Full Papers Caking of Urea Prills By Michael Wahl, Roman Kirsch, Ulrich Bröckel*, Stefan Trapp, and Michael Bottlinger DOI: 10.1002/ceat.200600067 To investigate the formation and growth of solid bridges between urea particles, the changes in the contact region between two discrete particles are measured. The work is carried out using a Double-Particle-System (DPS). The DPS was stored in a climatic chamber. The effect of load, temperature, relative humidity, and storage time on the formation of a bridge between these particles was analyzed. The objective was to describe the geometrical changes in the contact region and to measure the strength of the resulting interparticle bridge. 1 Background In the chemical, pharmaceutical and food indus- tries, time consolidation and caking of bulk solids during storage is of importance. These effects will cause handling problems in subsequent process steps. During transport or storage the products are often exposed to temperature and humidity changes. In practice, the storage time of urea may range from one month up to one year [1]. In conse- quence of these effects, the bulk solids may form lumps or solidify completely [2]. These phenomena are known as caking. For further use of these caked products a de-agglomeration step is necessary. Through this de-agglomeration process 2–10 % of the prod- uct is lost [3]. This process requires time and manpower. Ad- ditional equipment is needed resulting in additional costs. To minimize these costs the caking mechanism has to be identi- fied in order to avoid conditions leading towards caking. 2 Description of the Experimental Setup This work focuses on the caking behavior of urea prills. Thompson [4] has carried out some experiments with dou- ble-particle systems. Those experiments had been done un- der room conditions, not at defined temperatures or humid- ities. With the experimental setup described in the following chapters, specific temperature and humidity can be applied. A “Device to Apply Pressure on Particles” (DAPP) (see Fig. 1) was used for the research on double-particle-systems. On each pin one of the particles is fixed with a glue. One pin is guided in a fixed bearing, the other one in a floating bearing. The floating bearing locks the degree of freedom in the y-direction and prevents the particles from being pushed one upon the other. The fixed bearing can be used to adjust the fixed pin in a way that ensures a horizontal position of the torsion arm. A force equivalent to the pressure in the bottom layer of bulk storage can be applied on these parti- cles by mounting weights on the hook at the end of the tor- sion arm. The required load can be calculated by the adapted equation of Rumpf 1) [5]: r  1  e e  F H x 2 (1) For research on the caking behavior of two particles, the application of real storage conditions is very important. Five parameters have a major effect on the particles during stor- age: moisture, temperature, pressure, particle size, and stor- age time. Using several DAPPs, sets of two particles can be stored in a climatic chamber. The examination of the Dou- ble-Particle-Systems after at least 1 day, 3 days, 7 days, and 14 days gives information on time dependent effects. As one Double-Particle-System (DPS) represents two individual 674 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2006, 29, No. 6 Figure 1. Device to apply pressure on particles. – [*] Dr.-Ing. M. Wahl, Dipl.-Ing.(FH) R. Kirsch, Prof. Dr.-Ing. U. Bröckel (author to whom correspondence should be adressed, u.broeckel@um- welt-campus.de), Prof. Dr. rer.nat. S. Trapp, Prof. Dr.-Ing. M. Bottlinger, Institute for Micro-Process Engineering and Particle Technology, Umwelt-Campus Birkenfeld, Postfach 1380, 55761 Birkenfeld, Germany. – 1) List of symbols at the end of the paper.