as a depropanizer. As with any technology, the limitations in use must be understood.

FCC gas plants

As is seen in the processing schemes for the gas plants shown, the gas plants can be designed in a similar fashion. The primary difference is driven by processing saturates or a combination of saturates and olefins, the value of the products, capital cost limitations and the overall plant design configuration. The compositional difference for the processing of light ends with olefins changes the downstream processing of the gas plant products and leads to variations in the gas plant design.

Consider the similarities and differences in design for an FCC gas plant that processes olefins compared to an SGP. A simplified PFD of a common, conventional FCC gas concentration plant configuration is shown in FIG. 4. Those familiar with FCC gas concentration plants know that numerous deviations in the PFD have been used over the years. Depropanizers have been installed upstream of the debutanizer, the primary absorber has been stacked on top of the deethanizer, depentanizers have been installed in front of the debutanizer, intercoolers have been used on the primary absorber, refrigeration systems to supply cold absorption oil have been used, and other deviations in the process flow scheme have resulted in variations of this basic flow scheme.

FIG. 4. Conventional FCCU gascon simplified PFD.

Regardless of which conventional process flow scheme has been used, the conventional designs require five to six columns to fractionate the FCCU naphtha and lighter compounds into the streams shown for sales or further processing. The FCCU gascons utilizing the DWC process significantly reduces CAPEX. The DWC process can produce the same product streams shown in the conventional processing scheme of FIG. 4, with the same or improved purity and recoveries in two vessels. FIG. 5 is one configuration of a proprietary technologya for FCC gas concentration plants. The sponge absorber, primary absorber, deethanizer and C3/C4 splitter are combined into a single vessel, and the debutanizer and naphtha splitter are also combined into a single vessel. The conventional process flow configuration with six columns has been combined into two vessels by utilizing DWC technology.

FIG. 5. FCCU gas concentration plant utilizing proprietary technologies.b,c

As with the SGP, similarities exist in the design of the DWC technology compared to a conventional FCCU gas concentration plant design. The proper design takes advantage of these similarities, which have been proven by numerous FCCUs in operation for years. Using a stabilized naphtha stream as a source of lean oil enables improvement in propylene recovery. It is more common to use debutanized naphtha as the lean oil; however, naphtha that has been more deeply stabilized is more effective for propylene recovery.

As with the previously described SGP design, the composition of the lean oil stream, along with other operating conditions, determines the vapor/liquid equilibrium at the top of the absorber. Using lean oil that has only been debutanized results in enough gasoline-range components in the primary absorber offgas that a sponge absorber is normally used downstream to recover these components. Sometimes, when utilizing lean oil that has been more thoroughly stabilized, the sponge absorber may be eliminated. The gasoline components in the offgas are sufficiently low to allow for this design.

The wet gas compressor discharge cooler and high-pressure receiver provide for the removal of heat of compression, separate the vapor and liquid phases prior to the column, and combine the deethanizer offgas and the primary absorber bottoms with the wet gas compressor section discharge, which provides another stage of absorption, thereby improving propylene recovery. Separating the phases prior to the column helps eliminate issues with premature flooding of the column, especially if significant swings occur in the proportion of C2– components in the feed to the gas plant. The high-pressure receiver also provides the settling time necessary to remove water from the system that otherwise can cause water entrapment problems. These and other similarities are utilized to provide an effective and reliable design.

Experienced operators of FCC gas concentration units understand the problems associated with water entrapment. Some have utilized inferior design configurations that have operated adequately; however, as FCC shifts toward maximum propylene operation and design to increase propylene recovery, the design of the system becomes more important. Inferior designs will not work. As refiners shift from maximum gasoline mode to maximum propylene production, the problem with water entrapment becomes worse.

The DWC design utilizes two advanced separation units.b,c These units are based on well-proven designs that optimize plant CAPEX and maximize product recovery and purity for maximum recovery. Six conventional columns are combined into two vessels using DWC technology to provide significant savings in CAPEX and plot space. A significant reduction in column insulation, ladders, platforms, etc., is seen. The number of condenser systems (exchangers, drum, pumps, controls) is reduced from three to two, and the number of reboiler systems is reduced from four to two.

To better understand the reasons for the reduction in CAPEX, a design combining the primary absorber and sponge absorber columns is studied. In this design, the primary absorber, sponge absorber, deethanizer and C3/C4 splitter are combined, which reduces the number of foundations from four to one. For simplicity, however, it can be assumed that only the primary absorber and sponge absorber are being combined. In this case, two columns have been combined, with a common wall separating them. Typically, gas plants are designed for operating pressures of 180 psig–450 psig (12.66 kg/cm2–31.64 kg/cm2). This requires thick vessel walls, whereas the pressure differential across the wall separating these two columns is only about 2 psi (0.14 kg/cm2), resulting in a thin wall for construction.

Combining the two columns in this manner results in an approximately 30% reduction in metal. The quantity of column insulation is reduced by a similar amount. The vessel ladders and platforms can be reduced essentially by half. Only one instead of two foundations needs to be designed and constructed. The reduction in vessel weight reduces the foundation load, which reduces the construction cost of the foundation. These and numerous other reasons are why DWCs are characterized by significant CAPEX savings.

The DWC FCC gas concentration unit design can easily operate over the typical operating range of FCCUs and is a lower-CAPEX solution. The simplistic design provides for achieving equal or better product recoveries and purities as a conventional system at lower CAPEX. FCC gas concentration design can be used without the addition of a high-cost refrigeration system to provide propylene recovery of greater than 97% in FCCUs designed for maximum gasoline mode.

Changes in global refinery product demand will continue to drive changes in the optimum configuration and operation of FCCUs. Petrochemical market demand will continue to rise, with an expected declining or flat market demand for refined products. As high-compression engines become more prevalent, an increase in the percentage of high-octane gasoline blending components required for the gasoline pool will be necessary. Shifting the FCCU operation to increase propylene and butylene for petrochemical feedstock and higher-octane gasoline production is a piece of the puzzle. Proprietary technologya offers a lower-CAPEX means of attaining these demands.

Takeaway

The distillation technologya discussed in this article provides designs that can significantly reduce CAPEX and OPEX while improving refinery molecular management and lowering the impact on the environment. This technology not only improves business economics, but also helps reduce fossil fuel consumption, carbon emissions and impacts to the environment. HP

NOTES

        a GT-DWC Advanced Distillation
        b GT-LPG Max
        c GT-DWC

 

 

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