Seeding slurry addition

Four samples of faujasite were synthesized at Si/Al ratios of 2.61, 2.80, 2.97 and 3.03 using published methods from seeded slurries (8-9) and using proprietary methods. One additional sample of Si/Al ratio 2.58 was purchased from Union Carbide. The samples were characterized by X-ray powder diffraction, by surface area measurements, and by wet chemical analysis. The results of these measurements are contained in Table I. 

The build-up of scale on the evaporator plates reduces the heat transfer efficiency and may present a maintenance problem or increase operating cost. However, it has been demonstrated that fouling on the heat transfer surfaces can be avoided or minimized for certain dissolved solids by maintaining a seed slurry, which provides preferential sites for precipitate deposition. In addition, low-temperature differences in the evaporator will eliminate nucleate boiling and supersaturation effects. 

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. 

Several authors indicate the importance of the condition of the surface of seed crystals, and some qualitative suggestions can be made. These include (1) making a seed slurry to help condition the surface by dissolution of shards and providing some surface activation (which is also a good method of seed addition) and (2) avoiding the use of dried seed. Milled seed may be necessary for particle size control, but milled seed should either be reslurried or the milling should be done by wet-milling methods. 

Figure 9-14 Antisolvent addition procedure for Example 9-3, Option 2. Both batch and antisolvent are charged simultaneously to the seed slurry.

Good design practice typically includes dedicated thickener underflow recirculation pumps to deliver adequate volumes of precipitate as seed, and blending of seed slurry and feed liquor ahead of limestone addition to the first neutralization vessel. 

The addition of the seed slurry can be done at different stages onto the liquid surface (the crystals tend to agglomerate if they are not dispersed) near the stirrer, where the liquid is well mixed or by one of the feed streams in a bypass or loop in the crystallizer. The best way is to add seeds to the solution beneath the liquid level or to add them to a feed stream at the beginning of the batch or, in case of a semibatch system, continuously. In reactive batch crystallization, the nucleation rate can be decreased by the dilution of the feed streams or by the addition of fine crystals into one of the feed solutions or the reactor. The surface area and growth rate should be large enough to quickly reduce the supersaturation. The recirculation of the crystal suspension creates an... 

The flow chart of full-scale production is shown in Figure 6. CET slurry at a scale of about 100 L per batch containing 28 kg of CET is turned into a supersaturated solution by the above-mentioned equipment. The solution is filtered by ultrafiltration to remove endotoxins and sterilized by a membrane filter. Seed crystal is added in the sealed condition and then subdivided into vials. All of these procedures are carried out at around 5°C. The vials are frozen in the chamber of the freeze-dryer at 0°C and freeze-dried. Three batches are achieved per day. The mean times required are 25 min (at the longest) from the preparation of the supersaturated solution and 15 min (at the longest) from seeding to freezing, respectively. In our preliminary experiment adequate products were obtained even when the samples were left for 2 h before addition of seed crystal and for 2 h after seeding at standstill condition. No product defect occurred in the first batch of full-scale production. 

For antisolvent addition methods, the seed can be added to a smaU part of the antisolvent, which is then added as slurry as the saturation point is approached. Although some of the seed may dissolve, this technique is considered to be more reliable than adding the seed all at once. 

Seeding with dry powder seed through a vessel head nozzle, while widely practiced in the past, is now limited because of safety and exposure considerations. Slurry seed additions by pump or from seed tanks are preferred and are superior to powder addition for the reasons discussed above. The advantages of retaining the seed in the system, as utilized in continuous operation or in heel recycling are also indicated above. 

All of the above control factors are issues, as well as the local concentration gradients at the point of addition of the second solvent (also referred to as the antisolvent). This key point will be discussed in Chapter 9. Also discussed in that chapter is the method of seed introduction as a slurry with the antisolvent. This method can be useful in this case as well. 

In a separate feed vessel, a homogeneous solution with compound dissolved in ethanol was prepared. The homogeneous ethanolic solution was charged continuously to the crystallizer at a rate which was balanced by the vacuum distillation rate of ethanol in the crystallizer. The rate of batch addition and distillation was designed so that the overall crystal growth rate in the crystallizer was approximately 1% per hour, i.e., growing 1 gm of material from 100 gm of seed bed per hour. The crystallizer slurry was periodically harvested. 

Seeding is addressed throughout this book, and many of the issues apply to antisoivent addition. The primary issue of when to seed is again critical to avoid excessive nucleation. The seed can be added as a powder or as a slurry in the antisoivent. The latter is preferred for... 

In the crystallization of RRR, the initial solution in ethanol is seeded (2%) and aged for 1 hour at 40°C. After aging, the slurry is cooled linearly to 20°C, followed by a linear addition of antisolvent (water) over 5 hours. A final cool-down to O C is performed to minimize the mother liquor loss. The final ethanol/water composition is 55/45. 

The process for the final step in a multistep synthesis produces a sodium salt of free phenol. Figure 10-9 shows the flowsheet of the crystallization process. Two streams—stream A containing free phenol dissolved in n-methyl pyrolidone (NMP) and stream B containing ethanolic sodium ethoxide solution—ai e charged simultaneously to a slurry seed bed of sodium-salt of phenol. The addition time is several hours. At the end of each batch, a portion of the batch slurry is wet-milled and used as seed for the next batch. The amount of seed varies from 5% to 20% of the size of the batch. 

Pilot plant vessel to include variable-speed drive, subsurface addition at impeller, online measurement devices, variable-temperature jacket services, controlled seed addition as slurry, antisolvent/reagent addition at controlled rate. 

The greatest concern with this approach is lack of nucleation of the most stable form. If the starting API does not contain seeds of the stable form, a successful screen will produce nucleation of the stable form within the duration of the slurry experiments in at least one of the solvents. Lack of adequate solubility, or inhibition of nucleation due to solvent-solute interactions, may preclude transformation to the stable polymorph (Gu et al., 2001). Moreover, impurities or additives, even in trace amounts, can dramatically affect the rate of solvent-mediated polymorphic transformation (Gu et al., 2002 Okamata et al., 2004 Mukuta et ah, 2005). Thus, it is important to have starting API with the highest possible purity. Recrystallization prior to starting this or any type of polymorph screen can be used in an attempt to purify the API. While this could reduce an impurity that might stifle solvent-mediated transformation to the most stable polymorph, it should be noted that recrystallization from solution does not... 

In any within-scrubber-loop forced oxidation system, irrespective of whether it is additive promoted or not, the possibility exists for calcium sulfite blinding of limestone because the recirculated slurry lacks the solid CaSC>3 crystal seeds. 

FigureS.S Microstructureofthesilicon nitride fabricated through dispersing 2vol% of [1-seed crystals into the raw powder slurry of a-phase and sintering additives and tape-casting it (a) parallel to and (b) perpendicular to the...

Subsequent to hydrolysis, NaOH (2.1 moles per mole of HD) was added to the hydrolysate to neutralize the HCl. The final pH of the hydrolysate was approximately 11. This was diluted three-fold with water and amended with 1.5 g/L NH4CI, 0.28 g/L KH2PO4, and 10 ml/L Wolin Salts and fed to aerobic Sequencing Batch Reactors (SBRs) seeded with activated sludge from the Back River Wastewater Treatment Plant in Baltimore, MD. NaOH addition caused dissolved iron to precipitate out of solution the precipitate was fed to the SBRs as a slurry along with the dissolved organics and salts. 

---