Dilution of sodium hydroxide solution
Thread Content
In production, a sodium hydroxide solution with a concentration of 30% is prepared into one with a concentration of 20%. Currently, this is done on a batch basis, but it is to be changed to a continuous process. How can this be achieved? What method should be used for control?
A variable-flow meter is used to calculate the volume ratio of 30% sodium hydroxide solution to water required to prepare a 20% sodium hydroxide solution; the desired ratio is achieved by controlling the flow rates of the 30% sodium hydroxide solution and water
First, calculate the flow rates of the raw materials and diluent water according to the required ratio, and then mix them thoroughly using a mixer.
This control system should be fairly mature and is often used in continuous production processes. A flow control valve and a flow meter are employed in series for the water stream and the 30% NaOH stream respectively; the two streams pass through a mixer, and a sampling point can be installed at the outlet for regular analysis in order to carry out calibration.
In production, the process of preparing a 20% sodium hydroxide solution from a 30% sodium hydroxide solution on a batch basis has been replaced by a continuous preparation method. Typically, option A can be used: automatic control valves are employed to regulate the flow rates of the concentrated alkali and water, which are then fed into a mixer. After mixing, the mixture is cooled in a cooler before being sent to the next processing stage. Option B involves replacing the mixer with a reactor, using overflow discharge, and cooling with water.
Personally, I think this method is a good way to meet the requirements; by controlling the amounts of alkali and water, and using an automatic flow meter
A flow meter is installed at the outlet of the metering pump or centrifugal pump to keep the ratio of alkali to water within the desired range. A pipe mixer, typically a static mixer, is used to ensure uniform mixing of the alkali and water. An analysis sampling point is placed after the pipe mixer for determining the concentration of the mixed solution, which facilitates adjustment of the alkali-to-water ratio. Since the mixing of alkali and water is an exothermic reaction that causes a rapid rise in temperature, a cooler can be installed after the mixer, or cooling water can be used through jacketed tubes for cooling.
Considering the precision requirements for the 20% NaOH solution as specified by the poster, if high precision is needed, water and 30% NaOH are regulated using diaphragm control valves. A density meter installed in the dissolution tank (or in the outlet pipe of the static mixer) is used to monitor density changes, which in turn controls the opening degree of these two valves. If low precision is required, two pneumatic valves along with a densitometer and two-position control can suffice.
A classmate upstairs mentioned that cascade control is a good option; it’s actually just proportional control, where two different flow values are regulated using proportional principles to meet the desired requirements
We use this one too; unfortunately, the pump keeps having problems
This type of control is quite commonly used these days; there are two streams, water and 30% NaOH. The flow rates are controlled using flow meters and flow control valves. The flow rates of these two streams are regulated through cascade control, and the two streams are mixed together using a mixer