Small Size Colum Design

1. Table of Contents

1. Table of Contents 1

2. Purpose 1

3. General 1

4. Random Packing 2

4.1 FRI Design Practice Handbook Values 2

4.2 Body Flange Locations 2

4.3 Unloading Nozzles 3

4.4 Packing Size 3

4.5 Choice of Random Packing Type 4

4.6 Experience Based HETP Values 5

4.7 Calculated HETP Values 5

4.8 Metallurgy 5

4.9 Project Specification 307 for Packing 6

4.10 Project Specification 301 for Small Packed Columns 6

5. Cartridge Trays 6

5.1 Special Notes for 301 Drawing Sheets 6

5.2 Feed Point Issues 7

5.3 Sizing 7

2. Purpose

This procedure provides guidelines for design of small size columns. For this procedure small size columns are considered to be any vessel less than 2’-6” (750) in diameter. This procedure is a general discussion of this topic. The appropriate Technology Specialist or Process Specialist should be consulted about the design of any specific column.

3. General

Columns with an ID of less than 750 mm are considered to be too small to allow personnel entry for installation of internals. Access to allow installation of internals is mainly accomplished through the use of full body flanges on the column. Packing (both random and structured) and cartridge trays are all possible choices for the internals. However the use of random packing is generally considered the best choice due to customer acceptance and ease of design and installation.

Pipe sections are normally used for the fabrication of small columns. A nominal pipe size is used for the specification of the diameter. The largest size is normally 24 inches. If a diameter larger than this is required it is be best to specify to an ID of 750 mm or larger to allow personnel entry. In some cases there will be customer preferences that will override this guideline.

4. Random Packing

4.1 FRI Design Practice Handbook Values

Section 3.07 of the FRI Design Practice Handbook covers “Design of Small-Scale Columns”. Listed below are some of the general guidelines that this document recommends. These guidelines are normally appropriate. However some variation may be possible after consultations with the appropriate specialists and vendor.

• The nominal packing size should be no more than 1/10 of the column ID.
• The maximum ratio of a packed bed depth to Vessel ID to should be no more than 10:1.
• The maximum number of theoretical stages should be 10-12 per bed.
• The maximum bed depth should be no more than 20 ft (6100 mm).
• No flood derating factor is needed due to the column’s small size. Appropriate foam derating factors should still be applied.

Some additional discussion is needed on the limit of stages per bed. Vendor reported base HETP values are normally based on columns that contain 10 or fewer stages. As the stages per bed increases above 10 there is increasing possibility that the HETP value will increase. When the stage count per bed is more than 10 but less than 15 a small (10%) increase over the base HETP value should be considered. When the stage count per bed is more than 15 but less than 20 a moderate (25%) increase over the base HETP value should be considered. The use of more than 20 stages per bed is not acceptable.

4.2 Body Flange Locations

As the diameter is too small to allow personnel entry, body flanges are required at most locations where access is required for installation of internals.

The top feed (reflux) normally enters the top side of the column close to the top tangent line. The feed pipe brings the feed through the vessel wall and routes the feed into the distributor. The feed pipe may be a bayonet type or may be simply welded to the vessel wall. Typically there is a body flange below this point. The liquid distributor is normally located just below this body flange. The liquid distributor may be supported off of lugs or perhaps the flange face.

Below each packing support plate is normally a body flange. This body flange will allow installation of the above packing support plate and when appropriate the packing and distributor that is below this body flange. If there is a liquid or two phase feed pipe, at this point, it will normally be located above the body flange. This feed pipe normally needs to be a bayonet type to allow installation of the packing support plate above this point.

4.3 Unloading Nozzles

Packing unloading nozzles (when required) are specified near the bottom of each packing bed. Generally these are not required as most customers prefer to unload the packing with the use of vacuum equipment. This point should be confirmed with BEDQ and/or the customer.

4.4 Packing Size

Section 4.1 includes the guideline that the nominal packing size should be no more than 1/10 of the column ID. Tabulated below are the maximum packing sizes for IMTP and RSR packing in various column sizes. Extra capital expense occurs when a column’s height/diameter becomes large. FRI suggests that a height/diameter value greater than 22 is large. Choosing a smaller packing typically increases the column diameter and decreases its height. In some cases choosing a smaller packing size may be attractive to improve the height/diameter ratio value.

	Recommended maximum packing size	Recommended maximum packing size
Nominal Pipe Size	Use IMTP or equal (m2/m3)	Raschig Super-Ring (m2/m3)	Standard Wall Column ID
24”	#50 (97)*	No. 2 (100)	(Sch 20) 23.25” (591 mm)
20”	#50 (97)*	No. 2 (100)	(Sch 20) 19.25” (489 mm)
18”	#40 (153)*	No. 1.5 (120)	(--) 17.25” (438 mm)
14”	#25 (223)	No. 1 (150)	(Sch 30) 13.25” (337 mm)
12”	#25 (223)	No. 0.7 (180)	(--) 12.00” (305 mm)
10”	#25 (223)	No. 0.7 (180)	(Sch 40) 10.02” (254 mm)
8”	#15 (281)	No. 0.5 (250)	(Sch 40) 7.981 (203 mm)
6”	#15 (281)	No. 0.5 (250)	(Sch 40) 6.065” (154 mm)
4”	Use ProPak or Structured Packing	Use ProPak or Structured Packing	(Sch 40) 4.026” (102 mm)
* When a 24” or a 20” or a 18” pipe is used it is common to select the use of IMTP #25 rather than a larger packing. The justification for this is to produce a lower L/D ratio for the packing beds.	* When a 24” or a 20” or a 18” pipe is used it is common to select the use of IMTP #25 rather than a larger packing. The justification for this is to produce a lower L/D ratio for the packing beds.	* When a 24” or a 20” or a 18” pipe is used it is common to select the use of IMTP #25 rather than a larger packing. The justification for this is to produce a lower L/D ratio for the packing beds.	* When a 24” or a 20” or a 18” pipe is used it is common to select the use of IMTP #25 rather than a larger packing. The justification for this is to produce a lower L/D ratio for the packing beds.

4.5 Choice of Random Packing Type

For most columns a particular vendor’s packing should be specified to help establish the desired capacity and efficiency. Two good choices are KG IMTP random packing and Raschig Super-Ring (RSR) random packing. RSR has better performance than IMTP and should generally be specified when this is considered to be important. However for small columns performance is frequently not considered to be critical. The patent protection on IMTP packing has expired and most vendors produce an IMTP look alike. For small columns, many customers are willing to except a slightly larger column size and the use of a generic packing (IMTP look alike). IMTP packing is available in Tray2 for column sizing calculations.

4.6 Experience Based HETP Values

It is best to use experience based efficiencies, as is done with trays. It is critical that the appropriate Technology Specialist be consulted to determine the correct design HETP value that should be used for a packed column design. In cases where there is little or no Inflection Point Engineering experience, consultation with a packing supplier may provide valuable input.

There are some services that normally use trays with a standard tray count. The following method can be considered to convert this to a packed column design. In most cases 2 ft of IMTP #50 packing should provide the equivalent stage count to a single tray. Bed depth adjustments for other packing sizes can be based on relative calculated HETP values, see below.

4.7 Calculated HETP Values

When an experience based HETP value is not available, consult with the appropriate specialists to obtain recommendations on available calculation methods.

The most common calculation method is the Koch-Glitsch method for IMTP packing. This method allows prediction of HETP values for each the IMTP sizes, making use of surface tension and liquid viscosity. It is restricted to distillation and reboiler stripping services that are non-aqueous, non-reactive and have a relative volatility less than 3. The Koch-Glitsch IMTP brochure recommends that the calculated System Base HETP value be multiplied by 1.13 to obtain a design HETP value. However, for most designs, use a value of 1.2 for this multiplication factor.

In general different packing types that have similar specific surface areas have similar HETP values. This fact can be used to assist in estimating HETP values for packings other than IMTP. For a given packing type, e.g. IMTP, a plot of HETP versus the inverse of the specific surface area produces a fairly linear plot. This plot can assist in estimating HETP values for packings with any known specific surface area.

4.8 Metallurgy

Always consult with a metallurgist, but in general the following is expected.

For most hydrocarbon services the vessel will be KCS but the packing should be SS to prevent wet air oxidation during installation and turnarounds. Type 304, 316, 410, 409 and 430 are all normally acceptable. The assemblies sheet of the 307 specification is used to specify the metallurgy of the packing. The stainless steel abbreviation “SS” is a common specification. Clarification of the meaning of SS is provided on the general note sheet of the 307 specification. If Cl- ions are present the 300 series are not used as stress corrosion is a concern.

Any presence of CO3, SO4 or SO3 ions is of special concern and requires review by the Technology Specialist and the Metallurgist.

Other packing related internals are normally matched to the packing metallurgy. This includes feed pipes, distributors, bed limiters and support plates.

4.9 Project Specification 307 for Packing

Please refer to , section 6.2 for data which need to be included in this specification.

4.10 Project Specification 301 for Small Packed Columns

Please refer to Attachment 1 for a sample 301 specification for a small packed column.

5. Cartridge Trays

The use of cartridge trays is normally limited to columns that have a diameter that is no more than 24” and no less than 12”.

Cartridge trays are provided by the proprietor as a bundle or group of bundles. Each bundle is a number of trays, frequently 8 to 10, that have been assembled using tie rods and spacers. Envelope type downcomers are used so that downcomer sealing against the vessel wall is not required. The bundles are designed to be inserted into the column after the erection of the column. Each tray will have a gasket to provide a sufficient seal between it and the vessel wall. Each bundle, once installed, will either bolt to the bundle below it or attach to clips that are welded to the vessel wall. If trays are required below these clips then an additional body flange is also required. A full body flange is always required at the top of these columns.

5.1 Special Notes for 301 Drawing Sheets

During installation, visual verification that each cartridge-clip attachment is properly completed is required. A pair (one for viewing and one for a light source) of handholes is required below but close to these clips. A similar pair of handholes is also required near the bottom of the column to allow inspection for possible debris.

The number of required body flanges is set by the design of the cartridge trays.

The vessel shell may have to be fabricated with a special roundness tolerance (smaller tolerance than code) to ensure the cartridge bundle can be fully inserted into the vessel without binding or damaging the bundle.

Refer to Attachment 2 for a sample of the above notes.

5.2 Feed Point Issues

An all liquid feed that goes to the top tray is normally introduced behind a false downcomer. As such there is no expected need for a feed distributor. However a stab-in distributor is normally shown on the 301 drawing sheets to allow for this possibility.

Any middle feed will normally have a stab-in inlet pipe to allow the cartridges to pass this feed point.

5.3 Sizing

Sizing of cartridge trays is normally done using the Sulzer SulCol program. When the column diameter is small enough the geometry details panel will allow selection of a “Cartridge” downcomer type. This in turn allows for selection of the “Index”. Each index is a Sulzer standard cartridge tray design. Each index is only allowed over a certain range of column diameters. SulCol is available on the Inflection Point Engineering Citrix server or from the Sulzer Chemtech web site.

Figure 1: Clip from the SulCol Program

5.4 Project Specification 301 for Cartridge Tray Columns

Please refer to Attachment 2 for a sample 301 specification for a cartridge tray column.

• Atttachment 1
• Sample 301 for Packed Column

• Attachment 2
• Sample 301 for Cartridge Tray Column