• The system re-concentrates weak caustic lye from 40 to 50 gm/ltr to 250 to 450 gm/ltr as required by the process viz. dry-in-wet or wet-in-wet mercerizing.
• Fresh caustic in the process is required only for concentration variation and make-up.
• Payback period of the Plant is less than 4 months for a Triple Effect Caustic Recovery plant with Surface Condenser and Thermo-compressor.
• The energy of heating steam can be completely transformed into hot water. Hot Water is generated at:
  • The uncontaminated steam condensate from the first heat exchanger may be reused as boiler feed water.
  • At surface condenser hot water is generated by soft water feed to the surface condenser for cooling vapour of the last effect. The result is eventually soft water of a high temperature, which saves cost in water softening before use in mercerizing machines or in initial washing.
  • Alkaline condensate is generated as the vapour condensate, which can be used as hot wash water in the mercerizing machines or in initial washing.
• Financial savings in neutralizing agents and waste water treatment costs.

Plant Configuration Criteria

The Caustic Recovery Plant is fundamentally a multiple effect evaporation plant that works on the principal of raising film causing natural circulation. The lye circulates in the evaporators in such a way that when it boils in the heat exchanger it moves upwards. The vapour is released in the evaporator causing the weak lye to concentrate.

Multiple effect evaporation uses the steam produced from evaporation in one effect to provide the heat to evaporate product in a subsequent effect which is maintained at a lower pressure and temperature.

The vacuum is produced by the condenser, and partly by means of the vacuum ejector or water ring vacuum pump. This creates a pressure and temperature gradient from the first effect to the last effect, which accounts for the economical multiple effect evaporation.

The following table shows amount vapour generated from unit steam consumption for different effect of plant.

The quantity of heating steam required decreases with higher-staged evaporation plant. The selection of required number of effects is subject to budget cost and the quantity of weak lye to be evaporated. For optimum cost effective results, an evaporation of approximately 48,000 kg/day of water requires a triple effect evaporation plant, as the increase in capital expenditure for a higher-staged evaporation plant cannot be recovered through lower steam and cooling water amounts.

Thermal Vapour Re-compression:
Further improvement in economy is achieved by means of thermal vapour recompression across one or more effects. A portion of vapour discharged from one effect is compressed by direct steam injection and the resulting mixture is used as the heating medium in the first effect.

They operate according to the steam jet pump principle. They have no moving parts and are therefore not subject to wear and tear.

This ensures maximum operational reliability. The use of a thermo-compressor gives the same steam/energy saving as an additional evaporation effect.

The availability of steam at a pressure of about 100 psig (7 bar), permits the efficient use of thermo-compressor on a multiple effect evaporator. This is the simplest and least costly technique for enhancing evaporator efficiency.

Adiabatic Evaporation:
Adiabatic Cooling or Evaporative Cooling is the working principal of cooling towers. In adiabatic evaporation the wash liquor is fed to cool the vapors in surface condenser, then the hot wash liquor is sprayed in a cooling tower designed to handle caustic wash liquor and its fumes.

This significantly reduces steam consumption as water from the wash liquor is evaporated in adiabatic evaporator.

Gyratory Vibro Screen Filter

It removes lint, fluff, and other fibre from wash liquor. This improves the efficiency of plant by reducing the fouling of heat transfer material.

It is basically a vibrating screen device with S.S. Screen of 100 mesh and an electric motor with eccentric load to generate vibration. This removes the solid from wash liquor and is collected for disposal. It is an automatic working device, not requiring any human intervention.

Hydrogen Peroxide Lye Cleaning System

Different types of impurities are left in the lye during the mercerizing process like oils, paraffin, sizes and starches. Hydrogen Peroxide being a strong oxidizing agent reacts with OH-groups of organic materials. The lye viscosity is reduced and impurities float to the top and sludge sink down in the tank. Synthetic sizes cannot be removed by this process.

Specifically designed mixing and dosing tanks are provided for maximizing the residence time in settling tank with intricate baffles and piping inside to remove the floating impurities as well as settled sludge. This treatment also bleaches the dirt and reactive dye particals finely distributed in the lye. The exact peroxide dosage is set during plant operation and varied periodically.

The evaporation plant works with a fully-automatic operation sequence. This means that the operating stage of the plant can be monitored from the flow sheet on the front of the switchboard. The level of lye in each stage and therefore the weak lye input is monitored and controlled. Final concentration of lye is monitored and controlled by PLC.

The start up and stop of the plant can be automatically initiated from the liquid level in weak lye tank. All significant temperatures and pressures are digitally set at the control panel.