POO Test Prep

Topics 3-5

Why the need for energy conservation?

To minimize & optimize energy usage since:

• energy consists of 30-40% of the plant operating cost

• scarcity of energy sources

What is the Approach to Energy Conservation

Energy Usage Survey and apply 80/20 rule

Energy Usage Survey: Ask on what are the equipments with high energy usage, reasons for high consumption, how to improve and how it will impact operating cost?

80/20 rule: Focus on the top 20% of the highest energy consuming equipment which consumes 80% of the plant energy usage.

Importance of Energy Conservation for Air Systems and How to identify air wastages and leakage?

Importance of Energy Conservation for Air Systems
Air systems refer to plant and instrument air supplied from air compressors, where wastage of air leads to higher electricity consumption.

How to identify air wastage and leakage

• Perform an air balance.

• Significant differences between design requirement and actual air consumed is also an indication.
  
  
  

Common causes of higher than design air consumption

• Excess air being used to purge the instrument air dryer for water removal.
  

• Oversupply of purging air to instrumentation panel.
  

• Leakage in Air compressors/Coolers’ water condensate drain trap or manual drain valve open more than necessary for water condensate removal. (air leaks out alongside the water being removed)
  

• Leaking from the instrument air supply headers.
  

• Leakage of instrument air tubing
  

• Improper control setting of centrifugal air compressor discharge pressure (for surge protection)

Importance of Energy Conservation for Steam System and How to identify steam wastage & leakage?

Importance of Energy Conservation for Steam system

• Steam system refers to steam produced by direct-fired boilers and process waste heat steam generators

• Steam wastage & leakage causes more fuel and boiler feed water consumption

Identifying steam wastage & leakage

• Steam balance

Areas to reduce steam wastage

• Steam leak from holes on corroded steam distribution piping.

• Steam leak from faulty steam traps.

• Damage or insufficient steam lines insulation
  

• Inefficient boilers operations such as too much excess air, fouling at inside/outside of boiler tubes or at boiler feed water preheating tubes.
  

• Excessive amount of steam used in steam strippers.
  

• Steam pressure step down control valve instead of step down from turbines.
  

• Imbalance in steam supply and demand.

Ways to reduce steam wastage

• Implement maintenance program (to identify and repair steam leak from steam pipings, steam traps and damaged steam line insulation that lead to higher heat loss)

• Close operations monitoring (to address inefficient boilers, operations, excessive stripping steam and inefficient steam pressure step down)

• Switch rotating equipment drivers from motor to turbine (to balance steam supply and demand)

• Install air fin coolers (to recover excess low pressure steam as condensate to completely eliminate venting of steam to atmosphere)

Energy Conservation for Cooling Water System

• Review Cooling tower heat load

  • Reveal Opportunity to turn off fans in some towers to reduce electricity consumption

• Control blow down

  • Water purged from the system to control TDS and chemicals added to prevent fouling, bacterial growth and control corrosion

  • Monitor and avoid excessive blowdown rate to reduce water usage and chemical wastage
    

Objectives of Stripping Section

• Increases yield of lighter product

• Control IBP of heavier product
  
  Note: FBP of heaviest, residual product cannot be controlled.

Objectives of Rectification Section

• Increase yield of heavier product

• Control FBP of lighter product
  

  Note: IBP of lightest product cannot be controlled.

Advantages of pump-around heat removal

• Vapor and liquid loadings more evenly distributed, savings in equipment cost due to even column diameter

• Heat removal from tower at higher temperatures used to heat other process streams rather than being rejected to cooling water or air.

Rank the 3 trays from highest to lowest turndown

Bubble-cap > Valve > Sieve

Bubble-cap: most suitable to handle extremely low liquid flow

Rank the 3 trays in terms of cost

Bubble-cap > Valve > Sieve

Bubble-cap: 2-3x cost of Sieve trays
Valve: 20% higher than sieve trays

Which tray have the least fouling tendency, effects of corrosion and maintenance

Sieve Trays

When are bubble-cap trays used?

• where extremely high turndown is required

• when liquid leakage from one tray to another must be eliminated

Description and usage of Random or Dumped packings

Description

• Discrete pieces of a specific geometrical shape

• randomly packed into the column shell

• designed to provide good vapor-liquid contact

Usage: commercial applications

Description and Usage of Structured Packings

Description

• Crimped layers of wire mesh or corrugated sheets, stacked orderly in the column

Usage: Vacuum distillation (less common due to cost)

Description and Usage of Grids packings

Description

• Systematically arranged packings

• Open-lattice structure

Usage

• Heat transfer (pump around section)

• wash section of a tower

• section where high fouling resistance is required

Why are packings less commonly used compared to trays?

• Cost of installation more expensive

• Less tolerant to fouling

Usage of packings

• Applications where pressure drop across the internals is critical

• Corrosive but non-fouling services

• In towers that are less than 3ft in diameter

• In de-bottleneck situation

Definition and Types of Flooding

Excessive accumulation of liquid inside column

Types:

• Jet flooding

• Liquid entrainment flooding

• Downcomer backup flooding

• Downcomer choke flooding

apekah indications of flooding

• Unsteady column bottom level

• Falling bottom products flow

• high or erratic column pressure drop

• loss of fractionation efficiency

Actions to take when flooding occurs

• Reduce vapour and/or liquid load in column by reducing:

  • feed rate

  • reflux rate

  • reboiler duty or

  • combination of these activities