Chemical Technology Equipment, Chemistry tutorial

Introduction  

In units 1 and 2, heat and mass transfer were discussed in relation to industrial chemical technology respectively. In chapter 3, various unit operations were discussed and it is specifically stated that every industrial chemical procedure is based on unit operations (physical treatment) and unit procedure (chemical treatment) to produce economically a desired product from specific raw substances. The raw materials are treated through physical steps to make them suitable for chemical reaction and this involved the use of different types of equipment for the objectives to be realized. In this chapter, several of the equipment available in carrying out such unit operations will be discussed. 

Chemical technology  

Chemical technology can be described as the scientific techniques used in the chemical industry.  It is the science of those operations which convert raw materials into desired products on an industrial scale, applying one or more chemical conversions. The route along which a raw material is converted to products is a logical coupling of interconnected operations terms industrial processes which involve unit operation and unit process. 

Chemical Technology Equipment  

These are equipments used in facilitating or involved in the transformation of raw materials into products. The equipment helps in transfer of heat and mass from one point to another in order to achieve production processes. The first steps in a process, such as the mechanical operations of grinding or crushing, are often followed via physical treatments such as mixing, heating,   evaporating, distilling, and condensing and so on. are carried out via chemical technology equipment.  Several of these equipments are discussed below.

Heat Exchangers  

In common, a heat exchanger is a device that is utilized to facilitate the exchange of heat between 2 mixtures, from the hotter one to the cooler one. It is any equipment that shifts the energy from a hot fluid to a cold fluid, through maximum rate and minimum investment and running costs. Heat exchangers extremely frequently engage steam since steam is extremely good at carrying heat via convection, and it as well has a high heat capacity so it won't transform temperature as much as another working fluid would. In addition, though steam can be expensive to produce, it is probable to be comparatively less expensive compare to other working fluids since it comes from water. Instances of heat exchangers are: 

i) Intercooler and pre-heater 

ii) Condenser and boilers in steam plant 

iii) Condensers and evaporators in refrigeration units 

iv) Regenerators 

v) Radiators 

vi) Oil coolers and so on. 

Tubular Heat Exchangers  

A tubular heat exchanger is basically a jacket around a pipe. The working fluid (frequently steam) enters the jacket on one side of the heat exchanger and leaves on the other side. Inside the pipe is the mixture which we want to heat or cool. Heat is exchanged through the walls of the device in accordance to the 2nd law of thermodynamics that needs that heat flow from higher to lower temperatures. Thus, if it is desired to cool off the fluid in the pipe, the working fluid must be cooler than the fluid in the pipe. 

Tubular heat exchangers can be set up in 2 ways: co-current or counter-current. In a co-current setup, the working fluid and the fluid in the pipe go into on the similar side of the heat exchanger. This setup is somewhat inefficient because as heat is exchanged, the temperature of the working fluid will approach that of the fluid in the pipe. The closer the two temperatures become, the less heat can be exchanged. Worse, if the temperatures become equal somewhere in the middle of  the heat exchanger, the remaining length is wasted since the 2 fluids are at thermal equilibrium (no heat is liberated). 

To assist counteract such results, one can utilize a counter-current setup, in that the working fluid enters the heat exchanger on one end and the fluid in the pipe penetrates at the other end. As a clarification for why this is more proficient, suppose that the working fluid is hotter than the fluid in the pipe, so that the fluid in the pipe is warmth up. The fluid in the pipe will be at its elevated's temperature when it exits the heat exchanger and at its coolest when it enters. The working fluid will follow the similar trend since it cools off as it travels the length of the exchanger. Since its counter-current, though, the fact that the working fluid cools off has less of an consequence since it's exchanging heat through cooler, rather than warmer, fluids in the pipe. 

Distillation Towers  

Huge scale industrial distillation applications comprise together batch and continuous fractional, vacuum, azeotropic, extractive, and steam distillation. The most extensively utilized industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical and chemical plants and natural gas developing plants. 

Industrial distillation is classically executed in huge, vertical cylindrical columns recognized as   distillation towers or distillation columns through diametres ranging from about 65 centimeters to 16 metres and heights ranging from about 6 to 90 metres or more. When the procedure feed has a diverse composition, as in distilling crude oil, liquid outlets at intervals up the column permit for the extraction of dissimilar fractions or creations having different boiling points or boiling ranges. The lightest products (those by the lowest boiling point) outlet from the top of the columns and the heaviest products (those through the highest boiling point) way out from the bottom of the column and are frequently described the bottoms. A characteristic industrial distillation tower diagram is revealed in Fig. 

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Fig: Diagram of a Typical Industrial Distillation Tower  

Industrial towers utilize reflux to attain a more absolute division of products. Reflux refers to the portion of the reduced overhead liquid product from a distillation or fractionation tower, which is returned to the upper part of the tower as revealed in the schematic diagram of a characteristic, large-scale industrial distillation tower. Inside the tower, the down flowing reflux liquid gives cooling and condensation of the up flowing vapours thereby is raising the effectiveness of the distillation tower. The more reflux that are supplied for a specified number of theoretical plates, the better the tower's division of lower boiling substances from higher boiling materials.  Alternatively, the more reflux that is provided for a specified desired separation, the fewer the number of theoretical plates needed. 

These industrial fractionating towers are as well utilized in air division, producing liquid oxygen, liquid nitrogen, and high purity argon. Distillation of chlorosilanes too enables the production of high-purity silicon for utilizes as a semiconductor. 

The efficiencies of the vapour-liquid contact machines (termed to as plates or trays) utilized in distillation towers are classically lower than that of a theoretical 100% efficient equilibrium stage. Therefore, a distillation tower requires more trays than the number of theoretical vapor-liquid symmetry stages.

In present industrial utilizes, generally, a packing substance is utilized in the column instead of trays, particularly when low pressure drops across the column are needed, as when operating under vacuum. 

This packing substance can either be random dumped packing (1-3" wide) these as Raschig rings or structured sheet metal. Liquids tend to wet the surface of the packing and the vapours pass   across this wetted surface, where mass shift takes place. Unlike conventional tray distillation in that every tray symbolizes a divide point of vapour-liquid equilibrium; the vapour-liquid equilibrium curve in a packed column is continuous. Though, when modeling packed columns, it is helpful to calculate a number of theoretical stages to signify the division competence of the packed column through respect to more traditional trays. Differently shaped packings have dissimilar surface areas and void space between packings.

Both of such features influence packing performance. An additional feature in addition to the packing shape and surface area that affects the performance of random or structured packing is the liquid and vapour distribution entering the packed bed. The number of theoretical stages needed for a specified division is computed using an exact vapour to liquid ratio. If the liquid and vapour aren't evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapour ratio won't be correct in the packed bed and the needed division won't be attained. The packing will show not to be working correctly. The height equivalent of a theoretical plate (HETP) will be greater than imagined. The trouble isn't the packing itself but  the  mal-distribution  of  the  fluids  entering  the  packed  bed. 

Liquid mal-distribution is more frequently the difficulty than vapour. The plan of the liquid distributors utilized to initiate the feed and reflux to a packed bed is dangerous to making the packing execute to its maximum efficiency. Techniques of evaluating the effectiveness of a liquid distributor to evenly distribute the liquid entering a packed bed can be originate in references. Considerable work has been done on this topic through Fractionation Research, Inc. (commonly known as FRI).  

Multi-effect distillation  

The goal of multi-effect distillation is to enhance the energy competence of the procedure, for utilize in desalination, or in several cases one stage in the production of ultrapure water. The number of results is proportional to the Kw*h/m of water recovered figure, and terms to the volume of water recovered per unit of energy evaluated through single-effect distillation. One consequence is approximately 636 Kw*h/m3

  • Multi-stage flash distillation can attain more than 20 effects through thermal energy input.

Vapour compression evaporation. Commercial large-scale units can attain around 72 effects through electrical energy input, according to producers. There are many other kinds of multi-effect distillation procedures, including one termed to as simply multi-effect distillation (MED), in that multiple chambers through intervening heat exchangers are employed. 

Reactors

Chemical reactors are vessels planned to enclose chemical reactions. The design of a chemical reactor deals through multiple aspects of chemical engineering. Chemical engineers intend reactors to maximise net present value for the given reaction. Designers ensure that the reaction proceeds through the highest efficiency towards the desired output product, producing the highest yield of product while requiring the least amount of money to obtain and operate. Normal operating expenses include energy input, energy removal, raw material costs, labour, and so on. Energy transforms can come in the form of heating or cooling, pumping to raise pressure, frictional pressure loss (these as pressure drop across a 90° elbow or an orifice plate), agitation, and so on. 

Both kinds can be utilized as continuous reactors or batch reactors. Most usually, reactors are run at steady-state, but can too be operated in a transient state. When a reactor is 1st brought back into operation (after maintenance or in operation) it would be considered to be in a transient state, where key procedure variables alter by time. Both kinds of reactors might as well accommodate one or more solids (reagents, catalyst, or inert materials), but the reagents and products are classically liquids and gases. 

There are 3 major essential models utilized to approximation the most significant procedure variables of dissimilar chemical reactors:

  • Batch reactor model (batch)
  • Continuous stirred-tank reactor model (CSTR)
  • Plug flow reactor model (PFR).

Furthermore, catalytic reactors require divide treatment, whether they are batch, CST, or PF reactors, as the many assumptions of the simpler models aren't valid. 

Key procedure variables include: 

  • residence time (τ, lower case Greek tau) volume (V)
  • temperature (T)
  • pressure (P)
  • concentrations of chemical species (C1, C2, C3, ... Cn)
  • Heat transfer coefficients (h, U).

A chemical reactor, typically tubular reactor, could be a packed bed such as multi-bed reactor, multi-tube reactor and tubular high pressure reactor. The packing inside the bed may have catalyst to catalyse the chemical reaction. A chemical reactor may also be a fluidized bed.    

Chemical reactions occurring in a reactor may be exothermic, meaning giving off heat, or endothermic, meaning absorbing heat. A chemical reactor vessel may have a cooling or heating jacket or cooling or heating coils (tubes) wrapped around the outside of its vessel wall to cool down or heat up the contents. 

Types of Reactor  

a)  Continuous Stirred-Tank Reactors (CSTRs) and Fluidized Bed Reactors (FBs)  

A continuous stirred-tank reactor is an idealized reactor in which the reactants are dumped in one large tank, allowed to react, and then the products (and unused reactants) are released out of the bottom. In this way the reactants are kept relatively dilute, so the temperatures in the reactor are generally lower. This also can have advantages or disadvantages for the selectivity of the  reaction, depending on whether the desired reaction is faster or slower than the undesired one. The schematic descriptions of both the continuous stirred-tank (CSTR) and fluidized bed (FB) reactors are illustrated in Fig.

1543_Schematic descriptions of Continuous Stirred-Tank Reactors.jpg

Fig: Schematic descriptions of Continuous Stirred-Tank Reactors (CSTRs) and Fluidized Bed Reactors (FBs)  

CSTRs  are  generally  more  useful  for  liquid-phase  reactions  than  PFRs  since less transport power is required. However, gas-phase reactions are harder to control in a CSTR. 

In a CSTR, one or more fluid reagents are introduced into a tank reactor equipped with an impeller while the reactor effluent is removed. The impeller stirs the reagents to ensure proper mixing. By simply dividing the volume of the tank by the average volumetric flow rate through the tank provides the residence time, or the average amount of time a discrete quantity of reagent spends within the tank. Using chemical kinetics, the reaction's supposed percent completion can be computed. Several significant aspects of the CSTR are: 

i. At steady-state, the in-flow rate must equivalent the mass out-flow rate, or else the tank will overflow or go empty (transient state). While the reactor is in a transient state the model equation must be derived from the differential mass and energy balances. 

ii. The reaction proceeds at the reaction rate connected through the final (output) concentration. 

iii. Frequently, it is inexpensively helpful to activate several CSTRs in series. This permits, for instance, the 1st CSTR to activate at a higher reagent concentration and thus a higher reaction rate. 

In these cases, the sizes of the reactors may be varied in order to minimise the total capital investment required to implement the procedure. 

iv.  It  can be seen that an infinite number of infinitely small CSTRs operating in series would be equivalent to a PFR. 

The behaviour of a CSTR is often approximated or modeled by that of a Continuous Ideally Stirred-Tank Reactor (CISTR).  All calculations performed with CISTRs assume perfect mixing. If the residence time is 5-10 times the mixing time, this approximation is valid for engineering purposes. The CISTR model is often used to simplify engineering calculations and can be used to describe research reactors. In practice it can only be approached, particularly in industrial size reactors. 

A fluidized bed reactor is, in essence, a CSTR which has been packed with catalyst. The same analogy holds between an FB and CSTR as does between a PFR and a PBR. Unlike CSTRs  though,  fluidized  beds  are  commonly  used  with  gasses;  the  gas  is  pumped  in  the  bottom  and  bubbles through the catalyst on the way to the top outlet. 

b) Plug Flow Reactors (PFRs) and Packed Bed Reactors (PBRs)  

A plug flow reactor is a (idealized) reactor in which the reacting fluid flows through a tube at a rapid pace, but without the formation of eddies characteristic of rapid flow. Plug flow reactors tend to be relatively easy to construct (they are essentially pipes) but are problematic in reactions that work better when reactants (or products) are dilute. 

Plug flow reactors can be joined through membrane separators in order to enhance the yield of a reactor. The schematic diagram is shown in Fig. The products are selectively pulled out of the reactor as they are made so that the equilibrium in the reactor itself continues to shift towards making more products. 

1865_Schematic descriptions of Continuous Stirred-Tank Reactors.jpg

Fig: Schematic Diagram of Plug Flow and Membrane Reactors  

In a PFR, one or more fluid reagents are pumped through a pipe or tube.  The chemical reaction proceeds as the reagents travel through the PFR. In this type of reactor, the changing reaction rate creates a gradient with respect to distance traversed; at the inlet to the PFR the rate is very high, but as the concentrations of the reagents decrease and the concentration of the product(s) increases the reaction rate slows down. An important feature of PFR is that a packed bed reactor is essentially a plug flow reactor packed with catalyst beads. They are used if, like the majority of reactions in industry, the reaction requires a catalyst to significantly progress at a reasonable temperature. 

Bioreactors  

A bioreactor is a reactor that utilises either a living organism or one or more enzymes from a living organism to accomplish a certain chemical transformation. Bioreactors can be either CSTRs (in which case they are known as Chemostats) or PFRs. 

Certain characteristics of a bioreactor must be more tightly controlled than they must be in a normal CSTR or PFR because cellular enzymes  are very complex and have relatively narrow ranges of optimum activity. These include, but are not limited to: 

1. Choice of organism. This is similar to the choice of catalyst for an inorganic reaction. 

2. Strain of the organism. Unlike normal catalysts, organisms are very highly manipulable to produce more of what you are after and less of other products. However, also unlike normal catalysts,  they  generally  require  a  lot  of  work  to  get  any  significant  production at all. 

3. Choice of substrate. Many organisms can utilize many different carbon sources, for example, but may only produce what you want from one of them. 

4. Concentration of substrate and aeration. Two inhibitory effects exist which could prevent you from getting the product you are after. Too much substrate leads to the glucose effect in which an  organism  will  ferment  regardless  of  the  air   supply,  while  too much air will lead to Pasteur Effect and a lack of fermentation. 

5. pH and temperature: Bacterial enzymes tend to have a narrow range of optimal pH and temperatures, so these must be carefully controlled. 

However, bioreactors have several distinct advantages. One of them is that enzymes tend to be stereo-specific. For example, you don't get useless D-sorbose in the production of vitamin C, but you get L-sorbose, which is the active form. In addition, very high production capacities are possible after enough mutations have been induced. Finally, substances which have not been made artificially or which would be very difficult to make artificially (like most antibiotics) can be made relatively easily by a living organism. 

All calculations performed with PFRs assume no upstream or downstream mixing, as implied by the term "plug flow".  

Reagents may be introduced into the PFR at locations in the reactor other than the inlet. In this way, a higher efficiency may be obtained, or the size and cost of the PFR may be reduced. A  PFR  typically  has  a  higher  efficiency  than  a  CSTR  of  the  same  volume. That is, given the same space-time, a reaction will proceed to a higher percentage completion in a PFR than in a CSTR. 

For most chemical reactions, it is impossible for the reaction to proceed to 100% completion.  The rate of reaction decreases as the percent completion increases until the point where the system reaches dynamic equilibrium (no net reaction, or change in chemical species occurs). The equilibrium point for most systems is less than 100% complete. For this  reason  a  separation  process,  such  as  distillation,  often  follows  a  chemical  reactor  in  order  to  separate  any  remaining  reagents  or  byproducts from the desired product. These reagents may sometimes be reused at the beginning of the process, such as in the Haber process. 

c)    Continuous Oscillatory Baffled Reactor (COBR)  

It is a tubular plug flow reactor. The mixing in COBR is achieved by the  combination  of  fluid  oscillation  and  orifice   baffles,  allowing  plug  flow  to be achieved under laminar flow conditions with the net flow Reynolds  number just about 100. 

d)    Semi-Batch Reactor  

A semi-batch reactor is operated with both continuous and batch inputs and outputs. A  fermenter,  for  example,  is  loaded  with  a  batch,  which  constantly  produces carbon  dioxide,  and  has  to  be  removed  continuously.  Analogously, driving a reaction of gas with a liquid is usually difficult, since the gas bubbles off. Therefore, a continuous feed  of  gas  is  injected  into  the  batch  of  a  liquid.  One chemical reactant is charged to the vessel and a second chemical is added slowly. 

e)    Catalytic Reactor

Although catalytic reactors are often implemented as plug flow reactors, their analysis requires more complicated treatment. The rate of a catalytic reaction is proportional to the amount of catalyst the reagents contact. With a solid phase catalyst and fluid phase reagents, this is proportional to the exposed area, efficiency of diffusion of reagents in and products out, and turbulent mixing or lack thereof. Perfect mixing can't be assumed. Furthermore, a catalytic reaction pathway is often multi-step with intermediates that are chemically bound to the catalyst; and as the chemical binding to the catalyst is also a chemical reaction, it may affect the kinetics. The behaviour of the catalyst is also a consideration. Particularly in high-temperature petrochemical processes, catalysts are deactivated by sintering, coking, and similar processes. 

Separators          

The term separator in oil field terminology designates a pressure vessel used for separating well fluids produced from oil and gas wells into gaseous and liquid components. A separator for petroleum production is a large vessel designed to separate production fluids into their constituent components of oil, gas and water. A separating vessel may be referred to in the following ways: oil and gas separator, separator, stage separator, trap, knockout vessel (knockout drum, knockout trap, water knockout, or liquid knockout), flash chamber (flash vessel or flash trap), expansion separator or expansion vessel, scrubber (gas scrubber), filter (gas filter). These separating vessels are normally used on a producing lease or platform near the wellhead, manifold, or tank battery to separate fluids produced from oil and gas wells into oil and gas or liquid and gas. An oil and gas separator generally includes the following essential components and features: 

1.  A vessel that includes (a) primary separation device and/or  section,  (b)  secondary  "gravity"  settling  (separating)  section,  (c)  mist  extractor  to  remove  small  liquid  particles  from  the  gas,  (d)  gas outlet, (e) liquid settling (separating) section to remove gas or  vapor  from  oil  (on  a  three-phase  unit, this section  also  separates  water  from  oil),  (f)  oil  outlet,  and  (g)  water  outlet  (three-phase  unit). 

2.  Sufficient volumetric liquid capacity to handle liquid surges (slugs) from the wells and/or flow lines. 

3.  Sufficient vessel diametre and height or length to allow most of the liquid to separate from the gas so that the mist extractor won't be flooded. 

4.  A means of controlling an oil level in the separator, which usually includes a liquid-level controller and a diaphragm motor valve on the gas outlet. 

5.   A back pressure valve on the gas outlet to sustain a stable pressure in the vessel. 

6.   Pressure relief machines.

Separators work on the principle that the three components have different densities, which allows them to stratify when moving slowly with gas on top, water on the bottom and oil in the middle. Any solids these as sand will as well settle in the bottom of the separator.   

Other instances of separators are centrifuge, hydrocyclone, liquid- liquid separators, gas- liquid separators. 

Mixers  

The several available mixers that are normally used for the continuous mixing of low viscosity fluids include inline mixer such as simple mixing tee, and injection mixers. They are static devices which promote turbulent mixing in pipelines. In static mixer, materials flowing are mixed solely by redirecting fluid flow to follow the geometry. The only power required for static mixers is the external pumping power that propels the material through the mixer. Static mixers employ the principle of dividing the flow and recombining it in a geometric sequence. It is also called a Motionless mixer.  

Other kinds of mixers are stirred tanks that are mixing vessels fitted through several form of agitator for blending liquids and preparing solutions. The overall system design is dependent on many variables including the physical properties of fluids, mixer length, tube inner diameter, the number of elements and their geometrical design. 

Crushers

A crusher is a machine designed to decrease huge rocks into smaller rocks, gravel, or rock dust. Crushers might be utilized to decrease the size, or change the form, of waste substances so they can be more easily disposed of or recycled, or to diminish the size of a solid mix of raw substances (as in rock ore), so that pieces of different composition can be differentiated. Crushing is the process of transferring a force amplified by mechanical advantage through a material made of molecules that bond together more strongly, and resist deformation more, than those in the material being crushed. Crushing devices hold material  between two parallel or tangent solid surfaces, and apply sufficient force to bring the surfaces together to produce enough energy within the material being crushed so that its molecules divide from (fracturing),  or change alignment in relation to (deformation), each other. The earliest crushers were hand-held stones, where the weight of the stone supplied a improve to muscle power, used against a stone anvil. Querns and mortars are kinds of such crushing devices.

In industry, crushers are machines that utilize a metal surface to break or compress substances.  Mining operations utilize crushers, usually classified through the degree to which they fragment the starting material, through primary and secondary crushers handling coarse substances, and tertiary and quaternary crushers reducing ore particles to finer gradations. Each crusher is designed to work through a certain maximum size of raw substance, and frequently delivers its output to a screening machine which sorts and directs the product for further processing. Typically, crushing stages are followed via milling stages if the substances require to be further decreased. In addition rock-breakers are typically placed next to a crusher to decrease oversize material as well huge for a crusher. Crushers are utilized to decrease element size enough so that the substance can be processed into finer particles in a grinder. 

In operation, the raw substance (of various sizes) is generally delivered to the primary crusher's hopper via dump trucks, excavators or wheeled front-end loaders. A feeder device these as an apron feeder, conveyor or vibrating grid manages the rate at that this substance penetrates the crusher, and frequently encloses a preliminary screening machine that permits smaller  material to bypass the crusher itself, therefore improving effectiveness. Primary crushing reduces the huge pieces to a size that can be handled via the downstream machinery. 

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