JPH0350573B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to flow rate or pressure control in substance separation operations using semipermeable membranes.

Conventionally, the main parts of this type of apparatus are constructed as shown in the flow sheet of FIG. FIG. 1 shows an example of seawater desalination. Seawater is taken from a water intake device (not shown) and pretreated. The seawater is sucked into a pump 1, pressurized by the pump 1, and discharged.
through a pressure regulating valve 2 controlled by a flow meter 3.
Detection part 3' of a, pressure detector 2 in front of semipermeable membrane 4
It passes through the pressure measuring point 2' in a, enters one side of the semipermeable membrane 4, resists the osmotic pressure, passes through the fresh water, and is sent to the water tank 6 via the flowmeter 5. The concentrated seawater is passed through a water wheel, such as a Pelton water wheel. 7. The opening degree of the inlet nozzle 7' of the Pelton turbine 7 is controlled by a flow meter 3a. That is, the flow control valve 3 is constituted by the nozzle 7' and the flow meter 3a. The energy obtained by the water wheel 7 is used to assist the pump drive motor 8.

In the above configuration, the discharge pressure P ₀ of the pump 1 is 50 kg/cm 2 compared to the osmotic pressure of seawater at a normal concentration of approximately 25 kg/cm ² .
cm ² , and for the inner semipermeable membrane of pump 1 with a discharge volume of Q ₀
20-40% is desalinated by reverse osmosis.

Now, the fresh water output from the semipermeable membrane 4 is expressed as pressure P ₁ , flow rate Q ₁ ,
The flow rate adjustment of the device that adjusts the concentration C ₁ and the fresh water flow rate Q ₁ is performed in the following two ways.

(1) Adjusting the flow rate Q ₁ in a narrow range Keeping the discharge volume Q ₀ of the pump 1 constant, operate the pressure regulating valve 2 to adjust the pressure at the pressure measurement point 2', and adjust the semipermeable membrane inlet pressure P. ′ ₀ ,
Increasing the pressure P′ ₀ increases the freshwater output flow rate Q ₁ , and decreasing pressure P′ ₀ increases the freshwater output flow rate Q 1
Q ₁ decreases. Now A _M Area of the semipermeable membrane K Constant determined by the type of semipermeable membrane and temperature P _M Pressure on the seawater side of the semipermeable membrane π _M Osmotic pressure of the feed liquid (seawater) _{1 1} of the dilute liquid (fresh water) In terms of osmotic pressure, it is determined by Q ₁ =A _M K {(P _M −P ₁ )−(π _M −π ₁ )} (1). P ₁ , π ₁ , are almost constant, and π _M is almost constant in a narrow range, so the freshwater flow rate Q ₁
is approximately proportional to the pressure P _M on the pressure side of the semipermeable membrane, and this pressure P _M is equal to the semipermeable membrane inlet pressure P' ₀ at the fixed point 2' on the pressure side.
This is because it is proportional to

(2) Adjusting the flow rate over a wide range For example, if you want to increase the flow rate Q ₁ of the fresh water output, fix the flow rate control valve 3 and fix the flow rate of the detection section 3', and hence Q ₀ .

Semipermeable membrane inlet pressure P′ ₀ by pressure regulating valve 2
and increase the freshwater output flow rate Q ₁ based on equation (1).

If the freshwater recovery rate Q ₁ /Q ₀ exceeds the allowable value determined from the composition of seawater and the properties of the semipermeable membrane 4, open the flow rate control valve 3 to increase the flow rate Q ₀ and then use the pressure regulating valve 2 to reduce the The membrane inlet pressure P′ ₀ is increased and the flow rate Q ₁ is increased. and recovery rate
If Q ₁ /Q ₂ is within the allowable value, the operation is completed.

The conventional example described above has the following drawbacks.

(1) Excessive power was required because the pump discharge pressure was reduced to the membrane inlet pressure using a valve.

(2) Flow rate adjustment in a narrow range is possible only with the pressure regulating valve, but wide range flow rate regulation requires alternate operation of the pressure regulating valve and flow rate regulating valve, resulting in poor operability.

(3) Flowmeters, pressure detectors, and pressure regulating valves were required, and the cost of instrumentation components accounted for a large portion of the price, especially in small-capacity equipment.

An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional membrane separation apparatus and provide a simple and inexpensive flow control apparatus that requires less power and has good operability.

The present invention utilizes a performance curve at each rotation speed of the pump and a relational expression between the system pressure and the flow rate of a dilute (or concentrated) solution, which is determined from the relationship between the solute concentration and osmotic pressure. By operating the means, it is possible to select the operating point that requires the least number of operations with a simple operation.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a flow sheet including a control block diagram.

Seawater taken by a seawater intake pump (not shown) is sucked into a centrifugal pump (hereinafter simply referred to as pump) 1 through a pretreatment process. Seawater pressurized and discharged by the pump 1 has a pressure P ₀ , a flow rate Q ₀ , and a concentration C ₀ . Seawater in this state is subjected to reverse osmosis pressure higher than osmotic pressure to the semipermeable membrane 4, which outputs fresh water with pressure P ₁ , flow rate Q ₁ , and concentration C ₁ , and concentrates it with pressure P ₂ , flow rate Q ₂ , and concentration C ₂ The converted seawater is supplied to the Pelton turbine 7, recovers energy, and assists the motor 8.

The control device 9 has its output end connected to the control input end of the flow control valve 3, which comprises a valve drive device 3b and an inlet nozzle 7' whose stroke is adjusted by the valve drive device 3b.

The contents of the control device 9 will be described.

The flow rate Q ₁ on the dilute side of the semipermeable membrane is Q ₁ = A _M K△P ……(2) However, the area K of the A _M semipermeable membrane is the coefficient determined by the type of membrane and temperature △P≒P _M −π _M … ...(2′) P Pressure of the supplied seawater near the semipermeable membrane π _M is the osmotic pressure of _the supplied seawater.

The control device 9 performs the following calculations.

(1) Diluent flow rate Q ₁ is input for setting.

(2) The rotational speed N of the pump 1 is assumed. In block 11, Q corresponding to each rotational speed of pump 1 is
The discharge pressure P ₀ of the pump 1 is determined according to the QH curve 21 corresponding to the assumed rotational speed N and the rotational speed N of the −H curve.

Block 11 of the control device 9 shows the water head on the vertical axis and the flow rate on the horizontal axis. In the figure, curve 21
shows the performance curve (Q-H curve) of pump 1,
Curve 22 shows the input characteristic curve to the water turbine 7 at the inlet nozzle 7', and curve 23 shows the dilute flow rate of the semipermeable membrane 4 corresponding to the performance curve 21 of the pump 1.
Shows Q ₁ .

Assuming pump discharge pressure P ₀ , pump discharge amount Q ₀ can be found.

(3) Concentrate flow rate Q ₂ = Q ₀ - _{Q 1} , so it can be found by subtracting the set value Q ₁ from Q 0 obtained in item (2).

(4) Block 12 shows the osmotic pressure π on the vertical axis and the concentration C _M of the solution on the horizontal axis. Curve 24 shows the relationship between solute concentration and osmotic pressure. The liquid concentration C _M on the supply side of the semipermeable membrane 4 is approximately determined by C _M ≈(C ₀ +C ₂ )/2. As long as the recovery rate Q ₁ /Q ₀ does not change significantly, the above approximate formula may be used for C ₀ and C ₂ . Therefore, C ₀ and C ₂
can be considered a constant, especially during normal operation of the device. From this relationship, the osmotic pressure π _M can be determined.

(5) When the temperature of the supplied seawater changes significantly, a temperature detector 1 is installed to detect the temperature of the supplied liquid in the supply side piping.
4, K=K ₀ (D _W /T) where K ₀ is a constant determined by the type of membrane D _W is the diffusion coefficient of water in the membrane T The coefficient K is calculated based on the temperature of the supplied liquid. If the temperature change of the supplied liquid is small, it may be set as a constant. The relationship between T and D _W /T is shown in block 13 by curve 28.

(6) The vertical axes of blocks 11 and 12 are shown on the same scale, and the supply side pressure P _M of the semipermeable membrane 4 is shown in item (2).
It is calculated by subtracting the loss head P _L1 due to the fluid conduit of the supply side pipe from the semipermeable membrane 4 to the discharge pressure P ₀ of the pump 1 assumed in the above. The pressure P ₁ on the dilute side of the semipermeable membrane 4 is almost constant, and the concentration of the dilute solution can be considered to be constant, so the osmotic pressure π ₁ of the dilute solution can be assumed to be constant. Therefore, reverse osmotic pressure ΔP=(P _M −P ₁ )−(π _M −π ₁ ) (3) is calculated. This relationship is illustrated between blocks 11 and 12.

(7) Block 15 has the same vertical axis scale as blocks 11 and 12, and the vertical axis shows the pressure exceeding the osmotic pressure π, and the horizontal axis shows the dilute solution flow rate _Q1 . The line 25 represents Q ₁ =A _M KΔP (2), and the dilute fluid flow rate Q ₁ that changes with the pressure ΔP at which the dilute side flow rate of the semipermeable membrane 4 exceeds the osmotic pressure is shown in linear proportion. Q ₁ calculated by formula (2)
Let Q _1CALC be.

(8) Compare Q ₁ and Q _1CALC set in item (1). If this error is large, return to item (2), reassume pump discharge pressure P ₀ , and repeat items (2) to (8) in a loop until the error between Q ₁ and Q _1CALC becomes small. return.

Here, as shown in block 11, the discharge pressure P ₀₁ of pump 1 corresponding to the maximum diluent flow rate Q _1nax
If the pressure P ₀ assumed in item (2) before becomes Q _1CALC −Q ₁ > 0 in a small range, make the re-assumed P ₀ smaller than the initially assumed P ₀ and Q _1CALC −Q ₁
When <0, the re-assumed P ₀ is made larger than the initially assumed P ₀ .

First, the discharge pressure of pump 1 assumed in item (2)
Q _1CALC −Q ₁ > ₀ in the range where P 0 is greater than the discharge pressure P ₀₁ of pump 1 corresponding to the dilute liquid flow rate Q _1nax
When Q _{1 CALC} −Q ₁ <0, the re-assumed _{P 0 is} made smaller than _{the initially assumed P 0 .}

In the above control device 9, when the dilute solution flow rate Q ₁ is set, the performance curve of the pump 1 is determined, and the relationship between concentration and osmotic pressure is also determined depending on the type of solution, so the discharge pressure P ₀ of the pump 1 and dilution liquid flow rate
The relationship in Q ₁ is uniquely determined. Therefore, the steps (2) to (8) described above can be omitted by compiling them into a numerical table.

From the above, pump discharge pressure P ₀ and diluted liquid flow rate
Find the relationship _Q1 . In this case, the rotation speed N of the pump 1 is assumed to be constant and calculated.

Fig. 3 takes out block 11 in Fig. 2 and shows the relationship among rotational speed, diluted liquid flow rate, and required operation for more detailed explanation, with flow rate Q on the abscissa and pump discharge pressure P ₀ and pump rotation on the ordinate. number N,
The required operating HP of the pump is shown. In Figure 3A, the pump performance curve (Q-H curve)
21 is the pump performance curve at the specified rotation speed N _R , 21-1 is the pump performance curve when the rotation speed is 0.8 N _R , and 21-2 is the pump performance curve when the rotation speed is 0.9 N _R. The dilute liquid flow rate curve 23 representing the relationship between the pump discharge pressure P ₀ and the dilute liquid flow rate Q ₁ shows the case where the rotation speed of the pump 1 is a specified rotation speed N _R ,
The dilute liquid flow rate curves 23-1 and 23-2 show cases where the rotational speed of the pump 1 is 0.8N _R and 0.9N _R , respectively.
Dilute liquid flow rate curves 23, 23-1, 23-2,...
A dilute liquid maximum flow rate curve 29 is obtained by connecting the points indicating the respective maximum flow rates Q _1nAX . 31 is a valve control characteristic curve showing the output characteristic of the pressure regulating valve 2 when the rotation speed of the pump 1 is a specified rotation speed _NRR when controlling the membrane separation apparatus as shown in FIG. 32 is a maximum efficiency control line showing the relationship between the pump discharge amount P ₀ and the discharge amount Q ₀ at the operating point of the pump 1 to obtain Q _1MAX .

FIG. 3B shows the relationship between the diluent flow rate Q _1MAX and the rotation speed of the pump 1 when the rotation speed of the pump 1 changes.

Figure 3C shows the relationship between the theoretical discharge amount Q ₀ of the pump 1 and the theoretical required operation HP, and the rotation speed N _R of the pump 1,
The operating characteristic line 34 corresponds to 0.8N _R and 0.9N _R , respectively.
34-1 and 34-2 are shown.

As shown in FIGS. 3A and 3D, the maximum value Q' _1MAX of the dilute liquid flow rate _Q1 at a rotation speed N' smaller than the specified rotation speed N _R , for example 0.9N _R , is the dilute liquid flow rate curve 23- Point A on pump performance curve 21-2, which has the same ordinate as point D indicating Q' _1MAX , is the operating point of pump 1. At this point, the pump has a rotational speed of 0.9N _R , a discharge pressure of P _0a , and a discharge volume of
Q It is operated at _0a . Point A is selected for the purpose of explanation so that the valve control characteristic curve 31 of the pressure regulating valve 2 intersects with the point A. In a range where the rotational speed is greater than 0.9N _R , for example, at a specified rotational speed N _R , the point equal to Q′ _1MAX is the pressure P _0b above and below the membrane surface pressure P _0a that obtains Q′ _1MAX ,
At P _0c , there are two points E and F with the same flow rates Q _R1 and Q _R2 . The semipermeable membrane 4 has an allowable recovery rate Q ₁ /Q ₀ (industrially 20 to 40%), and the dilute liquid flow rate curve 23
It cannot be operated at flow rate Q _R2 at point E where the pressure is higher than the maximum point of . This is because at point G of the pump performance curve 23 having the same ordinate as point E, the pressure is P _0b and the discharge amount Q _0b , so Q _R2 /Q _0b ≈0.6.

The operating point for obtaining the flow rate Q _R1 is point C on the pump performance curve 21, and the pump is operated at a pump discharge pressure P _0c and a discharge amount Q _0c . Therefore, as shown in FIG. 3C, the required power is the power H _C at the point C that cuts the power characteristic line 34. Pump performance curve 21 mentioned above
The required power in the case of operating point A on -1 corresponds to point A on the power characteristic line 34-2 on the same abscissa, and the required power is H _a . In the conventional example _shown in _FIG . The discharge pressure P ₀ of is controlled to P _0d . Therefore, the pressure loss due to the pressure regulating valve 2 occurs between points A and B in FIG. 3A. Therefore, the required power Hb becomes as shown in Fig. 3C and _B. Therefore, Fig. 3C,
H _b − H _a between A and B rotates pump 1 at a rotation speed of 0.9N _R
If the discharge pressure of the pump 1 is controlled to P _0a by the inlet nozzle 7' of the pressure holding means, it becomes a power saving part. This value is approximately 3 times the conventional power requirement.
It can be seen that the required power can be reduced significantly. The same is true for other pump rotation speeds.
Dilute liquid flow rate Q ₁ on dilute liquid maximum flow rate curve 29
It will be understood that the membrane separator can be operated with the lowest power by setting the operating point and selecting the operating point on the curve 32 connecting pump operating points corresponding to the setting. In the above description of FIG. 3, the pressure loss in the flow path from the discharge pressure of the pump to the semipermeable membrane has been ignored.

Using Fig. 2 and assuming the rotation speed N, P ₀ and
The relationship of Q ₁ was obtained by looking at the above-mentioned section in _FIG . There is a high probability that the pump 1 is often operated at the operating point C.

As already mentioned in items (1) to (8), assuming the rotational speed N, it is possible to calculate the discharge pressure _P0 of the pump ₁ and therefore the discharge amount _Q0 for the set dilute liquid flow rate Q1. Beta. Here, since the dilute liquid flow rate Q ₁ is a curve with a maximum point, if this maximum point is Q _{1 MAX} , the power will be saved the most if the pump is operated at the rotation speed N that obtains Q _{1 MAX} .

Therefore, following item (8), perform the following calculation.

(9) Assuming the rotation speed N, Q _1MAX in Figure 3
Find the value of

(10) Therefore, Q _1MAX obtained in item (9) and Q ₁ obtained in item (8) are both relative to the assumed pump rotation speed N, and the difference in their absolute values is β, and |Q ₁ − If Q _1MAX | β, then Q ₁ > Q _1MAX , then N'> N. Q ₁ < Q _1MAX , then N'< N. Reassuming the rotation speed N', the above item (2)
Recalculate from If |Q ₁ −Q _1MAX |≦β, then the valve opening degree is adjusted.

(11) Find P ₂ as P ₂ = P ₀ −P _L1 −P _L2 . However, P _L2
is the pressure loss due to resistance in the flow path from the semipermeable membrane 4 to the inlet nozzle 7' of the Pelton turbine 7.
This means that the block 16, which is on the same scale as block 11 of the control device 9, represents the concentrate pressure P ₂ on the vertical axis, and the characteristic curve 26 of the nozzle 7' on the horizontal axis.
shown in between. The characteristic curve 26 is v=α√2 ₂ where α is a constant and g is the acceleration of gravity.
Block 16 determines v.

(12) In block 17, the horizontal axis represents the valve opening A _v of the nozzle 7', and the vertical axis represents the stroke S of the flow control valve 3.
Characteristic curve 2 of valve opening per valve stroke
7 is shown. When v is determined by block 16, the valve opening degree of the flow rate control valve 3 is determined by A _v =Q ₂ /v. When the valve opening degree A _v is determined, the flow control valve 3
The stroke S is calculated.

Since this valve stroke S is output as a signal from the control device 9, it is amplified by the driver 18 to operate the flow rate control valve 3.

The above procedure is uniquely determined once the pump performance is determined, so summarize the relationships between Q ₁ ~ N and Q ₁ ~ S in a table to find the valve stroke S for items (2) to (12). You may omit the steps up to.

FIG. 4 is a control block diagram of the apparatus of the present invention. In the above-mentioned case, when the dilute liquid flow rate _Q1 is set, the pump rotation speed N is determined, and the amount of movement of the stroke S is also determined. Therefore, when the set value _Q1 of the dilute liquid flow rate setting device 35 is input, the pump rotation speed function generator 36 sets the pump rotation speed N=f as described above.
(Q ₁ ) is determined and a signal corresponding to N is sent to the control device 38 of the pump 1, and the control device 38 controls the motor 8 to set the number of revolutions of the pump 1 to N. On the other hand, when the set value Q ₁ of the dilute liquid flow rate setting device 35 is input to the valve stroke function generator 37, a signal corresponding to the valve stroke S is output based on S=f(Q ₁ ), and the driver 18 is The signal is amplified and input to the valve drive device 3b, where the valve stroke S is adjusted.

Based on the above, the pump rotation speed N and the valve stroke S
is adjusted.

FIG. 5 shows a closed loop version of the open loop control shown in FIG.
In the control shown in the figure, the pump discharge pressure P ₀ and concentrated liquid pressure P ₂ detected by the pressure detectors 2b and 19 are sent to the function generators 36 and 37.
The difference between the pump discharge pressure P ₀ and the concentrated liquid pressure P ₂ calculated in 1 is controlled so as to be on the diluted liquid maximum flow rate curve 29 so as to approach 0. Fifth
In the figure, only one of the pressure detectors 2b and 19 may be provided, and the structure may include an open/close loop.

The maximum diluted liquid flow rate Q _1MAX determined above may exceed the allowable recovery rate γ=Q ₁ /Q ₀ . In this case, an allowable recovery rate setting device is provided. If Q _1MAX /Q ₀ > γ, the diluent flow rate is determined to be a value q _1MAX that is slightly smaller than Q _1MAX , and the allowable dilute solution flow rate q _1MAX is calculated using the formula Q _1MAX /Q ₀ ≦ The allowable maximum diluent flow rate q _1MAX may be reduced so as to satisfy γ.

If the allowable recovery rate of the semipermeable membrane is determined by the manufacturer in consideration of the raw water structure, the pump rotation speed N is uniquely determined once the allowable maximum dilute liquid flow rate q _{1 MAX} is determined, so N = f (q _{1 MAX} ) to find the pump rotation speed N.
The valve stroke S is calculated using the flow rate q _1MAX as S=f
(q _1MAX ) may be used to control the flow rate control valve 10.

As described above, the flow rate control valve 3 is adjusted to obtain the required diluted liquid flow rate _Q1 . In the embodiment, a nozzle whose opening degree can be adjusted at the downstream end of the pump discharge side and a Pelton water wheel operated by the liquid ejected from the nozzle are used as a system pressure holding means, but the invention is not limited to this, and a simple flow control valve is used. Alternatively, other energy recovery means such as a flow control valve and a reversing pump may be provided. The above description of the present invention has been based on the state quantities of the supply solution, pressure P ₀ and flow rate Q ₀ , but the pressure P ₂ and flow rate Q ₂ of the concentrated liquid are based on the curve 22 of block 11 in FIG. It goes without saying that this can be explained based on the state of the concentrated liquid since it corresponds to Q ₀ and Q ₁ as shown in .

The present invention provides a membrane separation device for separating solutes in a solution using a semipermeable membrane to obtain a dilute solution and a concentrated solution.
A centrifugal pump that pressurizes the solution, a semi-permeable membrane, and a control device for maintaining the system pressure downstream of the semi-permeable membrane on the concentrated liquid side, and the control device is based on the performance curve of the centrifugal pump and the Equipped with a function that calculates the theoretical internal pressure in the system corresponding to the flow rate of a dilute or concentrated solution from the relationship between concentration and osmotic pressure and supplies it to the pressure holding means.It has the function of operating the pressure holding means based on the concentrated liquid pressure and concentrated liquid flow rate. Since the control means is provided to set the pump rotation speed to a maximum dilute solution flow rate with a small amount, the number of components of the device is small and the piping is simple and inexpensive. If the diluted liquid flow rate is set, the pressure within the system will be automatically controlled, resulting in good operability.

[Brief explanation of drawings]

FIG. 1 is a flow sheet of a conventional example, FIG. 2 is a flow sheet of an embodiment of the present invention, FIG. 3 is a pump control diagram, and FIGS. 4 and 5 are control block diagrams. 1...Pump, 2a, 2b...Pressure detector, 3
...Flow rate control valve, 3b... Valve drive device, 4... Semipermeable membrane, 7... Water wheel, 8... Drive motor, 9... Control device, 19... Pressure detector, 36, 37... Function Generator, 38...control device.

Claims (1)

[Claims] 1. In a membrane separation device that uses a semipermeable membrane to separate solutes in a solution and obtain a dilute solution and a concentrated solution, a centrifugal pump that pressurizes the supplied solution and a centrifugal pump whose rotational speed is controlled. A means for controlling the rotation speed of the pump, a semipermeable membrane, a system pressure holding means disposed downstream of the semipermeable membrane on the concentrated solution side, and a control device for the system pressure holding means. The control device of the pressure holding means determines the pump rotation speed and system theoretical pressure corresponding to the dilute or concentrated solution flow rate set from the performance curve at each rotation speed of the centrifugal pump and the relationship between the solute concentration and osmotic pressure in the solution. A substance separation device using a semipermeable membrane, characterized by being equipped with a function to calculate relationships. 2. In a membrane separation device that uses a semipermeable membrane to separate solutes in a solution and obtain a dilute solution and a concentrated solution, a centrifugal pump that pressurizes the supplied solution, a means for controlling the rotation speed of the centrifugal pump, and a supply It is equipped with a liquid or concentrated liquid pressure detection means, a semipermeable membrane, an internal pressure holding means disposed downstream from the semipermeable membrane on the concentrated solution side, and a control device for the internal pressure holding means, and controls the rotation speed of the pump. The system and the control device for the means and system pressure holding means are designed to match the pump rotation speed and system to the dilute or concentrated solution flow rate set based on the performance curve at each rotation speed of the centrifugal pump and the relationship between the concentration of solute in the solution and osmotic pressure. A substance separation device using a semipermeable membrane, which is equipped with a control circuit that calculates the internal theoretical pressure and adjusts the pressure holding means so that the indicated value of the pressure detection means is equal to the internal theoretical pressure.