Tray Design for Sidedraw with Measured Gravity Flow Reflux

1. Purpose

This procedure describes the normal guidelines that are used for designing an accumulator tray with a sidedraw and measured gravity flow reflux.

2. General

Many heavy oil columns (plus a few select additional columns) contain an accumulator tray that supplies both a sidedraw and reflux. The sidedraw most frequently goes to a side stripper. The reflux can be supplied in one of three ways.

2.1 Internal Overflow Reflux

While an internal overflow for the reflux has the advantage of simplicity it has the disadvantage that the reflux can not be metered. For most heavy oil columns the ability to see the reflux rate is considered critical for column control.

2.2 Pumped External Reflux

One very effective method to meter and control the reflux rate is to bring liquid from the accumulator tray to a pump and then return it to the column through a control valve and FI. An LIC on the accumulator tray is normally used to control the reflux rate. While this method is very effective it does require a set of pumps, along with significant piping and instrumentation.

2.3 Gravity Flow External Reflux

A simpler and less expensive option to meter the reflux is to have gravity flow from the accumulator tray with the flow rate controlled by an overflow weir on the accumulator tray.

3. General Guidelines

3.1 Sample PID

The below Figure 1 shows a section of the PID module F215080 M (Unicracking) and displays how the gravity reflux is normally specified.

Figure 1

Typical PID Detail

The baffle on the upper level connection is only needed when there is a feed at this location.

3.2 Standard Drawings and Dimensions

Use the 301 specification to indicate the location of the total accumulator tray with two draw nozzle wells and a vapor riser. The well for the gravity flow reflux shows an overflow weir. The 307 specification indicates the use of standard drawing 3-288 along with the type and dimensions that are needed for that standard drawing.

3.3 Weir Height

The height of the weir is normally been set to 1 ft (300 mm). While additional weir height and hence surge time can be specified there is little advantage as the level is tightly controlled by the overflow process. A 1 ft (300 mm) weir height is sufficient to prevent vapor from entering the sidedraw line.

3.4 Level Instrumentation

The level instrumentation is determined by the appropriate process specialist. The normal liquid level will be slightly above the weir and is the result of the overflow rate and the weir length. The sole purpose of any level instrumentation is to allow identification of a problem. When the accumulator tray is operating normally there is no value in measuring the level.

A level below the top of the weir indicates that the leakage plus the primary (not the reflux) draw rate is at least equal to the liquid entering the accumulation area. For this case the ability to see very low levels is of interest.

A level that is considerably above the top of the weir indicates that overflow reflux hydraulics are preventing the required flow rate. For this case the ability to see a level as high as the top of the vapor riser is of interest. A level higher than this will likely result in froth formation that will flood the tray or packing above the accumulator tray.

The appropriate process specialist sets the standard for the desired instrumentation based on the need and value of trouble shooting the above difficulties. The following choices are possible.

• No level instrumentation.
• A LG spanning the distance of interest.
• A LG with a LI spanning the distance of interest.
• A LG with a LI spanning the distance of interest with alarms.

An LI or LG will normally use a top-side connection to cover the distance of interest between the level nozzles. The lower level nozzle is normally 3” (80 mm) above the accumulator tray floor. The upper level nozzle is normally 12” (300 mm) below the tray above.

3.5 Clearance Above Weir

The elevation difference from the top to the weir to the floor of any above downcomer trap needs to be at least 6 in (150 mm).

3.6 Riser Height above Weir or Downcomer Trap Floor

The riser height needs to be at least 6 in (150 mm) above top of the weir and the floor of the downcomer trap when present. This elevation is normally specified in the 307 specification in combination with a reference to a standard drawing.

3.7 Hat Height above Riser

The standard 3-430 calls out the exit area to be at least 125% of the riser area. Inflection Point Engineering does not specify the number, location or shape of the risers.

3.8 Riser Hat to Hardware Above

The distance from the hat to the hardware above needs to be at least 12 in to allow for vapor flow into this area. Inflection Point Engineering does not specify this distance.

3.9 Reflux Line Size

Size the external reflux line for a maximum pressure drop of 0.1 psi/100 ft.

3.10 Requirements

The FE is to be 7 pipe diameters below the return nozzle. This provides required straight pipe upstream of the FE and insures no vapor to the FE. The bottom of the loop is 3 pipe diameters below the FE. This provides required straight pipe after the FE.

3.11 Type of Flow Meter

Indicate on the PID the use of an averaging pitot FI.