Apr-2013

Customised tower design

To save project costs, the distillation towers for a new refinery were designed to meet process design objectives using best available technology

DARIUS REMESAT and MICHAEL KRELA
Koch-Glitsch Canada

Article Summary

The first new refinery in North America in over 30 years is located in Alberta, Canada, and is currently under construction. The feedstock is Athabasca bitumen, API 7-9, with a product slate of transportation-grade diesel, naphtha-based diluent for bitumen transport, and vacuum gas oil for further processing.

The project philosophy for the refinery’s design and construction is to leverage technology, service and equipment suppliers early in the design process to streamline the engineering stage. Reducing the iterative process for technology and equipment selection, specification and detailed design can yield significant cost savings. These can be achieved by engaging and embedding vendors prior to and during the detailed engineering process. From past project experience, this can save the equivalent of the cost of the technology and equipment purchased, which can translate to tens of millions of dollars per vendor. Over 20 sole source agreements were initiated for this project, one of them being for mass transfer internals for the entire refinery.

Koch-Glitsch performed preliminary validation simulations coupled with a design customisation review for all units for the new refinery in support of the project mandate. The purpose of the customisation review is to provide input at the early stages of the project with regard to vessel size and selection of mass transfer internals. Design improvements positively impacting separation performance and capacity can be factored into the tower dimension specifications through the proper selection of mass transfer equipment compared to the specified design by the various process licensors. This can reduce the overall cost of the 
decision-making process and actual execution of the work. The customisation effort differs from a cost-cutting review in that the distillation column is designed to meet requirements and does not suffer from a linear approach to costs, which may or may not be an optimal performance solution.

The customisation study also provides the following future value:
• An understanding of the capabilities of current mass transfer technology and the relative capacity gain these devices provide
• A starting point for Phase II and Phase III grassroots tower sizing.

The selected engineering contractors expressed concern with the customisation approach. However, the recommended reductions in tower diameter provided a tangible design margin (typically 10%) in addition to leaving room, in some cases, for a further debottleneck with the use of even higher capacity equipment. The customisation study serves as a guide to understanding where available vessel cost savings can be captured due to excessively conservative sizing practices. The study uncovered that the overall average reduction in diameter for towers that could have smaller diameters across the entire plant is 20%. This reduction has resulted in substantial savings in vessel and tower internals, along with the savings from reduced engineering hours, while still positioning the refinery for future rate increases. 

Inside-Out design approach
The typical approach to grassroots mass transfer equipment is:
1. Engineering company or licensor performs simulation
2. Engineering company or licensor performs preliminary design calculations to generate vessel data sheets; vessel diameter is to be confirmed by equipment vendor.

An issue often arises when recommendations are made during step 2 in cases where the recommended diameter is smaller than that provided on the vessel data sheet. A conservative approach is taken to general equipment sizing, and often the engineering company or licensor is satisfied knowing that the specified diameter is sufficient rather than seeking to minimise cost by reducing the design margin or employing a different type of equipment technology. This occurs even though the preliminary design that set the vessel diameter often uses large safety factors, rule-of-thumb hydraulic calculations or design guidelines that do not 
incorporate the latest mass transfer technology and knowledge. In fact, many tower internals design manuals in use today were published in the 1980s and earlier, and pre-date the past 20 years of high-capacity tray and packing development.

The goal of the Inside-Out design approach1 (see Figure 1) is to use the actual hydraulic calculations to optimise/refine/improve the tower simulation. This is done by accurately inputting the pressure drop at each stage (rather than an assumed constant pressure drop). At the same time, tray efficiencies are confirmed, and vessel diameter reductions can be found by matching the best available equipment options to the application. Koch-Glitsch used the Inside-Out design approach in evaluating all of the internals for the new refinery.

Mass transfer internals
A benefit of the Inside-Out approach is the ability to incorporate equipment characteristics, such as pressure drop and separation efficiency, into the simulation. For the customisation study, high-
performance equipment, such as Superfrac trays and Proflux severe service grid, were used.

Superfrac technology
The Superfrac tray comprises a suite of features that together form a high-performance, single-pass, cross-flow tray that has the highest combined capacity and efficiency of all cross-flow trays tested at FRI. The features that can make up a Superfrac tray are:
• High-capacity and high-efficiency valves available in different sizes
• Vapour tunnel or truncated vapour tunnel downcomers with various downcomer outlet shapes to maximise tray capacity and efficiency
• Inlet weir and bubble promoters
• Push valves and other directional devices
• Multi-pass arrangements
• Special features to deal with fouling
• Mechanical installations to simplify installation.