MATERIAL BALANCE

MATERIAL BALANCE


GIVEN:

TO DESIGN A 100 TPD CAPACITY H2SO4 ACID PLANT

BASIS:
1 HOUR OF OPERATION.

PURITY:
Product which is to be manufactured is assumed to have strength of 98% acid.


100 TPD implies that we have 100 x 103 / 24 = 4166.67 Kg/hr of Acid

With 98% purity, the acid that is produced per hour = ( 98 x 4166.67) / 100
= 4083.34 Kg/Hr

Kmoles of Sulphuric acid to be produced = 4083.34 / 98
= 41.667 Kmoles/Hr

It’s assumed that overall absorption of the acid is 100 %

Then, SO3 required = 41.6667 / 1.0
= 41.6667 Kmoles/Hr

Also its assumed that the overall conversion of SO2 to SO3 in the reactor is 99.8%

Then SO2 required = 41.6667 / 0.998
= 41.75 Kmoles/Hr
= 2672 Kg/ hr

Assuming 100% combustion of Sulfur,

Then S required = 41.75 Kmoles/Hr
= 1336.0 Kg/ hr

Amount of oxygen required to convert 1Kmole of S to SO3 = 1.5 Kmoles

Then, O2 required = 41.75 x 1.5
= 62.625 Kmoles/ hr


As cited in the literature that some amount of excess oxygen must be used,
Using 40% excess,
O2 required = 62.625 x 1.4
= 87.675 Kmoles/ hr
= 2805.6 Kg/ hr

From this the total dry air that is coming in can be calculated as.

Dry air in = ( 87.675 x 100) / 21
= 417.5 Kmoles/Hr

At 300C, assuming 65% Relative Humidity,

Humidity as calculated from the psychometric chart is,

Humidity = 0.0165 Kg water/ Kg dry air

Then, water entering with dry air = 417.5 x 28.84 x 0.0165
= 198.672 Kg/Hr
= 11.037 Kmoles/Hr

Total weight of entering air = 417.5 x 28.84 + 11.037 x 18
= 12239.336 Kg/hr

Then,
Weight of sulphur required = 1336.0 Kg/ hr
Weight of air entering = 12239.366 Kg/ hr


DRYING TOWER:
















Making a Mass balance around the Drying Tower

P + R = Q + S

As water is being removed from the incoming air to make it dry, the 98% acid that is being recycled to the tower, decreases in concentration and let this concentration be assumed as 97%, then we can write,

0.02 x R + 198.672 = S x 0.03 (1)

H2SO4 Balance will give,
R x 0.98 = S x 0.97 (2)

Solving the above equations

Amount of 98% H2SO4 entering R = 19271.184 Kg/hr
Amount of 97% H2SO4 leaving S = 19469.856 Kg/hr

SULFUR BURNER:











The combustion reaction takes place inside the burner, where Sulphur is oxidized to Sulphur Dioxide
Moles of Sulfur coming in = 41.75 Kmoles/ hr = 1336.0 Kg / hr
Moles of Oxygen coming in = 87.675 Kmoles/ hr = 2805.6 Kg / hr

As mentioned before we have assumed 100% combustion of sulphur,

Sulfur Dioxide formed = 41.75 Kmoles/ hr = 2672.0 Kg/ hr

Oxygen leaving = 2805.6 – 1336.0
= 1469.6 Kg/ hr

Nitrogen leaving = (87.675 x 79) / 21
= 329.825 Kmoles/ hr = 9235.1 Kg / hr


REACTOR:























As cited in the reference by author NORMAN SHREVE et al Pg 337, the temperature and conversions in each stage of a Monsanto Converter is given as follows:


Location Temperature 0C Equivalent conversion (%)
Gas entering I pass
Gas leaving I pass
Rise in temperature 410.0
601.8
191.8
74.0
Gas entering II pass
Gas leaving II pass
Rise in temperature 438.0
485.0
47.30
18.4
Gas entering III pass
Gas leaving III pass
Rise in temperature 432.0
444.0
12.0
4.3
Gas entering IV pass
Gas leaving IV pass
Rise in temperature 427.0
430.3
3.30
1.30

Total rise
254.4
98.0

As we see from the table that the overall conversion in the reactor is 98% but to validate our assumption that was made earlier, we assume that the conversion in the last stage of the reactor is 3.1% instead of 1.3% so that the assumption of 99.8% as overall conversion remains unaffected and thus temperature for the gas leaving the fourth pass is then assumed to be 4370C.

Component
I stage II stage III stage

S 74 %
conversion 18.4%
conversion 4.3%
conversion



For II stage:

Sample calculation for the 2nd stage is shown as follows:

Components:

SO2 INLET = 694.72 Kg/ hr
SO2 OUTLET = 694.72 – 41.75 x 0.184 x 64
= 203.072 Kg/ hr

N2 INLET = 9235.1 Kg/ hr
N2 OUTLET = 9235.1 Kg/ hr

O2 INLET = 975.28 Kg/ hr
O2OUTLET = 975.28 – 41.75 x 0.5 x 0.184 x 32
= 852.368 Kg/ hr

SO3 INLET = 2471.6 Kg/ hr
SO3 OUTLET = 2471.6 + 41.75 x 0.184 x 80
= 3086.16 Kg/ hr




components I stage
(Kg/ hr) II stage
(Kg/ hr) III stage
(Kg/ hr)

Inlet
Outlet Inlet Outlet Inlet Outlet
N2 9235.1 9235.1 9235.1 9235.1 9235.1 9235.1
SO2 2672.0 694.72 694.72 203.072 203.072 88.176
SO3 0.0 2471.6 2471.6 3086.16 3086.16 3229.78
O2 1469.6 975.28 975.28 852.368 852.368 823.644
Total (Kg/ hr) 13376.7 13376.7 13376.7 13376.7 13376.7 13376.7
After the passage from the 3 stages or after the first contact, the gases are let into the interpass absorber where the absorption of the SO3 takes place. After the contact with H2SO4 in the tower, the gases are returned back to the 4thstage for the second contact.

We can write from the reaction for sulfur dioxide oxidation to give sulfur trioxide that,

KP = (PSO3) / (PSO2) (PO2)1/2 (A)

And available in the reference by author NORMAN SHREVE et al Pg 333, the equilibrium constants for the Sulfur Dioxide Oxidation are given at different temperatures.

Now for KP at the entering temperature of 4th stage i.e. 4270C,

We have KP = 270.2


INTERPASS ABSORBER:

From the equation (A), SO3 present are calculated by the following

Let
X1 = Moles of SO2
X2 = Initial moles of SO2 entering the reactor
X3 = Moles of O2
X4 = Moles of SO3

Then
KP = (X4 + 0.031 x X2) x (X1 + X3 + X4)0.5 (B)
(X1 - 0.031 x X2) x (X3 – 0.05 x 0.031 x X2)0.5

The above equation is once again mentioned in the literature. Calculating the value of the unknown X4, we have, with KP = 270.2, we get

X4 = 15.8 Kmoles/ hr (or) 1264 Kg/ hr


Then Moles of SO3 removed in the interpass absorber is given as

(40.373 – 15.8) = 24.573 Kmoles

SO3 + H2O H2SO4

As from the stoichiometric coefficients of the reaction given, we can find out the weight of sulfuric acid to be absorbed as

( 24.573 x 98 ) = 2408.154 Kg/Hr
Also mentioned in the literature that “its required to take the strength of the solvent H2SO4 for absorption of SO3 not to increase by more than 1-2%, and the best absorption will occur when the absorbing acid has the strength between the range 97.5 to 99%”.
























SO3 absorbed in this stage = 40.373 – 15.801
= 24.572 Kmoles
= 1965.76 Kg/ hr

From stoichiometry

H2SO4 required for absorption of 1965.76 Kg of SO3 = 1965.76 x 98 / 80
= 2408.056 Kg/ hr

The recycle stream contains 98% H2SO4

0.98 X = 2408.056 Kg/ hr
X = 2457.2 Kg/ hr

Weight of the recycle stream = 2457.2 Kg/ hr
Weight of H2S2O7 produced = 1965.76 x 178 / 80
= 4373.816 Kg/ hr

Weight of H2O accompanying = 491.44 Kg/ hr
DILUTION TANK I:















Weight of H2O required to dilute 4373.816 Kg of H2S2O7 = 4373.816 x 18 / 178
= 442.296 Kg/ hr

H2SO4 produced from H2S2O7 = 4373.816 x 196 / 178
= 4816.112 Kg/ hr

The above H2SO4 contributes 98% of X, then
0.98 X = 4816.112
X = 4914.4 Kg/ hr

H2O actually to be added = 4914.4 x 0.02 + 442.292 – 49.144
= 491.44Kg/ hr


REACTOR:
















For the second contact we have the following details,

Components
N2 SO2 SO3 O2 Total

Inlet (Kg/ hr)

9235.24
88.1792
1264.08
823.648
11411.21

Outlet (Kg/ hr)

9235.24
5.344
1367.68
802.944
11411.21


FINAL ABSORPTION TOWER:

SO3 + H2SO4 H2S2O7

SO3 entering = 1367.68 Kg/ hr
H2SO4 required =1367.68 x 98 / 80
= 1675.408 Kg/ hr

0.98 X = 1675.408 Kg/ hr

Recycle stream, X = 1709.6 Kg/ hr

In the exit stream,

H2S2O7 produced = 1675.408 x 178 / 98
= 3043.088 Kg/ hr

H2O leaving = 34.192 Kg/ hr


DILUTION TANK II:

H2S2O7 + H2O 2 H2SO4

H2O required to dilute H2S2O7 = 3043.088 x 18 / 178
= 307.78 Kg/ hr

H2SO4 produced = 3043.088 + 307.78
= 3350.816 Kg/ hr

This contributes 98% of the acid
0.98 X = 3350.816

X = 3419.2 Kg/ hr

Then, H2O to be added = 3419.2 – 3043.088
= 376.112 Kg/ hr

Then, H2O added from dryer = 19469.856 – 19271.184
= 198.67 Kg/ hr

H2O added from final absorption tower = 34.91 Kg/ hr

H2O to be added = 376.112 – 198.67 – 34.91
= 143.24 Kg/ hr

H2SO4 produced = 3419.2 – 1709.6
= 1709.6 Kg/ hr


STORAGE TANK:












H2SO4 from dilution tank I = 2457.2 Kg/ hr
H2SO4 from dilution tank II = 1709.6 Kg/ hr
Total sulphuric acid produced = 4166.8 Kg/ hr
= 100 TPD

METHODS OF MANUFACTURING

METHODS OF MANUFACTURING

As described above, sulfuric acid is an important raw material for phosphate fertilizer production and to a much lesser extent for nitrogen and potassium fertilizers. World production of sulfuric acid was about 121 million tons in 1977 and about half of this production was used in the fertilizer production.

About 58% of the worlds production was based on elemental Sulfur, 25% on Pyrite and 17% on other sources. Of the other sources, the principle one was the by-product sulfuric acid recovered from smelting operations.

In general terms, the sulfuric acid is produced by catalytic oxidation of sulfur dioxide to sulfur trioxide, which is subsequently absorbed in water to form sulfuric acid. In practice the sulfur trioxide is absorbed in sulfuric acid which is kept at a controlled concentration (usually 98%) by the addition of water.

There are no major variations of commercial interests on this mentioned chemistry. There are alternatives as to source of Sulfur dioxide and method of conversion to sulfur trioxide. The two most common methods for the conversion of sulfur dioxide to sulfuric acid are

1. Lead Chamber Process:
2. Contact Process


LEAD CHAMBER PROCESS:

This is an old process and was introduced in Europe in near the middle of 18th century. This method uses nitrogen oxides as oxygen carrying catalysts for the conversion of sulfur dioxide to sulfur trioxide. The reactions which produce the sulfur trioxide and sulfuric acid take place within the huge lead chambers or in packed towers which may be substituted for the chambers. Chambers process produced acid of concentration less than 80 %.The major disadvantage includes the limitations in throughput, quality and concentration of the acid produced. All known new plants uses the Contact process although some older Chamber process plants may still be in use.

CONTACT PROCESS:

In the contact processes, the sulfur dioxide is converted to sulfur trioxide by the use of metal oxide catalyst. Platinum was once widely used as catalyst but because of its excessive first cost and susceptibility to poisoning, it has been largely replaced by vanadium oxide. The vanadium pentaoxide is dispersed on a porous carrier in a pellet form. The characterstics of the catalyst which can be used are mentioned as follows:

1. Porous carrier having large surface area, controlled pore size and resistance to process gases at high temperature; in pellet form if used in fixed bed and powdered form if used for fluidized bed. Ex- Alumina, silica gel, zeolites.
2. Active catalytic agent:
Vanadium pentaoxide in this case. Preparations are generally kept secret for the competitive reasons but they usually consist of adding water soluble compounds to gels or porous substrates and firing at temperature below the sintering point.
3. Promoter:
Alkali and/or metallic compounds added in trace amounts to enhance the activity of the catalytic agent.

Advantages of the V2O5 catalyst
1. Relatively immune to poisons.
2. Low initial investment and only 5% replacement per year.

Disadvantages of V2O5 catalyst
1. Must use dilute SO2 input (7-10%).
2. As a catalyst it is less active and requires high oxygen or sulfur dioxide to give economic conversions
3. Requires larger converters and thus higher initial investment.

Now the SO3 gas is passed to an absorption tower where it is absorbed in recirculating concentrated acid. There are many variations in the contact process depending upon the types of raw materials available and other considerations; also a number of engineering variations are in use by many different design/construction firms offering services in this field.

Main disadvantages of the contact process are that concentrated acid (98%) of high purity can be produced directly and that compact plants of quite high capacity have now become rather common place.














DESCRIPTION OF CONTACT PROCESS
CHAPTER-5

THE PRODUCTION OF SULFURIC ACID BY CONTACT PROCESS:


RAW MATERIALS:

One of the early raw materials for the sulfuric acid was sulfate of iron or vitriol. By heating the solid sulfate and condensing the fume an oil of vitriol resulted. The rectified oil of vitriol (ROV) is concentrated acid and the brown oil (BOV) is about 77% Sulfuric Acid. The raw materials for sulfuric acid manufacture are chiefly Sulfur, Pyrites, Spent oxide, anhydrite and gases from the smelting of metalliferous ores, from the purification of natural gas and from refining operations.


CHEMISTRY OF SULFURIC ACID PRODUCTION:

The equations governing the production of sulfuric acid are:
S + O2 SO2 ΔH = -70 KCal
(solid) (gas)

SO2 + 1/2 O2 SO3 ΔH = -23.50 KCal

SO3 + H2O H2SO4 ΔH = -32 Kcal


The first reaction expressing the combustion of sulfur is strongly exothermic; sulfur on burning gives about one third of the heat of combustion of coal, and this heat raises the temperature of combustion gases roughly in accordance with the graph as shown



This heat is high in temperature and there is plenty of it, consequently it is worth utilizing and the hot gases are led across pipes through which the water passes. The water is heated, steam is raised and the gases are cooled. This is the arrangement in the water tube boiler. In the fire tube boiler the hot gases pass through the tubes which are surrounded with water.

The second equation is also exothermic and it’s apparent that the equation gives a decrease in volume, three volumes become two volumes and this reaction would be aided by pressure. High conversions are however, obtainable with catalysts at 400 to 500 0C with a small excess of oxygen and the use of pressure.

The third equation represents the absorption of sulfur trioxide to form sulfuric acid. It is exothermic and the absorbing sulfuric acid has to be cooled continuously; the heat is available at a relatively low temperature and is not worth recovering. Sulfuric acid is used for the absorption of sulfur trioxide as it has been found in practice that sulfur trioxide and water form a mist, which is difficult to separate from the gas and that under these conditions the absorption, is not complete. The strength of the acid is best about 98%.


SULFUR HANDLING AND STORAGE:

Sulfur used for the production of sulfuric acid is practiced to handle as solid in bulk, from ship to wagon and from wagon to cool off and solidify; it can then be broken up and shovelled into wagons for disposal. It consists of carbonaceous matter and inorganic ash. Although commercial sulfur is over 99% sulfur, the impurities present as dust in the plant gases tend to be filtered out by the catalyst and a blanket or layer of hardened dust on the catalyst detracts from the efficiency of conversion. In consequence some manufacturers filter the molten sulfur through leaf filters to remove some of the impurities and so obtain a longer period before the plant has to be shut down for cleaning away the dust and sieving the top layers of the catalyst from it. However, the filter leaves have to be removed for the replacement of the filtering medium and for the removal of the
accumulated sludge, and these unpleasant operations, together with the installation cost of the equipment, have to be weighed against the benefits of having slightly less dusty gases.


SULFUR BURNING:

There are several types of burners for sulfur. One is revolving cylinder containing pool of molten sulfur which is combusted by the passage of air over its surface. Another type is a brick lined vertical cylindrical vessel in which is erected a pile of fire brick in the form of a pyramid, and on to this structure molten sulfur is pumped to be met by a stream of concurrent dry air for its combustion. A third variety is in the form of a burner similar to an oil burner.

The quantity of air is regulated to give between 8 to 10% SO2.

In the starting up the plant with a vertical burner the fire-brick is first heated by the burning of a fuel gas. The gas, when a sufficiently high temperature has been reached is cut off and the liquid sulfur is pumped over the brick-work. A measured amount of air is passed down the burner and the sulfur burns to sulfur dioxide providing sufficient heat in normal operation to raise the temperature of the gases to some 810-900 0C. If the air contains 21% oxygen and sulfur is burned to give 10% sulfur dioxide gas, then 11% oxygen and 79% nitrogen will form the residual gases. Some excess oxygen is necessary over and above that required to combine with the sulfur and the sulfur dioxide. Out of the 11% oxygen, 5% will be required to combine with the dioxide to form the trioxide leaving an excess of 6%. This is adequate but if attempts were made to have a 14% sulfur dioxide gas then 14% from the 21% oxygen would be taken up in forming sulfur dioxide and another 7% would be required to convert the dioxide to trioxide. This would leave no excess oxygen and an excess has shown to be essential for a good conversion of the dioxide.

When sulfur burns, the gases rise to a temperature depending on the dioxide concentration; with 8% sulfur dioxide the temperature is about 750 0C. To withstand these temperatures the burner is brick lined and the area of the brickwork radiates heat and helps to burn the sulfur completely in the time given by the volume of the burner for the passage of the gases.

The composition of the gas can be varied by altering either the air or the sulfur to the burner. The very hot gases containing the ash from the sulfur are led straight into the waste heat boiler.


THE WASTE HEAT BOILER:

The object of the waste heat boiler is to utilize the heat in the gases to generate steam. A water tube boiler consists of tubes among which gases pass, the tubes being full of water. The gases heat the tubes which in turn raise the temperature of the water. The boiler is in the form of cylinders connected by hairpin shaped tubes arranged across the path of gases. The tubes are kept filled with water (to prevent burning) and are connected at the top to a steam drum, a cylindrical vessel in which water is kept at a constant level by an automatic feeding device. The steam drum, where the water boils, is above the tubes and serves to supply water to the boiler and for the release of steam. The surface area of the water inside the drum must be sufficient to minimize the carryover of the spray with steam which is led off from the top of the drum and then through super heater tubes by which the steam is heated several degrees above its condensation temperature to give it superheat and make it free from droplets, dry and suitable for use in turbines or other steam engines. The water circulates from the steam drum to the sludge drum, another cylindrical shaped vessel at the bottom of the boiler, from which the solids, deposited from the evaporating water, are sludged out and the solid content of the boiler water controlled. The water circulates through the hairpin tubes upwards to another cylindrical drum and then passes from this intermediate drum upwards again through another set of hairpin tubes to the steam drum. The pressure under which steam is generated depends upon the purpose for which it is to be employed.

There are the usual auxiliaries which are associated with a boiler, the feed pumps for pumping the feed water into the drum against the boiler pressure, the feed water preparation tank where phosphates and alkali are added to the water to prevent boiler corrosion, and the economizer which heats the boiler feed water near to the temperature of the water in the boiler drum. The feed water is sometimes preheated by exhaust steam from the boiler feed pumps before being heated in the economizer. Preheat may be to 1000C and the final temperature of the feed water say 2150C. The economizer and the super heater obtain their heat from the sulfurous gases at convenient points in the process usually from the converter after the second and third stages. Cold water in the economizer tubes could cause local condensation from the sulfurous gases and result in corrosion. After the first stage of conversion there is another waste heat boiler similar in construction to the first boiler but smaller and the two boilers use the same steam drum. The inlet gas temperature to the converters should be 380-4000C and the waste heat boiler is designed to take away the heat from the gases until their temperature is in this range. This reduction in temperature is about 4000C and corresponds to several tons of steam an hour form the moderate size sulfur burning plant.

The temperature of the exit gases is controlled by a by-pass on the waste heat boiler. The amount of heat evolved is dependent upon the quantity of sulfur and the temperature on the proportion of sulfur dioxide in the gases. The higher the temperature, the higher is the proportion of heat which is to be removed. It is best to gauge this so that the waste heat boiler by-pass in normal operation is almost shut.

The waste heat boiler design takes into consideration the following factors. The area of the tubes must be adequate to take the requisite amount of heat from the gases. This is dependent upon the amount of heat transferred per unit of tube area, which itself is dependent upon the velocity of gases over the tubes and the temperature difference between the gases and the boiler water


GAS DRYING:

It has been found in practice that if moisture is present in the gases before conversion, a sulfur trioxide mist will form after the converters, which is extremely difficult to absorb in the acid absorbers. There are several theories to account for this. It may be that the sulfur trioxide particles are surrounded by a film of acid and the aggregates are sufficiently small to pass through the absorbers and out into the atmosphere, giving rise to inefficient absorption and causing a local nuisance. There mists can be destructive to vegetation, damaging to buildings and extremely unpleasant to life in the vicinity. It is therefore essential to use dry air for sulfur burning or install special plant for absorbing the mist, which is a difficult proposition.

Air is dried in a drying tower which uses strong sulfuric acid for this purpose. The tower consists of a mild steel vertical cylinder lined with acid-resisting brick and packed with ceramic rings. The acid is distributed down the tower and air is blown upwards countercurrent to the acid by a blower which also serves to give the air sufficient for it to pass through the whole of the sulfuric acid plant to atmosphere after the absorbers. The vapor pressure of water above acids of high concentrations at ordinary temperatures can be extremely low and consequently under the right conditions (95-98% H2SO4 at 35 0C or less) the acid removes nearly all the water vapor in the air that is down to 30mg/m3.

The tower has to be of a sufficient diameter not to require a significant pressure drop for the gases to pass through it and to have an adequate surface area of packing for the absorption of water from the air to take place in the time which the gases take to pass through the tower volume. Consequently the gas velocity up the tower must be small, and this will determine the minimum diameter of the tower. The amount of acid used in the tower has to be significant to wet the surface of the packing without flooding and not to become so dilute that its vapor pressure becomes appreciable. The amount of water in air at a given temperature and relative humidity is known and hence the minimum quantity of 98% acid on the drying tower can be calculated.

It is common practice to use acid from the absorption section on the drying tower; the heat of dilution is then removed on the absorption coolers but coolers are sometimes provided on the drying section with a bleed-off to the absorption circulation system.

GAS FILTRATION:

The gas from the burners after passage through the waste heat boiler contains ash from the sulfur and some scale from the waste heat boiler and gas lines. These solid impurities are best removed before the gases enter the converter; otherwise the dust accumulates on the layers of catalyst and causes channeling through the catalyst layers, irregular contact and pressure drop.

The filter consists simply of the wide diameter vessel filled with the filtering medium which is commonly the lumps of quartz. The vessel is of squat cylindrical shape in mild steel. The gases pass downwards to assist in the removal of the solids at a velocity which is slow because of the wide diameter of the vessel. When the filter is first put into service, the pressure drop is several m atmospheres, but when it is ready for opening and cleaning, this pressure drop rises to some 100m atmosphere. The interval between removals of dust depends on the ash content of the sulfur; a filter usually lasts three to six months and this period would be extended if the molten sulfur were also filtered before burning.

In many cases the gas filtration unit may not be present and thus this step may be treated as the auxiliary unit, depending upon the requirement. In the flow sheet given, the gas filtration unit is not shown.

CONVERSION:

The converter is a reactor and its objective is to combine the sulfur dioxide with the residual oxygen in the gases to form sulfur trioxide. The conversion is aided by a catalyst and the more sensitive the catalyst the lower the temperature at which the conversion takes place and more favorable the equilibrium but in general sensitive catalyst are more readily poisoned. In practice it is necessary to have a catalyst which is sufficiently robust to resist poisoning but is active enough to give good conversion at about 400 0C. The converter consists of a tall cylindrical vessel of sufficient diameter (generally 3.5 to 5.5m) to give a low gas velocity, inside which there are three or four trays for quantities of catalyst. Between the catalyst sections there are devices for cooling the gases to keep the temperature entering the later catalyst sections in the region of 405 to 440 0C. The first catalyst pass contain relatively little catalyst because the reaction is rapid and the temperature rises sharply; the second a little more, and the last stages most of the catalyst, where both the sulfur dioxide and oxygen are less concentrated.

After the passage through the first catalyst tray when the gas temperature has risen from about 4100C to over 6000C, the gases pass into an external waste heat boiler to raise steam and bring the gas temperature down to 4300C and at this temperature the gases enter the second catalyst tray. On passing through the catalyst the temperature again rises but this time not so much, and after the second tray sufficient heat can be removed by superheating the steam raised in the waste heat boilers. The super heater tubes are led from the boiler into a space underneath the catalyst bed in the path of the gases. The temperature is again brought down to about 4300C and after the third pass the gases are similarly cooled. In the final section, which contains most of the catalyst, the temperature rise is small as the reaction has been brought near the equilibrium value in the previous passes and only relatively small amounts of sulfur dioxide and oxygen remain to react. After leaving the catalyst the gases are at 400 to 4500C; they are passed through the economizer where the temperature is reduced to a lower value. The gases then pass through an air cooler to the absorbers.

The catalyst consists of vanadium in the form of small pellets or cylinders. The total volume is arranged to give the time of contact necessary for the reaction to take place. The speed of the reaction depends on the activity of the catalyst. A conversion of sulfur dioxide to trioxide of between 98 and 99 % is achieved.

The equilibrium is given by

KP = (PSO3) / (PSO2) (PO2)½

(PSO2) (PO2)½ should be as high as practicable to give a good value for (PSO3). If there is excess of oxygen (PO2)½ will increase in value but too great an excess will diminish (PSO2) initially. On some converters, air is introduced between the converter stages which acts as a cooling medium and provides the additional excess of oxygen.


Below 4000C the reaction is very slow but above 6300C the reaction is fast but the equilibrium is becoming unfavorable, the reaction goes more quickly the higher the temperature, but the equilibrium becomes unfavorable. The aim in running the converter is to maintain a pattern of temperature which experience has shown will give the optimum conversion. These temperatures depend on the catalyst activity, gas strengths and other factors.

The running of the converter consists in close observance of temperatures, the pressure drops and the sulfur dioxide conversion. The temperature rise across the catalyst is the measure of the amount of reaction taking place. Pressure drops across the bed of catalyst, if they are abnormally high, indicate a partial blockage and channeling through the catalyst, which would be accompanied by a small temperature rise. A high pressure drop across the first pass may necessitate shutting down the plant and screening the top layer of the catalyst which has possibly become choked with dust.

ABSORPTION:

The gas leaving the reactor is cooled further in a heat exchanger as mentioned above and before entering the absorption tower where the Sulfur trioxide is absorbed in a recirculated stream of concentrated sulfuric acid. The sulfuric acid is maintained at desired concentration (usually 98% H2SO4) by the addition of water and its temperature is controlled in the desired range of 70 to 900C measured at the tower inlet by cooling the recirculated acid.

Some of the acid goes to the air drying tower mentioned previously where the moisture from the incoming air supplies some of the water needed in the reaction. Since, the heat released in this step is at a low temperature level, little use can be made of it. In the above mentioned Single Absorption process, the recovery of the sulfur as sulfuric acid is 97-98% and the remainder is lost to the atmosphere as Sulfur dioxide. In many countries, the discharge of this amount of Sulfur dioxide to the atmosphere is environmentally unacceptable. Therefore most of the plants use a Double Contact Double Absorption Process (DCDA)

The gas after passing through three catalyst bed goes to the first absorption tower where the Sulfur trioxide is removed. The gas is then reheated to about 4200C, passed through the fourth catalyst bed, then cooled and sent to a second absorption tower.

In the reaction
2SO2 + O2 2SO3

removal of the reaction product sulfur trioxide facilitates more efficient conversion in the last catalyst bed. The DCDA process reduces the sulfur dioxide loss to less than 2Kg of sulfur dioxide/ ton of the sulfuric acid. High efficiency mist eliminators are also required to limit the loss of sulfuric acid mist to less than 0.05Kg/ton of sulfuric acid. Thus the recovery in a DCDA plant should atleast be 99.8%.


THE TAIL GAS:

The gas from the absorption section contains about 0.15 % sulfur dioxide which oxidizes in part to sulfur trioxide and forms mist. At this concentration, corresponding to a conversion efficiency of over 98%, the effluent is tolerable and no further treatment of gas is required. In exceptional cases where the oxygen content is low or for other reasons where the conversion is down, the gases can be scrubbed with ammonia liquor and then treated by electrostatic precipitator.

STORAGE:

The last part of the sulfuric acid plant is the storage and the pumping system. The tanks are large flat cylinders which are sometimes of more than 100 tons capacity. The pumping of acid is commonly done by the centrifugal pumps, the submerged glandless type on smaller tanks where the shaft can be less than about 10ft in length. The absence of gland leaks makes for a neat and clean pumping section. The storage installation should be calculated to maintain continuity of supply, if this is required, during the shutdowns of the acid plant and to cater for peak demands.

DEVELOPMENTS:

The use of the DCDA system adds 10 to 15% to the cost of the plant in comparison with the older Single Absorption Process. It also uses more energy and produces less steam or other recoverable energy. An alternative which is less expensive is to recover the sulfur dioxide from the single absorption plant by ammonia scrubbing. Scrubbing the gas with the ammonia solution produces an ammonium sulfite solution which is then acidulated with sulfuric acid. The liberated sulfur dioxide is returned to sulfuric acid plant and a concentrated ammonium sulfate solution remains which may find use in a fertilizer industry.

Operation of the sulfuric acid plant has some advantages,
1. The equipment is smaller and less expensive
2. Less Catalyst is required.
3. Equilibrium condition and reaction rates are more favorable in the conversion and absorption steps.

The solubility of sulfur dioxide in sulfuric acid increases with the increase in pressure and with the decrease in temperature. In a conventional plant operating at 1 atm and with an acid temperature 1100C, the solubility of sulfur dioxide in sulfuric acid is only 8ppm. However, increasing the pressure to 8 atm and lowering the temperature to 490C, the sulfur dioxide solubility is increased to 190ppm.Under these condition a substantial amount of sulfur dioxide can be transferred in the acid stream to the air drying tower and then to the incoming air stream. Many authors have pointed out that there is no theoretical limit to the amount of sulfur dioxide that can be recycled or recovered; it depends on the rate of re circulation of the acid between the absorber and the air drying towers.

PROPERTIES AND USES

CHAPTER-3
PROPERTIES AND USES

SULFUR:-

Sulfur is one of the most important raw materials used for the production of sulfuric acid. It’s insoluble in water and soluble in organic solvents. It’s available in the nature in the form of rocks, lumps, ground powder, sublimed powder etc. The various physical properties are mentioned in the table shown

CHEMICAL FORMULA S
ATOMIC WEIGHT 32.07
MELTING POINT: RHOMBIC
MONOCLINIC 112.8 0C
119.0 0C
BOILING POINT 444.6 0C
SPECIFIC GRAVITY:
SOLID – RHOMBIC
MONOCLINIC
LIQUID
2.07
1.96
1.803


SULFUR TRIOXIDE:-

Sulfur trioxide at room temperature and atmospheric pressure is a colourless liquid that fumes in air. Trace amounts of water of sulfuric acid can catalyze the formation of polymers. However the polymerization apparently proceeds at a negligible rate so long as the liquid is maintained free of solids ( that is above its freezing point ). Once the solid polymers are present, temperatures of 50 to 75 0C are required to fully convert polymers back to liquid monomers.

There is some controversy about the nature of the polymeric sulfur trioxide. Apparently, polymers can have various molecular weights and degrees or types of cross-linking. The literature generally reports that solid sulfur-trioxide can exist in three trimorphic phases as shown in the table

PHASES MELTING POINTS
ALPHA SO3 62.3 0C
BETA SO3 32.5 0C
GAMMA SO3 16.8 0C

The alpha and beta forms melt to five liquid gamma SO3. The alpha - SO3 phase has a polymeric cross-linked structure that forms crystals resembling ice-needles, beta - SO3 consists of polymeric molecules that form white asbestos-like crystals with a silky lusture and gamma - SO3 is in a colloidal form that consists of cycle trimer and monomer molecules.



The various properties of Sulfur trioxide are summarized in the following table:-

PROPERTY NUMERICAL
VALUE
CRITICAL TEMPERATURE, 0C 217.8
CRITICAL PRESSURE, KPa 8208
CRITICAL DENSITY, g/cc 0.630
TRIPLE POINT PRESSURE (γ phase), KPa 21.13
TRIPLE POINT TEMPERATURE (γ phase) 0C 16.80
NORMAL BOILING POINT TEMPERATURE, 0C 44.80
MELTING POINT, 0C 16.80
TRANSITION TEMPERATURE, 0C -183.0
LIQUID DENSITY (γ phase at 20 0C), g/cc 1.922
SOLID DENSITY (γ phase at 10 0C), g/cc 2.290
LIQUID COEFFICIENT OF THERMAL EXPANSION, / 0C 0.002
LIQUID HEAT CAPACITY (at 30 0C), KJ/Kg0C 3.222
HEAT OF FORMATION OF GAS (at 25 0C), MJ-Kg/mole -395.7
FREE ENERGY OF FORMATION (at 25 0C) MJ-Kg/mole -371.1
ENTROPY OF GAS (at 25 0C) MJ-Kg/mole/ 0C 0.256
HEAT OF DILUTION MJ/Kg 2.109
HEAT OF VAPOURISATION (γ phase), MJ-Kg 0.058
DIFFUSION IN AIR (at 80 0C), m2/s 0.000013
LIQUID DIELECTRIC CONSTANT (at 18 0C) 3.11


SULFURIC ACID:-

Sulfuric acid is a strong acid that is in aqueous solution, it is largely changed to hydrogen ions (H+) and sulfate ions (SO4-). Each molecule gives two hydrogen ions and thus sulfuric acid is dibasic. The general physical properties of the sulfuric are given below:-


EMPIRICAL FORMULA H2SO4
MOLECULAR WEIGHT 98.08
MELTING POINT 10.5 0C
BOILING POINT 340 0C (Decomposition)
SPECIFIC HEAT 1.4435 KJ/Kg
SPECIFIC GRAVITY 1.8357
HEAT OF DILUTION 9.304 KJ/Kg water

Commercial sulfuric acid is sometimes colorless but, it is often yellow and its color ranges from pale to dark brown shades. In dilute solution, it is highly corrosive and attacks nearly all metals.

Sulfuric acid, oily, corrosive colorless liquid when mixed with water releases considerable amount of heat. Unless the mixture is well stirred the added water may be heated beyond its boiling point and the sudden formation of steam may blow the acid out of the container. The concentrated acid destroys the skin and flesh and can cause the blindness, if it gets into eyes. The best treatment is to flush away the acid with large amount of water.

Dilute solution of sulfuric acid show all the behavior characteristics of acids. They taste sour, conduct electricity, neutralize alkalies and corrode active metals with the formation of hydrogen gas. From sulfuric acid, one can prepare both normal salts containing the sulfate group (SO4)- and acid salts containing hydrogen sulfate groups (HSO4)-. Sulfuric acid is not one–function or one purpose product. Its used as drying agent, acidifying agent (pH control), hydrolyzing agent, neutralizing agent, dehydrating agent, oxidizing agent, absorbing agent, purifying agent, leaching agent, catalyst and active reagent is petrochemical industries.

Sulfuric acid need not be one time use product. It can be recovered easily from some phases in the refining of the petroleum and in the manufacture of explosives, petrochemicals, detergents, and dyes. It is often recovered in the form unsuitable for reuse in the same process but of strength and quality suitable for use in another process. Sulfuric acid can also be returned to the producer for the fortification with sulfur trioxide or for the regeneration to strong virgin acid.

Sulfuric acid is not a one-quality product. It is produced and supplied in exact purity for storage batteries, rayon, textiles, dyes, food and pharmaceutical industries, in less pure grade for steel, heavy chemicals, petrochemicals, fertilizers, super phosphates and ammonium sulfate industries.

Sulfuric acid is used in wide range of concentrations from very dilute for pH control to the strong fuming acids used in dyes, explosives, detergents, pharmaceuticals and petrochemical industries.

Standard strengths available in the market are listed below:-


SPECIFIC GRAVITY – 1.250 33.33%
SPECIFIC GRAVITY – 1.400 50.08%
SPECIFIC GRAVITY – 1.500 59.80%
SPECIFIC GRAVITY – 1.550 93.19%
60 BAUME 77.67%
66 BAUME 93.19%
20% OLEUM 104.5%
40% OLEUM 109.0%








GRADES OF SULFURIC ACID:-

CHEMICAL
IMPURITIES COMMERCIAL
GRADE
(Max ppm) E-GRADE,
BATTERY ACID
(Max ppm) FEDERAL
SPECIFICATION
CLASS – I
(Max ppm)
Ammonium 10.0 10.0 10.0
Antimony 0.02 0.02 1.0
Arsenic 1.0 1.0 1.0
Chloride 10.0 10.0 10.0
Copper 1.0 1.0
Iron 100.0 50.0 50.0
Manganese 0.2 0.2 0.2
Nickel 1.0 1.0 1.0
Nitrate 20.0 5.0 5.0
Platinum None None None
Selenium 0.05 0.05 20
Sulfurous acid 400 40 40
Zinc 10.0 10.0 40
Cadmium 1.0 1.0 -
Cobalt 1.0 1.0 -
Chromium 0.75 0.5 -
Lead 0.2 1.0 -
Mercury 1.0 1.0 -
Organic matter None None None
Fixed residue 300.0 300.0 300.0

GRADES OF ACID % H2SO4 SPECIFIC GRAVITY
(At 18 0C)
50 Be (Fertilizer acid) 62.2 1.525
60 0 Be (Oil of Vitriol) 93.2 1.833
95 % Acid 95.0 1.840
98% Acid 98.0 1.843
Monohydrate Acid 100.0 1.834
20 % Oleum (fuming) 104.5
(20 % free SO3) 1.924
40 % Oleum (fuming) 109.0
(40 % free SO3) 1.963
65 % Oleum (fuming) 114.6
(65 % free SO3) 1.987

In the past it was popular to report the concentration of sulfuric acid as specific gravity in degrees Baume. In United States, the Baume scale is calculated using the formula
0 Be = 145- (145 / Sp gr)
The Baume scale only includes the sulfuric acid concentration in the range of 0 to 93.19% H2SO4. Higher acid concentrations are not covered because of the great difficulty in differentiating between acid concentrations in the range of 93 to 100% H2SO4 by specific gravity measurements.


APPLICATION:

Sulfuric acid is widely used in industry because of its most important chemical and physical properties. Other acids have similar properties but the relative low cost of sulfuric acid makes it the most economical choice for wide variety of chemical application and these operations can be classified by the particular property of sulfuric acid involved.

1. Sulfuric acid is an active acid with a high boiling point. The manufacture of halogen acids namely HCl, HF etc and pickling of still make use this high boiling point. Leaching ores in the manufacture of a metal pigment is more effective with sulfuric acid because high leaching temperatures can be used without loss of acid by volatilization.
2. Sulfuric acid has great affinity for water. It is widely used for drying gases containing moisture (Ex-Cell Chlorine). Virtually complete removal of water vapor from these gases is accomplished by simple scrubbing operation.
3. Sulfuric acid forms hydrolysable sulfates with many organic compounds. Many alkylation operations of petroleum and petrochemical industries depend on the ability of this acid to react with hydrocarbons to form intermediate compounds. Aromatic alkylamines important to dye, photographic and pharmaceutical industries are manufactured with sulfuric acid. The production of industrially important synthetic alcohols is also based on this sulfuric acid property.
4. Sulfuric acid has special catalytic properties, probably related to its affinity for water. These catalytic properties account for its large volume use in the manufacture of aviation gasoline.
5. Oleum is used in the manufacture of organic sulfonates. These materials used in large quantities are major ingredients of the household detergents. Smaller quantities of special sulfonates are used as lubricants and as additives to automotive lubricants.
6. Teamed with relative low cost as a marked advantage of sulfuric acid is the availability. Sulfuric acid in the strengths (99 to 99%) common to commerce, doesnot reacts appreciably with steel. Special containers are needed to transport commercial grades of Hydrochloric acid and Nitric acid, but sulfuric acid can be transported in steel tank cars and tank trucks or shipped in steel drums.
7. Sulfuric acid is widely used in the acidulation and neutralization processes because it is frequently the most economical acid available for a particular purpose. The widespread use of Sulfuric acid for pH control, which can be performed satisfactorily by any acid, is a direct result of its low cost and its availability. The manufacture of phosphate fertilizers is the single largest use of sulfuric acid. Large amounts of sulfuric acid are used in acid Coagulation Process (Ex- GRS Synthetic Rubber) and in the regeneration of cationic exchange equipment. Other major use include as the neutralizing agent is the production of synthetic fibers

Thus the uses of sulfuric acid are so varied that the volume of its production provides an approximate index of general industrial activity. American production of Sulfuric acid exceeded 29 million tons annually in the early 1970, a figure corresponding to a daily production of 1/ 3 Kg per person throughout the year. The largest single use of sulfuric acid is for making fertilizers, both superphosphate and ammonium sulfate, organic products, refining petroleum, making paints and pigments, processing metals, making rayon, as car energizer etc. and hence it is difficult to imagine the human survival in absence of Sulfuric Acid.

SULPHURIC ACID MANUFACTURE

ABSTRACT

Sulphuric acid has always been considered as the foundation for inorganic chemicals. It is the wide spread use of sulphuric acid through out industry rather than tonnage that has caused consumption of sulphuric acid to be considered as a dependable barometer of general business conditions which includes manufacturing, utility suppliers and environmental technologies.

Sulphuric acid is manufactured by the combustion of sulphuric acid to produce sulphur dioxide gases, that is being converted to sulphur trioxide and absorbing the gas in water or sulphuric acid. This is the basic principle for all the processes. However we selected the Double Contact Double Absorption process for the manufacture of sulphuric acid.

This project deals with the manufacturing of sulphuric acid at a capacity of 100 tonnes per day. This project involves studying and gathering all the preliminary information required for sulphuric acid manufacture, generating material and energy balances for the plant, designing the equipments involved in the process of manufacturing, estimation of the economics of the plant, layout of the plant and analyzing the safety and environmental aspects.