METHOD AND SYSTEM FOR MINIMIZING ENERGY CONSUMPTION DURING REVERSE OSMOSIS UNIT OPERATION

A method is disclosed for estimating an optimal individual product water flow rate for a RO train in an RO unit. The RO unit includes a plurality of RO trains. The method can include providing a desired overall product water flow rate for the reverse osmosis unit followed by obtaining one or more dynamic characteristics for each RO train in the plurality of RO trains; estimating a minimal specific energy consumption value for each RO train using the one or more dynamic characteristics; and subsequently obtaining an optimal individual product water flow rate for each RO train.

DETAILED DESCRIPTION

As used herein and in the claims, the singular forms “a,” “an,” and “the” include the plural reference unless the context clearly indicates otherwise.

As used herein, the Reverse Osmosis (RO) means a filtration process that involves forcing a liquid through one or more membranes at a pressure, wherein the membrane is designed to allow only the liquid to flow through while retaining the solutes. Other filtration techniques, such as nanofiltration or microfiltration or ultrafiltration methods, also involve similar principles and consequently the methods and systems described herein, while described with respect to reverse osmosis, are applicable in these situations as well.

As noted herein the invention provides, in one aspect, a method for estimating an optimal individual product water flow rate for a RO train in a RO unit. As used herein, the phrase “RO plant” is also meant to encompass the phrase “RO unit”, and vice versa.FIG. 1shows exemplary steps of the method of the invention10in a flowchart representation. The method comprises providing a desired overall product water flow rate for the reverse osmosis unit12. The desired overall product water flow rate is a direct reflection of the productivity of the unit. It is measured in the form of standard units known to those of ordinary skill in the art, and may include, for example, litres/unit time, kilograms/unit time, kilolitres/unit time, tonnes/unit time, and the like. The overall product water flow rate is distributed among the RO trains in the RO unit. Each RO train consequently has an individual product flow rate that is expected as an output from it. In a standard RO unit, the desired overall product water flow rate is usually distributed equally among all the RO trains. However, as already noted, each RO train may be at different stages of fouling at a given point of time. Hence, equal distribution of desired overall product water flow rate is a highly inefficient and energy consuming method of distribution.

The method then comprises obtaining one or more dynamic characteristics for each RO train in the plurality of RO trains, depicted by numeral14inFIG. 1. Dynamic characteristics of each RO train include various prevalent parameters relevant to the operation of the RO train, such as, but not limited to, pressure of high pressure pump, pressure of booster pump, feed liquid flow rate, liquid feed flow rate to booster pump, extent of fouling, fouling rate, temperature of RO train, and the like, and combinations thereof. Some of these dynamic characteristics may be obtained through some measurement techniques using gadgets such as pressure sensors and thermometers. Other dynamic characteristics may be obtained from estimation techniques such as mathematical models applicable to the RO trains. In the case of the use of mathematical models, some kind of historical data may be necessary to estimate predicted values, such as fouling rates. Such models are known in the art, and are described in, for example, WO2009/104035 and references therein.

The method then involves estimating a minimal specific energy consumption value for each RO train using the one or more dynamic characteristics shown inFIG. 1as numeral16. In a RO train, and the RO unit as a whole, the energy consuming components may be identified in a facile manner by those skilled in the art. The method of the invention involves estimating the specific energy consumption by each RO train, as a combination of the energy consumed by the aforementioned identified components, which in some instances may be the sum of the energy consumed by the components per unit volume of product water produced. Subsequently, the method of the invention involves estimating the minimal specific energy consumption value for all RO train. This may be achieved by the use of pareto-optimal set between specific energy consumption and one or more dynamic characteristics subjected to constraints like limit on product concentration, recovery etc. In some embodiments, the optimization functions may be a polynomial functions. The optimization functions comprise constraints such as minimum and maximum bounds of certain dynamic characteristics, such as, for example booster pump pressure, feed flow rate, and the like. The optimization functions are designed to minimize the specific energy consumption while maximizing individual product output volume using appropriate mathematical methods, such as a multi-objective optimization technique. A polynomial function may be derived for each RO train. This estimation and derivation is repeated for all the RO trains in an RO unit.

With the above polynomial model equation the optimization problem for optimal load distribution between the RO trains is formulated as given below

Subject to

Product flowTrainlrepresents individual product water flow rate, and demand Flow represents desired overall product water flow rate. This step is represented by numeral18inFIG. 1for the method of the invention.

Subsequently, the method represented by numeral20inFIG. 1is used to generate one or more set points for the operation of each RO train based on the optimal individual product water flow rate obtained from step18. The one or more set points include those required for the operation of the RO train which includes booster pump flow rate, reject stream pressure, high pressure pump speed, supply pump speed, and the like, and combinations thereof. These set points for each RO train are calculated using the optimization problem given below

Subject to

In another embodiment, the optimization problems as described herein can be formulated into a single optimization problem as given below

The method of invention can be used as an off-line application wherein the optimization problem is solved separately using the necessary computing requirements, and subsequently, the solution applied to the operation of the RO unit. Alternately, the method of the invention may also be advantageously used as an on-line application, wherein the computing equipment required to solve the optimization problem is also connected to the RO unit. In another embodiment, the method of the invention also includes monitoring the dynamic characteristic of each RO train and estimating the minimal specific energy consumption for all trains, and accordingly, if necessary, adjusting the one or more set points dynamically to ensure minimization of energy consumption during the course of operation. One skilled in the art will recognize that operating an RO unit using the method of the invention will result in optimized energy consumption, thus resulting in considerable savings in costs while maintaining productivity and quality of product water.

As noted herein, in another aspect the invention provides an RO system used to purify an input water.FIG. 2shows a schematic of the RO system of the invention22configured in one kind of operation.FIG. 3shows a schematic of the RO system of the invention22configured in another kind of operation, wherein a plurality of RO units are comprised within the system of the invention. The following description is given with respect to a single RO unit as part of the system of the invention for ease of explanation, however, one can easily extend this explanation theFIG. 3as well. Other configurations may also be possible, and are contemplated to be within the scope of the invention.

The input water may be from any input source24such as sea water, brackish water, ground water, spent water from a processing unit, and the like. The RO system comprises a single or plurality of RO trains30, that is used for the purification to yield a product water, wherein the product water flow rate is characterized by a desired overall product water flow rate. The input source is coupled to a supply pump26, and a high pressure pump28for increasing an input pressure for the input water to yield a pressurized input water stream. The pressurized input water stream is then fed into the single or set of RO trains30, which is then connected to a product outlet32, and a waste outlet34.

Further, the RO system22comprises a booster pump36, a control valve38to control the flow of the reject stream, an energy recovering device40to recover energy from the reject stream. These components are well-known to one of ordinary skill in the art, and may be made available from a variety of commercial sources. Further, other components associated with a RO system may become obvious to one skilled in the art, and is contemplated to be encompassed within the scope of the invention. Such additional components may include, for example, sensors for pressure, temperature, flow rates, and the like, that may be placed at strategic locations along the flow lines, to obtain real time information of various parameters in the RO system.

Each RO train yields an optimal product water flow rate into the product outlet based on the functioning of an optimizer module42for estimating a minimal specific energy consumption value for all RO train using one or more dynamic characteristics for the RO train using the method of the invention. Subsequently the optimizer module42is also used to calculate the optimal individual product water flow rate for each RO train based on the corresponding to the minimal specific energy consumption value. It will be understood that the sum of the optimal individual product water flow rate for each RO train yields the desired overall product water flow rate.

The optimizer module42is also used to generate one or more set points for each RO train based on the optimal individual product water flow rate. The optimizer module42is shown inFIG. 2to be connected to all of the components shown therein. The optimizer module is configured to accept inputs for the one or more dynamic characteristics, and then used to estimate the minimal specific energy consumption, followed by estimating the one or more set points. The optimizer module may be connected to any of the additional components, such as sensors, to obtain more real time inputs of the operation.

One skilled in the art will understand that it need not be connected to all of the components, or in some situations, none of the components at all. In the latter case, the dynamic characteristics are manually input or estimated through other means, and then used in the model to obtain the individual product water flow rate.

The optimizer module may be made available as a software on a hardware in the form of a distributed control system (DCS) or standalone software works with control system or other microprocessor based embedded systems. The optimizer module may be made available as a dedicated hardware or may be installed as a software tool on an existing programmable system, such as a computer with sufficient computing capabilities. Thus, in yet another aspect, the invention provides a tool that uses the method of the invention.

In a further aspect, the invention provides a RO unit that comprises the RO system of the invention as described herein.

Example

In one example, two dynamic characteristics reject water pressure and booster pump flow rate are used to optimize the productivity for a given desired overall product water flow rate in an RO unit comprising a set of 3 RO trains.FIG. 4shows the effect of varying reject water pressure and booster pump flow on the individual product water flow rate. For operation of the RO unit, one of ordinary skill in the art will realize that it is advantageous to maximize the individual product water volume.

FIG. 5shows the effect of varying the reject water pressure and booster pump flow on the specific energy consumption (abbreviated inFIG. 5as SEC). Once again, one of ordinary skill in the art will understand maximizing the individual RO train product water flow rate may increase the combined specific energy consumption for three RO trains.

FIG. 6shows a pareto-optimal set between the specific energy consumption and the individual product water flow rate for the given set of data points for the 3 RO trains in consideration in the example, wherein the top line is for an older and more used RO train, the lower line is for a newer and less used RO train, and the middle line is intermediate between the two RO trains. The specific energy consumption for the older RO train is greater than that for the others, which is reflected here in the graph.

FIG. 7shows a comparative example of operating an RO unit comprising 3 RO trains in current as-is scenario, wherein the individual product water flow rate from each RO train is given without optimization for minimal specific energy consumption of all three trains i.e. the RO train with newer membrane is loaded to its full capacity, RO train with older membrane is loaded to less capacity and RO train with intermediate membrane is loaded in-between to old and new membrane train capacity. Here, LB stands for Lower Boundary value for product flow rate, UB stands for upper boundary value for product flow rate. Qp_1, Qp_2and Qp_3stands for product water flow rate from RO train1, RO train2and RO train3, respectively. It can be seen that the specific energy consumption is considerably high.

In direct contrast,FIG. 8shows an exemplary situation of operating an RO unit wherein the individual product water flow rate is estimated using the method of the invention. It can be seen that the magnitude of the specific energy consumption, represented by SECOptin the figure, is considerably lower than that of the corresponding values of as-is scenario inFIG. 7.