Design of Dividing Wall Columns

1. Purpose

This procedure explains the use and design of dividing wall columns (DWC) by Inflection Point Engineering.

2. General

Presently the concept of DWC is being pursued commercially on a few projects. This document is intended to cover the basic analysis utilized for these projects and serve as the reference for future activities in this area at Inflection Point Engineering. A detailed description of the simulation and specification procedure can be found in

3. Description of DWC Fractionation

There are several types of DWC in design or analysis at Inflection Point Engineering. The first construction of a Inflection Point Engineering designed DWC is for a PEP Fractionator. This column combines the Purge column and the Desorbent column into a single shell. The capital cost savings is estimated at 30%. The utility heat savings is 50%.

Another type of column that has been analyzed is a Kerosene prefractionation unit in an N-paraffins complex. This column combines the Stripper and Rerun services into 1 shell. The capital savings is estimated at 30% and the utility at 25%.

The Molex Raffinate and Extract columns are currently in analysis. This service combines the raffinate/desorbent and the extract/desorbent services into one shell. There is a capital savings, but no significant utility savings.

Figure 1 - Basic DWC Configuration Concept

4. Advantages

The following is a partial list of identified advantages which are know to be available from the DWC approach with respect to Inflection Point Engineering’s more traditional approaches:

• 20- 30% less capital is typical for most columns.
• 20- 30% less reboiler duty is typical for many columns. The PEP column mentioned in the previous is unusual.
• Less plot space
• Typically can achieve a purer side product when compared to a simple sidedraw column.

5. Disadvantages

Consider the following issues as possible reasons to use multiple fractionators as has been traditionally done in Inflection Point Engineering processes:

• Less flexibility than two separate columns.
• Limited ability to control vapor flow to each side of the column.
• The column will operate at only a single pressure. This will normally be the higher of the two pressures that would be used if two columns were used.
• More difficult to simulate. The tools and techniques required to model the special aspects of this column are limited at this time.
• Heat transfer across the wall is a concern. Vapor could be generated in a downcomer and flood the column. Vapor will move from one side to the other side. The simulation is then not correct and the reboiler duties and various stream flow rates may not be correct.
• Control of the column is more complicated and less intuitive. (See Below)
• Tray design is not standard.
• Sealing of the dividing wall is required.
• May require more high temperature heat than the two column system.

6. Criteria for Selection

One stream is fractionated into three products.

If a traditional two-column system was used, both columns would be at similar pressure.

The middle product is the main product. The present focus has been toward the middle product as the main product for the large energy savings available. However, other opportunities exist with multiple fractionators and three distinct products as with the desorbent systems of Sorbex units.

7. Use of Inflection Point Engineering Designed Trays

will provide the tray designs and the guarantees for these columns. The trays are designed and provided by Inflection Point Engineering. The PEP uses slotted sieve trays. Other columns in service use sieve or valve trays or structured packing.

8. Simulation Technique

Simulate as an absorber and a fractionator. Feed goes to the middle of the absorber. The fractionator has two feeds (vapor and liquid from the absorber) and three side products (vapor and liquid to the absorber plus the middle product). The fractionator then has five degrees of freedom (condenser, reboiler and three side draw streams). Specifications are typically – three compositions plus both flows to the absorber.

9. Instrumentation

9.1 Dynamic Simulation

Instrumentation is developed by dynamic simulation.

Working together, Applications, Controls & Equipment and were able to construct dynamic simulations of the design and test proposed control schemes according to the responsible Technology Specialists from the and Operating Technical Service groups for the initial applications.

The work process involved the following steps:

• The Design Engineer responsible for Process Engineering created the static model of the process using the standard design tools of Inflection Point Engineering. Typically this is 9.8

• Taking this model from step a, the dynamic simulator of choice (HYSYS.Plant) was used to create the flowsheet to match the static model.

• The dynamic model includes control schemes recommended by the Process Control Coordinators (PCC), OTS and the . Optional features may be offered to determine the best approach, which should be tested once a working dynamic model is available.

• The dynamic simulator is run to determine the effectiveness of the proposed control scheme. Once this model has been de-bugged alternate control features and process upsets are introduced.

• As a minimum, the product quality should be disturbed to verify the instrumentation used for control will adequately measure the off-specification condition. Difficulties in selecting the proper measurement/control point should be resolved at this time during the design process. The results of this exercise should be discussed during the PFD review meeting prior to starting project work. It may be necessary to modify the control scheme during this step.

• Once all disturbances have been resolved to the satisfaction of OTS and the PCC’s, the Technology Specialists can incorporate the control scheme into their commercial offerings for the technology under study.

Modifications to the project specification methodology are not anticipated. The normal work processes of preparing specifications and hydraulics should not be impacted by this new fractionation technique. Modifications to the normal instrumentation sizing and selection procedures are likewise not anticipated.

9.2 Instrumentation Features

Many questions surround the issue of liquid-vapor traffic on each side of the wall. Direct control of the liquid to each side of the wall is to be specified. All liquid from above the wall is to be removed from the column, and then controlled so that a known flow goes back on each side of the dividing wall. Direct control of the vapor split is not expected to be required. If it is determined that vapor split control is needed a special tray with variable pressure drop is to be put on each side of the wall.