Volume XXXI: ASME Pressure Vessels

ASME PRESSURE VESSELS

The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.

COLUMN INTERNALS

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General Considerations

  • A tower design is normally divided into two main steps a process design followed by a mechanical design.
  • The purpose of the process design is to calculate the number of required theoretical stages, column diameter and tower height.
  • On the other hand, the mechanical design focuses on the tower internals and heat exchanger arrangements.

Steps:

  • Determine the separation sequences, which depends on the relative volatility and concentration of each component in the feed
  • Performing a material balance for the column
  • Determining the tower operating pressure (and/or temperature)
  • Calculating the minimum number of theoretical stages using the Fenske equation
  • Calculating the minimum reflux rate using the Underwood equations
  • Determining the operating reflux rate and number of theoretical stages
  • Selection of column internals (tray or packing)
  • Calculating the tower diameter and height

The Selection of Column Internals:

  • The selection of column internals has a big impact on the column performance and the maintenance cost of a distillation tower.
  • There are several choices of column internals and the two major categories are trays and packing. The choice of which to utilize depends on the
  • Pressure
  • Fouling potential
  • Liquid to vapor density ratio
  • Liquid Loading
  • Life Cycle Cost

CRITERIA FOR A TRAY COLUMN

Trays can be divided into many categories, such as baffle trays, dual flow trays, conventional trays, high capacity trays, multiple downcomer trays and system limit trays. According to some rules of thumb, trays should be selected is:

  1. The compounds contain solids or foulants
  2. There are many internal transitions
  3. Liquid loads are high
  4. There is a lack of experience in the service
  5. Vessel wall needs periodic inspection
  6. There are multiple liquid phases

CRITERIA FOR A TRAY COLUMN

On the other hand, packing divisions include grid packing, random packing, conventional structured packing, and high capacity structured packing. The rules of thumb for selecting packing are:

  1. The compounds are temperature sensitive
  2. Pressure drop is important (vacuum service)
  3. Liquid loads are low
  4. Towers are small in diameter
  5. Highly corrosive service (use plastic or carbon)
  6. The system is foaming
  7. The ratio of tower diameter to random packing is greater than 10

Important Thumb rule:

  • Tower operating pressure is determined most often by the temperature of the available cooling medium in the condenser or by the maximum allowable reboiler temperature.
  • Economically optimum reflux ratio is about 120% to 150% of the minimum reflux ratio.
  • The economically optimum number of stages is about 200% of the minimum value.
  • A safety factor of at least 25% about the reflux should be utilized for the reflux pumps.
  • Reflux drums are almost always horizontally mounted and designed for a 5 min holdup at half of the drum's capacity.
  • Limit tower heights to 175 ft (53 m) due to wind load and foundation considerations.
  • The Length/Diameter ratio of a tower should be no more than 30 and preferably below 20.
  • A rough estimate of reboiler duty as a function of tower diameter is given by:
  • Q = 0.5 D2 for pressure distillation
  • Q = 0.3 D2 for atmospheric distillation
  • Q = 0.15 D2 for vacuum distillation

Where,

  • Q : Energy in Million Btu/hr
  • D : Tower diameter in feet.
  • Overall column height depends on tray spacing. Tray spacing should from 18“ to 24“ (ease of maintenance to be kept in mind).
  • For tower dia > 4 ft, Tray spacing ~ 24 “ and for tower dia < 4 ft, Tray spacing is ~18”.
  • Peak tray efficiencies usually occur at linear vapor velocities of 2 ft/s (0.6 m/s) at moderate pressures, or 6 ft/s (1.8 m/s) under vacuum conditions.
  • A typical pressure drop per tray is 0.1 psi (0.007 bar).
  • Tray efficiencies for aqueous solutions are usually in the range of 60-90% while gas absorption and stripping typically have efficiencies closer to 10-20%

Tray vs. Packing. The difference in cost between plate and packed columns is not too great, although packings are generally more expensive than plates. In addition, the difference in column height is not usually significant if the flow rates are such that efficiencies are near maximum.

This is presented to you as a service from BOARDMAN INC. located in Oklahoma City, Oklahoma.

Since 1910, Boardman has been a respected custom fabricator. We take pride in our ability to take the most stringent specifications and requirements to provide a high quality solution to our customers. With more than 75 years of ASME Section VIII, Division I engineering experience, we have the unique ability to provide custom solutions to our customers.

Fabricated Projects Include:

  • Trayed Towers & Columns
  • ASME Pressure Vessels
  • Molecular Sieves
  • Rotary Dryers & Kilns
  • API Tanks
  • Acid settlers
  • Stacks, Scrubbers
  • Thermal Oxidizers
  • Accumulators, Condensers
  • Crystallizers
  • Ducting
  • Bins
  • Large Diameter Piping

The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick.

BOARDMAN INC. is available for shop tours and Pressure Vessel and Static Equipment Fabrication Classes.

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