Membrenas de osmosis

Chapter 9 - Multiple port vessels.
Hydraulic design of side-ported RO vessel arrangements (How to get the best out of the side-ported vessels)
Generally any RO vessels arrangement has two primary elements: the vessel row and the pipe manifolds (Fig.1)

Fig. 1 - Basic side-ported RO vessels arrangement elements From hydraulics point of view the side-ported vessels and pipe manifolds may beconnected in a number of ways (Fig.2)

Fig.2 - Hydraulic classification of the side-ported vessels and pipe manifolds connections (in clockwise direction): 1. S-type row; 2. U-type row; 3. S-type manifold; 4. U-type manifold;

Technical Manual Version 3, Rev. D / page 1 of 8 / 01 February 2006

In the S-type row of vessels the feed inlet and brine outlet for the row are in the oppositevessels. For U-type row the feed inlet and the brine outlet are in the same vessel. The former type is rarely used. The S-type and U-type manifold arrangements consider the feed and brine streams distribution to the vessel rows and collection from the rows. In the S-type manifold connection to the vessel rows the feed and brine streams in the dividing and collecting manifolds are in the same direction.In the U-type manifold connection the afore-mentioned streams are in opposite directions (Fig.2). In designing the vessel arrangement three hydraulic phenomena shall be taken into account: 1. Pressure losses in connections of vessels and manifolds, 2. The feed flow maldistribution between the vessels in the same row, 3. The feed flow maldistribution between the vessel rows connected to the samedividing and collecting manifolds. The pressure losses in the feed and brine streams have direct effect on the process energy consumption and productivity. They can’t be neglected even at the preliminary design stage of the RO desalination unit as the loss in production may be surprisingly high – up to 3…8%. The flow maldistribution may be measured by the ratio of minimum flow rate of the feed troughthe vessel to the feed flow maximum value. For example, in the U-type row the first vessel accepts the maximum fed flow, while the vessel farthest from the manifold receives the minimum flow. In the U-type manifold the picture is quite the opposite: the row nearest to the inlet receives the minimum feed flow rate. This behavior is inherent and has a theory-grounded explanation. In a dividingmanifold, the flow diverted to the vessel causes the main stream to decelerate and its static pressure to increase stepwise. Thus, the pressure drop across the vessel row increases progressively with its distance from the inlet. This results in hydraulic non-uniformity with the maximum flow in the row farthest from the manifold inlet. In a combining manifold, the velocity in the main stream increasesas the branch streams from the vessel rows progressively merge into it. It causes the static pressure in the manifold to drop downstream, its minimum being at the manifold outlet. Minimum and maximum pressure areas in both manifolds define extreme flows through the vessels. Generally the S-type manifolds perform better than the U-type ones. The feed flow maldistribution results in the recoverymaldistribution working in the opposite direction; the higher the flow rate the lower the recovery. Fig. 3 gives fair indication of the predicted maldistribution effect in a row containing 6 vessels. Two cases are compared – the 2-inch and 3-inch side ports for feed and brine.

Technical Manual Version 3, Rev. D / page 2 of 8 / 01 February 2006

Fig.3 - The vessel feed flow rate and membranerecovery maldistribution in the U-type row for seawater of 62 Bar, at the maximum permeate flux of 27 l/(m2*h), recovery of 45%, and fouling of 80%. In hydraulically faulty designs the flow maldistribution may reach 15% (abs), the maximum recovery maldistribution being below 5%. It leads to a less effective usage of the RO membranes and shortens their life. To limit the extents of the “in-row”...