ESEL Paper Review_20110306
 
By Hong Guo

1,Title and Author

Title: Effect of Stream Mixing on RO Energy Cost Minimization

Authors:  Aihua Zhu1, Panagiotis D.Christofides, Yoram Cohen
1. Department of Chemical and Bimolecular Engineering, University of California, Los Angeles, CA 90095,USA
2. Department of Electrical Engineering, University of California, Los Angeles. CA 90095, USA

2, Summary of Paper

In this paper, it studies about Various mixing operations between the feed, retentate and permeate streams are studied to access their potential effectiveness for decreasing the SEC (specific energy consumption) of single-stage, two-pass and two-stage RO desalination processes operated at the limit of the thermodynamic restriction.

 2.1 Effect of partial recycling operation on the SEC of single-stage RO desalting at the thermodynamic limit

    Here the paper only studies about the partial recycling because full recycling of either the retentate or permeate streams are not possible for a continuous process.
                                  
    The conclusion is that in a single stage RO operation, permeate recycling increases the SEC, while retentate recycling doesn’t change the SEC.

2.2 Effect of second-pass retentate recycling to the first-pass feed in a two-pass membrane desalting process

    After getting the general equation for the SEC, the paper also talks about the critical water recovery. By expressing the SEC with the ERD (for the second and first pass process) efficiency, Y (water recovery) and R (salt rejection), it could get the relationship between the Y and efficiency. This three important parameters and those relationship are used to discuss the SEC under the different conditions in this paper.

    2.2.1 Two- passes desalting with complete retentate recycling and non-ideal energy recovery.

    1,If the Y1 (first water recovey, 15%)< Yt (critical water recovery, 15.4%)，
   The two-pass process has lower SEC than a sing pass process at the same water recovery.

  2, If the Y1 (first water recovery, 16%)< Yt (critical water recovery, 15.4%)
  The two-pass process has lower SEC than a sing pass process at the same water recovery.

2.2.1 Two- passes desalting with complete retentate recycling without energy recovery.

(a) is the situation that the Y1 (first water recovey, 15%)< Yt (critical water recovery, 15.4%) and (b)is the opposite side. In the (a), the two-pass process has a lower SEC than s single-pass when operated at the same water recovery.  (b) is shows that the opposite one.

Contribution:
  
The paper studied the mixing methods for the RO process under the different conditions. It may be useful for the optimization design for the desalination with various reasons and provide an advantage from the viewpoint of energy reduction.

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