Theoretical analysis and experimental study on a low-temperature heat pump sludge drying system

Highlights

• A heat pump sludge drying system was designed, constructed and experimentally studied.

• Theoretical models for circulations of heat pump and moist air are developed.

• The coupling model is validated by the experimental data.

• Sensitivity analysis of operating parameters is conducted.

• Air mass flow rate ranging between 0.8 and 1.0 kg/s is suggested.

Abstract

A low-temperature heat pump sludge drying system was constructed and experimentally studied in the present study. The initial and final moisture contents on dry basis of sludge are 1.53 and 0.2, and the drying duration is 5 h. Theoretical models for heat pump circulation and moist air circulation are established and validated with the experimental results. Based on the validated theoretical models, sensitivity analysis of evaporating temperature, condensing temperature, and air mass flow rate on the temperature and humidity of moist air at the outlet of drying closet, drying rate and specific power consumption (SPC) are carried out. The temperature of moist air at the outlet of drying closet increases with increasing evaporating temperature and condensing temperature, while decreases with increasing air mass flow rate. The drying rate decreases with increasing evaporating temperature while increases with increasing condensing temperature, and first increases and then decreases with increasing air mass flow rate. The appreciated air mass flow rate ranges between 0.8 and 1.0 kg/s, and the drying rate around 6.0 kg/10min can be achieved under these conditions. The relative humidity of moist air at the outlet of drying closet shares a similar trend with the drying rate while SPC shows a contrary trend.

Introduction

Daily life and industrial production are the main sources of urban sewage. During the sewage treatment process, sludge is inevitably generated. Sludge, which is composed of organic fragments, bacteria and inorganic particles, is a heterogeneous body with high moisture content [[1], [2], [3]]. If sludge is directly discharged into the environment without treatment, serious pollution will be caused. The sludge will damage the soil environment and cause pollution to the groundwater due to the harmful organic compounds, pathogenic microorganisms and heavy metals. Meanwhile, the inorganic salts will disrupt the ionic balance in the soil, leading to nutrient imbalances in the plants [4,5].

At present, several methods are popular in sludge dewatering treatment, which are mechanical dewatering, pressure filtration dewatering, high temperature drying and so on [[6], [7], [8]]. These traditional sludge heating drying technologies maybe lead to many problems, such as high investment, high operation cost, large energy consumption, low safe, and secondary pollution [9]. The low-temperature drying technology, which is energy-saving, environmental protection, operational safety, has become an emerging drying technology. In recent years, employing low-temperature heat pump for sludge drying has been studied and popularized with the development of heat pump [10,11]. With the help of moist air and the differential temperature between the condensing side and evaporating side, low-temperature heat pump technology achieves the purpose of drying by cycling the moisture absorption and dehumidification [12,13].

Several studies can be found on the sludge drying. Le’onard [14] established models for heat transfer rate and water evaporation rate of two sludges with different properties; effects of temperature and relative humidity of air and flow rate on the drying process were analyzed. The results showed that temperature was the main factor affecting the convective drying. Hossain [15] established a theoretical model for the heat pump sludge drying system. Theoretical and empirical formulas were employed in the governing equations of mass and heat transfer and energy efficiency. The simulated results showed that the average coefficient of performance (COP) of the proposed system was 5.45, the mass rate of dehumidification was 140.03 kg/h, the power consumption ratio of dehumidification was 0.038 kg/kWh and the drying efficiency was 78.23%. Minea [16] proposed a method to improve the devices design and control strategies for the high temperature heat pump and established a theoretical model to optimize the drying effect. The results showed the migration and control strategy of refrigerant using liquid valve and artificially cooled suction accumulators can prevent the liquid refrigerant from entering the suction port of compressor and further avoid damaging of compressor. Employing time-based and intermittent methods for optimizing dehumidification rates can improve the final quality of the dried material and reduce the total drying cycle.

A novel low-temperature heat pump sludge drying system was designed and constructed in the present study. Experimental studies were carried out to reveal its practical performance. Meanwhile, theoretical models for heat pump circulation and moist air circulation are established to validate and evaluate the operational performances of the proposed system. Based on the validated model, the effects of evaporating temperature, condensing temperature and air mass flow rate on the temperature and relative humidity of moist air, drying rate and specific power consumption (SPC) are analyzed. The simulated results can be referenced for practical application.