Seeding seeded cooling crystallization

Design a batchwise, seeded cooling crystallizer for a solute. A, crystallizing from solvent B. The concentrations are expressed on a solute-free basis. The crystallization is accomplished by cooling a feed solution at an initial concentration of 0.25 kg solute A per kg solvent B from from 50°C to 20°C. The operating conditions and the physico-chemical properties of the system are provided in the Table. Determine ... 

Seeded Cooling Crystallization. It is assumed that a certain number N of seed crystals of a uniform size Lj are introduced to the crystallizer at the saturation temperature T. No nuclei are formed spontaneously and the seeds are allowed to grow, at a constant growth rate G, until the final size Lf has been reached after the batch time ly. In the absence of spontaneous nucleation, Eq. (10.46) becomes simplified to... 

The shape of the equilibrium line, or solubility curve, is important in determining the mode of crystallization to be employed in order to crystallize a particular substance. If the curve is steep, i.e. the substance exhibits a strong temperature dependence of solubility (e.g. many salts and organic substances), then a cooling crystallization might be suitable. But if the metastable zone is wide (e.g. sucrose solutions), addition of seed crystal might be necessary. This can be desirable, particularly if a uniformly sized product is required. If on the other hand, the equilibrium line is relatively flat (e.g. for aqueous common salt... 

Figure 7.2 Batch cooling crystallization, (a) Uncontrolled rapid cooling without seed crystals, (h) Controlled cooling with seed crystals added after Griffiths, 1928)...

Although programmed cooling crystallization clearly results in a larger mean crystal size than that from natural cooling it is also evident that some fines i.e. small crystals are also present in the product. Since the solution was seeded these fine crystals must clearly have arisen from crystal attrition or secondary nucleation (see Chapter 5). 

Doki, N., Kubota, N., Sato, A., Yokota, M., Hamada, O. and Masumi, F., 1999. Scaleup experiments on seeded batch cooling crystallization of potassium alum. American Institution of Chemical Engineers Journal, 45(12), 2527-2533. 

This oil Is kept supercooled at the ambient temperature. Crystallization may be obtained by cooling or by seeding with crystals of the product. The melting point is 34°C (instantaneous on the Maquenne block). 

The sorbitol solution produced from hydrogenation is purified in two steps [4]. The first involves passing the solution through an ion-exchange resin bed to remove gluconate and other ions. In the second step, the solution is treated with activated carbon to remove trace organic impurities. The commercial 70% sorbitol solution is obtained by evaporation of the water under vacuum. The solid is prepared by dehydration until a water-free melt is obtained which is cooled and seeded. The crystals are removed continuously from the surface (melt crystallization). The solid is sold as flakes, granules, pellet, and powder forms in a variety of particle size distributions. 

In the Bayer process, the bauxite is leached with hot sodium hydroxide, thus forming a solution of sodium aluminate. After purification this solution is seeded with crystals of gibbsite and cooled. The process steps are summarised in Eqs. (1) and (2) ... 

H. Davy obtained crystals of phosphorous acid by evaporating the aq. soln. to a thin syrupy liquid, and cooling it P. L. Dulong, and 0. A. Wurtz evaporated the soln. until decomposition began, and allowed the mass to stand in vacuo for a few days. H. Rose, and 0. F. Rammelsberg found that the thin syrup crystallized easier than the thick one. J. Thomsen evaporated the liquid with the temp, slowly rising to 180°, and found that the syrupy liquid on cooling crystallized in a few hours—often in a few minutes. If the liquid be seeded with a crystal of the acid, the cold or undercooled liquid crystallizes very quickly. 

The solution was seeded. The crystal slurry was cooled to 2°C. The crystals were collected by centrifugation and were washed with acetone followed by isobutyl methyl ketone. The product was not dried. The moist crystalline product was extracted with isobutyl methyl ketone (3.60 L) and diluted NaOH (2.60 L) at pH>ll. The phases were separated. The organic phase was washed with water (0.6 L) and was used directly in the next step. Yield (calculated on the dry basis) about 0.39 kg of (S)-pipecoloxylidide (about. 90%). 

K2C03 (0.32 kg), Nal (catalytical amount), 1-bromopropane (0.28 kg) and water (1.70 L) were added to the organic phase from the previous step. The mixture was heated to reflux to complete the reaction. The excess of bromopropane was removed by distillation. The reaction mixture was separated. Acetone (1.70 L) was added to the organic phase followed by HCI (aq) to pH about 2. The solution was seeded. The crystal slurry was cooled to... 

The seeds have to be added to a saturated if not supersaturated solution. The saturation temperature corresponding to a feed concentration of 0.25 kg/kg can be calculated to be 48 C from the solubility relationship. It is therefore necessary to cool the feed from 50"C to 48 C, seed the crystallizer with 62.5 g of... 

Doki, N. Seki, H. Takano, K. Asatani, J. Yokota, M. Kubota, N. Process control of seeded batch cooling crystallization of the metastable a-form glycine using an in-situ ATR-FTIR spectrometer and an in-situ FBRM particle counter. Crystal Growth Design 2004, 4 (5), 949-953. 

Adding the seed as slurry in the proper solvent composition is one of the best methods to control a batch cooled crystallizer. The slurry addition is started before reaching saturation and is continued until it can be determined that the seed is no longer dissolving. Although this method can increase the probability that seed will be present at the start of crystallization, the amount of seed actually remaining may be subject to excessive variation. 

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