Select Design of Column Devices

Purpose and General Comments

This procedure provides the fundamentals for determining column diameters and also identifies the variables that Inflection Point Engineering specifies for column contacting devices. The form F-PSD-57 Tray Design Job Aid is of value during the design of column internals.

Types of Internals

Column contacting devices are divided into tray and packing categories. Each category is divided into several sub-areas.

Tray design sub-areas include:

Sieve Cartridge

Valve Dual Flow

Bubble Cap MD Tray

Disc and Donut Slotted Sieve

Side to Side Screen

Packing design sub-areas include:

• Random
• Structured (sheet metal)
• Grid

Vapor/Liquid Loads

• The chosen device must have enough open area for vapor flow so that there is no significant entrainment of liquid up the column. Frequently, the parameters capacity factor, C, and Vload are used to evaluate liquid entrainment by the vapor. These parameters are defined below:

• Where:

• QV : Vapor volumetric flow rate

• A : For trays, typically the bubbling area but sometimes the free area or the superficial area. For packings, this is the superficial area.

• V : Vapor density

• L : Liquid density
• Internals shall be specified for each column section between any location where material or energy is removed or added to the column. Additional sections shall be defined so that each section shall have a minimum Vload at least equal to 20% of the maximum Vload for the governing case of that section. An example of this is a product Fractionator bottom steam stripper section. Normally, the bottom tray is a separate section based on a loading from one tray above the bottom tray.

Sizing and Choice of Sieve and Valve Trays

4.1 Capacity Ratings

• For sieve trays, the Inflection Point Engineering tray design program, Tray2, uses the FRI correlations for jet flood. Tray2 sizes the downcomers for sieve trays using a modified Koch method and reports the value from of the FRI correlation for downcomers.

• For valve trays, choose the KG Valve Tray method in Tray2. Tray2 uses the Koch-Glitsch method for jet flood and sizes the downcomers using the modified Koch method

• See Procedure , for further design information.

4.2 Valve Trays

While valve trays are typically about 20% more expensive than conventional sieve trays, it is common that valve trays are specified. Some of the common justifications for the specification of valve trays follow

• The customer requests valve trays.
• For the governing case of a particular section, the lowest Vload is less than 60% of the highest Vload.
• The Vload for alternate feed cases are less than 60% of the Vload for the governing case of the controlling stage.
• The customer expects reflux and duties to turn down to the hydraulic turndown.
• Cartridge trays are normally specified as valve trays.
• Valve trays are believed to have a small capacity advantage over sieve trays in low pressure columns. Columns are to be considered low pressure if the receiver is no more than 50 psig.
• Normally, if valve trays are required for one section of a column, valve trays are then specified for all sections of that column.

While normally not important, valve trays are expected to require more pressure drop than sieve trays.

5. Sizing of Packed Columns

• There is no single Inflection Point Engineering design method for packing. Consult with the Technology Specialist (TS) for the process of interest to see if any Vendors have established a predominate position in the particular service. The TS will indicate what sizing method to use.

• The 307 Packing Specification lists the packing used in the column design. It normally states that any “or equal” packing may be used.

• Random packing is often used in services where the required column diameter is too small for conventional trays. See Procedure for guidelines on selecting and specifying random packing for small columns.

6. Sizing of MD and Slotted Sieve Trays

• Normally the sizing of these trays is completed by the Inflection Point Engineering-Tonawanda group.

7. Sizing of Cartridge Trays

• Normally these are sized using the Sulzer program SulCol. This program is located on the Citrix server or an individual copy can be requested from the Sulzer web site. See Procedure on how to size and specify cartridge trays.

8. Specifications

• For all contacting devices, specify the maximum vapor and liquid loading for each section of the column. Report the operating temperature and pressure of the tray from which the loadings came.

• Turndown loadings are not typically listed. Specify the column diameter and the type of contacting device for each section of the column. Report the system’s tendency to foam via a derating factor. Normally, specify the column pressure drop used for the material and energy balance to be the maximum allowable overall pressure drop for a vessel. Refer to Procedure , for guidelines on tray pressure drops.

• Refer to Procedure for clarification of the specification for metric jobs.

8.1 Valve and Sieve Trays

• Specify the following for all valve and sieve trays:

• Column Diameter
• Tray spacing
• Type of downcomer
• - straight
• - sloped/broken
• Number of flow paths
• Chordal dimensions of downcomers and bubbling areas
• Use of froth initiators (if required)
• Type of accumulators and distributors

Specify these additional items for sieve trays:

• Number and size of holes
• Weir height
• Downcomer clearance

Do not specify these for valve trays:

• Number of valves
• Weir height
• Downcomer clearance

• If notched weirs are required, either simply note the requirement or specify the following:

• The use of notched weirs
• The effective weir length (the sum of all notch lengths)
• The height of weir between the notches (half of the tray spacing)

• If the tray requires baffles to channel liquid down to the middle of the tray, either simply note the requirement or specify the following:

• Draw baffles on tray layout sketch
• Baffle height (half of tray spacing)
• Channel width (acceptable jet flood)
• Distance from downcomer outlet to baffle (3 in/75mm)

• Reference Attachment 1, 2 and 3 for sample loading sheets for the 307 specification.

8.2 Packing

On the 307 process data sheet specify the following hardware information for each of the packing beds:

• Column Diameter
• Type and size of packing – A trademark name is desired to assist with indicating the needed capacity, efficiency and pressure drop. In general the packing type should be indicated as “or equal” in the row header.
• Bed depths
• Distributor Type - In general, indicate the type as either “Gravity” or “Spray Nozzle”. Gravity type distributors are normally required when efficiency (HETP) is important. Spray nozzle type distributors are normally appropriate when heat transfer is the main function of the packing, such as a pump around zone or a contact condenser.

The “Column Assemblies” sheet of the 307 spec should normally have the following line items for each of the packing beds.

• Distributor – For gravity type distributors the description should contain two lines, “Feed Pipe” and “Distributor”. The metallurgy for both of these is normally matched to the packing. For a spray nozzle distributor the description should contain two lines, “Distributor Piping” and “Distributor Spray Nozzles”. This is to allow separate metallurgy specifications for these two components. For many of these services the “Distributor Piping” can be CS (carbon steel) and the “Distributor Spray Nozzle” can be SS (one of several possible stainless steel that are listed in the general note section of the 307 specification).
• Bed Limiter – In some cases the bed limiter may be an integral part of the distributor. Regardless of this it should still be listed as a separate line item. In most cases the metallurgy matches the packing metallurgy.
• Packing – The metallurgy of the packing is based on consultation with the Technology Specialist and/or a metallurgist. In many cases SS (one of several possible stainless steel that are listed in the general note section of the 307 specification) is appropriate. This will prevent rusting when exposed to moist air.
• Bed Support – In most cases the metallurgy matches the packing metallurgy.

Reference Attachment 4 for a sample loading sheet of the 307 Project Specification for packing.

The packed column General Project Specification Templates and allow for specification of the loadings at the top and bottom of the bed. If the maximum load is neither of these, report the maximum load in place of the bottom load. Label it as the maximum bed load.

When a Spray Nozzle distributor is specified there is generally no need to issue a 911 specification. Assume a 15 psi nozzle pressure drop for hydraulics.

8.3 MD and Slotted Sieve Trays

After consulting with Inflection Point Engineering Tonawanda, specify only the following for MD and Slotted Sieve Trays:

• Column Diameter
• Tray spacing
• Type of tray

Reference Attachment 5 for a sample loading sheet of the 307 Project Specification.

8.4 Cartridge Trays

Specify only the following for cartridge trays:

• Column diameter
• Minimum downcomer area
• Minimum bubbling area
• Use of valve type cartridge trays

Reference Attachment 6 for a sample loading sheet of the 307 Project Specification.

8.5 Existing Internals

If the internals are existing, Contractor or Vendor drawing numbers are to be noted for conventional valve, sieve and cartridge trays.

Attachment 1 – Typical Valve Tray

Attachment 2 – Notched Weir

Attachment 3 – Baffled Bubbling Area

Attachment 4 A - Packing

Attachment 4 B – Packing

Attachment 5 – MD Tray

Attachment 6 – Cartidge Tray