Unit Operations, Chemistry tutorial

Introduction 

Every industrial chemical procedure is depends on unit operations (physical treatment) and unit procedure (chemical treatment) to create economically desired product from exact raw substances.  The raw materials are treated through physical steps to build them appropriate for chemical reaction. So, knowledge of unit operations like 'mixing and agitation of liquid' and 'heat flow' is very much necessary. The subject Unit Operations is based on fundamental laws   and physicochemical principles. Unit operation provides thought about science related to exact physical operation; different equipments-its design, material of construction and operation; and computation of diverse physical parameters (mass flow, heat flow, mass balance, power and force and so on). Instances of chapter operations are: heat flow, fluid flow, mixing, drying, absorption, evaporation, adsorption, distillation, condensation, crystallization, vaporization, leaching, separation, extraction, sedimentation, filtration, crushing and so on. 

Unit operation  

A unit operation is any part of potentially multiple-step procedure that can be considered to have a single function. It is a necessary step in a procedure. Unit operations engage bringing a physical change these as separation, crystallization, evaporation, and filtration and so on. For instance, in milk processing: homogenisation, pasteurisation, chilling, and packaging are entity unit operations that are connected to create the overall process. A process may have many unit operations to obtain the desired product. 

Historically, the different chemical industries were regarded as different industrial processes and with different principles. Arthur Dehon Little (1916) propounded the concept of "Unit Operations" to clarify industrial chemistry processes.., William H.Walker, Warren K. Lewis and William H. McAdams (1923), explained that assortment of chemical industries have processes  which follow the similar physical laws. They summed-up such similar processes into unit operations.  Each unit operation follows the equivalent physical laws and might be utilized in all chemical industries. The unit operations shape the fundamental principles of chemical technology. 

After preparing raw substances via physical treatment, such undergo chemical conversion in a reactor. To perform chemical conversion basic knowledge of stoichiometry, reaction kinetics,   thermodynamics, chemical equilibrium, energy balance and mass balance is necessary. 

Many alternatives may be proposed to design a reactor for a chemical process. One design may have low reactor cost, but the final materials leaving the unit need higher treatment cost while separating and purifying the desired product. Therefore, the economics of the overall process also play a vital role to select a suitable alternative design. Each chemical process consists of series of assembly that are organized systematically to achieve the goal. The physical and chemical steps in a procedure are set through the assist of joined knowledge and experience of engineers, industrial chemists, technologists and economists to create a product. The individual operations have several general phenomena and are depend on the similar scientific principles for instance heat transfer is the general phenomenon in evaporation, drying and crystallization. 

A process designer designs a chemical procedure depend on the subsequent:  

(1)    Efficiency of procedure and equipments   

(2)   Safety by respect to the procedure, raw chemicals, finished products and long term consequence on environment 

(3)   Financial viability of the products as demanded through the purchaser. 

The subsequent are several instances of physical processes: 

(a)   Sugar Manufacture:  

Sugar cane → crushing → sugar extraction → thickening of syrup → evaporation of water → sugar crystallization → filtration → drying → screening → packaging. 

(b)   Pharmaceutical Manufacture: 

Formulation of chemicals, mixing, granulation → drying of granules → screening → pressing tablet → packaging. 

(c)   Salt Manufacture: 

Brine  transportation  →  evaporation  →  crystallization  →  drying  → screening → conveying → packaging. 

On the other hand conversion of starch to dextrose through the assist of acid catalyst is a typical chemical reaction which involves transportation of raw materials, physical steps of mixing the reactants, heat transfer, reaction kinetics, fluid flow, separation of products, product purification, drying, screening, conveying and packaging. 

In pharmaceutical curriculum, knowledge of unit operations is very much relevant particularly with respect to formulated drug products and basic drugs.  

Unit process  

A unit procedure is a step in manufacturing in which chemical reaction takes place, for example the oxidation of para xylene to terephthalic acid is a unit process, and the hydrogenation of vegetable oil to ghee is a unit process.  Others include, nitration, sulphonation, hydroformylation e.t.c. 

Unit operations and unit processing form the main principles of all kinds of chemical industries and are the foundation of designs of chemical plants, factories, and equipment used.

Industrial processes  

Large processes are broken into unit operations in order to make them easier to analyse. The key thing to remember about them is that the conservation  laws  apply  not  only  to  the  process  as  a whole but also to each individual  unit  operation.  Examples of such processes are discussed below. 

Mixing   

Mixing can be termed as the preparation of mixture of substances either in solids, liquids, or gaseous form. It is an essential part of most production processes in chemical and allied industries covering all processing stages from the preparation of reagents to the final blending of products. The equipment used depends on the nature of the materials and the degree of mixing required.  Mixing process can be considered under three stages. 

(a)  Gas Mixing: Specialized equipment is seldom needed for mixing gases, which mix easily because of their low viscosities. The mixing given by turbulent flow in a length of pipe is usually sufficient for most purposes. Turbulent promoters, such as orifices or baffles can be used to increase the rate of mixing. In most cases, in-line mixers are always used. 

b)     Liquid Mixing: The following factors must be taken into account when choosing equipment for mixing liquids: 

(i)    Batch of continuous operation. 

(ii)    Nature of the process whether is miscible or immiscible liquids or dispersion liquid. 

(iii)   Degree of mixing required.  

(iv)   Physical properties of the liquid, particularly the viscosity. 

(v)    Whether the mixing is associated with other operations such as reaction, heat transfer and so on. 

(c)   Solids and Pastes:  Mixing of solid is carried out by using various types of highly specialized equipment. Such equipment include cone blenders which are  used  for  free  flowing  solids,  ribbon  blender for dry solid and for blending liquids with  solid  while  Z-  blade  mixers  and  pan  mixers  are  used  for  kneading  heavy pastes and dough.  

Agitation 

Agitation refers to the induced motion of a material in a specified way, usually in a circulatory pattern inside some sort of container. Agitation involves both homogeneous and heterogeneous phases while mixing is used for heterogeneous phase. For example, water in a cup can be agitated but cannot be referred to as mixed.  Thus, agitation isn't the same as mixing.

Separation processes

There are different types of separation processes. These include: distillation, extraction, absorption, membrane filtration, and so on. Each of these can also be used for purification, to varying degrees. 

(a)   Separation by Flashing  

A mixture of 2 liquids or a liquid and vapour can be separated by passing it to a flash drum at a fixed temperature and pressure. The mixture is allowed to reach equilibrium (or near it), and then the vapour exits the top and the liquid exits the bottom of the drum. This separates the  components somewhat, provided that the temperature is chosen between the boiling  temperatures  of  the  components of  the mixture  at  the  pressure  of  the  drum.  The degree of separation depends on the composition of the mixture, the concentrations of the species in the mixture, and the temperature and pressure. Having data such as fugacity data or even vapour pressures for simple modeling like Raoult's Law are invaluable when choosing the operating conditions.  

When a solution boils, the resulting gas is still a mixture, but the gaseous mixture will in  general  have  more  of  the  lower-boiling  compound  than  the  higher-boiling  compound.  Therefore,  a  higher- boiling compound can be separated from the lower-boiling compound by simply allowing a part of the solution to boil and another part to remain as liquid. 

(b)  Distillation

Distillation is the most widely used separation process in the chemical industry. It is also known as fractional distillation or fractionation. It is normally used to separate liquid mixtures into two or more vapour or liquid products with different compositions.  

Distillation is an equilibrium stage operation. In each stage, a vapour phase is contacted with a liquid phase and mass is from vapour to liquid  and from liquid to vapour. The less  volatile, heavy or high-boiling components concentrate in the liquid phase; the more volatile, light components concentrate in the vapour. By using multiple stages in series with recycle, separation can be accomplished. 

Operating Principles  

931_Schematic  Representation  of  the  Operating  Principles  of.jpg

Fig:  Schematic Representation of the Operating Principles of Distillation  

The feed to a distillation column may be liquid, vapour, or a liquid- vapour mixture. It may  enter  at  any  point  in  the  column,  although  the  optimal  feed  tray  location  should  be  determined  and used. More than one stream may be fed to the system, and more than one product may be drawn.  

A column is divided into series of stages. These correspond to a cascade of equilibrium stages. Liquid flows down the column from stage to stage and is contacted by vapour flowing upward.   Traditionally, most columns have been built from a set of distinct trays or plates, so these terms end up being essentially interchangeable with stages. Each tray in a distillation column is designed to promote contact between the vapour and liquid on the stage. Distillation can be conducted in a packed column (just as absorption can be done in a trayed column), but we will focus on trayed columns for the present. A schematic illustration of the operating principles of distillation is shown in Fig. 

The product leaving the top of the column is said the overhead product, the overhead, the top product, the distillate, or distillate product. Distillate product might be liquid or vapour (or occasionally both) depending on the kind of condenser utilized. Most of the time the distillate flow rate is allocated the symbol D, and the composition xD or yD

The product leaving the bottom of the column is said the bottom product or bottoms, and specified the symbol B, through composition xB.  

In several conditions, particularly petroleum refining, one or more intermediate or "sidedraw" products might be eliminated from the column. Vapour leaving the top of the column passes through a heat exchanger, the condenser, where it is partially or totally condensed. The liquid that a consequence is temporarily held in the accumulator or reflux drum. A liquid stream is reserved from the drum and returned to the top tray of the column as reflux (R or L) to endorse division.  

The portion of the column above the nourish tray is said the rectification section. In this section, the vapour is enriched via contact through the reflux. The portion of the column below the feed tray is said the stripping section. The liquid portion of the feed serves as the reflux for this section.  

The operating pressure of the column is typically controlled by adjusting heat removal in the condenser.  

The base of the column is typically used as a reservoir to hold liquid leaving the bottom tray. A heat exchanger, the re-boiler, is used to boil this liquid. The vapour which results, the boil-up (V) is returned to the column on one of the bottom three or four trays. In normal  operation, there are 5 handles that can be adjusted to manipulate the behaviour of a  distillation column-the  feed  flow, two product flows, the reflux flow, and the boil-up flow (or  re-boiler  heat  input). A normal column has a temperature gradient and a pressure gradient from bottom to top. Distillation, like flashing, is a procedure that is usually utilized to divide a mixture of two or more liquids depend on their boiling points. 

Though, what occurs in a distillation column is essentially a series of flashes that are joined through recycle loops. The liquid from each tray comes to equilibrium (ideally) through the vapour, and the vapour increases up to the next tray and the liquid drops to the tray beneath it. Each tray has a different temperature because a re-boiler on the bottom and a condenser at the top sustain a temperature gradient across the column (in certain division setups one of these components is skipped). 

The net consequence is, as flashing, more of the lower-boiling compound(s) will exit at the top of the column, and more of the higher-boiling compound(s) will fall to the bottoms. Since   distillation is multiple flashes in a row, it is typically more effectual than a single flash, although the latter might be adequate depending on the purpose. 

Distillation columns are standard for many kinds of divisions since it is comparatively inexpensive for its efficacy. Distillation has a limit, though; non-ideal mixtures can form azeotropes. An azeotrope is a point at that when the solution boils; the vapour has the similar composition as the liquid. Thus no further division can be done with no another technique or without using several special tricks. 

Two instances of distillation processes are petroleum distillation and the production of alcoholic beverages. In the first case, oil is separated into its many components, with the lightest at the top and the heaviest on the bottom. In the latter, the gas is enriched in ethanol, which is later recondensed. 

(c)   Gravitational Separation  

Gravitational division takes benefit of the well-known result of density differences. Something that is less dense will float on something that is denser. Thus, if 2 immiscible liquids have significantly different densities, they can be separated by simply letting them settle, then draining the denser liquid out via the bottom. As we know that the key word here is immiscible; if the liquids are soluble in each other, then it is impossible to separate them by this method. 

This process can as well be utilized to divide out solids from a liquid mixture, but again the solids must not be soluble in the liquid (or must be less soluble than they are as present in the solution). 

(d)    Extraction  

Extraction is the common practice of taking something liquid and forcing it to become dissolved in another liquid. This is done via taking benefit of the relative solubility of a compound between 2 liquids. For instance caffeine must be extracted from coffee beans or tea leaves in order to be utilized in beverages these as coffee or soda. The general technique for doing this is to utilize that is able to dissolve caffeine as if it were a liquid. Then, in order to obtain the caffeine out, the temperature is lowered (lowering in carbon dioxide) and water is infused. The system is then permitted to attain equilibrium. Because caffeine is more soluble in water than it is in carbon dioxide, the majority of it goes into the water.

Extraction is as well utilized for purification through a pollutant; the pollutant can be removed by another, clean stream. Even if it isn't very soluble, it will still extract pollutant. 

Another kind of extraction is acid-base extraction that is helpful for another kind of extraction is moving a basic or acidic compound from a polar solvent (such as water) to a non-polar one. Frequently, the ionized form of the acid or base is soluble in a polar solvent, but the non reverse is true for the non manipulate where the majority of the compound will finish up, we modify or adjust the pH of the solution via adding acid or base.

For instance, suppose we wanted to extract fluoride (F-) from water into benzene. First, we would add acid, because when a strong acid is into benzene. 1st, we would an added to the solution it undergoes the subsequent reaction through fluoride that is virtually irreversible:

F- + H3O+ -> HF + H2O

The hydrogen fluoride is more soluble in benzene than fluoride itself, so it would move into the benzene. Solutions could then be divided via density because they are immiscible.

The term absorption is a generalization of extraction that can involve liquid instead of liquid-liquid). Though, the ideas different phases (gas-liquid instead of liquid are still the same. 

(e)    Membrane Filtration  

A membrane is any barrier that permits one substance to pass through it more than another.  There are 2 common kinds of membrane separators: those that divide depend on the size of the molecules and those which separate based on diffusivity. 

An instance of the 1st kind of membrane separator is us everyday vacuum cleaner. Vacuum cleaners work via taking in air laden through dust from our carpet. A filter inside the vacuum then traps the dust particles (which are relatively large) and allows the air to pass through it (since air particles are comparatively small). A larger-scale operation that works on the same principle is said a fabric filter or 'Baghouse', which is utilized in air pollution control or other applications where a solid must be eliminated from a gas. 

Several fancy membranes exist that are able to divide hydrogen from a gaseous mixture via size. Such membranes have extremely tiny pores that permit hydrogen (the smallest possible  molecule,  through molecular weight) to pass throughout via convection, but other molecules can't pass  through the pores and must resort to diffusion (which is moderately slow). Therefore a purified hydrogen mixture consequence on the other side. 

Membranes can divide substances via their diffusivity also, for instance water might diffuse through an indeed kind of filter faster than ethanol, so if these filter existed it could be utilized to enrich the original solution through ethanol.   

Purification methods  

In order to bring any product to market, it is necessary to purify it adequately. Without purification, people could get sick from eating such foods; side effects could occur in industry which would cause safety concerns, or a scientist's research could be invalidated. Fortunately there are several methods that are commonly used for purification. The separation processes mentioned above are often used for this purpose, as are the following two processes. 

(a)   Adsorption  

This should not be confused through absorption; adsorption is a procedure that divides components via their relative adhesiveness to a surface. An adsorption column is basically a pipe filled through an indeed substance. When the contaminant flows via, it will bind to the substance, and in this way the fluid flowing via is cleaned. Adsorption is then a surface phenomenon.

A main drawback to this technique is that the substance will always have a saturation point after that no more contaminant can latch on to it. At this point, the procedure of purification stops and thus the spent substance must be swapped via new material. 

(b)   Re-Crystallization  

Re-crystallization is the purification of substances by taking advantage of changes in solubility with respect to temperature. We take advantage of this by dissolving an impure compound and then lowering the temperature slowly.  The solubility of most solid substances increases with temperature, so decreasing the temperature will cause the solubility of both the impurities and the substance to be purified to decrease. However, since there is likely much more of the impure substance present than impurities, the impure substance will eventually crystallize out long before the impurities. Therefore, as long as the temperature isn't lowered too quickly, the impure substances will crystallize out in a purer form, while most impurities will remain in solution.  A disadvantage to this method is that it takes a long time to perform, but it is often the most effective method for attaining a pure model of a product. 

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