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May-2011

Dividing wall column technology as a way of removing benzene from refinery gasoline streams

The primary separation method in the petrochemical industry is distillation, or fractionation, which is an energy-intensive process.

Bill Townsend and Bhari Bhujan Singh
KBR

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Article Summary

Dividing wall column (DWC) technology represents a mechanical implementation of fully thermally coupled distillation that not only saves significant amounts of energy but is both more efficient and cost effective than conventional distillation methods.

What is DWC and how does it work?
Simple distillation sequence (eg, two-distillationcolumn-in-series or distillation column with side-stripper configuration) is the commonest way of separating a multicomponent feed stream into three or more product streams (see Figure 1 and Figure 2). The same task can be performed with DWC technology while using less energy and plot space compared with simple distillation sequence, which requires two separate distillation columns along with their ancillaries. It also utilises more energy because it boils the same material twice to perform the same separation task as DWC technology.

DWC technology utilises the customised design of a single column in performing the combined separation task of two conventional columns. The pre-fractionator is integrated within the shell of the main fractionator by vertically dividing a conventional distillation column with a gas and liquid-sealed wall as shown in Figure 2. This permits the low-energy separation of the low and high boiling fractions in the feed section, while the medium boiling fraction is concentrated  in the draw-off section of the column. The wall separates the feed and pre-fractionator section from the main fractionator section and side-draw. The main fractionator and pre-fractionator are linked by vapour and liquid streams.

It’s nothing new

The concept of DWC technology has existed for more than 50 years (first patent 1949). But lack of design experience, fear of operational and control problems, and availability of inexpensive energy have prevented its widespread use. It’s now been commercialised with great success and no operability and control problems. It’s also finding wider acceptance as plant operators are seeking intelligent solutions to increase their profitability by reducing their operating expenditure with low cost investments.

Towers of power

In March 2011, KBR announced the successful start-up of its DWC tower design for Valero Energy at three of the refiner’s US-based refineries. The start-ups marked the first successful implementation of DWC towers in the US. The units were designed to concentrate and remove benzene from gasoline streams to meet US regulatory mandates limiting the benzene content of gasoline. The DWC towers were designed and optimised by KBR for each unique project and allowed Valero to meet the regulatory requirements.

The project objective was to provide a design that not only complied with the new stringent gasoline benzene content specifications in the US, but to do so in the most cost-effective way. The challenge provided an opportunity to apply the efficient, elegant solution that is DWC technology. As previously mentioned, it is thermodynamically more efficient, with studies proving that DWC applications can save up to 30% capital investment and use approximately 30% less energy compared with conventional two-column configurations for reformate benzene management.

Historical studies
KBR began developing DWC technology in the late 1980s through pilot-plant studies. Since then, the KBR London Technology Centre has performed several studies to identify potential grassroots and revamp applications in the refining and chemical industries. KBR designed and commissioned two commercial DWC units before offering the technology to Valero. In April 1998, in co-operation with BP, the first tower was commissioned at BP’s UK Coryton refinery. It was a revamp of an existing stabiliser column that produced a gasoline-blend compound. In 2004, a grassroots tower was designed and commissioned for an Asian client for production of pure iC5, light naphtha and heavy naphtha.

Custom made
Where applicable, a DWC installation can be designed to meet all product specifications as per a client’s requirements. A DWC unit can be designed to increase system throughput using the same energy as a conventional twocolumn sequence, thus saving in steam and fuel demand and reducing the overall energy requirement per unit of feed processed. A new DWC system can easily be incorporated within current refinery infrastructure; a unit requires no more plot space than a conventional single-column system and can be designed to utilise the available site’s heating and cooling media, meaning less capital outlay. The mechanical revamp of existing distillation towers with DWC technology is straightforward and can be done within a normal turnaround period.

Pros, cons and uses
The design process for a DWC unit is far more complex than for a conventional distillation column system, but within 1-2 years the savings achieved from a reduction in capital and energy cost or revenue generated from increased productivity definitely outweighs any complexities in the design efforts.

There are diverse possible applications of DWC technology in the chemical industry and the key is to identify the specific separation options where it will provide an advantage over simple distillation sequence.

Gasoline benzene specification in Europe is not as strict as in the US but European refiners are under pressure to operate more efficiently to boost margins. Also, refiners are exploring ways to reduce energy consumption and greenhouse-gas emissions. DWC units offer an efficient separation solution to refiners to reduce energy consumption or increase productivity. KBR has identified several applications for DWC technology in Europe where these benefits can be realised, such as straight-run naphtha fractionation upstream or downstream of reformers, full-range FCC naphtha fractionation, FCC debutanizer, C4 isomer separation and BTX separation.

We are confident, then, that it won’t be long before DWC technology becomes a common sight in refineries all around the world.


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