Crystallisation process

Crystallisation processes in PEEK have been the subject of many academic papers [10-13]. However, the crystallisation of PEEK generally matches the classic behaviour of other polymers. The effect of time is described by Avrami kinetics ( 3) and secondary crystallisation occurs after the spherulites have impinged. This secondary crystallisation results from an increase in the crystallinity within the spherulites and is probably related to the existence of the low-temperature melting peaks (LTMP) described later. A number of non-isothermal crystallisation models have been developed. 

The crystallisation rate of PAEK reaches a maximum between Tg and T. Crystallisation is driven by the rate of nucleation and the rate at which polymer chains can reel onto a nucleated crystalline surface. 

At low temperatures the rate of nucleation is very high (it is easy to form a nucleus of critical size in a low-energy environment) but the rate of chain movement is slow. At high temperatures chains move very fast but it is difficult to nucleate new highly energetic chains onto a crystalline surface. Accordingly there is a maximum rate at intermediate temperatures. In the case of PEEK the maximum occurs at around 225 °C and the minimum crystallisation half-life is of the order of six seconds [1]. 

The rate of crystallisation will vary with molecular weight and nucleation density. Other PAEK will show similar generic behaviour but rates of crystallisation can very widely. In particular any unit in the chain that does not readily crystallise will retard the rate of crystallisation, reduce overall crystallinity and limit crystal size -reducing the melting point. This effect can be used to limit while maintaining Tg (as in PEKK) but excessively reduced crystallinity and crystallisation rate can cause problems with mould cycle times or properties such as fatigue and chemical resistance. 

Crystallisation rate can also be affected by a failure to melt the polymer at a high enough temperature. For example, PEEK that is melted below 390 °C shows faster crystallisation rates than when melted above 390 °C. This is because residual crystalline regions can persist up to the thermodynamic melting point (typically reported to be around 390 °C) and result in self-nucleation [15]. 

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The purification of solids by crystallisation is based upon differences in their solubility in a given solvent or mixture of solvents. In its simplest form, the crystallisation process consists of (i) dissolving the impure substance in some suitable solvent at or near the boiling point,... 

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

In the Sulser-MWB process the naphthalene fractions produced by the crystallisation process are stored in tanks and fed alternately into the crystalliser. The crystalliser contains around 1100 cooling tubes of 25-mm diameter, through which the naphthalene fraction passes downward in turbulent flow and pardy crystallises out on the tube walls. The residual melt is recycled and pumped into a storage tank at the end of the crystallisation process. The crystals that have been deposited on the tube walls are then pardy melted for further purification. Following the removal of the drained Hquid, the purified naphthalene is melted. Four to six crystallisation stages are required to obtain refined naphthalene with a crystallisation point of 80°C, depending on the quaHty of the feedstock. The yield is typically between 88 and 94%, depending on the concentration of the feedstock fraction. 

The crystallisation process for concentrating bismuth ia lead by squeesiag the eutectic (high ia bismuth) Hquid out of the solidified high lead portion at a temperature within the melting range of the alloy is seldom used. 

There are essentially four steps or unit operations in the manufacture of fatty acids from natural fats and oils (/) batch alkaline hydrolysis or continuous high pressure hydrolysis (2) separation of the fatty acids usually by a continuous solvent crystallisation process or by the hydrophilisation process (J) hydrogenation, which converts unsaturated fatty acids to saturated fatty acids and (4) distillation, which separates components by their boiling points or vapor pressures. A good review of the production of fatty acids has been given (1). 

F. B. White, Fan Crystallisation Process, American Oil Chemists Society Meeting, Chicago, lU., 1983. 

In thermodynamic language it is said that a reaction will occur if there is a decrease in the free energy, i.e. AF is negative. Since at the melting point melting and crystallising processes are balanced AF is zero and the expression may be written... 

Woinaroschy, A., Isopescu, R. and Fillipescu, L., 1994. Crystallisation process optimisation using artificial neural networks. Chemical Engineering Technik, 17, 269-272. 

Jenike, A. W. and Johnson, J. R. (1970) Chem. Eng. Prog. 66 (June) 31. Solids flow in bins and moving beds. Jones, R. L. (1985) Chem. Engr., London No 419 (Nov.) 41. Mixing equipment for powders and pastes. Jones, A. G. (2002) Crystallisation Process Systems (Butterworth-Heinemann). 

These experimental results could not be confirmed by Lahav and co-workers they suggest that impurities in the starting materials have a much greater effect on the crystallisation process than the PVED (Parity Violating Energy Difference). Extensive experimental studies indicate the importance of small quantities of impurities, particularly in early phases of crystallisation nucleus formation. Amino acids from various sources were used, and the analyses were carried out using the enan-tioselective gas chromatography technique (M. Lahav et al 2006). 

The crystallisation process, whether from the melt or a concentrated solution, is also an interesting process that has a bearing on the... 

Industrial solutions invariably contain dissolved impurities that can increase or decrease the solubility of the prime solute considerably, and it is important that the solubility data used to design crystallisation processes relate to the actual system used. Impurities can also have profound effects on other characteristics, such as nucleation and growth. 

Secondary nucleation can, by definition, take place only if crystals of the species under consideration are already present. Since this is usually the case in industrial crystallisers, secondary nucleation has a profound influence on virtually all industrial crystallisation processes. 

In simple terms, the crystallisation process may be considered to take place in two stages—a diffusional step in which solute is transported from the bulk fluid through the solution boundary layer adjacent to the crystal surface, and a deposition step in which adsorbed solute ions or molecules at the crystal surface are deposited and integrated into the crystal lattice. These two stages which are shown in Figure 15.13, may be described by ... 

Crystallisation processes employing DCC have been used successfully in the de-waxing of lubricating oils 63, the desalination of water(64), and the production of inorganic salts from aqueous solution. 

Semi-continuous crystallisation processes which often combine the best features of both batch and continuous operation are described by Nyvlt(35), Randolph(37), Robinson and Roberts(70) and Abbeg and Balakrishnam(7I ). It may be possible to use a series of tanks which can then be operated as individual units or in cascade. Mullin(3) suggests that for production rates in excess of 0.02 kg/s (70 kg/h) or liquor feeds in excess of 0.005 m3/s, continuous operation is preferable although sugar may be produced batch-wise at around 0.25 kg/s (900 kg/h) per crystalliser. 

A melt is a liquid or a liquid mixture at a temperature near its freezing point and melt crystallisation is the process of separating the components of a liquid mixture by cooling until crystallised solid is deposited from the liquid phase. Where the crystallisation process is used to separate, or partially separate, the components, the composition of the crystallised solid will differ from that of the liquid mixture from which it is deposited. The ease or difficulty of separating one component from a multi-component mixture by crystallisation may be represented by a phase diagram as shown in Figures 15.4 and 15.5, both of which depict binary systems — the former depicts a eutectic, and the latter a continuous series of solid solutions. These two systems behave quite differently on freezing since a eutectic system can deposit a pure component, whereas a solid solution can only deposit a mixture of components. 

A single-stage crystallisation process may not always achieve the required product purity and further separation, melting, washing, or refining may be required. Two approaches are used ... 

As described in Section 15.2.1, eutectic systems can be purified in theory by single-stage crystallisation, whereas solid solutions always require multistage operations. Countercurrent fractional crystallisation processes in column crystallisers are described in Section 15.4.3. 

All freeze separation processes depend on the formation of pure solvent crystals from solution, as described for eutectic systems in Section 15.2.1. which allows single-stage operation. Solid-solution systems, requiring multistage-operation, are not usually economic. Several types of freeze crystallisation processes may be designated according to the kind of refrigeration system used as follows . 

The principle of operation is illustrated in Figure 15.37 which shows the pressme-volume relationship. Curve a shows the phase change of a pure liquid as it is pressurised isother-mally. Crystallisation begins at point Ai and proceeds by compression without any pressure change until it is complete at point A2. Beyond this point, the solid phase is compressed resulting in a very sharp rise in pressure. If the liquid contains impurities, these nucleate at point Bi. As the crystallisation of the pure substance progresses, the impurities are concentrated in the liquid phase and a higher pressure is required to continue the crystallisation process. As a result, the equilibrium pressure of the liquid-solid system rises exponentially with increase of the solid fraction, as shown by curve b which finally approaches... 

AIChESymp. Ser. (a) 65 (1969) no. 95, Crystallization from solutions and melts (b) 67 (1971) no. 110, Factors affecting size distribution (c) 68 (1972) no. 121, Crystallization from solutions Nucleation phenomena in growing crystal systems (d) 72 (1976) no. 153, Analysis and design of crystallisation processes (e) 76 (1980) no. 193, Design, control and analysis of crystallisation processes (f) 78 (1982) no. 215, Nucleation, growth and impurity effects in crystallisation process engineering (g) 80 (1984) no. 240, Advances in crystallisation from solutions. 

Beale, A.M. and Weckhuysen, B.M. (2007) Understanding the crystallisation processes leading to the formation of microporous aluminophosphates, in Zeolites to Porous Materials-The 40th Anniversary of International Zeolite Conference, Studies in Surface Science and Catalysis, vol. 170, Elsevier Science Publishers B V, Amsterdam,... 

It is easy to see how this technology might develop in the fumre to allow better monitoring, and ultimately control, of continuous crystallisation processes. 

The number of inputs which are available for controlling crystallisation processes is limited. Possible Inputs for a continuous evaporative crystallisation process are, crystalliser temperature, residence time and rate of evaporation. These Inputs affect the crystal size distribution (CSD) through overall changes in the nucleatlon rate, the number of new crystals per unit time, and the growth rate, the increase in linear size per unit time, and therefore do not discriminate directly with respect to size. Moreover, it has been observed that, for a 970 litre continuous crystalliser, the effect of the residence time and the production rate is limited. Size classification, on the other hand, does allow direct manipulation of the CSD. 

The average radius is deduced. Figure 3.4.2 shows a change in the size of the particle with the molar ratio of cadmium and sulfide ions concentrations. The largest size particles are obtained for [Cd2+]/[S2-] = 1 and the smallest for [Cd2+]/[S2 ] = 2. It can be noticed that the size of CdS is always smaller when one of the two reactants are in excess ([Cd2+]/[S2 ] = 3,5, 2). This confirms that the crystallisation process is faster when one of the reacting species is in excess (56). 

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