Journal of Nanotechnology

Volume 2015 (2015), Article ID 518015, 7 pages

http://dx.doi.org/10.1155/2015/518015

## Analysis of Thermal Properties on Backward Feed Multieffect Distillation Dealing with High-Salinity Wastewater

^{1}College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China^{2}School of Architecture and Engineering, Qingdao Binhai University, Qingdao, Shandong 266555, China^{3}Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

Received 7 May 2015; Accepted 25 June 2015

Academic Editor: Zhongwei Zhu

Copyright © 2015 Jianliang Xue et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Theoretical investigations on thermal properties of multieffect distillation (MED) are presented to approach lower capital costs and more distillated products. A mathematical model, based on the energy and mass balance, is developed to (i) evaluate the influences of variations in key parameters (effect numbers, evaporation temperature in last effect, and feed salinity) on steam consumption, gained output ratio (GOR), and total heat transfer areas of MED and (ii) compare two operation modes (backward feed (BF) and forward feed (FF) systems). The result in the first part indicated that GOR and total heat transfer areas increased with the effect numbers. Also, higher effect numbers result in the fact that the evaporation temperature in last effect has slight influence on GOR, while it influences the total heat transfer areas remarkably. In addition, an increase of feed salinity promotes the total heat transfer areas but reduces GOR. The analyses in the second part indicate that GOR and total heat transfer areas of BF system are higher than those in FF system. One thing to be aware of is that the changes of steam consumption can be omitted, considering that it shows an opposite trend to GOR.

#### 1. Introduction

Wastewater is the by-product of petrochemical enterprises, including oily wastewater, sulfur-containing wastewater, saline wastewater, and high-concentration ammonia-nitrogen wastewater [1, 2]. The wastewater will be greatly harmful to the environment if they are untreated and discharged directly. Petrochemical enterprises have been plagued by saline wastewater treatment owing to its high salt and biotoxicity. The MED is one of the most successful traditional desalination technologies [3], which presents a number of advantages: low scale formation, easy operation, high performance ratio (PR), and operating with any available source of heat energy (e.g., waste heat from petrochemical enterprises and power plants) [4–6]. In each effect of MED system, pure water is produced at slightly lower pressure than the previous effect. The water evaporates at lower temperatures with the pressure decreasing; therefore, the produced vapor of the first effect evaporator serves as the heating steam for the second effect and so on [7]. An increase of effect numbers can lead to a higher PR. On the basis of energy consumption and heat transfer obtained, MED has been found to be more efficient than MSF [8].

Jernqvist et al. [9] and Ettouney [10] developed a simulation code for the MED system with shell and tube evaporators; subsequently the influences of different design parameters on PR were studied. Shakib et al. [11] developed a thermodynamic model for MED desalination with thermal vapor compression (METVC) and its main objective was optimization of METVC from economical and thermodynamic point of view. Although the general computer code and optimal model for thermoeconomic optimizations of MED desalination systems [4, 11–13] had been studied by many reports, few studies have been reported on the thermal properties of MED so far. The purpose of this work was to analyze the thermal properties of BF system concentrating high-salinity wastewater. Therefore, a mathematical model was developed based on mass and energy balance. In this paper, the work consisted of two parts: at first, the influences of effect numbers, evaporation temperature in last effect, and feed salinity were studied. These specifications include steam consumption, GOR, and total heat transfer areas. Furthermore, the performance comparisons between FF system and BF system were conducted in the second part.

#### 2. Mathematical Model

The MED system usually consists of some evaporators, several flashing chambers, and a condenser. The mathematical model is developed for MED concentrating saline wastewater based on mass and energy balance. In the mathematical model, at first mass and energy balance equations have been developed for the system and then evaporator heat transfer areas balance equations are designed [14–16].

##### 2.1. Mass Balance

In the evaporator, the mass balance can be considered as follows:

In the flashing chamber, the mass balance can be considered as follows: where and are the mass flow rate of feed brine water and mass flow rate of condensed brine water, respectively, .

and are the salinity concentration of feed brine water and mass flow rate of condensed brine water, respectively, %.

##### 2.2. Energy Balance

In the evaporator, the energy balance can be considered as follows: : specific heat capacity, J/(kg·°C), : heat utilization efficiency, : enthalpy;

The BPE is the boiling point elevation and is estimated as follows:

The heat transfer equation is as follows: : heat transfer coefficient in condensing surface, W/m^{2}°C; : thermal resistance of tube, W/m^{2}°C; : thermal resistance of furring, W/m^{2}°C; : falling film evaporation heat transfer coefficient, W/m^{2}°C.

In the condenser, the calculation of energy balance is as follows:

The heat transfer equation is as follows:

In the flashing chamber, the mass balance can be considered as follows:

##### 2.3. Total Heat Transfer Areas Balance

In the first evaporator, the heat transfer area is calculated as follows:

In the other evaporator, the heat transfer area is calculated as follows:

So the total heat transfer area is equal to

##### 2.4. Calculation Parameters

Traditionally, salinity wastewater from petrochemical enterprises often contained a large percentage of organic matter (including oil type matter) and suspended matter [16]. The stability of MED process would be influenced if they were not removed. So it was firstly pretreated by biological treatment facilities, RO, and ultrafiltration (UF) to remove the organic matter and suspended matter and then entered the MED to desalinate. Therefore, the saline wastewater in this study was obtained from a typical refinery in China. In order to investigate the influences of key parameters on performance of MED, the calculation parameters of MED were shown in Table 1.