Introduction
Vapor that passes the thermometer condenses in the condenser, a double-walled tube that is cooled by water flowing through the outer layer, and drips into the receiver. Before moving on, let us look at benefits and drawbacks to a simple distillation versus a fractional distil- lation.
1. Distillation
Distillation is a purification technique for a liquid or a mixture of liquids. We utilize the difference in bolloing points of liquids as a basis of separation. The core of a distillation process, is selective evaporation and condensation of particular components. Our overall goal is to evaporate and condense only one component from a mixture, but to attain this goal, we must allow many, many cycles of evaporation and condensation to take place. This process gradually enriches the vapor phase in favor of the most volatile component.
After a sufficient number of evaporation and condensation cycles have taken place, the final condensate contains a liquid that is en- riched in the more volatile component.
Distillation is easier to understand if we envision a spesific mixture of two liquids, say diethyl ether and ethanol. The boiling points of the two liquids are 36°C and 78°C, respectively.
When we boil this mixture, we observe the following: the entire mixture (both compounds) boils, but the vapor phase is enriched in the more volatile component (diethyl ether). As this vapor mixture rises, cools, and condenses, the resulting liquid is enriched in diethyl ether too. If we attach a column to the flask so that the vapor enters this column, the condensing liquid will be heated by rising vapors, and it will boil again producing a vapor that is even more enriched in diethyl ether.
The higher the column, the more times this cycle of evaporation-condensation can be repeated, and the higher up we sample the vapor, the more enriched the vapor phase will be in the more volatile component (diethyl ether). Ideally, with a long enough column, one could obtain a vapor that is nearly pure diethyl ether, and leave behind a liquid that is nearly pure ethanol, the less volatile component.
2. Distillation Column
Cosolvent Machines
John B DurkeeII, in Cleaning with Solvents: Methods and Machinery, 2014
This distillation column shown above is not the one shown in Figure 1.8, which separates solvent mixtures from soil – as the water has already been removed in the decanter.
This distillation column is designed based on the understanding that all components of the SA mixture are less volatile than the RA (water). So, the distillation column shown above separates water from the solvent mixture, and then as much of the SA mixture as possible from all of the soil components.
3. Process DesignIn Lees’ Loss Prevention in the Process Industries (Third Edition), 2005
11.8.4 Distillation columns
Distillation columns present a hazard in that they contain large inventories of flammable boiling liquid, usually under pressure. There are a number of situations which may lead to loss of containment of this liquid.
The conditions of operation of the equipment associated with the distillation column, particularly the reboiler and bottoms pump, are severe, so that failure is more probable.
The reduction of hazard in distillation columns by the limitation of inventory has been discussed above. A distillation column has a large input of heat at the reboiler and a large output at the condenser. If cooling at the condenser is lost, the column may suffer overpressure. It is necessary to protect against this by higher pressure design, relief valves or HIPS.
On the other hand, loss of steam at the reboiler can cause underpressure in the column. On columns operating at or near atmospheric pressure, full vacuum design, vacuum breakers or inert gas injection is needed for protection. Depostition of flammable materials on packing surfaces has led to many fires on opening of distillation column for maintenance.
Another hazard is overpressure due to heat radiation from fire. Again pressure relief devices are required to provide protection.
The protection of distillation columns is one of the topics treated in detail in codes for pressure relief such as API RP 521. Likewise, it is one of the principal applications of trip systems.
Another quite different hazard in a distillation column is the ingress of water.
The rapid expansion of the water as it flashes to steam can create very damaging overpressures.
4. Using the second law of thermodynamics
Yaşar Demirel, in Nonequilibrium Thermodynamics, 2002
5.2. Distillation
Since it requires a large amount of heat in reboiler and discharges a similar amount of heat at the condenser, distillation column resembles a heat engine that requires an optimum operating condition. Therefore a distillation column is an energy intensive process, and heat causes the work for separating the components of a feed stream into products
The thermodynamic analysis may be an effective tool for identifying the possible improvements in distillation column design by understanding the thermodynamic inefficiencies in a column. Considering the distillation column as a heat engine, the lost work profiles can be determined to quantify the inefficiency in terms of the pressure drop, heat and mass transfer, and coupling between heat and mass transfer.
The column efficiency may be related to the optimal feed conditions including the feed plate location, leading to the minimum irreversibility based on the utility requirements.
The thermodynamic optimization of a distillation column should lead to producing more uniform irreversibility distributions.
This may be achieved through the column modifications, such as feed condition, feed stage location and use of intermediate exchangers in order to reduce irreversibility in sections with large driving forces and to increase irreversibility in sections with small driving forces. The pinch point calculations may also be used for determining the minimum energy demand for distillation with distributed components and side-product withdrawals.
5. Distillation
James G. Speight, in The Refinery of the Future, 2011
4.2.3 Columns
- Distillation columns(distillation towers) are made up of several components, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation column consists of several major parts:
- A vertical shell where separation of the components is carried out.
- Column internals such as trays, or plates, or packings that are used to enhance component separation.
- A reboilerto provide the necessary vaporization for the distillation process.
- A condenser to cool and condense the vapor leaving the top of the column.
- A reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycled back to the column.
- The vertical shell houses the column internals, together with the condenser and reboiler constitutes a distillation column.
6. DISTILLATION COLUMN DYNAMICS AND CONTROL
A thesis presented for the· degree of Doctor of Philosophy in Chemical Engineering in the University of Canterbury Christchurch, New Zealand by GRANT WILSON p”‘~ 1979
1. ABSTRACT
A pilot plant scale, atmospheric pressure, sieve plate distillation column was constructed and fully instrumented. Novel speed controllable pumps were used to control liquid flows. A microcomputer was constructed to provide local and hierarchical control of the column. The microcomputer included an operator console, a 16 channel data acquisition uni·c, a 4 channel control output unit, and a hardware arithmetic processor.
A software development system was assembled by linking the microcomputer to a minicomputer. Software written for the development system included a cross-assembler, a transfer program, and a microcomputer control program. A binary steady state distillation column model was developed, solved on a digital computer, and verified _against experimental data using a binary mixture of methanol and water.
Two control schemes were investigated using only the microcomputer resources. A multi-loop system using digital PI controllers was found to give excellent control within the accuracy of the instrumentation. An adaptive feedforward controller was proposed and verified using a steady state model, and experiments.
The results were good, but because of the relatively simple dynamics of the experimental column, the feedforward controller was no better than the feedback controllers. A microcomputer control system has been shown to be an effective replacement for conventional analog control on a distillation column. The computing power of the microcomputer has enabled a sophisticated control scheme to be implemented at low cost.
Many thanks to Dr •. R.M. Allen for his help and advice during this work.· Thanks also to all the technicians of the Chemical E.ngineering Department for their efforts iri constructing equipment. This work was financed by a University Grants Committee grant CC76/25)_.
Glassware was kindly donated by Ivon Watkins-Dow Ltd. ii The author acknowledges financial support from a B.P. Postgraduate Scholarship, .a University Grants Committee Postgraduate Scholarship, and a University of Canterbury Teaching Fellowship. Thanks to Mrs. D.E. Ball for typing, and my parents for proofreading this thesis from my illegible writing. Finally, to my parents, who provided endless support and.help, my most grateful thanks.
2. INTRODUCTION
Distillation processes have been extensively studied because they occur frequently in the chemical processing industries, are large consumers of energy, and are often critical in determining product purities.
The objectives of previous studies have been to improve the understanding of the dynamics of distillation columns, and consequently to improve performance by maximising production and minimising costs. This work represents a continuation of this theme with the use of new technology, and an alternative control system.
A range of approaches have been adopted in,the literature including complex computer simulations, simple black box models, and experimental tests using analog and digital hardware. Each approach may have its merits, but there is no single approach which can be applied to all distillation processes, in fact the control problems of a particular distillation column may be unique.
The overriding philosophy behind all investigations of the performance of various control schemes will ultimately be profit.
3. REVIEW
Process control has been extensively studied and the investigation of the behaviour and control of distillation columns has been no exception. Every conceivable control strategy has been applied to controlling simulated and experimental distillation columns, but production units have in general relied upon the less exotic control strategies such as feedforward/feedback control.
The introduction of minicomputers into process control produced a centralisation of control functions over the previously used distributed analog systems. The big failing of the minicomputer system was the need to provide analog backup in case of a computer failure .(Bruce and Fanning (1964), Guisti et al (1962), Rosenbrock et al (1965}).
The current trend is back to distributed control schemes with microcomputers providing dedicated control on one piece of plant and perhaps connecting with a hierachical control scheme (Tao et al (1977)). The major control equipment vendors are now marketing distributed control systems, e .. g. Honeywell TDC2000 system.
4. Distillation column control
There is a vast quantity of material on distillation column control in the literature. Rademaker et al (19751 have summarised the major contributions in this field up to the late 1960’s. Control schemes have generally been examined theoretically on experimental columns or practically on production columns and there is a large. gap between these two groups.
Experimental columns are, in general, pilot scale atmospheric pressure binary columns, and hence avoid a lot of the problems of large production columns, e.g. liquid flow lags, and the need for pressure controls. Much. work needs to be done in bridging this gap and extending the scope o:l; industrial control schemes using economic criteria to justify the changes.
The major practical contributions to distillation control have been summarised by Shinskey (1967) using single loop controllers with variations. These schemes include the direct and indirect material balances, pressure control loops, and feedforward schemes based on a constant product to feed ratio. Systems based on these approaches have been the basis of most.
- Industrial distillation control
Many researchers have looked at the application of multivariable techniques to distillation columns. An excellent review of this approach is given by Edgar and Schwanke (1977) • Some of the simple multivariable techniques such as decoupling have been studied but generally only on experimental columns.
The effect of decoupling single loop controllers operating on product compositions has been shown to produce improvements, but this is to be expected because of the lags and delays caused by the large liquid holdups in the reflux accumulator and reboiler, and the consequently slow responses of the composition loops. Indirect camposition control by controlling internal tray liquid temperatures is superior even without decoupling because of the faster loop dynamics.
Other schemes using optimal multivariable controllers require process models which are generally inaccurate because of linearisation, model order reduction, incomplete measurements, and changing operating conditions (Schwanke et al (1976)). The results of these inaccuracies are suboptimal and in some cases unstable control.
Some work on the use of adaptive controllers (Sastry et al (197 7)_) has shown promising results for a single loop, but the overall performance is no better than can be achieved by feedforward/feedback techniques.
6. DISTILLATION COLUMN HARDWARE
A pilot plant scale, atmospheric pressure distillation column had been used in the Department of Chemical Engineering for teaching in undergraduate laboratories. For the reasons noted below, this column was unsatisfactory both in its performance and in its possible application to this project:
- reboiler capacity was excessive
- condenser capacity was inadequate
- reflux/distillate split was by a swinging bucket reflux divider giving pulsed flows
- poor tray efficiency ( < 30% overall)
- insufficient and ineffective control equipment
- noon-tray sampling and temperature measurement
- no composition measurement
- no centralised control station
The column was completely dismantled and rebuilt using such parts of the original as were required. The reboiler capacity was reduced, and the condenser capacity increased. The trays were redesigned and rebuilt to allow feed/drawoff and temperature measurement.
The reflux divider was replaced by a reflux accumulator. A control system was devised constructed and installed using speed controllable motors driving positive displacement vane pumps for liquid flows, and a flow control loop for the reboiler steam supply. The distillation column was instrumented with temperature, pressure, level and composition sensors, and interfaced to a microcomputer-based control system.
7. Production Zone Method: a New Non-ideal Shortcut Method for Distillation Column Design
Abstract
This work describes a new non-ideal shortcut method for distillation design and includes a graphical representation. Based on the operation leaves of Castillo et al. (1998) the method uses production segment rather than completely specified product which eliminates any sensitivity to the composition of the minor products. It has two aims.
The first is to determine if a specified separation respects the mass balance and the thermodynamic feasibility. The second one is to find the minimum reflux ratio and a preliminary design of the column.
The following mixtures are investigated: an ideal mixture of ethanol, n-propanol, and n-butanol; a non-ideal mixture of acetone, water, and acetic acid; and an azeotropic mixture of acetone, isopropanol, and water. Designs obtained with this new method lead to purity and recovery rate close to specifications which is sometimes impossible with conventional ideal shortcut like the well-known Fenske-Underwood-Gilliland shortcut.
8. Assessment of Chemical and Mechanical Properties of Polymers Aiming to Replace the Stainless Steel in Distillation Column
Abstract and Figures
Small scale ethanol production process faces a number of challenges that negatively impact productivity and economic costs. The present work aims at finding alternative materials with lower cost and suitable resistance to replace the stainless steel commonly used in distillation columns in small scale businesses.
For that we analyzed the chemical and mechanical behavior of nine different polymeric materials. An important parameter to consider in this case is the compatibility of polymer with the ethanol, which was determined according to the ASTM D543, as well as the swelling degree and ethanol diffusion in material being also studied in this work.
Three-point flexural tests were performed because the material when in service is subjected to forces or loads. The results show that some of the materials under test present chemical resistance and satisfactory mechanic performance after the contact with the ethanol in temperatures compatible with the distillation process.
Question
1. What glassware is used in distillation?
Borosilicate Glass
Based on this, the quality of your laboratory glassware for distillation can fall into the following groups: Borosilicate Glass – This is favored because of its durability as well as chemical resistance.
2. What is a distilling column used for in laboratory?
A distillation column is an essential item used in the distillation of liquid mixtures to separate the mixture into its component parts, or fractions, based on the differences in volatilities. Fractionating columns are used in small scale laboratory distillations as well as large scale industrial distillations.
3. Why are porcelain pieces of glass beads used in distillation?
They are mainly added to the mixture for two reasons i.e., to prevent the superheating of liquid and also to make the distilling flask more stable. Porcelain chips are also substituted with boiling chips also known as boiling stones or anti-bumping granules.
4. What is the principle of distillation column?
The column works on the principle of differential distillation, where a mixture is heated to create vapours. These vapours then ascend the column, which is cooler at the top than at the bottom. As the vapours rise, they cool and condense at different levels according to their boiling points
5. What is the purpose of distillation?
Distillation is used to separate liquids from nonvolatile solids, as in the separation of alcoholic liquors from fermented materials, or in the separation of two or more liquids having different boiling points, as in the separation of gasoline, kerosene, and lubricating oil from crude .
6. Why are glass rods used in fractional distillation?
Note: A fractionating column is fitted with glass beads to increase the surface area through which the evaporated liquids travel. This enables better separation of the liquids. If the boiling points of the liquids are extremely close then the length of the fractionating column should be more.