Mini Project Process Instrumentation

January 1, 2015

MINI PROJECT PROCESS INSTRUMENTATION

1.0 Introduction Sulphuric acid (H2SO4) is one of the most important compounds made by the chemical industry. It is used to make, literally, hundreds of compounds needed by almost every industry. There are two major processes in production of sulphuric acid, they are by contact process and lead chambers and it is available commercially in a number of grades and concentration. The contact process produces a purer, more concentrated acid but require purer raw materials and the use of expensive catalyst. The lead chamber process is used to produce much of the acid used to make fertilizers. It produces a relatively dilute acid (62% - 78% H2SO4). In both processes Sulphur dioxide is oxidized and dissolved in water. Some sulphuric acid is also made from ferrous sulphate waste solutions from pickling iron and steel and from waste acid sludge from oil refineries. They are many uses of Sulphuric Acid (H2SO4) in industries. By far the largest amount of sulfuric acid is used to make phosphoric acid to make the phosphate fertilizers, calcium dihydrogenphosphate and the ammonium phosphates. It is also used to make ammonium sulfate, which is a particularly important fertilizer in sulfur-deficient.

hydrofluoric acid 3%

paints, pigments 3%

pulp paper 1%

fibres 6%

metal processing 13% phosphates 8%

phosphate fertilizers 66%

Figure 1: Uses of Sulphuric Acid (H2SO4) 1|Page

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Figure 1 showed the percentage of uses if sulphuric acid in industries. It is widely used in metal processing for example in the manufacture of copper and the manufacture of zinc and in cleaning the surface of steel sheet, known as 'pickling', prior to it being covered in a thin layer of tin, used to make cans for food. It is also used to make caprolactam, which is converted into polyamide 6 and in the manufacture of titanium dioxide, used, for example, as a pigment.Amongst its many other uses is in the manufacture of hydrofluoric acid and phenol with propanone all of which are used in many industries.

1.1 Manufacture of sulfuric acid The process for producing sulphuric acid has four stages: a) Extraction of Sulphur. b) Conversion of Sulphur to sulphur dioxide. c) Conversion of sulphur dioxide to sulphur trioxide. d) Conversion of sulphur trioxide to sulphuric acid. 1.1.1 Extraction of Sulphur Easily the most important source of sulfur is its recovery from natural gas and oil. These contain sulfur compounds, both organic and hydrogen sulfide both of which must be removed before they are used as fuels or chemical feedstock. Another important source of sulfur is as sulfur dioxide from metal refining. Many metal ores occur as sulfides and are roasted to form an oxide and sulfur dioxide, for example in the manufacture of lead:

Other metals manufactured from their sulfide ores include copper, nickel and zinc. Worldwide about 35% of the sulfur is obtained as sulfur dioxide from sulfide ore roasting and this is increasing, as plants which traditionally passed the sulfur dioxide to atmosphere are recovering it as sulfuric acid. In particular, China makes most of its sulfuric acid from pyrites, an iron sulfide ore. Sulfuric acid is also obtained from ammonium sulfate, a by-product in the manufacture of poly (methyl 2-methylpropenoate) and also recovered from 'spent' sulfuric acid. 2|Page

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1.1.2 Conversion of Sulphur to Sulphur dioxide If sulfur is the feedstock, it must first be converted to sulfur dioxide. Molten sulfur is sprayed into a furnace and burnt in a blast of dry air at about 1300 K. The sulfur burns with a characteristic blue flame:

As excess air is used the emerging gas contains about 10-12% sulfur dioxide and 10% oxygen, by volume. The gases are very hot and so are passed through heat exchangers (waste heat boilers). The gases are cooled to about 700 K and the water in the surrounding boiler pipes is converted into steam. In manufacturing one tonne of sulfuric acid, one tonne of high pressure steam is also produced. 1.1.3 Conversion of sulphur dioxide to sulphur trioxide (The Contact Process) A typical plant contains one cylindrical vessel which acts as a fixed bed reactor with four separate beds of catalyst, known as a converter, heated to 700 K, through which the sulfur dioxide and air pass:

The catalyst, vanadium(V) oxide on silica, is generally in the form of small pellets, to which caesium sulfate has been added as a promoter (Figure 2). The function of the promoter is to lower the melting point of vanadium(V) oxide so that it is molten at 700 K. 1.1.4 Conversion of sulphur trioxide to sulphuric acid The sulfur trioxide formed from the third bed (and the small amount from the fourth bed) are now converted tosulfuric acid. Sulfur trioxide reacts with water and the reaction can be expressed as:

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However, water itself cannot be used for absorption as there is a large temperature rise, and a sulfuric acid mist is formed, which is difficult to handle. Instead, sulfuric acid of about 98% concentration is used. This is kept at this concentration by addition of water and removal of acid at that concentration. To keep the temperature at about 400 K, the heat is removed by heat exchangers, Figure 2.

Figure 2: A line diagram illustrating a heat exchanger used in the manufacture of sulfur trioxide. The gases not absorbed contain about 95% nitrogen, 5% oxygen, and traces of sulfur dioxide. The gas stream is filtered to remove any traces of sulfuric acid mist and is returned to the atmosphere using a high stack.

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1.2 Significance of Process instrumentation in industry Industry has entered into a new age. This can be seen in the advancements of instrumentations that have brought a very important changes in industry. For example, today, most products are manufactured using some type of automatically controlled processing equipment. This equipment is often complex, demanding a variety of skilled personnel to keep it in operation. At one time, most industrial equipment could be easily placed into operation with just a few simple tools and some basic common sense. Today the situation differs in that a large part of equipment and processes contain numerous control devices vital to the performance of precise automatic operations.The instrumentation calibration techniques and troubleshooting methods is needed in order to keep things in a good state of repair and reliability. Equipment breakdown is common problem will be faced in industry. Preventative maintenance and operational efficiency of plant instrumentation have become more important with the increased dependency quality and maximum production. The industrial instrumentation is a rather broad area of study that deals with the measurement, evaluation and control of process variables such as temperature, pressure, flow rate, fluid level, force, light intensity and humidity. These variables are usually involved in a manufacturing operation of some type that eventually leads to a finished industrial product. Automation production operations are largely responsible for a major part of all instrumentation applications. The primary areas of concern in instrumentation are pneumatics, electronics, mechanics and hydraulics. Each of these areas is individually unique, but they are all very similar in many respects.

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2.0 PFD and P&ID Diagram 2.1 PFD PFD is depicting a given system’s flows, temperature, pressure and mass balance for various operating conditions. Such information is typically presented in the form of tables. PFD present functional information about a system or subsystem. The components such as piping, pumps and valves are represented by standard schematic symbols that illustrate their function in the system, as opposed to their relative sizes, locations or physical shapes. The instrumentation and control information is not included. PFD are typically the first drawing developed by mechanical processes, often in the pre-conceptual or conceptual design place.

2.2 P&ID P&ID are typically developed from PFD. P&ID communicated detailed information on how to operate, troubleshoot and repair or modify the system or subsystem . In addition to the mechanical components, they include instruments, signal modifiers, controllers and their inter-relationship. They typically do not include tabular parameters as PFD do. P&ID is a very common term used in the world of process industries. A process engineer in a manufacturing plant need to create new and/or modify the already existing P&ID to the as-is plant modifications. A P&ID is a detailed graphical representation of a process including the hardware and software (piping, equipment, instrumentation) necessary to design, construct and operate the facility.

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3.0 Process Description 3.1 Sulphuric Acid A sulphuric acid with a chemical formula of H2SO4 is a strong acid and it is widely used for many applications. Sulphuric acid has become one of the most important industrial chemicals and causing a high demand in sulphuric acid manufacturing locally and globally. However it is odorless, colorless and extremely corrosive. Sulphuric acid at a high concentration can cause very dangerous on contact and safety precautions should be strictly observed when handling it. In general, there are several process in sulphuric acid production such as contact process, wet sulphuric acid process and others. Among all of the processes, only contact process are being focused in this mini project.

3.2 Contact Process Now a days, sulphuric acid is prepared by contact process all over the world. Preparation of sulphuric acid by contact process is based upon the catalytic oxidation of SO2 to SO3.

3.3 Details Of Contact Process Following steps are involved in the preparation of H2SO4. i.

Preparation of SO2.

ii.

Purification of SO2.

iii.

Oxidation of SO2.

iv.

Absorption of SO3.

v.

Dilution of Oleum.

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3.3.1 Step I - Preparation of SO2 SO2 is obtained by burning sulphur or by heating iron pyrite (FeS2) in pyrite burner. S + O2 4FeS2 + 11O2

SO2 2Fe2O3 + 8SO2

3.3.2 Step II - Purification of SO2 SO2 contains a number of impurities such as dust particles, Arsenous oxide, vapours, sulphur etc. These impurities must be removed otherwise catalyst loses its efficiency (catalyst poisoning). 3.3.2.1 Dust Chamber SO2 is first passed through the dust chamber where steam is spread over the gas to remove dust particles, which settle down. Fe(OH)3 also sprayed over to remove oxides of Arsenic. 3.3.2.2 Washing Tower SO2 is then passed through a washing tower after cooling. Here it is sprayed by water to remove any other soluble impurities. 3.3.2.3 Drying Tower The gas is now dried by passing through drying tower where concentration H2SO4 (dehydrating agent) is sprayed. H2SO4 removes moisture from SO2.

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3.3.3 Step III - Oxidation of SO2 To SO3 3.3.3.1 Contact Tower Oxidation of SO2 is carried out in contact tower where V2O5 is filled in different pipes. SO2 here reacts with air (O2) to produce SO3. Under above conditions 98% SO2 is converted into SO3. 2SO2 + O2

2SO3

3.3.3.2 Conditions Necessary For Maximum Yield Of SO3 Oxidation of SO2 is a reversible and exothermic process in which volume of product is less than the volumes of reactants. In order to obtain maximum amount of SO3, according to LeChatelier’s Principle following conditions are necessary. 3.3.3.2.1 Temperature A decrease in temperature favours reaction in forward direction. Optimum temperature for this process is

450oC to 500oC.

3.3.3.2.2 Pressure Since volumes of reactants are greater than the product (3:2), therefore, according to LeChatelier’s Principle a high pressure is favourable. Optimum pressure is about 1.5 to 1.7 atmosphere. 3.3.3.2.3 Use of Catalyst At low temperature, rate of reaction was decreases. To increase rate of reaction a catalyst vanadium pentaoxide (V2O5) is used.

3.3.4 Step IV - Absorption of SO3 in H2SO4 SO3 is not directly passed in water, because a dense fog of minute particles of H2SO4 is produced. It is therefore, dissolved in concentrated H2SO4 to form pyrosulphuric acid (oleum). SO3 + H2SO4

H2S2O7 (OLEUM)

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3.3.5 Step V - Dilution of Oleum Oleum is now diluted with water to form H2SO4 of required concentration. H2S2O7 + H2O

2H2SO4

3.4 Outline of Contact System Sulphuric Acid Plant In process drawing (Figure 3) sulphur dioxide (SO2) gas having a temperature of 300 to 500 oC generated from roasting furnace is relieved of large-grained dust particles and impurities by dust Cottrell. Fine dust and impurities that are still not removed are washed off by the next washing tower and cooled down to 30 to 40 oC through cooling tower. In the cooled gas a large quantity of mist is present, removed by first mist Cottrell, intermediate tower and second mist Cottrell. In the gas passed through mist Cottrell, 20 to 40 g/m 3 moisture is contained ; if this is passed on to the next blower heat exchanger, corrosion will be increased ; so in order to reduce moisture to about 0.1 g/m3, it is dried by a drying tower. Gas that is dried is passed through a filter by a blower and sent to heat exchanger and converter. Gas sent into theconverter produces catalytic reaction through catalysis and is oxidized into sulphur trioxide (SO3). Temperature necessary for catalytic reaction is constantly maintained by the heat exchanger. Gas coming out from the converter by way of the heat exchanger is cooled, but is still too high in temperature (200 oC) for the absorption tower, so it is futher passed through a cooler to bring the temperture down to 100 oC. Sulphuric acid concentration most suitable for sulphur trioxide absorption is that point where total vapour tension is minimum; and below this, concentration component pressure of water will be great, making sulphur trioxide form a mist with water and difficult to absorb. Sulphur trioxide which has become about 100 oC will be absorbed by 93.8% sulphuric acid through the absorption tower, and sulphuric acid of 98% concentration will be produced. In the case of two absorption towers, by circulating fuming sulphuric acid in the first tower, it will be possible to produce fuming sulphuric acid.

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4.0 Instrumentation In Contact System Sulphuric Acid Plant

Figure 3: Process flow diagram (PFD) Contact System Sulphuric Acid Plant

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4.1 Selective Process The process that is selected for the study is involving the reaction in the absorption tower. The drying acid absorbs the water vapor remaining in the gas after it leaves the Gas Cleaning Section of the plant. Drying of the gas is necessary: 

To avoid corrosion caused by wet SO2 gas before the converter and by wet SO3 after conversion



To avoid loss of production due to the formation of acid mist in the absorption tower



To keep a clear stack,



To avoid acid condensation during shut-downs and thus protect the catalyst from degradation The absorber acid absorbs the SO3 formed in the converter. Absorption of SO3 from the

gas is necessary to: 

Remove all SO3 and acid mist from the gas stream before it exits the tower.



Produce 98.5% acid in the absorption tower The Absorption Towers equipped with a ceramic packing to increase the contact surface

between the SO3 gas and sprayed acid. It is also equipped with mist eliminator to prevent scabbing of any mist drops from the tower.

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Figure 4: Process Equipment and Instrumentation layout of the process

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Table 1: Abbreviation of various instrument and physical variables List of Abbreviations & Specifications Equipments

Instruments

HP : Hoper for sulfur feed SMT : Sulfur Melting Tank

Level All having 0 to 10 ft level

DST : Dirty Sulfur Tank

L1 : SMT Tank Level

SF : Sulfur Filter

L2 : DST Tank Level

CST : Clean Sulfur Tank

L3 : CST Tank Level

AB : Air Burner with Ash Filter inside

L4 : Boiler Water Level

DR : Dryer

L5 : ACT Tank Level

AF : Air Filter

L6 : AST Tank Level

WHB : West Heat Boiler 4SCR : Four Stage Convertor/Reactor

Temperature

HE : Heat Exchanger

T0 : SMT tank temp. 120 to 160oC

2SAR : Two Stage Absorber

T2 : AB temp. 800 to 900 oC

ACT : Acid Circulation Tank

T3 : SO2 line to boiler temp. 800 to 900 oC

AST : Acid Storage Tank

T4 : Boiler temp. 700 to 900 oC T5 : SO2 line to 4SCR temp. 600 to 700 oC

Pumps All Having 0 to 500 rpm speed

T6 : 1st stage temp. of 4SCR , 825 to 945 oC

P1 : MST pump to DST

T7 : 2nd stage temp. of 4SCR, 625 to 725 oC

P2 : DST pump to CST

T8 : 3rd stage temp. of 4SCR, 450 to 550 oC

P3 : CST pump to AB

T9 : 4th stage temp. of 4SCR, 400 to 450 oC

B1 : Air Suction Draft for dryer P4 : ACT to 2SAR pump

Switches

P5 : ACT to AST pump

S1 : SF pressure switch

P6 : AST pump for final product out

S2 : DR humidity switch S3 : DR pressure switch S4 : AF pressure switch CS1 : Venting SO3 concentration switch 30% = low & 60% = high

Concentration

Control Valves

C1 : concentration of SO3 from 4SCR 60% to

V1 : Steam for melting sulfer tank 14 | P a g e

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90% C2 : Concentration of sulfuric acid in 1st stage

V2 : Air to bumer control valve

of 2SAR 97% to 99% C3 : Concentration of sulfuric acid in 2nd stage

V3 : SO2 flow control to boiler

of 2SAR 92% to 94% V4 : Boiler feed water control Pressure

V5 : SO2 flow control to 4SCR

PT1 : AB pressure 0 to 20 kgpcme

V6 : Coolant flow control to HE

PT2 : Boiler Pressure 30 to 50 kgpcm2

V7 : Air control for 3rd stage 4SCR

PT3 : Vapor pressure in ACT

V8 : Air control for 4th stage 4SCR

PT4 : Vapor pressure in 2SAR

V9 : SO3 flow control from 4SCR V10 : Recycle SO3 flow control for 2SAR

Flow

TV1 : Three way valve for vent or recycle of SO3

F2 : CST to AB flow of sulfur 0 to 10 CFPM

V11 : Dilution water control to ACT

F3 : SO2 gas flow to boiler 0 to 10 CFPM

V12 : Air control for ACT

F4 : SO2 gas flow to 4SCR 0 to 10 CFPM

V13 : Vent control of sulfuric acid vapor

F6 : Sulfuric acid flow from 4SCR to ACT 0 to 10 CFPM

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5.0 Description of instrument used in Contact Process The Absorbing Acid System absorbs the sulphur trioxide (SO3) that is formed in the converter to produce sulphuric acid. The reason for absorbing the SO3 is to achieve the desired production of sulphuric acid and to avoid environmental problems that would result if the SO3 were allowed to escape to the environment. Efficiently absorbing the SO3 will also help to maintain a clear stack by avoiding the formation of sub-micron mist particles that would form the moment the SO3 containing gas leaves the stack and reacts with the moisture in the air.

5.1 Equipment of Process A typical Absorbing Acid System consists of the following items of equipment: a) Absorbing Tower b) Acid Pump Tank c) Acid Pump d) Acid Cooler e) Piping f) Instrumentation and Control 5.1.1 Absorbing Tower A typical Absorbing Tower is a vertical cylindrical vessel designed to contact process gas and strong sulphuric acid (98.5% H2SO4) for the purpose of absorbing SO3. The tower may be constructed of specialty alloys or the more traditional brick lined carbon steel. The tower will be equipped with a packing support, packing, acid distributor, and mist eliminator. 5.1.2 Acid Pump Tank An acid pump is required to circulate the acid from the pump tank up to the distributor inside the Absorbing Tower. The acid flows by gravity over the packing and drain out the bottom or side of the Absorbing Tower back to the pump tank. The pump can be a vertical submerged centrifugal pump in which case it would be mounted in the pump tank. External vertical centrifugal pumps are also available as well as horizontal centrifugal pump types, although the latter is uncommon. 16 | P a g e

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5.1.3 Acid Pumps An acid pump is required to circulate the acid from the pump tank up to the distributor inside the Absorbing Tower. The acid flows by gravity over the packing and drain out the bottom or side of the Absorbing Tower back to the pump tank. The pump can be a vertical submerged centrifugal pump in which case it would be mounted in the pump tank. External vertical centrifugal pumps are also available as well as horizontal centrifugal pump types, although the latter is uncommon. 5.1.4 Acid Cooler The absorption of water into sulphuric acid is an exothermic reaction and will cause the temperature of the acid to rise unless the heat of absorption is removed. The heat is typically removed in either a plate heat exchanger or anodically protected shell and tube heat exchanger. 5.1.5 Piping Acid piping is required to carry the acid from one piece of equipment to the next. Materials vary considerably depending on the acid concentration, temperature and cost. 5.1.6 Instrumentation and Controls Instrumentation and controls are required to monitor the operation of the system and control its operating parameters. Acid concentration, flow and temperature are the most important factors in ensuring the system is performing its function of gas drying as efficiently as possible.

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5.2 Absorption The absorption of SO3 from the gas relies on equilibrium factors as described in the section Absorption and Stripping. There are many physical factors that affect the degree of absorption that will occur in a tower. One of the primary factors is the vapour pressure of sulphur trioxide/sulphuric acid above the acid. Absorber towers operate in a relatively narrow range of concentration and temperature for effective absorption of SO3. This is apparent when you look at a plot of the total vapour pressure versus acid concentration curve. In a concentration range of 97.5% to 98.5% the equilibrium vapour pressure above sulphuric acid reaches a minimum. Operation in this acid concentration range will result in the maximum absorption of SO3. The vapour pressure curves (better curve to follow) indicate that the lower the acid temperature, the lower the vapour pressure above the acid. This would imply that operating the absorber system at lower temperatures would result in better absorption, however, this is not the case. At lower temperatures other physical properties such as density and viscosity begin to affect the absorption process in a negative manner. The specific operating conditions of a plant’s absorber system will depend on the design of the tower, packing characteristics, distributor design, acid flow, mist eliminator efficiency, etc. in addition to the acid concentration and temperature. The optimum operating conditions can be determined by measuring the amount of SO3/H2SO4 leaving the tower or in the case of a final absorber, observing the opacity of the stack.

The quality of the stack

emissions are observed as the operating conditions are varied. Operating conditions must reach steady state and be held for a period of time before the affects of the new operating conditions are recorded.

A systematic program of varying operating conditions and

observation will allow the optimum operating conditions to be determined.

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6.0 Conclusion and Recommendation Sulphuric acid (H2SO4) is one of the most important compounds made by the chemical industry. It is used to make, literally, hundreds of compounds needed by almost every industry. There are two major processes in production of sulphuric acid, they are by contact process and lead chambers and it is available commercially in a number of grades and concentration. The process for producing sulphuric acid has four stages which is extraction of sulphur, conversion of sulphur to sulphur dioxide, conversion of sulphur dioxide to sulphur trioxide and conversion of sulphur trioxide to sulphuric acid. The process that is selected for the study is involving the reaction in the absorption tower. The drying acid absorbs the water vapor remaining in the gas after it leaves the Gas Cleaning Section of the plant. The absorber acid absorbs the SO3 formed in the converter. Absorption of SO3 from the gas is necessary to remove all SO3 and acid mist from the gas stream before it exits the tower. Besides that, it is to produce 98.5% acid in the absorption tower. The Absorption Towers equipped with a ceramic packing to increase the contact surface between the SO3 gas and sprayed acid. It is also equipped with mist eliminator to prevent scabbing of any mist drops from the tower. Therefore, it is a recommendation for the future process in production of sulphuric acid by contact process. Firstly, it is about single absorption plant with a tail gas scrubber and bypass gas direct to tail gas scrubber during extended period of low or no SO2. It is recommended because single absorption plants are better able to handle varying operating conditions than double absorption plants. It has lower auto thermal limit than double absorption plants. Besides that, the tail gas scrubber was able to handle varying inlet SO2 levels and still provide low SO2 emissions. The tail gas scrubbers are also a backup during startups and upset conditions. Lastly, acid plants can tolerate short periods of no gas and still retain sufficient heat in the catalyst to restart without using the preheater. Bypassing gas direct to tail gas scrubber avoids having to use the preheater to maintain catalyst temperatures.

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7.0 References Carey, F. A. "Reactions of Arenes. Electrophilic Aromatic Substitution". On-Line Learning Center for Organic Chemistry. University of Calgary. Retrieved 27 January 2008. Pelletreau, K.; Muller-Parker, G. (2002). "Sulfuric acid in the phaeophyte alga Desmarestia munda deters feeding by the sea urchin Strongylocentrotus droebachiensis". Jones, Edward M. (1950). "Chamber Process Manufacture of Sulfuric Acid". Industrial and Engineering Chemistry 42 (11): 2208–2210. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 653.

Maltis J.P, Renovance, Bashahr: “ Instrumentation in Producing Sulphuric Acid”, published 2008. Pp:38-pp 67.

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