How to calculate the performance ratio (PR) for MED-TVC desalination

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Dear Members,

I want to know how I can check/calculate the performance ratio for MED-TVC desalination process if I have the PFD (process flow diagram) of desalination plant? Is there any standard process or charts available for this?

Do I need to consider the energy of steam entered to TVC also?

I would be very grateful if I can see a sample calculation for any MED-TVC desalination plant.

Thank you in advance.

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5 Answers

  2. Thanks to all

    At first I should correct my question. Indeed I meant that "do I need to consider the energy of steam entered to  vacuum ejectors  also?"

    And next, thank you Hamid for your useful information. By the way, I'm not sure to use the cooling water enthalpy instead of return condensate enthalpy (although the deference is negligible, I think). 

    In fact I did this calculation in this way:   Energy level = Sigma [M * (H_Stream- H_return condensate)]

    And the results have a good accordance with the thermo flow results.

  3. Hi Reza
    How are you?
    As you know, general definition of performance ratio is kg of water produced per 2326 kJ of heat consumed. For each project client, supplier and engineers, have to agree which water and steam streams are considered in the calculation. I am familiar with your current project so this is my advice based on my previous experiences and current research.

    You need to calculate energy level of steam enterered desaliantion for TVC and ejectors. You have mass flow rates and temperatures for steam streams in PFD and only you have to caculate enthalpy change. Entalpy change is difference between entalpy of cooling water (condencer inlet) and entalpy of steam streams. We can not decrease entalpy of steam lower than reference entalpy which is cooling water entalpy, that's why.

    Energy level= Sigma [M * (H_Stream- H_Cooling Water)]
    and finally,
    PR= (Net Distillate mass flow rate) / ( Energy level / 2336 Kj).

    You have to consider ejector consumption and condensate in first effect in your calculation. To put it another way, desalaintion as a black box in your calculation. I am interested in your current project and don't hesitate to ask if you have any qestion.

    1 Comment

  4. Dear reza Behrami

    The process is relatively simple to operate and once set up, is stable in operation. Because of the thermal inertia of the plant and vacuum considerations, the process is best suited for continuous operation. As seawater is corrosive to carbon steel, there is an increasing tendency to construct plants, particularly small ones, using stainless steels and copper nickel alloys. The following is the description of process operation:

    1. The feed is pumped through a large duct, which contains the coarse screens. The screens remove large suspended solids. This is necessary to prevent fouling and blockage of the pumping units and the condenser tubes.
    2. The feed is deaerated to remove dissolved gases, i.e., oxygen, nitrogen, and carbon dioxide. If these gases are not removed, it is released in the flashing stages due to heating and reduction in pressure.
    3. The deaerator may have vertical or horizontal configuration, which is equipped with spray nozzles or trays. Deaeration is accomplished by heating steam, which results in increase in the feed temperature and as a result reduces the gas solubility in the feed water. Also, the heating steam contains no dissolved gases, this generates a gradient for desorption of the dissolved gases into the steam in order to achieve equilibrium.
    4. Other treatment chemicals are then added to the feed water. The chemicals include antiscalent, chlorine, and antifoaming. The antiscalent/antifoaming agents have to be added at the proper dosage otherwise scaling or excessive foaming may occur in the high temperature stage. The chlorine is added to the feed water to prevent biofouling inside the condenser tubes.
    5. The deaerated feed water flows through the condenser tubes starting from the cold end or the last stage. The feed seawater temperature increases as it recovers the latent heat of the flashed off vapor. The feed seawater is heat to the desired top brine temperature in the brine heater.
    6. The feed seawater flows on the tube side of the brine heater. This is necessary to simplify cleaning and removal of fouling and scaling material. This is achieved through use of on-line ball cleaning as well as acid cleaning.
    7. The heating steam flows on the shell side; where more than one inlet is used to achieve uniform temperature distribution within the heater. The steam condensate is collected in a small well at the bottom of the heater. The well generates sufficient hydraulic heat to prevent vapour flushing within the condenser pump.
    8. The hot feed seawater then flows through the stages, where vapor flashing takes place.
    9. The distillate product flows in the distillate trays across the flashing stages.
    10. In the last stage the brine blow down and distillate are collected, where the brine blow down is rejected back to the sea and the distillate is treated further through chlorination and adjustment of its pH value.                                                                          Regards                                                                                                                                             Prem Baboo