I. solution with prepared crystals it encourages Secondary

I. IntroductionCrystallization is a process of forming solid materials from a liquid solution, where the solid that forms has a crystalline structure (Hallas, 2011).  The purpose of crystallization is to purify products in the food and pharmaceutical industry. As mentioned above the purpose of crystallization is to purify products, this is also an advantage.  Another advantage of crystallisation is the amount of energy it requires is considerably lower in comparison to the evaporation process which is why it is a very popular technique used by industries as its more efficient. I.INucleation is the initial formation of a crystal from a solution from either a liquid, solution or a vapour (Jaiswal A.K (2017)) there are two steps in nucleation known as Primary nucleation and secondary nucleation.Primary nucleation is the process which takes place as soon as the molecules come together to form a crystal structure. Secondary nucleation is the process which only takes place if there are some crystals present in the solution. Secondary nucleation can happen at a lower level of supersaturation (Jaiswal A.K, 2017) and therefore, companies tend to use secondary nucleation as it’s more cost effective. As to reach the level of supersaturation in Primary nucleation a large quantity of a solute may be needed, therefore by “spiking” the solution with prepared crystals it encourages Secondary nucleation at lower financial costs.Figure 1. Displaying Nucleation I.II The two common methods of crystallization are gradual cooling and diffusion (Food, 2011).Crystallization by gradual cooling is where the solid is liquefied in a minimal amount of a very hot solvent and allowed to cool down at room temperature “gradually”. A limitation of gradual cooling is the time taken to cool down the solution gradually.Crystallization by diffusion is when the solvents are not heated. Therefore, solubility is of the utmost importance in this method. A good quality solvent is used to dissolve the solid in a beaker or test tube. Then an insoluble layer is carefully placed on top of this. The diffusion of both the good quality and the insoluble layer will eventually cause crystals to grow at the border of the two liquids.I.IIIThere are many different types and shapes of crystals, these include cubic, hexagonal, tetragonal. A carbon crystal is polymorphic as there is three different types; diamond, graphite, allotropes. (Jaiswal A.K (2017))Ideal solution for the production of crystals is supersaturated solution due to the unstable environment solid materials known as crystals form. This can be seen in figure 2 below.I.IVThere are two types of supersaturated solutions; metastable and labile.Metastable is where crystallization is possible but not spontaneous.Labile is where crystallization is possible and spontaneous.In order to carry out the crystallization process the correct equipment is needed. There are three different types of crystallizers batch crystallizer, forced circulation type or continuous type.I.VBatch process is when a bulk of materials are used in “batches” to get the desired product. An advantage of batch processing is that the crystals do not grow on the side on the equipment, while with continuous the crystals have a tendency to grow on the side of the equipment. With forced circulation the solution is heated until all of the solvent is removed, then these crystals are removed and are passed back through the body of the crystallizer. (A.K, 2017) Figure 2.  displaying saturation zones(Snell EH, 2017) II. Current State of the Art. II.I  Gradual cooling.Gradual cooling involves dissolving the impure solid in a minimum amount of a hot solvent and allowing the resulting solution to cool as slowly as possible to room temperatureIn this method, the supersaturation (supersaturation refers to a state in which the liquid (solvent) contains more dissolved solids (solute) than can ordinarily be accommodated at that temperature, (Reciprocal Net, 2004) is created by a decrease in temperature. Depending on how the solution is cooled, a different end product is obtained- i.e. a wide distribution of crystal sizes may be expected for example. The crystals are gathered by vacuum filtration (vacuum filtration is a technique used to separate a solid product from a liquid product. The combined mixture of solid and liquid is poured through filter paper in a Buchner funnel. The solid is trapped by the filter paper, whereas the liquid is pulled through the funnel into a flask placed below the funnel, by a vacuum. (Croatian-English Chemistry Dictionary & Glossary, 2017) The simplest way of performing a cooling crystallization process is by letting the warm solution cool to the surrounding room temperature.In an industry based procedure, the first step in this crystallization process is to find a suitable solvent. This is done by testing a small amount of the impure solid. The desired compound should be very soluble in the solvent when it is hot/near boiling, but only ever so slightly soluble at room temperature. Sometimes a solvent mixture (2 or more solvents combined) is required to achieve this temperature dependent solubility. At the same time, it is essential for the dissolved impurities (by-products and unreacted starting material) to be highly soluble in the solvent at BOTH high and low solvent temperatures. The compound must precipitate out of the solution at cooler temperatures to allow its separation from the solvent and the dissolved impurities by filtration.The next step is to dissolve the solid to be purified in a minimum amount of hot solvent. This step should be performed quickly, with stirring if necessary. (Note: The solid and solvent should not be combined when heated- this may cause decomposition of the desired end product. The solvent should always be heated first, and then added to the solid.) To achieve the largest and purest possible crystals to form, the final solution should be placed in a location to cool in its own time whereby it will not be disturbed. (UCI Faculty Websites, 2012)II.II Diffusion.Crystallization by diffusion is an alternative method to gradual cooling. What differentiates both these methods is that diffusion does not use heated solvents. This is preferable when the desired compound devalues at increased temperatures of solvent boiling points. Furthermore, this method is applicable to mg quantities of sample that are air and/or solvent sensitive. The impure solid is dissolved in the minimum amount of a solvent required in which the solid is highly soluble in. (University of Washington, 2006)To begin with, the solution is placed in a test tube anchored at a 45° by a clamp. Next, carefully layer a solvent in which the desired compound is insoluble in on top of the solution already placed in the test tube. The less dense, insoluble solvent is slowly placed by a pipette that is in contact with the lowest corner of the test tube opening. Slow diffusion of the insoluble solvent into the soluble solvent will cause a slight layer, allowing crystals to slowly grow at the point where the two solvents merge. For diffusion to occur successfully, the impurities must be soluble in both solvents. (UCI Faculty Websites, 2012)II.III Vapour diffusion is the most common method used in protein crystallisation. Protein crystallisation is the method of forming a protein crystal. As already mentioned previously, molecules can be encouraged to form crystals when the solution in which they are being dissolved in becomes supersaturated, and proteins are not exempt from this. Proteins can be fragile molecules and to solve this protein crystallization can be used. The crystallization provides these fragile molecules with structure and depth in their three dimensional structure. The growth of these protein crystals can be affected by the following factors, temperature, concentration of solution, osmotic pressure and pH. (Heyman, 2017) To carry out vapour diffusion, a small droplet containing the purified protein, pH buffer and a precipitant are allowed to equilibrate with a much larger reservoir containing similar buffers and precipitants to the droplet but in a much higher concentration. To begin with, a low precipitant and protein concentrations is observed in the droplet in comparison to the reservoir, however as the drop and reservoir begin to equilibrate, the precipitant and protein concentrations undergo an increase in the droplet. This method is performed because it allows for a more steady change in the concentration of protein solution and precipitant concentration, with large and uniform crystals as an end product.Vapour diffusion may be performed in either “hanging-drop” or “sitting-drop” format. Hanging-drop involves a drop of protein solution placed in a cover slip, which is then lifted above the large reservoir. Sitting-drop involves a drop of protein solution placed on a pedestal that is separated from the reservoir. Sealing of the surrounding environment is essential in order for equilibration between the drop and reservoir to occur. (Collaborative Crystallisation Centre, 2012)Figure 3. Sitting-drop and hanging-drop vapour diffusion methods, where the pink represents protein solution and the blue represents reservoir. (Collaborative Crystallisation Centre, 2012)Case studyI will discuss the principle of crystallization in the food industry. I will outline the techniques of crystallization; which instruments are used and how heat transfer is calculated. I contacted several companies such as Pfizer, Kerry Group, Danone, Glanbia and Achill salt to investigate the systems they have in place in terms of crystallization in the food engineering  section of the companies. I spoke directly to staff over the phone, they said they would get back to me answering my questions via email. I contacted several people on LinkedIn also. Unfortunately, no one has replied. Seen as this is out of my control, I decided to research the common methods and instruments of crystallization in the food industry. (Anon., 2016)In the food industry, controlling crystallization is a key factor in quality as it relates to texture, with some foods requiring the promotion of crystallization and others its prevention. Crystallization is used in the food industry to purify solids. Crystallization is often a four step process that includes: ·    Generation of a supersaturated state·    Nucleation·    Crystal growth·    Continued crystal growth Controlling crystallization in food processing requires control of the relative rates of nucleation and growth (Krauss, 2015).  For example, to make the appropriate number and size of ice cream requires that the proper conditions are met during ice cream manufacture. To make smooth texture of ice cream, many small crystals must be formed during processing. Two general methods which are used widespread are gradual cooling and diffusion. Gradual cooling involves dissolving the impure solid in the minimum amount of a hot solvent and then allowing the solution to cool to room temperature. During the cooling process, crystals will form and are then collected by vacuum filtration providing the pure substance (Sica, 2017). Another method used in the food industry is, Vapor diffusion, a popular technique for the crystallization of macromolecules such as protein. A drop containing a mixture of sample and reagent is placed in vapor equilibration with a liquid reagent which contains a higher reagent concentration. IV. Future technology developments of Crystallizers:There are three main types of crystallizers in today’s world, batch crystallizers, forced circulation crystallizers and continuous crystallizers. The more commonly used in the food and pharmaceutical industries are continuous and batch crystallizers. Both of which have there advantages and disadvantages. Continuous processes tend to be more efficient and effective in production and can reduce product variability without disruptions. The continuous manufacture of vertex and the change from batch to continuous of Darunavir (Janssen) manufacturing is perhaps set to lead the way in crystallization in manufacturing. Currently being developed and future crystallizers include microfluidic crystallizers. The advantages of this type of crystallizer are plentiful and include the ability to adapt to trace amounts of a sample and will give good heat and mass transfer. It will also be a lot more efficient and more accurate. It will give the potential to grow big single crystals and may lend itself to gaining a deeper understanding of crystallisation. It may not be fully suitable to use in large scale industry but this is to be revised and will hopefully be overcome.(Gao et al., 2017)Other crystallizers which will improve different methods of crystallization are being developed. These include, jet crystallizer which can make mixing a solution and an anti-solvent reagents together much easier and always gives a supersaturated solution, they can run continuously and are no trouble to scale up. Microwave-assisted crystallizers can rapidly dissolve small particles and can shorten the length of a heating cycle which are necessary to remove fine crystals in batch crystallization. Ultrasound-assisted crystallizers are very good at triggering nucleation and can narrow the metastable zone. Both microwave- and ultrasound-assisted crystallization will be used widely and throughout both the food and pharmaceutical industries in the future as they can efficiently reduce batch time and improve product quality. Another future focused crystallizer is an airlift crystallizer, this will use air to stir the solution and solvent together rather then stirring probes or internal moving parts. These crystallizers should be more effective in reducing crystal collisions and suppressing secondary nucleation.A forward thinking idea has been suggested to combine an anti-solvent tube with a kenics static mixer. This promotes homogeneous mixing in a double-jacket tubular crystallizer. Another futuristic type of crystallizer is a continuous oscillatory baffled crystallizer (OBC) which uses periodically spaced restrictions which will enable oscillatory mixing. The advantages of an OBC combine the advantages of most other types of crystallizers like the microwave-assisted, ultrasound-assisted and the airlift crystallizer, they are enhanced heat and mass transfers, shortened residence time and induction time and a narrower metastable zone width. More commercialized crystallizers from NiTech Solutions (based in Scotland) and other companies are majorly contributing to pushing forward the use and application of the continuous OBC in industry. (Gao et al., 2017)


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