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Chemical Plants

The design of a chemical plant is sometimes as much an art as a science. The overall aim is to increase production while minimising costs, and at the same time keeping a good margin for safety. It is the chemical engineer who has the task of balancing a number of criteria to ensure the most efficient running. For example, chemicals tend to react faster at high temperatures. However, if the reaction is exothermic and involves an equilibrium, a high temperature will hinder the formation of products. Often a compromise must be reached. Even if a high temperature is best, for chemical reasons, for reasons of economy it may be best to work at a lower temperature. It may cost more to provide the energy to increase the temperature than is returned by the profit on the sale of the extra chemical produced.

Similarly, gaseous reactions may be most effective at high pressures, but it is far more costly to build a plant to withstand high pressures. Indeed, a high pressure plant is also more expensive to run.

Energy costs are one of the most important variables in the design and running of a plant. Large chemical sites are often run as integrated concerns. This means that, for example, ‘the beat from an exothermic reaction in one part of the plant may be used to produce steam that drives turbines, compressors or pumps used in another part of the plant. Likewise an endothermic reaction may be used to cool fluid, which in turn cools gases from an exothermic reaction.


 Running a chemical plant

There are many different types of chemical plant, but they usually use one of the following two methods of production.

(i) Making chemical continuously

(ii) Making a chemical in batches.

Continuous reactors are best when there is large and dependable demand for a product. For example, oil refineries work on a continuous basis. Crude oil is fed into the distillation towers, and the various products are directed into the catalytic crackers, polymerization reactors and so on. On the other hand if the requirement of the chemical is less then a speciality chemicals firm makes small amounts of chemicals for research establishments in industry or universities. Thus, a chemical would be made in small batches.

The economics and physical scale of batch and continuous methods will be different. but they have many features in common. In particular, safety is of absolute importance, as is quality control.

With the advances in automation it is possible to run a huge chemical plant with no more than a dozen workers. Temperature and pressure changes can be measured by instruments directly connected to computers. The computers can be programmed to control pumps, heaters and the like so that the plant is kept running under the best conditions.

way the reactors are controlled has a direct bearing on safety. Dangerous situations can be caused if the sensors respond too slowly, or not at all. Especially, exothermic reactions that are not cooled properly can increase their rate of reaction extremely quickly. If gases are produced, a sudden rise in pressure may fracture pipes and cause fires or explosions. Similarly, if the plant is not regularly inspected for metal fatigue, leaks can give rise to a poisonous cloud of gas escaping into the atmosphere, or to an explosion taking place.
 

SOME CHEMICAL PLANTS IN GHANA

Crude oil refinery

Crude oil, like many other natural resources, is not found in all parts of the world. For example, at present, Nigeria and, to a lesser extent, the Cote d’Ivoire are the only countries in West Africa that produce oil. Offshore oil deposits have been found in Ghana, but oil wells have yet to be sunk to obtain the oil in commercial quantities.

Ghana imports both refined and unrefined crude oil, much of it from Nigeria.

 

Composition of crude oil

Petroleum (or crude oil) comes from Latin words meaning ‘oil from rock’. It has already been stated that crude oil is a complex mixture. On heating to 500 K {-230°C), crude oil can yield ·at least 500 different compounds. Crude oil refining means separating crude oil into usable and useful fractions.

Initial separation is based on the different boiling points (and volatilities) of the compounds present in the crude oil mixture.

Fractional distillation or fractionation is used to separate the petroleum into fractions. One fraction differs from another in the temperature range over which its components boil. The boiling ranges and composition of the various fractions are shown in Fig. 49.1.

 

Industrial refining of crude oil

Solids such as sand are separated from the impure crude oil by filtration. The filtrate of crude oil and water is allowed to stand in a reservoir for some time. The oil separates as the upper layer and is then pumped through pipes to a refinery such as the one at Tema in Ghana (Fig. 49.1).

At the refinery, the oil is vaporized in a furnace at 570 K (-300°C). The hot vapour is pumped into a fractionating tower such as the one shown in Fig. 49.2.

The fractionating tower is about 45 m high; it contains 30 to 40 perforated horizontal trays, regularly spaced throughout its height. The trays provide a large surface area on which evaporation and condensation can take place. Each tray contains a number of bubble caps. The bubble caps force the vapour through the liquid which has condensed on the tray. This results in continuous condensation and reevaporation.

The vaporization of the crude oil allows the more volatile fractions to rise up the tower. Since the temperature of the vapour falls as it goes up the tower (i.e., there is a temperature gradient), the less volatile fractions condense first, near the base. The more volatile fractions, which contain molecules of low molecular mass and are gases at room temperature, are collected from the top of the tower. Fractions with middle range boiling temperatures and molecular mass condense in the middle of the tower. The process goes on continuously, vapour being ejected from the furnace and the various fractions being removed for storage or further processing.

All the fractions are redistilled before they become usable and useful products. The composition of the fractions varies from refinery to refinery, since the crude oil varies in its composition and the methods of distillation differ in their detail.

 

Flow diagram of typical refinery

The image given on the next page is a schematic flow diagram of a typical oil refinery that depicts the various unit processes and the flow of intermediate product streams that occurs between the inlet crude oil feedstock and the final end products. The diagram depicts only one of the literally hundreds of different oil refinery configurations. The diagram also does not include any of the usual refinery facilities providing utilities such as steam, cooling water, and electric power as well as storage tanks for crude oil feedstock and for intermediate products and end products.

There are many process configurations other than that depicted above. For example, the vacuum distillation unit may also produce fractions that can be refined into end products such as: spindle oil used in the textile industry, light machinery  oil, motor oil, and steam cylinder oil. As another example, the vacuum residue may be processed in a coker unit to produce petroleum coke.

Refineries also produce pure chemicals, called feedstocks, from crude oil. These feedstocks are sold to the petrochemical industries, where a great variety of products are made.

Feedstock                                              Products

Methane                                  Fertilizers        Polyethylene

(Natural Gas)

Ethylene                                  Plastics            Explosives

Propylene                                Rayon              Synthetic Rubber

Butylene                                  Vinyl               Polystyrene

Naphthene                               Nylon              Dyes

Dacron            Drugs

Teflon              Antifreeze

 

GOLD

Mining of gold in Ghana began long before the arrival of the Europeans, The metal was extracted from alluvial materials deposited on the banks and bottoms of rivers. Ghana’s pre independence name of ‘Gold Coast’ shows the importance of the metal in the economy and history of the country.

Gold is an inert metal and, it is always found uncombined And in association with copper and silver. The extraction of the metal is done from the concentrated are.

To concentrate the ore, it is ground up into a very fine powder. It is then dissolved by leaching in sodium cyanide solution in the presence of oxygen and water. A soluble goldcomplex is formed.

4Au(s) + 8NaCN(aq) + O2(g) + 2H2O(1) → 4NaAu(CN)(aq) + 4NaOH(aq)

The gold reacts with the cyanide solution, leaving the impurities behind. Products are a gold-rich slurry. The slurry is passed through a series of thickeners in a filtration process. Addition of metallic zinc dust to the concentrated filtrate precipitates the metal:

2NaAu(CN)(aq) + Zn(s) → 2Au(s) + Na2Zn(CN)4 (aq)

 

Refining of Gold

The metal is refined by electrolysis. The anode is impure gold. The cathode is a strip of pure gold coated with a thin layer of graphite. The electrolyte is an aqueous solution of gold trioxonitrate(V) and trioxonitrate(V) acid. The gold ions are reduced at the cathode on electrolysis:

Au+ +e-  → Au

Crude mining and extraction of gold and diamond, is carried out on a small scale in Ghana. It is popularly called ‘Galamsey’. The ores or tailings, i.e., waste from previous ore treatment, is first concentrated by washing several times with water in a pan or other suitable container. This helps to remove slime (mud and other filth)

The product is then poured on a jute bag or any wide mesh cloth laid on an inclined table and repeatedly washed with water to remove the lighter gangue. The heavier gold material, trapped in ¢e sack or cloth, is washed off into a pan with water and then decanted. The solid gold product is rubbed well with mercury to form an amalgam leaving the remaining gangue.

In the final stage, the gold/mercury amalgam is put in a white handkerchief and squeezed hard to let out as much mercury as possible. The gold product looks white due to mercury contamination. This is washed with water in a pan. The heavier mercury falls to the bottom and the gold remains on top. The gold is harvested (collected) ready to be sold The gold may be also be recovered by distillation.

The gold produced by both Ashanti Goldfields company Limited and ‘Galamsey’ operators is not totally pure-; purer gold is obtained by refining their products outside Ghana.

 

Soap Making

Soap making is an activity that the Rural Enterprises Project (REP) supports through running training sessions on how it’s made and business training for startup businesses run out of the Business Advisory Centre (BAC).

 

Process of Making Bar Soap in Ghana:

1. The materials used are (coconut oil, buckets, scoops, caustic soda, water, soda ash, perfume, mixing spoons, hydrometer, rubber gloves, moulds, plastic bags, tables, and cutting equipment.

2. The moulds are set on the table and lined with plastic

3. A solution is made by mixing caustic soda (NaOH) and water. Measure out the volume and ensure correct level of acidity (pH) with the hydrometer

4. Mixing 10 L of coconut oil, 1 scoop of perfume, 1 scoop of soda ash, and the measured volume of caustic soda and water into a large mixing bowl

5. The contents are mixed for about 10 minutes or less, in the same direction at an average speed

6. The contents are poured into the moulds (caution, heavy lifting!)

7. These are left to dry for approximately 2.5-3 hours until they are white in colour. The tops are leveled out with a scrapper as the cooling occurs.

8. The moulds are removed, and the scrapper is used to clean up all sides of the block of soap. The block of soap is brought to cutting board, and pushed through to slice it into 10 long rectangles (caution, very difficult!)

9. These are pushed through another cutter to create square blocks these blocks are carried back to the table, and all 6 sides are stamped with the logo (time intensive)

10. The blocks are placed outside to dry in the sun

11. The bars of soap are collected and stored inside

12. Bars of soap are distributed to various sellers to sell at the market and in villages.

CEMENT INDUSTRY

Portland cement was first produced and patented in 1824 by a British stonemason. Today around 1700 million tonnes of cement are used every year, with different types manufactured to meet various chemical and physical requirements. To produce these requires a clear understanding and careful control of the manufacturing processes.

 

Key stages in cement production

Cement production is one of the world’s most energy intensive industries. Key production stages as used in Ghana can be summarized as:

 

1. Raw Materials

These are generally combinations of limestone, shells or chalk, and shale, clay, sand or iron ore, usually mined from a quarry close to the plant where they undergo reduction using primary and secondary crushers. When the reduced materials reach the cement plant they are proportioned to create a cement of specific chemical composition. Much work is being done on the use of alternative raw materials-often the by-products of other industrial processes. These can minimize the effects of quarrying, reduce the impact of the cement plant on the local environment and enable the cement industry to become a major player in materials recycling.

There are two basic methods used in Portland cement production-wet and dry.

In the dry process dry materials are proportioned, ground to a powder, blended and fed into the kiln dry.

The wet process involves adding water to the proportioned raw materials and completing the grinding and blending operations in slurry form. Increasingly alternative fuels are being used. These include materials such as scrap types, processed sewage sludge and packaging waste.

 

2. Cooling/finish grinding

Clinker is discharged from the lower end of the kiln and transferred to various types of coolers. Cooled clinker is combined with gypsum and ground to a fine powder in a ballmill to produce the final grade cement.

 

EXTRACTIVE METALLURGY OF ALUMINIUM

Aluminium is the most abundant metal in Earth.

It occurs in the nature in form of aluminium oxide and other combined forms. The ore containing aluminium compound, which is commercially used in the extractive metallurgy is called bauxite. Bauxite is a hydrated aluminium oxide. The extraction of aluminium is expensive in terms of energy consumption owing to the large amount of electrical energy required. Between 13000 and 17000 kWh are needed to produce 1 tonne of aluminium. For example, the smelter at Tema, Ghana, owned by the Volta Aluminium Company, uses 2,760,000 MWh to produce nearly 200,000 tonnes of aluminium annually. The company’s power lines are connected to the Akosombo power station of the Volta River hydroelectric dam and the site of the company allows it to obtain relatively cheap and plentiful electrical energy otherwise the electrolysis operation would be too expensive.

Aluminium production makes a major contribution to the economies of some countries, particularly the members of the International Bauxite Association (IBA).

Extraction of aluminium from bauxite is carried out in three stages:

• Ore dressing: cleaning ore by means of separation of the metal containing mineral from the waste (gangue).

• Chemical treatment of bauxite for converting the hydrated aluminium oxide to pure aluminium oxide.

At this stage crushed and ground bauxite is mixed with hot sodium hydroxide solution, which dissolves the aluminium hydroxide, forming solution of sodium aluminate.

The residual impurities (oxides of silicon, iron and titanium), which called “red mud”, are separated from the sodium aluminate solution.

The solution is then treated in precipitator tanks, where aluminium trihydrate precipitates form the solution.

The aluminium trihydrate after separation from the sodium hydroxide is converted into pure aluminium oxide by heating to 1800F (1000°C).

• Reduction of aluminium from aluminium oxide by the electrolytic process.

As aluminium oxide is a very poor electricity conductor its electrolysis is carried out in a bath of molten cryolite (mineral, containing sodium aluminium fluoride-Na3AlF 6).

The technology is called Hall-Heroult process.

Schematically the process is presented in the picture:

The electrolytic cell for aluminium production consists of a pot with carbon lining, serving as negative electrode (cathode) and positive electrodes (anodes), connected to the current conductor (bus bar). The anodes are immersed into the bath of molten cryolite.

The aluminium oxide is added to the cryolite and dissolved in it. When electric current passes between the anodes and the cathode through the cryolite, aluminium oxide decomposes to metallic aluminium deposited at the cathode and oxygen liberated at the anode.

The molten aluminium is periodically tapped from the furnace into a crucible and cast into ingots.

 

STEEL MAKING

Steel is an alloy of iron, containing up to 2% of carbon (usually up to 1%).

Steel contains lower (compared to pig iron) quantities of  impurities like phosphorous, sulfur and silicon.

Steel is produced from pig iron by processes, involving reducing the amounts of carbon, silicon and phosphorous.

The main steel making methods are basic oxygen process (basic oxygen furnace, basic oxygen converter) and electric arc furnace.

 

1. Basic Oxygen Process (BOP)

The Basic Oxygen Process is the most powerful and effective method of steel manufacturing.

The scheme of the Basic Oxygen Furnace (BOF) is presented in the picture.

Typical basic oxygen converter has a vertical steel shell lined with refractory lining.

 

The furnance is capable to rotate about its horizontal axis on trunnions. This rotations is necessary for charging raw materials and fluxes, sampling the melt and pouring the steel and the slag out of the furnace.

Basic Oxygen is equipped with the water cooled oxygen lance for blowing oxygen into the melt.

The basic oxygen converter uses no additional fuel. The pig iron impurities (carbon, silicon, manganese and phosphorous) serve as fuel.

The steel making process in the oxygen converter consists of:

• Charging steel scrap.

• Pouring liquid pig iron into the furnace.

• Charging fluxes.

• Oxygen blowing.

• Sampling and temperature measurement.

• Tapping the steel to a ladle.

• De-slogging.

The iron impurities oxidize, evolving heat, necessary for the process.

The forming oxides and sulfur are absorbed by the slag.

The oxygen converter has a capacity up to 400 t and production cycle of about 40 min.

 

2. Electric-arc Furnace

on to the charge and heating it to the required temperature.

As the electric-arc furnace utilizes the external origin of energy (electric current), it is capable to melt up to 100% of steel scrap.

The steel making process in the electric-arc furnace consists of:

• Charging scrap metal, pig iron, limestone

• Lowering the electrodes and starting the power (melting)

• Oxidizing stage

At this stage the heat, produced by the arcs, causes oxidizing phosphorous, silicon and manganese. The oxides are absorbed into the slag. By the end of the stage the slag is removed.

• De-slagging

• Reducing stage

New fluxes (lime and anthracite) are added at this stage for formation of basic reducing slag.

The function of this slag is refining of the steel from sulfur and absorption of oxides, formed as a result of deoxidation.

• Tapping

• Lining maintenance

The advantages of the electric-arc furnance are as follows:

• Unlimited scrap quantity may be melt;

• Easy temperature control;

• Deep desulfurization;

• Precise alloying.

Ladle refining (ladle metallurgy, secondary refining) Ladle refining is post steel making technological operations, performed in the ladle prior to casting with the purposes of desulfurization, degassing, temperature and chemical homogenization, deoxidation and others. Ladle refining may be carried out at atmospheric pressure, at vacuum, may involve heating, gas purging and stirring.

Sulfur refining (desulfurization) in the ladle metallurgy is performed by addition of fluxes (CaO, CaF2 and others) into the ladle and stirring the steel together with the slag, absorbing sulfur.

In the production of high quality steel the operation of vacuum treatment in ladle is widely used.

Vacuum causes proceeding chemical reaction within the molten steel:

[C] + [O] =CO

This reaction results in reduction of the quantity of oxide inclusions.

The bubbles of carbon oxide remove Hydrogen, diffusing into the CO phase.

An example of ladle refining method is Recirculation. Degassing ( RH) vacuum de gasser, which consists of a vacuum vessel with two tubes (snorkels), immersed in the steel.

In one of the tubes argon is injected. Argon bubbles, moving upwards, cause steel circulation through the vacuum vessel. Additions of fluxes in the vacuum vessel permits conducting desulfurization treatment by this method.

 

USE OF FOREIGN RAW MATERIALS

The government of a country may sometimes act as a factor in influencing the siting of an industry in spite of better economic pulls elsewhere. This is done with a view to giving employment to people of the area where the industry is located. It may also be located to win the people’s political support or to check the movement of the area’s labour force to the urban centres. Ghana’s old Matchet Factory at Bibiani was sited for these reasons. Currently, it is the policy of governments in West Africa, for example, to promote the establishment of cottage industries through tax concessions. This is one of the attempts to check the concentration of industries and people in the urban areas.

Finally, another way of maximizing profit or producing low-cost but quality products is to avoid waste. Uses and hence markets must be found for the waste products-they should be turned into by-products.

Factors that influenced the location of some specific individual chemical industries will now be discussed.

Volta Aluminium Company (VALCO) Tema

The Volta aluminium company obtains electrical energy supply from the hydroelectric power at Akosombo. The energy supply is cheap and regular because Tema is only about 80 kilometers from Akosombo. Valco at present imports its raw materials. Alumina is one of them. Alumina is extracted from bauxite. Although there are bauxite deposits in Ghana, Valco does not use this raw material at present. This importation increases production cost and hence the aluminium product is expensive. On the other hand, the situation of the industry at Tema has the advantage that imported raw materials can easily be received and products exported. The industry aims at both internal and external markets. With an assured market. the aluminium sheets products need not be stored for long periods, hence much space is not needed by the industry.

 

Asbanti Limited

There is no question about the marketability of the processed gold and its importance in international trade. The industry is sited at Obuasi because this is one area in the country where the concentration of gold, more than 10 parts in a million, is considered enough for commercial exploitation. It has one of the richest gold ore deposits in the world. Good security is needed to prevent stealing of the precious metal. The Akosombo hydroelectric plant supplies electrical power.

Panbros salt industry, Accra

This industry is situated at Weija, a suburb of Accra. The  location of the industry is at an open space near the sea. The open space provides an area for the evaporation of the sea water is pans. The sea-water is an abundant raw material. Thus the industry does not face the problem of raw material transportation to the factory. The use of the salt for preservation and flavouring of food means it has a ready marked. Transportation of the products to market areas is not problem because of its location.

 

BASIC CHEMICAL REACTIONS TO TRANSFORM RAW MATERIALS TO PRODUCTS

• Saponification

Saponification is hydrolysis of esters of higher fatty acids in alkaline medium

 

RCOOR + NaOH(aq)             →      7 RCOONa + R’OH

Ester                                 Sodium             Alcohol

alkanoate

The alkaline hydrolysis of oils/fats, which are triesters of glycerol and higher fatty acids produces glycerol and sodium salts of fatty acids which are called soaps. This reaction used in soap making is:

• Electrolysis

It is a process of decomposition of electrolyte in fused state or dissolved state by the passage of electricity. This process is utilised in large number of industrial preparations. For example, production of aluminium is carried out by the electrolysis of molten alumina (~03) dissolved in cryolite (Na3AIF6).

Similarly, electrolysis of brine solution (NaCl) is used for the production of caustic soda (NaOH)

Electrolyis

2NaCl(aq) + 2H2O(l)              →    2NaOH(aq) +H2(g) + Cl2(g)

 

• Fermentation

Fermentation is a process of breaking down of complex organic material into smaller fragments by the action of living organism which secrete the enzyme catalyst suitable to the process. Some common examples are souring and curdling of milk, putrefaction of meat, convertion of fruit juices into wines, etc.

Alcohol is produced in brewing industry by the action of enzyme Zymase on glucose/fructose is found in sweet fruits. Zymase is secreted by the microorganisms called Yeast.

 

• Solvent Extraction

Solvent extraction is a technique used to separate a component of a mixture by making use of its different solubilities in two immiscible solvents. Immiscible solvents

cannot dissolve in each other, and so they form two layers when mixed. The desired component in a reaction mixture dissolves well in one solvent and is insoluble or only slightly soluble in the other at a constant temperature. When the reaction mixture is shaken well in the two immiscible solvents, the desired product is (dissolved) extracted into one solvent whilst the other components or impurities are (dissolved) extracted into the other solvent.

Solvent extraction is very useful in extracting natural organic products from their natural sources such as tree barks, leaves, roots, stems, and animal parts.

Caffeine is a heterocyclic base (alkaloid) found in cocoa, tea, coffee beans, cola nuts and drinks. It is an organic compound which is far more soluble in tetrachlormethane (CC14) than water. The solvent extraction technique can therefore be used to separate it from its aqueous solutions.

 

• Precipitation

The process of precipitation is employed in the extraction of metals from their ores. For example, in the refining of gold, the ore, it is ground up into a very fine powder. It is then dissolved by leaching in sodium cyanide solution in the presence of oxygen and water. A soluble gold complex is formed.

 

4Au(s) + 8NaCN(aq) + O2(g) + 2H2O(l)      → 4NaAu(CN)2 (aq) + 4NaOH(aq)

The gold reacts with the cyanide solution, leaving the impurities behind. Products are a gold-rich slurry. The slurry is passed through a series of thickeners in a filtration process.

Addition of metallic zinc dust to the concentrated filtrate precipitates the metal:

2NaAu (CN)2(aq) + Zn(s)→ 2Au(s) + Na2Zn(CN)4(aq)