Question

The steam used during the production of alumina in the Bayer process serves as both a heating and process fluid. Some steam is recycled, but some is condensed, clarified, and used for hot water in the plant before being cooled in a forced draft cooling tower and discharged.Most of worldwide bauxite mining and alumina production takes place in Jamaica. Therefore, the source of process water for most plants is the Caribbean Sea. Seawater is desalinized and sent as “slightly saline” water to the plant for use as makeup water.Bleedoff is removed from the tower when it has reached approximately thesaltconcentration of the seawater to which it is discharged.What is the makeup rate (in kg/h) of slightly saline water to a 75 kW-ratedcooling tower?

Answer 1

Solution:

1. Cooling Tower Approach:

The difference between the Cold Water Temperature (Cooling Tower Outlet) and ambient Wet Bulb Temperature is called as Cooling Tower Approach.

Approach = Cold Water Temperature - Wet Bulb Temperature

Cooling Tower approach is the better indicator of the performance.

2. Cooling Tower Range:

The difference between the Hot Water Temperature (Cooling Tower Inlet) Temperature and Cold water (Cooling Tower Outlet) temperature is called Cooling Tower Range.

Range = Hot Water Temperature - Cold Water Temperature

3. Cooling Tower Efficiency Calculation:

The calculation of cooling tower efficiency involves the Range and approach of the cooling Tower. Cooling tower efficiency is limited by the ambient wet bulb temperature. In an ideal case, the cold water temperature will be equal to the wet bulb temperature. This is practically not possible to achieve. This requires very large tower and results in huge evaporation and windage or drift loss resulting in a practically not viable solution. In practice, the cooling tower efficiency will be in between 70 to 75%.

Cooling Tower Efficiency = (Range/ (Range + Approach)) x 100

In summer the ambient air wet bulb temperature raises when compared to winter thus limiting the cooling tower efficiency.

4. Other Cooling Tower Calculations:

This includes determination of cycle of concentration, Evaporation loss, Drift or Windage Loss, Blow down water requirement makeup water requirement.

4.1 Cycle of Concentration:

The cycle of concentration is a dimensionless number. It is a ratio between parameter in Cooling Water to the parameter in Makeup water. It can be calculated from any the following formulae.

COC= Silica in Cooling Water / Silica in Makeup Water

COC = Ca Hardness in Cooling Water/ Ca Hardness in Makeup water

COC = Conductivity of Cooling Water / Conductivity of Makeup water

The cycle of concentration normally varies from 3.0 to 7.0 depending on the Process Design. It is advisable to keep the Cycle of concentration as high as possible to reduce the makeup water requirement of the cooling tower. At the same time, the higher cycle of concentration increases the dissolved solids concentration in circulating cooling water which results in scaling and fouling of process heat transfer equipment.

4.2 Draw off or Blowdown:

As the cooling water circulates the cooling tower part of water evaporates thereby increasing the total dissolved solids in the remaining water. To control the Cycle of Concentration blow down is given. Blowdown is the function of Cycle of concentration. Blowdown can be calculated from the formula:

B = E/ (COC-1)

B = Blow Down (m3/hr)

E = Evaporation Loss (m3/hr)

COC = Cycle of Concentration. Varies from 3.0 to 7.0 depending upon Manufactures Guidelines

4.3 Evaporation Loss Calculation:

Evaporation Loss in the cooling tower is calculated by the following empirical equation.

E = 0.00085 x R x 1.8 x C

Where,

E = Evaporation Loss (m3/hr)

R= Range

C = Circulating Cooling Water (m3/hr) (Using: Perry's Chemical Engineers Hand Book )

Alternatively, The Evaporation loss can be calculated from the heat balance across the cooling tower. The amount of heat to be removed from Circulating water according to Q = m Cp ΔT is C x Cp x R . The amount of heat removed by evaporative cooling is

Q = m x Hv is E x HV

On Equating these two, we get

E = C x R x Cp / HV

Where,

E = Evaporation Loss in m3/hr

C= Cycle of Concentration

R= Range in °C

Cp = Specific Heat = 4.184 kJ / kg / °C

HV = Latent heat of vaporization = 2260 kJ / kg

4.4 Windage or Drift Loss Calculation:

Drift loss of the cooling tower is normally provided by the cooling tower manufacturer based on the Process design. If it is not available it may be assumed as

For Natural Draft Cooling Tower D = 0.3 to 1.0 * C /100

For Induced Draft Cooling Tower D = 0.1 to 0.3 * C /100

For Cooling Tower with Drift Eliminator D = 0.01* C /100

Cooling tower mass balance gives an idea about make-up water requirement. Cooling Tower Makeup has to substitute the water losses resulting from Evaporation, Windage and Blowdown.

M = E + D + B

Where,

M = Make up water Requirement in m3/hr

B = Blow Down in m3/hr

E = Evaporation Loss in m3/hr

D = Drift Loss in m3/hr