Distillation is a significant commercial technique that is used to purify a wide range of materials. It is the procedure of selective boiling and condensation to separate the components or compounds from a liquid combination.
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Distillation can produce an almost complete separation (almost pure components) or a partial separation (increased concentration of selected components in the mixture). The technique uses changes in the relative volatility of the mixture's components in each situation. Distillation is a unit operation of virtually universal importance in industrial applications, although it is a physical separation process, not a chemical reaction.
Q1: Porcelain pieces are put into the distillation flask to avoid ____________
a) Overheating
b) Bumping of the solution
c) Uniform boiling
d) None of the mentioned options
Answer: b) Bumping of the solution
Explanation: To prevent the solution from bumping due to uneven heating, porcelain pieces are placed in the distillation flask.
Q2: What are the two processes in distillation?
a) Distilling and condensation reflux
b) Distilling and freezing
c) Freezing and condensation reflux
d) Only condensation reflux
Answer: a) Distilling and condensation reflux
Explanation: Distillation is a two-step process that includes distillation and condensation reflux. The gas-liquid two-phase flow over the countercurrent contact is commonly carried out in a distillation column.
Q3: How aniline and chloroform can be separated?
a) Sublimation
b) Condensation
c) Distillation
d) Evaporation
Answer: c) Distillation
Explanation: The distillation technique can be used to separate aniline and chloroform. The toxic organic molecule aniline has the formula C6H5NH2. The organic compound chloroform has the formula CHCl3. It's a colourless, sweet-smelling, thick liquid that's used as a precursor to PTFE and refrigerants on a huge scale.
Q4: Which of the following is not separated through the distillation process?
a) Acetone and water
b) Milk and water
c) Impurities in Sea water
d) Aniline and chloroform.
Answer: c) Impurities in Sea water
Explanation: The distillation technique does not separate milk from water. A distillation procedure can separate all of the other options.
Q5: The process of distillation is used for the liquids having ____________
a) Sufficient difference in their solubility
b) Sufficient difference in their melting point
c) Sufficient difference in their boiling point
d) None of the mentioned
Answer: c) Sufficient difference in their boiling point
Explanation: The distillation method is employed for liquids with a significant difference in boiling points. Distillation also allows you to separate the components of air.
Q6: Which salt is obtained as a result of the chemical reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH)? Which method will you use to separate it from the solution?
Answer:
The chemical reaction between HCl and NaOH gives rise to common salt (NaCl). The following is a representation of the chemical reaction:
HCl + NaOH ' NaCl + H2O.
The salt is separated from the solution using the distillation process.
Q7: Draw a labeled diagram showing the process of distillation.
Answer:
The distillation process is depicted in the diagram below.
Q8: Match the following:
Answer:
(a) Melting point ' 4. Change of state from solid to liquid.
(b) Boiling point ' 3. Change of state of a liquid into a gas.
(c) Fractional distillation ' 2. Obtaining diesel from crude oil
(d) Distillation ' 1. Separating common salt and water from their mixture.
Q9: Answer the following questions.
Answer:
Experiment: We'll put water in a flask with a round bottom and a single holed rubber stopper. We'll put a thermometer in the hole and start heating the flask by placing it on iron gauze on a tripod stand.
Observation: The increase in temperature in the thermometer will be observed. When the mercury within the thermometer reaches 100°C., water begins to boil, and the temperature falls.
Conclusion: The boiling point of water is defined as the temperature at which the temperature stops rising and the water begins to boil. As a result of the experiment, the boiling point of water is 100°C.
(i) State change: The candle's wax melts and then vaporises to generate wax vapours.
(ii) Combustion: Heat and flame are produced when wax vapors burn.
Q10: What is the difference between distillation and fractional distillation?
Answer:
The following table summarises the differences between distillation and fractional distillation:
Q11: Mention the various uses of Distillation.
Answer:
Some of the uses of Distillation are as follows:
Q12: Fractional distillation can be used to separate miscible liquids having a boiling point difference of less than 25 K. What part of the fractional distillation equipment makes it efficient and superior to a normal distillation process? Use a diagram to explain.
Answer:
Fractional Distillation of Miscible Liquids
Q13: Which tubes in Fig. 2.1 (a) and (b) will be more effective as a condenser in the distillation apparatus?
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Answer:
As a condenser in the distillation equipment, tube (a) will be more effective. The presence of marbles enhances the surface area of the vapours' contact area. This gives the vapours more time to condense, making the first column more effective than the second column without marbles.
Q14: Give an example of each mixture having the following features. Suggest a suitable method to separate the components of these mixtures
a) A volatile and a non-volatile component.
b) Two volatile components with appreciable differences in their boiling point.
c) Two immiscible liquids.
d) Two or more coloured constituents soluble in some solvents.
Answer:
(a) Example: Mixture of acetone and water.
A mixture of volatile and non-volatile components can be separated by simple distillation.
(b) Example: Mixture of kerosene and petrol.
Simple distillation can be used to separate two volatile components with significant boiling point differences.
(c) Example: Mixture of mustard oil and water.
Separating funnel is used to separate a mixture of immiscible liquids.
(d) Example: A mixture of different pigments from an extract of flower petals.
Two different compounds present in the same solution can be separated using the chromatography method.
Q15: Briefly describe Vacuum Distillation.
Answer:
Q1: What are the basic methods of distillation?
a) Fractional distillation and simple distillation
b) Steam distillation and destructive distillation
c) Steam distillation, simple distillation, and gas distillation
d) Fractional distillation, destructive distillation, and simple distillation
Q2: Define the following terms.
(a) Melting point
(b) Boiling point
(c) Distillation.
Q3: Give scientific reasons.
(a) A condenser has two taps.
(b) At dawn, in winter, we see dewdrops on the leaves of trees
Q4: Salt is dissolved in seawater. You're probably aware that you can distill pure water from seawater. This approach, however, is not used to obtain drinking water from seawater. What do you believe the reason is?
Q5: Two water samples, labeled 'A' and 'B,' are given to you. Sample 'A' boils at 100°C, while sample 'B' boils at 102°C. Which water sample will not freeze at 0°C? Comment.
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Question 1 - What is the basis for maintaining minimum wetting rates in vacuum column (whether based on vacuum charge or design condition?) What will happen if minimum wetting rates are not adhering to?
My Response -
The response for this question depends on a several parameters like the characteristics of the column internals as well as the mixture which will be separated.
Considering that we are dealing with a vacuum column, there is a great chance that the equipment is operating with packing internals which presents lower pressure drop than the perforated plates. There is a several correlations in the literature capable to give an estimative for the minimum wetting rate of a separation column which relies on the characteristics of the fluids like viscosity, density, and temperature and the characteristics of the packing like applied material, if is stacked or random, geometric form, atc.
An wetting rate below of the minimum will not conduct adequate mass and heat transfer rates, leading to poor performance of the separation column. In services with high temperature with hydrocarbons the low wetting rate can lead to premature coking deposition in the separation section leading a poor fractionating performance, high pressure drop and shorter operating lifecycle.
Question 2 - What is the impact in the product quality if circulating refluxes return temperatures are not maintaining at design temperatures? Is it wise to reduce heat recovery in pre heat network for maintaining design pump around return temp at the expense of pre-heat?
My Response -
The temperature profile of a separation column is a key parameter for an adequate fractionating, for this reason it's expected deleterious effects over the final quality of the products or side streams if the temperature profile is below or above the parameters recommend by design.
Reduce the heat recovery to maintain an adequate temperature profile in the distillation column can be interesting in some cases, bute reveals that you have a problem with your energy balance and recovery of the processing unit. Crude oil distillation units are the major energy consumer is a crude oil refinery and the energy is responsible for higher then 60 % of the operating costs of a crude oil refinery, furthermore the CO2 emissions is raised in an unefficient energy system, based on these data it's not recommended to deoptimize the energy balance of the processing unit even to improve the fractionating quality. In other words, if this is happened it's necessary a energy integration study (maybe through pinch technique) to identify bottlenecks and then propose alternatives to eliminate then.
Question 3 - We have a black sludge formation at the interphase of naphtha and water in OVHD accumulator. But all the OVHD paramters like pH, Iron and Chloride are normal. The crude unit has a partail condensation ovhd system and the black sludge is observed in the second boot (Cold reflux boot). There is a CI dosing in the accumulator upstream. What could be the reason for this sludge?
My Response -
This is a relatively common condition in overhead systems of crude oil distillation units. The black sludge observed in the overhead vessel is probably pickering emulsion stabilized by iron particles which is accumulated in the interface between sour water and naphtha, despite the information that the pH, Iron and Chloride content is controlled in the overhead system it's possible that this system and the atmospheric tower can operate under corrosion situation in the past. When the emulsion is formed in the vessel, this residue cant be removed without the shutdown of the processing unit or through draining the overhead vessel totally which requires a special procedure aiming to minimize the safety risks as well as the damage to the pumps of the overhead drum.
Regarding the corrosion control in the overhead systems it's important to analyze that the corrosion control parameters is under an adequate range, especially the operating temperature of the overhead system. There are some correlations in the literature which relates the ammonia and chloride concentration in the sour water to determine salt deposition temperature in the top of the tower and this needs to be considered to define the operating temperature of the system.
Question 4 - Our desalter is facing a rag layer issue when we process cabinda crude. The brine turns black. It seems like our current emulsion breaker can not solve this problem. Is there any ideas or recommendations?
My Response -
According to the datasheet of the Cabinda crude oil, this is a light and sweet crude oil which probably contains high amounts of paraffinic hydrocarbons. To realize an adequate analysis it's important to know if the refinery is processing only the Cabinda crude or under blending with heavier crudes, in this case we can saw chemical instability between the crudes leading to asphaltenes precipitation which stabilize emulsions reducing the separation efficiency in the desalter vessels and provoke the change in the brine colour. In this case, it's possible to solve the problem by applying a crude stabiliser agent which is dosed independently of the emulsion breaker agent.
Another approach is analyze the incompatibility between the Cabinda crude with the another crude oils processed by the refinery and take anticipatory actions like reduce the processed flow rate in the crude oil distillation unit to ensure a higher residence time in the dessalters when processing a crude blending with high incompatibility potential, or avoid to process crude oils chemical incompatible with the Cabinda crude.
Question 5 - We have low PH (3 to 4) in the CDU overhead but in same time we have low chloride values ( 3 to 10 ) and already we injected high values of neutralizing amine and corrosion inhibiter. What is the reason that causes this drop in PH value?
My Response -
It's important analyze the content of chloride salts (MgCl2 and CaCl2) in the processed crude, these salts can suffer hydrolysis and generate hydrogen chloride (HCl) which can cause drastic reduction in the pH. According to the concentration of chloride salts in the crude oil it's possible to minimize this problem injection sodium hydroxide (NaOH) upstream of the dessalting vessels aiming to neutralize the hydrochlorides compounds.
Question 6 - We have a problem in our Desalter. When we turn on the electric transformers, the electricity feeder trips off immediately several times. One of the transformers is damaged. The desalter contains only Crude Oil (Not mixed with water) What are the causes?
My Response -
Firstly, you need to check that the BSW of the Crude Oil is according to the design parameter of the desalting system as well as the interface level is adequate to avoid the risk of grounding the desalter leading to the trip of the power electric systems.
It's important to analyze if the desalting system suffered some instability event which caused internal damages to desalters like the electrodes breaking, for this it's necessary open the desalters and access the internals of the equipment. I suggest to start by the desalter that have a failed transformer. Furthermore, it's important to check the isolation of the electrode responsible for the electric alimentation of the desalter internals from the transformers, a failure in the electric insulation can cause the trip of the electric system. This can be caused by internal damages to desalting systems which can led to contact between shell of the desalter and the electrodes.
It's important to analyze if the desalting system suffered some instability event which caused internal damages to desalters like the electrodes breaking, for this it's necessary open the desalters and access the internals of the equipment. I suggest to start by the desalter that have a failed transformer.
Another important point is to check the isolators integrity of the transformers which are responsible to conduct the electricity to the desalters internals.
Question 7 - What's the philosophy of desalting system in Crude Distillation Unit, with respect to High Voltage & Demulsifier?
My Response -
The desalting of crude oil is one of the most important processes in a refinery to ensure the reliability and the operational availability of the refining hardware. During the crude oil extraction processes the petroleum drag sediments and water beyond inorganic salts (carbonates, calcium, chlorides, etc.) which are responsible for fouling heat exchangers leading to efficiency reduction, raise in energy consumption and reduce the operation campaign of the process units.
The presence of the dissolved salts in the crude oil is still responsible for catalysts deactivation in conversion process units (FCC and Hydrotreating), furthermore, these compounds can accumulate in the top of atmospheric crude distillation columns leading to corrosion and loss in separation efficiency. The desalting process involves the mixture of crude oil with water aiming at the dissolution of the salts considering the higher solubility of these compounds in the aqueous phase.
The crude oil is pumped from the storage tanks through the heating battery where it is heated and mixed with dilution water, the mixture is made by a mixing valve that promotes an intense mixture through pressure drop. The major part of water is under the free form and is removed by decantation due to the difference of density between the aqueous and oil phases, however, part of the water is emulsified in the oil phase and are required actions to broke the emulsion and allow the decantation of this water and the dissolved salts.
The emulsion breaking is carried out with the application of high-intensity electric field (close to 3,0 kV/cm) that provokes the polarization of water droplets, his agglutination and consequently his decantation. Desalting heavy crude oils is a greater challenge to refiners once the lower difference of density between the aqueous and oil phases makes the separation hard, beyond the higher content of compounds which stabilize the emulsions in heavier crudes (asphaltenes), in these cases the refiners operate under higher desalting temperatures and are used demulsifiers to facilitate the emulsion breaking.
Demulsifiers are normally a combination of surfactants with hydrophilic and hydrophobic bands in the same molecule which normally have their formulation protected by patents and his dosage needs to be accompanied by a specialist (chemical vendor). Regarding the electrical field, higher electrical intensity tends to improve the desalting efficiency considering the other variables fixed once improve the mixing effect and intensity of water droplets, collision with consequent coalescence and decantation, but it's necessary to consider that there is an optimal point for achieve this effect, once mixing in excess can promote collisions but without adequate conditions of coalescence.
It's important to consider the whole desalting process and all operating variables and not only the demulsifier and electrical field. The desalting temperature is a key parameter of the process once impact the viscosity of the crude and consequently the sedimentation velocity, it's important to realize an study including all operating variables like content of dilution water, pressure drop in the mixture valve, electrical field and desalting temperature. It's important to take into account the compatibility of the crude oils processed, which can lead to asphaltenes precipitation in some cases, especially in blends of high paraffinic crudes with heavier crudes.
Dr. Marcio Wagner da Silva is Process Engineer and Stockpiling Manager on Crude Oil Refining Industry based in São José dos Campos, Brazil. Bachelor's in chemical engineering from University of Maringa (UEM), Brazil and PhD. in Chemical Engineering from University of Campinas (UNICAMP), Brazil. Has extensive experience in research, design and construction to oil and gas industry including developing and coordinating projects to operational improvements and debottlenecking to bottom barrel units, moreover Dr. Marcio Wagner have MBA in Project Management from Federal University of Rio de Janeiro (UFRJ), in Digital Transformation at PUC/RS, and is certified in Business from Getulio Vargas Foundation (FGV).
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