EP1707909A2

EP1707909A2 - Percolation of hygroscopic bulk material

Info

Publication number: EP1707909A2
Application number: EP06007073A
Authority: EP
Inventors: Werner Soyez
Original assignee: Qatar Fertiliser Co (saq)
Current assignee: Qatar Fertiliser Co (saq)
Priority date: 2005-04-02
Filing date: 2006-04-03
Publication date: 2006-10-04
Also published as: EP1707909A3; EG24340A

Abstract

The phenomenon of caking of hygroscopic particulates is analyzed. Mechanical reconditioning of agglomerated material does not prevent the problem of caking to reoccur, either in bags or other storage areas. Percolation of dry air in the bulk hall utilizes the prolonged residence time in the bulk hall to improve the handling quality of the product once and for all. A simple grid arrangement (3) at the bottom of particulates piles (4) distributes dry air over a certain period, significantly improving the quality of the product.

Description

Field of Invention

The field of invention applies to all hygroscopic organic and inorganic particulates prone to caking and agglomeration. The invention is a simple, efficient and cheap way of upgrading product behavior during bulk storage preventing caking and re-caking from occurring.

Prior Art

Inorganic salts such as phosphates, carbonates, sulfates, nitrates, nitrites, etc. but also organic compounds such as urea, flour, sugar etc. and blends thereof are generally prone to caking. Caking complicates loading, bagging, spreading, dissolution and other activities associated with bulk material handling and storage. Mechanical breaking of resulting lumps leads to dust and fines formation, which in turn increases handling problems as well as the re-caking tendency of the product.
A big effort has been made to mitigate this problem. During production, prior to transferring to storage, steps can be taken to adjust the temperature and residual moisture content during and after particulation. Also, the selection of the correct solidification stage where the particulates are being formed can improve product quality in this regard: bigger particles (granules or big crystals) show less caking tendency - but are more costly to produce. On top of these technological means, a broad variety of conditioning measures are in use. They range from "inner" conditioning (to improve the crushing strength), coating for surface impregnation to finally sophisticated bulk hall ambient control and de-dusting stage after reclaiming.
All these measures can only reduce or delay the caking tendency. Caking as such is not prevented. It stays a function of hygroscopiscity, size distribution, residual moisture, ambient conditions, handling method, product purity, storage condition and product pile height. Nevertheless, there is a worldwide trend to further improve the product quality and to suppress caking as far as possible.

Background of the Invention

Looking closely at the phenomenon of caking, it is evident that very few physical and technical features stimulate the initial bridging between the single grains of salts or compounds. The first reason is the residual moisture. The salts and organic compounds mentioned initially are usually derived from aqueous solutions. Solidification and particulates formation requires the near complete removal of water prior or during the solidification stage. Further more, all these chemical compounds are hygroscopic. To dry a hygroscopic material completely, long residence times, high temperatures and a moisture free drying medium are required. This is in contradiction to realities. Long residence lines make the production process inefficient and capital-intensive, high drying temperatures often collide with the thermal stability of the product and a moisture free drying medium is costly to provide. Therefore, the finishing stage of any standard solidification unit leads to a product with a certain moisture level being a function of the product temperature, the moisture in the drying medium and the approach to the hygroscopic equilibrium. The moisture level in the product is above -or at best- close to the so-called "Critical Moisture". This critical moisture defines the particular pressure of H₂O in the ambient atmosphere around the single granule/crystal or particle. It is the partial pressure of H₂O, at which moisture release from the product and moisture absorption by the product are exactly in balance.
Once the product has left the final stage of production, it is usually conveyed into a bulk hall. There, the single particle becomes an element of the pile. From pile temperature measurements we know, that only the outer layer (some 50 cm depth) reacts to the changes in the bulk hall ambient conditions. The rest of the material does not respond at all and is only exposed to the microclimate around the single product particle, sealed inside the pile.
Typically in a pile, around 40-60% of the bulk is void. This "void" space consists of air and moisture at a certain humidity level, which is at or above the Critical Humidity. Since due to production constraints, residence times are never sufficient to allow particulates to completely stabilize, the product temperature and the elevated moisture content create a microclimate around the single particle inside the pile, which is optimal for bridging between particles and finally for caking.

Summary of the Invention

The proposed solution is to draw-up a process minimizing that ongoing, self-propagating bridging by replacing the humid microclimate inside the bulk material with a dryer one, thus hindering the cycle of moisture release, crystal growth, bridging, and moisture pick-up. This can be performed by percolating a pile of bulk material with a sufficiently dried air flow, injected and distributed for example by a simple system at the bottom of the pile, comprising for example a single twisting tube running at the bottom of the pile or a grid of tubes spreading at the bottom of the pile, with holes to allow dry air to flow.

Brief Description of the Drawings

Figure 1 shows a schematic perspective view of an arrangement to put the inventive process into practice.
Figure 2 shows a schematic representation of a grid of tubes to put the inventive process into practice.

Detailed Description

The sketch presented in figure 1 represents a pile 4 of bulk hygroscopic particulate material. A percolation grid is represented in the form of tubes 3 spreading at the bottom of the pile 4 with holes to allow dry air to flow through the enclosed core of the pile replacing the humidity entrapped in the void between the particles. The dry air is delivered to the interspersed tubes 3 through a header 2. The dry air source 1 may be any excess working air available in the plant or a dedicated dry air source.
The invention includes a grid of tubes 3 that could be arranged in a number of ways to maximize distribution of dry air at the bottom of a pile 4 of hygroscopic particulate material. The distribution of the holes on the tubes 3 should account for the shape of the pile 4 and the required amount of dry air needed per ton of material. If the bulk hall to be percolated is too large for one grid to be efficient, then air should be distributed using a number of grids connected through a header. If the amount of dry air needed is too large, then separate grids should be used to percolate sections of the bulk hall intermittently. The flow of dry air should be minimized since the diffusion of moisture from the inside of the particle to the void space in the pile is a very slow process.

Example 1: Percolation of urea granules in bulk hall storage.

During this experiment, the tubes 3 were designed to cover an area of 21 x 20 m². The tubes 3 were connected to two common feeders 1 as shown in figure 1. The airflow rate was 1000 Nm³/d. The relative humidity in the dried air was ~10% at 35°C. The results are summarized in the table below.

	Unconditioned	Conditioned
Initial moisture in product	0.23 wt.%	0.23wt.%
Moisture after 3 weeks time	0.18 wt.%	0.09 wt.%
Initial temperature	45°C	45°C
Temperature after 3 weeks time	48°C	42°C
Caking tendency (*)	63	22

(*) "Zero" caking: Baseline for free flowing material under test conditions is 15-20.

Example 2: Percolation of urea granules in bulk hall storage.

This was a second experiment set up using the grid arrangement shown in figure 2. The conditions were similar to those of the previous experiment, except if otherwise specified.

The area covered was around 1200 m², the product treated was around 7500 t. The air flow rate is 500 Nm³/h at 28.7°C and a relative humidity of 7.8%. The results are summarized in the following table:

	Unconditioned	Conditioned
Initial moisture in product	0.22 wt.%	0.22 wt.%
Moisture after 3 weeks time	0.19 wt.%	0.10 wt.%
Initial temperature	44°C	44°C
Temperature after 3 weeks time	47°C	41°C
Caking tendency (*)	65	24

The two examples above show, that with quite simple means and low costs, a significant quality improvement can be reached. The root cause of caking is basically eliminated. This also prevents any re-caking effects as long as the product is handled with due care.

Claims

A process for upgrading the bulk behavior of inorganic salts such as phosphates, carbonates, sulfates, nitrates, nitrites, etc. but also organic compounds such as urea and blends thereof by percolation of a product pile with dried air.
Process according to claim 1, characterized by an application rate of a dried air flow between 1000 Nm³/h and 10 Nm³/h per 1000 m³ of piled product.
Process according to claim 1 or claim 2,
characterized by application of air dried to a level of maximum 50% of the critical moisture (measured at bulk temperature).
Process according to claim 1 or claim 2,
characterized by application of air dried to a level below maximum 50% of the critical moisture of the product/blend in pile.
Process according to any one of claims 1 to 4, characterized by a percolation air temperature between 15°C and 45°C.
Apparatus to carry out the process of any one of claims 1 to 5, including at least one feeder (1) and a single twisting tube (3) running at the bottom of the pile, with holes to allow dry air to flow through the enclosed core of the product pile (4).
Apparatus to carry out the process of any one of claims 1 to 5, including at least one feeder (1) and a grid of tubes (3) spreading at the bottom of the pile, with holes to allow dry air to flow through the enclosed core of the product pile (4).
Apparatus according to claim 7, including at least one header (2) connecting said tubes (3).