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Project - Waste Heat Recovery:Sulphuric Acid Plant

Dr. M G Dastidar

Professor(M.Tech Co-ordinator)

Dr. S C Mullick

Professor Emeritus(Project Guide)

Project By

Gourav ChutaniEntry No. 2010JEN3552

Mid-Term Review Minor Project

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Objective

To analyze the Waste Heat Recovery System in Sulphuric Acid

Plant. The project will calculate the total waste heat recovery

utilization and also do comparative study for the following:

a. Energy Saving

b. Cost Saving

c. GHG Emissions Reduction

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Background

The Company is manufacturing Fertilizer - Single Super Phosphate (SSP). One ofthe key raw material for manufacturing of SSP is Sulphuric Acid.

The Company has own Sulphuric Acid manufacturing plant to cater the need of

fertilizer.

Sulphuric acid is produced by Double Contact Double Absorption (DCDA).

Sulphur is the main raw material.

Sulphur is melted, filtered and then oxidized to SO2.

SO2 is further oxidized to SO3 by passing through a catalytic convertor

having Vanadium Pentoxide V2O5 as catalyst.

The converted SO3 is absorbed in concentrated sulphuric acid to for oleum.

Then oleum is converted to sulphuric acid.

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DCDA

The DCDA process contains the following equipments

1. Melting pit

2. Furnace

3. Convertor

4. Absorber

5. Scrubber

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Process Flow Diagram

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Melting Pit

The sulphur obtained may contain impurities like ash, mud etc. therefore sulphur is to be

purified to about 100%. The purification is done by melting. When sulphur is melted the

impurities present in it remain in the molten sulphur itself. Sulphur has a melting point of

130 C.

The un-dissolved impurities are separated by settling in the melting pit itself. The overflowfrom the melting pit is pumped to the furnace.

The temperature of molten sulphur is maintained at a value of 130 C. If the temperature is

below 130 C the pumping of molten sulphur becomes difficult and above that temperature

causes firing of molten sulphur.

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Furnace

.Air is fed through air blower in the furnace. The molten sulphur reacts with compressed

air and give sulphur dioxide gas (SO2). This furnace is used for burning molten sulphur

and compressed air. The furnace is a horizontal, cylindrical vessel with sulphur guns. The

furnace is made up of carbon steel, acid proof bricks and fire resistant bricks. The molten

sulphur is burned with excess air to generate Sulphur di-oxide.

S+O2 -> SO2

Molten Sulphur is pumped from a storage tank through heated lines and sprayed into the

furnace. Dry air from air drying tower is introduced into the furnace. The temperature of

furnace is kept at about 900-950 C, otherwise leakage of sulphur takes place.

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Waste Heat Recovery

Before the gases are fed to the first stage of the converter, they are adjusted

to the minimum temperature at which catalyst rapidly increases the speed of

reaction, usually 450-500 C. For that purpose heat exchanger 1 is used. It

has shell and tube side.

SO2 gas at a temperature of around 900 C is passed through the tube side

and low pressure steam is passed through shell side. The temperature of SO2

gas is reduced to about 450-475 C and low pressure steam gets converted

into High Pressure Steam (HPSTM).

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Convertor

The chemical conversion of sulphur dioxide to sulphur trioxide is designed to maximize

the conversion by taking into consideration that

1. Equilibrium is an inverse function of temperature and a direct function of the oxygen

to the sulphur dioxide ratio

2. Gas composition and amount of catalyst affect the rate of conversion and thekinetics of the reaction.

3. Removal of sulphur trioxide formed allows more sulphur dioxide to be converted.

The commercialization of these basic conditions makes possible high overall conversion

by using a multi pass convertor.

A four pass convertor is used for conversion in the sulphuric acid plant. It has 4 beds of

ring and star type materials coated with vanadium pentoxide catalyst for better

conversion. The conversion takes place in two stages.

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Stage-1

Sulphur dioxide gas from the mixing chambers is given to the 1st layer of the 4 pass

convertor. This reaction is highly exothermic. Thus the temperature increases to 800 C. Ifthis conversion stream is directly given to the 2nd layer of the convertor then the catalytic

bed may be spoiled. In order to avoid that condition the outlet from the first layer is given

to a super heater.

Super Heater

The gaseous stream from the first stage of the convertor is given to the tube side of the

superheater (Heat Exchanger 2, HE2) and high pressure steam is passed through the shell

side. Then the temperature of gaseous stream is reduced to 550 C.

The outlet of the superheater is given to the second bed of the convertor where 28%

of conversion of SO2 to SO3 takes place. The conversion raises the temperature of gas to

620 C. For cooling this gas before it is given to the third layer another heat exchanger

(HE4) is used.

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The temperature of SO3 gas reduces to 520 C. The gas stream from this heat exchanger is

given to the third layer of the convertor. In the third layer 6% conversion of SO2 to SO3

takes place and the temperature rises to 580 C. This completes the first stage of

conversion of SO2 to SO3.

To further convert SO2 into SO3, arrangement is done for absorption of converted SO3.

The temperature of the gas is to be further reduced. Thus another heat exchanger is

used.

Inter-pass absorption tower (absorber 1)

The SO3 gas with reduced temperature is absorbed in an inter pass absorption tower. It is

cylindrical in shape and the gas is absorbed in 98.5% sulphuric acid. The acid is sprayed

from the top of the IPAT and gas is given through bottom. The temp of IPAT is 70 C.

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Stage-2

The unabsorbed SO3 gas from the IPAT is given to the 4 th bed of the catalytic convertor.

The temperature of gas is increased by Heat exchangers (HE3 & HE4) to a temperature

of 480 C and 5.75% conversion of SO2 to SO3 takes place in the 4th

bed and temperatureis increased due to exothermic reaction.

Another heat exchanger is used to reduce the temperature of gas before reaching Final

Absorption Tower (FAT)

Final absorption tower (absorber 2)

Second stage absorption of SO3 gas is done in the final absorption tower. It is similar to

IPAT. Sulphuric acid at 98.5% concentration is sprayed from the top of the FAT and SO3

gas from the heat exchanger is given to the bottom of FAT absorption of SO3 results.

The outlet of both the IPAT and FAT has the formula H2S2O7 (oleum)

SO3 + H2SO4 ->H2S2O7

The oleum is collected in acid pumping tank.

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Acid pumping

The oleum from FAT is pumped back to IPAT where make up water is mixed to form

Sulfuric Acid. Some portion is pumped back to FAT to produce oleum and rest is

collected as final product.

Inlet Temperature (Deg C) Outlet Temperature (Deg C)

Heat Exchanger 1 826 477






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Energy CalculationsInput Energy

Exothermic energy utilized reheating by Heat Exchanger 3 & 4 = 46588 GJ (13.1%)

Superheated Steam Generated

Quantity Units Pressure Units

Temperatur

e Units Enthalpy Units

Total

Energy

Recovered Units

66673 MT 15 kg/cm2 275 C 2981.2 Kj/Kg 198765 GJ

7.84 bar

HSD

Quantity 1200 L

CV 10000 Kcal/Kg

Energy 42.13 GJ

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Available Energy from Exothermic Reaction

S + O2 -> SO2 Heat of reaction: -297 KJ/mol

SO2 + 0.5O2 ->SO3 Heat of reaction: -99 KJ/mol

SO3 + H2SO4 -> H2S2O7

H2S2O7 + H2O -> 2(H2SO4)

SO3 + H2O -> H2SO4 Heat of reaction: -132.5 KJ/mol

Total H2SO4 produced by company = 65631 MT

= 669704082 mol

Energy (Exothermic) = 353939 GJ

Energy input (Calculated Earlier) = 42.13 GJ

Energy Recovered = 198765 GJ

Utilization of Waste Heat

For Generating Superheated Steam = 56.16 %

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Way Forward

The Project will further have following analysis:

Analysis of each Heat Exchanger Utilization

Scope for enhancement of waste energy utilizations

Cost savings & GHG emission reduction without enhancement

& with enhancement of system

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