No abstract

Fouling in phosphoric acid concentration is a persistent operational problem that compromises energy recovery in this process. Progress is hampered by the lack of quantitative knowledge of fouling dynamic effects on heat exchanger transfer. The object of this work is an experimental determination of the thermal fouling resistance in the tubular heat exchanger of phosphoric acid preheated installed in phosphoric acid concentration process. By measuring the inlet and outlet temperatures of phosphoric acid, steam temperature, suction and discharge pressure of the pump and acid density measurement, the overall heat transfer coefficient has been determined. The determination of the overall heat transfer coefficient with clean and fouled surfaces, allowed calculating the fouling resistance. The results from the heat exchanger studies showed that the fouling resistance increased with time and presented an asymptotic evolution in compliant with the proposed model by Kern and Seaton, with the existence of fluctuation. The poorly cleaned heat exchanger implied the absence of the induction period and caused, consequently, high values of the fouling resistance in a relatively short-time period.

In typical heat exchanger design methods it is generally assumed that the overall heat transfer coecient is constant and uniform; however, the heat transfer coecients on the hot and cold sides of the heat exchanger may vary with times. In this work, a large number of operating parameters were collected for three types of heat exchangers, namely stainless-steel tubular and two graphite blocks from dierent suppliers (A and B), in an industrial phosphoric acid concentration unit over a period of two years. An experimental study performed to determine both the overall heat transfer coecients in the clean state and the overall heat transfer coecients at time t. A statistical approach was proposed to determine the overall heat transfer coecients in the clean state as well as the retain cycles. The overall heat transfer coecients were evaluated at dierent times. Our results show that the overall heat transfer coecients decreased exponentially with the time and reach a minimum value ranging between 1419 to 2406 W m −2 K −1 according to the type of heat exchanger.

The production of phosphoric acid by dehydrated process leads to the precipitation of unwanted insoluble salts promoting thus the crystallization fouling build-up on heat transfer surfaces of the exchangers. During the acid concentration operation, the presence of fouling in heat exchangers results in reducing the performance of this equipment, in terms of heat transfer, while increasing energy losses and damaging the apparatus. To mitigate these adverse effects of fouling, it is necessary to forecast the thermal resistance of fouling to schedule and perform exchanger cleaning. In this context, artificial neural network and response surface methodology were used to estimate thermal resistance of fouling in a cross-flow heat exchanger by using the operating data of the concentration loop. The absolute average relative deviations, mean squared errors, root mean squared errors and correlation coefficients were used as indicators error between the experimental and estimated values for both methods. The best fitted model derived from response surface methodology method was second order polynomial while the best architecture topology, for the artificial neural network method, consists of three layers: input layer with six input variables, hidden layer with six hidden neurons and an output layer with single output variable. The interactive influences of operating parameters which have significant effects on the fouling resistance were illustrated in detail. The value of correlation coefficient for the output parameter from the response surface methodology is 0.9976, indicating that the response surface methodology as an assessment methodology in estimating fouling resistance is more feasible compared with the artificial neural network approach.

A remarkable decrease in olive production has been observed in Tunisia since 2000, particularly in the semi-arid region. This downfall was mainly due to a notable change in climatic conditions as a result of wind erosion and over cultivation. The aim of this work was to study the biological, physical, and chemical properties of several semi-arid soils from olive tree fields subjected to different farming practices, such as crop time and type of crop, olive mill wastewater (OMW) application rates, and tillage timing and depth (deep or conventional plowing). We noted that hydraulic conductivity (HC) was proportional to the age of the soil tillage, and the highest values were recorded in the soil cultivated for 100 years, with an average value of 33.05 ± 0.02 mm h −1. An important increment of the iron soil content was observed, especially after the fig tree introduction among the olive trees (12,094 ng μL −1) in 2007. Also, a significant increase of the organic matter (OM) content (up to2.6 mg-OM/g-soil) was identified in soil treated with OMW compared to the lowest OM content (0.83 mg-OM/g-soil) recorded in the soil cultivated since 1901. The bacterial communities of the different soils were characterized by 454 pyrosequencing technology, and showed an important diversity, mainly corresponding to Proteobacteria, Actinobacteria, and Acidobacteria. Many operational taxonomic units (OTUs) are raretons, indicating a high resilience of the soil bacterial communities. Statistical analyses showed significant correlations with the different soil parameters. However, an unexpected correlation was determined between soil respiration and OM (r = − 0.583*), suggesting that OM increases the retention of CO 2 , a greenhouse gas. The farming techniques analyzed in this study resulted in a reduction of the bacterial diversity, even though the total bacterial biomass augmented. Keywords Olive tree. Olive mill wastewater. Farming soil management. Semi-arid crop culture. 454 pyrosequencing * Haifa Rajhi