Heat and mass transfer in granular potash fertilizer with a surface dissolution reaction

Abstract

his paper concerns coupled heat and mass transfer in granular potash when it is stored and handled as a bulk material. Potash T fertilizer, comprised mostly of sylvite (KCI), with small mass fractions of halite (NaCI) and carnallite (KMgC13.6H,0), is highly soluble in water. When exposed in humid air, the potash particles will adsorb moisture. Subsequently, when the relative humidity (RH) drops, moisture will be released from the granular particles by drying process. If this occurs a t moisture contents greater than 0.3%, the small potash particles will cake to form larger particles or clumps, which impedes the handling and flow of this granular material and is especially unfavourable for uniform distribution by most agricultural machines. Pyne et al. (1996) studied the interaction between water vapour and granular potash fertilizers. They used an isothermal heat conduction calorimeter to measure the rate of water vapour uptake by potash as a function of water vapour pressure over a potash sample. Their results show that there are two transition reactions for moisture uptake with increasing relative humidity, one occurring at about 0.60 RH where significant moisture accumulations begin, and the other at about 0.84 RH. In a recent work, Hansen et al. (1998) used the same method to investigate the effect of anitcaking agents on the thermodynamics and kinetics of water sorption by potash fertilizers. Caking research is reported by Walker et al. (1998) for mixed granular nitrogen, phosphorus and potassium (NPK) fertilizers. Their research indicates that the caking process is dominated by free water through capillary adhesion and crystal bridging processes.Moisture-potash interactions depend on the surface composition of the particles. Potash is primarily composed of potassium chloride (KCI), but it contains internal and surface impurities. Internally, crystal particles of potassium chloride may trap some sodium and magnesium which will form sodium chloride (NaCI) and carnallite (KMgCI3.6H,O) microcrystalk inside potassium chloride crystals. Processing potash to remove impurities results in higher concentrations of impurities on the surface of every particle, since either flotation or recrystallation is completed in a brine which is a mixed solution of KCI, NaCI, and MgCI,, with very small amounts of organic materials in it. The potash particles produced from these flotation or recrystallation processes are subjected to a centrifugal de-brining process. There is typically 2% -4% residual brine on the particle surfaces after surface de-brining. In the subsequent drying process, the residual brine forms mixed crystals, comprised of Sylvite (KCI), Halite (NaCI) and Carnallite (KMgC13.6H,0), with small local compostitonal variations on the surface of each particle. Some of these impurities (i.e. NaCl and KMgC13.6H,0) may be covered by trace amounts of organic materials. Thus, only a part of the above water *Author to whom correspondence may be addressed. E-mail address: shiwen@ ualberta.ca Potash is a widely used granu...

Heat and mass transfer during silica gel–moisture interactions

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“…To theoretically model and numerically simulate this head and mass transfer process, the governing equations are listed here [8,11]:…”

“…The linear governing equations were solved and results were presented for selected values of dimensionless constants for uniform internal adsorbent temperatures. Peng et al [8] tested and modeled the adsorption and dissolution process due to air flow through granular potash beds at non-equilibrium conditions. They built a one-dimensional transient heat and mass transfer model in which the specific surface area was assumed constant in calculation of water vapour adsorption rate.…”

Heat and mass transfer in liquid desiccant air-conditioning process at low flow conditions

“…The equations below are somewhat similar to those used by Peng et al [10] for humid air flow in granular potash bed, but they differ in liquid/solid mixed energy equation because there is a convection term due to desiccant flow; and a water transport equation for liquid phase.…”

Measurement of convective heat transfer coefficients in a randomly packed bed of silica gel particles using IHTP analysis

“…Kar and Dobbs [8] investigated the internal heat transfer coefficient of porous metals experimentally and presented a correlation for Nusselt number versus Reynolds number for porosity up to 0.65 in the Darcian regime of the airflow (Re < 10). Peng et al [9] developed a correlation to find the convective heat and mass transfer coefficients in order to simulate the heat and mass transfer in a packed bed of potash particles that were exposed to humid airflow in the range of 20 < Re < 100. Due to large uncertainties in their experimental data at low Reynolds number, their correlation was not successful to correlate constant Nusselt number at Re < 5.…”