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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.

Wastewater is the by-product of petrochemical enterprises, including oily wastewater, sulfur-containing wastewater, saline wastewater, and high-concentration ammonia-nitrogen wastewater [

Jernqvist et al. [

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 [

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

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

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

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

The heat transfer equation is as follows:

^{2}°C;

^{2}°C;

^{2}°C;

^{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:

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

Traditionally, salinity wastewater from petrochemical enterprises often contained a large percentage of organic matter (including oil type matter) and suspended matter [

The calculation parameters.

Parameters | Values and unit |
---|---|

Feed salinity | 0.6% |

Discharged salinity | 3.8% |

Evaporation temperature in first effect | 92.5°C |

Cooling water temperature | 31.0°C |

Distillate flow rate | 5.5 m^{3}/s |

Overall heat transfer coefficient of the evaporator | 2.5 kw/m^{2}·°C |

Condenser effectiveness | 0.5 |

Boiling point elevation | 1.5°C |

Specific heat capacity | 4.0 kJ/(kg·°C) |

Latent heat | 2330 kJ/kg |

Figure

The influence of effect numbers on the evaporation capacity and heat transfer areas of each effect in MED.

Table

Comparison of steam consumption, GOR, and total heat transfer areas for different effect numbers of MED.

Effect numbers | Steam consumption, kg·s^{−1} |
GOR | Total heat transfer areas, m^{2} |
---|---|---|---|

2 | 2.94 | 1.87 | 188.69 |

3 | 2.09 | 2.64 | 296.38 |

4 | 1.61 | 3.42 | 406.56 |

5 | 1.34 | 4.10 | 525.60 |

6 | 1.17 | 4.70 | 650.14 |

7 | 1.05 | 5.24 | 783.45 |

Figure

Variation of steam consumption with evaporation temperature in last effect.

Variation of GOR with evaporation temperature in last effect.

Figure

Variation of total heat transfer areas with evaporation temperature in last effect.

Figures

Variation of steam consumption with feed salinity.

Variation of GOR with feed salinity.

Figure

Variation of total heat transfer areas with feed salinity.

Figure

Comparison of steam consumption and GOR between FF system and BF system.

As shown in Figure

Comparison of total heat transfer areas between FF system and BF system.

To analyze the influences of key parameters on thermal properties of BF system concentrating high-salinity wastewater system, a mathematical model was developed based on the energy and mass balance. The results indicated that the effect numbers were very important to keep the balance between lower costs and more distillated products in the MED system. More distillated products could be produced with high effect numbers than those with low effect numbers. Also, higher effect numbers led to higher capital costs and distillated product costs. GOR increased slightly with the evaporation temperature in last effect, but the total heat transfer areas rose greatly. The heat transfer temperature difference caused by boiling point increased with the feed salinity, which increased steam consumption and total heat transfer areas. Thereby, GOR decreased with an increase of feed salinity. Furthermore, it was observed that GOR and total heat transfer areas of BF system were higher FF system.

The authors declare that there is no conflict of interests regarding the publication of this paper.

The authors are very grateful to the National Nature Science Foundation (Grant nos. 51408347, 51178463, 21307149, and 51474140) and Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (Grant no. 2014RCJJ018) for financial support of this study. The authors’ deepest gratitude goes to the anonymous reviewers for their careful work and thoughtful suggestions that have helped improve this paper substantially.