Optimization of Energy and Resource Efficiency in a Multistage Drying Process of Phosphate Pellets

Valery Meshalkin,Maksim Dli,Vladimir Bobkov,Vincenzo Dovì

doi:10.3390/en12173376

Abstract

The roasting of phosphate pellets is an energy-intensive process the optimization of which can give rise to considerable efficiency improvements. To this purpose, the mathematical modeling and the computer-aided simulation of a multi-stage process of phosphate pellets roasting have been developed. This process includes drying, dissociation reaction of carbonates and a sintering process in a moving, dense, multilayer mass of phosphate pellets in a special horizontal grate apparatus. A theoretical model for the physical-chemical processes of a mass of phosphate pellets moving in a multistage process and subject to external operation variables (i.e., temperatures and flowrates of the drying gas at different positions of the conveyor length) has been developed. By letting these control variables vary over suitable ranges limited by technical constraints, it is possible to construct an algorithm that minimizes the overall energy consumption subject to the attainment of the required quality indicators of the pellets. Therefore, the set of constraints includes bounds on both the operational variables and quality indicators of the final product. A dynamic programming algorithm subject to suitable penalty functions for avoiding constraints violation has been used. The results obtained show that the scientifically based operation described in this paper achieves the goal of attaining considerable energy savings while assuring the quality of the finished pellets. Furthermore, the results highlight the convenience of extending the general strategy developed in this article to other chemical energy engineering processes.

Highlights

The rational use of energy and raw materials is a problem of vital importance for most complex chemical and metallurgical plants and provides an ecologically sound approach that reduces the environmental impact and the necessity of wastewater treatments or of remediation procedures
The theoretical physical model and the resulting optimization problem include several steps that are described by the following equations: 1. Drying of single pellets, the state variables of the drying process depending on one spatial coordinate and one temporal variable
The vapour pressure and the temperature at the evaporation front are established during the drying process and follow the usual saturated vapour pressure law [10]

Summary

Introduction

The rational use of energy and raw materials is a problem of vital importance for most complex chemical and metallurgical plants and provides an ecologically sound approach that reduces the environmental impact and the necessity of wastewater treatments or of remediation procedures. This is true for manufacture energy-intensive industrial products using thermal treatments. The process of roasting a moving multilayer mass of phosphate pellets in a horizontal grate apparatus (a widely used equipment in the chemical industries [1,2]) is considered. The markets for phosphates include important sectors such as fertilizers, foods, detergents, water treatment chemicals and others. The production of phosphorous requires the phosphate pellets to be dried before remelting, otherwise the functioning of the ore-thermal furnace would be seriously impaired and the consumption of energy would be higher than the one necessary to dry the phosphate pellets before entering the furnace.

Methods

Results

Conclusion