Application of IPE MD Trays

1. Purpose

This procedure outlines the applications where Inflection Point Engineering recommends using multiple downcomer (MD™) trays. Additionally, guidelines are provided for specifying MD trays in other applications.

2. General

MD trays are proprietary, high capacity fractionation trays originally patented by the Union Carbide, Linde Division. They employ a sieve deck in combination with multiple, terminated downcomers, thus eliminating the need for receiving pans. Basic design criteria, manufacture and supply of MD trays are presently controlled by Inflection Point Engineering-Tonawanda. Since their inception in 1965, MD trays have been employed in over 900 columns. Enhanced Capacity MD trays (or "ECMD trays") have 15 to 20% more capacity than MD trays and have been used in more than 190 columns.

3. MD Tray Information

3.1 General

MD trays have been successfully applied to many refinery applications. They are particularly useful in revamp applications that require 25 to 35% additional capacity and/or increased stages. In grassroots applications, MD trays have been installed where large columns are required because they provide for a more cost-effective column design.

3.2 Services

MD trays are best suited for high pressure (greater than 50 psig) or heavy liquid-loaded service. MD trays have been used in both fouling and foaming services. In non-fouling services, e.g., propane/propylene splitters tray perforations are normally 3/16". In moderate fouling services, e.g., Platforming debutanizers, tray perforations are normally 1/4" to 3/8". Tray perforations of 1/2” or larger are normally used in severe fouling services.

3.3 Efficiency

MD trays are generally less efficient than conventional trays. However, MD trays normally have close tray spacings (typically 12 to 14 inches) which allows for more MD trays to be placed within a given column height, thus compensating for the efficiency difference. In general, MD trays provide for a shorter column.

3.4 New Unit Design

On a new unit design, MD trays are competitive with conventional trays when the cost of the additional MD trays is offset by the savings due to the smaller column diameter and tangent length. These savings increase with column diameter, tangent length, and design pressure (i.e., shell thickness).

3.5 Revamp Design

For a revamp design, MD trays may be attractive in any column where an existing column with conventional trays would flood, but the same column with MD trays would not. MD trays may also be attractive in existing columns that have insufficient stages. This is due to the ability of MD trays to use small tray spacings.

4. Applications Where MD Trays Are Recommended By Inflection Point Engineering

There are several applications where Inflection Point Engineering recommends using MD trays. Make the customer aware of these recommendations as soon as possible. Clearly state in any Design Basis Memorandum that MD trays will be specified. If a customer requires a study to prove cost savings, inform Inflection Point Engineering-Tonawanda who may help provide one. The Project Manager/Lead Engineer shall keep Inflection Point Engineering-Tonawanda informed of the project schedule.

4.1 Propane-Propylene Splitters

MD trays are the Inflection Point Engineering recommended design for C3 Splitter Columns. Procedure ” covers the design procedure for these columns.

4.2 Deisobutanizer Column in MTBE Complexes

MD trays are the Inflection Point Engineering recommended design for the deisobutanizer (DIB) column in MTBE complexes. Most DIB columns in MTBE complexes are expected to be of similar size, based on capacities set by maximum single train Oleflex designs. When MD trays are used in this application, column diameters normally range from 18 to 21 feet.

4.3 Design Procedure for Deisobutanizer Columns

The Design Engineer doing process work determines the design reflux to feed ratio and the required number of theoretical stages.

For the main part of the column, use a tray efficiency of 75% for MD Trays and 80% for conventional trays.

The Design Engineer calculates the column tray loadings based on the design reflux.

Also, choose an appropriate number of theoretical trays, so that there is contingency that can accommodate a small shortfall in tray efficiency, regardless of the tray type used. Use care when assessing the amount of neo-pentane in the feed.

Provide Inflection Point Engineering-Tonawanda with tray loadings. Inflection Point Engineering-Tonawanda shall then provide the required diameter and tray spacing within approximately two working days. However, if circumstances require, the column diameter and tray spacing may be provided within several hours. Inflection Point Engineering-Tonawanda sets the diameter based on the provided tray loading and their normal design practices.

The Design Engineer assigned to project work shall confirm the column tray loadings, diameter, number of trays, and tray spacing with Inflection Point Engineering-Tonawanda. Within approximately three days of confirmation, Inflection Point Engineering-Tonawanda shall provide a vessel sketch that indicates requirements for items such as reflux and feed distributors, side draws, and space above reboiler return that dictates column tangent length.

4.4 Xylene Rerun and Xylene Splitter Columns

MD trays are the Inflection Point Engineering recommended design for a Xylene Rerun or Xylene Splitter Column in Parex units. MD trays reduce the diameter and height of these large towers, resulting in a more cost-effective design.

4.5 Design Procedure for Xylene Fractionators

The design procedure for Xylene Columns is the same as that of Deisobutanizer Columns (Section 4.3). Use a tray efficiency of 70% for MD trays and 80% for conventional trays.

5. MD Tray Guidelines

Consider using MD trays on columns that meet any of the following criteria:

• The diameter is greater than five feet.
• The operating pressure is greater than 50 psig. If the pressure is less than 50 psig, MD trays may still be considered on a case-by-case basis in consultation with Inflection Point Engineering-Tonawanda. More conservative MD design guidelines are applied at pressures, less than 50 psig.
• The required separation cannot be accomplished in a single column due to height or column length limitations.
• A large number of closely spaced trays are required in a single column.
• The new loads for a revamp would flood conventional trays in the existing column.
• Foaming or fouling are concerns.

6. MD Tray Applications In Inflection Point Engineering Schedule A Designs

Schedule A designs have specified MD trays in the following applications:

Unit Service

7. MD Tray Applications In Inflection Point Engineering Revamps (Non-Schedule A)

Revamp designs have specified MD trays in the following applications:

Unit Service