Influence of Water and Supercooling on Permeability and Compressibility of Acrylic Acid Crystal Beds

Abstract

The performance of a solid-liquid separation process is often limited by size distribution and morphology of the crystals. To predict the filtration behavior, a robust and applicable model based only on crystallization process parameters is necessary. Therefore, a model has been successfully developed for the industrial system of aqueous acrylic acid melt to predict compressibility and permeability of a crystal bed. For this purpose, common equations are transferred successfully to nonspherical particles. Chord length distributions (CLDs) obtained from an inline focused beam reflectance measurement (FBRM) probe and crystal aspect ratios have been used in combination to determine the major input parameters for the model. Integration of a complex mathematical restoration of crystal size distribution (CSD) from CLD data by the use of existing models is avoided. Considering acceptable fault tolerances of the model, a simple approach used as a robust industrial application is employed. The adjustment of all required constants used in the permeability model was based on simple filtration tests. IntroductionSuspension crystallization is a highly efficient separation technology for industrial applications. In the recent past, several developments have been made to push this promising technology successfully to industrial scale. The overall efficiency of the process is largely determined by an effective solid-liquid separation. To obtain high product purities, complete separation of the crystals and mother liquor is required along with an efficient washing step.Hengstermann et al. [1] showed that the amount of the water content in the melt and the driving force in terms of the supercooling are the main influencing factors in the crystallization of acrylic acid. With increase in the water content and supercooling, a cavity starts appearing at the crystal extremity, of face {002}, and goes along with a needle-like shape development. This morphology trend is often an indicator of problems coming up in downstream facilities like solid-liquid separation and washing devices. Simple tests at lab-scale confirm these issues and show challenging filtration conditions at higher water contents and supercooling.Therefore, the scope of the present work is to investigate the influence of water content and supercooling in the melt on the performance of the downstream solid-liquid separation. The filterability performance of different crystal suspensions was determined by simple filtration tests. In these tests, different compressive stresses were applied on the crystal bed by means of different liquid pressure drops. Later on, the experimental results were used as input parameters for an extended model which was developed previously to calculate permeability and compressibility. State of KnowledgeThe main scope of the required model is to describe the permeability performance of a crystal bed formed from nonspherical acrylic acid crystals under the influence of mechanical stress. Hengstermann et al. [1] identified the water content and ...

Solvent Screening and Measurement of Phase Diagrams for the Yield Maximization of an Acrylic Acid Crystallization

“…This could cause problems due to the high compressibility of the acrylic acid crystals. 1,2 2. The addition of butanoic acid to the aqueous feed does not result in any increase of the crystallization yield.…”

“…Increasing water content causes problems in the downstream solid-liquid separation, too. 1,2 Therefore, the general scope of the present work is to increase the yield by the addition of a third component and therefore by altering the eutectic limit. Further, the performance of the crystallization should be improved in that way that the maximum recovery of acrylic acid should be achieved at low water content to reduce compressibility problems.…”

“…Further, the performance of the crystallization should be improved in that way that the maximum recovery of acrylic acid should be achieved at low water content to reduce compressibility problems. 2 The work flow should be divided into two stages to save time and money. This results in a dedicated solvent screening for the component identification and ends in a concerted experimental work flow using fast thermal analysis methods.…”

“…The yield is limited by the eutectic point at w AA (mass fraction) = 0.62 and a temperature of 262.15 K. Additionally, the performance of the suspension crystallization unit is limited by the water content of the mother liquid. Increasing water content causes problems in the downstream solid−liquid separation, too. , …”

Computing Crystal Size Distribution by Focused‐Beam Reflectance Measurement when Aspect Ratio Varies

“…Therefore, it is essential for CSD to be obtained online in manufacturing processes. Process analytical technologies (PATs) such as optical reflectance measurement [11][12][13], online image processing [14][15][16][17], focused-beam reflectance measurement (FBRM) [18][19][20][21], etc. are increasingly used for in-line measurement of CSD.…”