We will explain the concept behind the design of water scrubbers .
It is useful as equipment for safely processing exhaust gas generated in chemical plants.
In response to recent environmental issues, the demand for exhaust gas treatment is increasing.
It would be great if you could design the system yourself, but there may be cases where you leave it up to the vendor.
Whether you do it yourself or leave it to a vendor, I will introduce the “thinking” part of the system that you should know as a mechanical engineer.
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Needs for exhaust gas treatment equipment
Water scrubbers play an important role in chemical plants as exhaust gas treatment equipment .
Chemical plants generate some kind of gas.
In the past, this depended only on atmospheric diffusion .
The idea is that it is OK as long as the concentration of various gases can be diluted to below the agreed value at the boundary of the business premises .
Recognizing that this alone was not a problem, progress was made in the development of a removal device.
Comprehensive abatement method
As expected, this alone is not a problem, so there is a method that collects the exhaust gas in one place and processes it using an absorption tower .
This is called the comprehensive abatement method .
The outline of the equipment is as follows.
Filling is placed inside the tower , and gas is poured from the bottom and liquid from the top .
We expect the following effects from an absorption tower that brings gas into contact with liquid.
- Neutralization by contacting acid (or alkali) gas with alkali (or acid) liquid
- Dissolve gas in water
Since all the gas in the factory is collected and processed in one place, functions cannot be divided, but it can function as a last resort .
That’s why when the comprehensive abatement tower stops, it immediately stops running.
Even in the event of a power outage, we often connect an emergency generator and run it anyway.
Local abatement method
The local abatement method is the opposite of the comprehensive abatement method.
If you write it as an image, it looks like the following.
In this example, we want to locally collect the gas emitted from the device A to detoxify it.
There are many cases where it is very dangerous, or the amount is very large, and you cannot rely on a single comprehensive abatement anyway.
As an easy-to-understand example, we often see cases where acid-based gases are treated with local abatement A and alkaline gases are treated with local abatement B.
The most popular local abatement device is the water scrubber system.
The configuration is as follows.
- Pressurized liquid (mainly water) is pumped into the scrubber .
- Gas is sucked by the force of liquid blowout pressure
- Gas and liquid come into contact at the outlet tank
- Increase the pressure of the liquid in the tank with a pump and return to 1
Depending on the amount and temperature of the gas, absorbed heat may become a problem, so the gas may be circulated while being cooled using a plate heat exchanger .
The advantages of this device are:
- No suction device (fan) required and can be configured with minimal source equipment
- Can be installed in explosion-proof areas
scrubber
We introduce how to design a scrubber as a local abatement device.
This is a vacuum device classified as a water ejector.
First, let’s start with the scrubber itself.
- Calculate suction air volume
- Calculate suction pressure
- Determine nozzle size
- Determine the compression
Suction air volume
The suction air volume is determined by the total amount of gas generated from the device .
In batch chemical plants, unless there is a special request, it can be determined based on the pipe diameter of the liquid flowing in and out of the tank.
Exceptionally, it is determined from the amount of gas that comes out in processes such as distillation and evaporation …
Since most of the air is condensed in a condenser, the amount of suction air is usually much less than the amount of generated gas.
suction pressure
In the case of the scrubber method, the suction pressure is usually slightly reduced .
The pressure inside the tank is normal and the primary side of the scrubber is slightly decompressed to suck gas.
The decompression degree itself of slight decompression cannot be so high, and the limit is about 5 kPa.
What should be designed here is the pipe diameter of the gas line.
The pipe diameter is determined by calculating the pressure loss from the suction air volume of each device that generates gas .
nozzle size
Once the suction air volume is determined, it is possible to determine the capacity of the scrubber.
It will appear in the form of determining the nozzle size .
Here, let’s also confirm that there is no difference from the pipe diameter obtained from the calculation result of pipe pressure loss.
If the pipe diameter is 200A and the nozzle size is 100A, pressure loss will occur at the scrubber outlet.
The size of the tip tip , which is the cornerstone of the scrubber , is also decided at this stage.
This area is the design range of the scrubber manufacturer.
Compression
In order to demonstrate the ability of the scrubber, the pressure must be determined.
When circulating with a pump, the following factors should be considered for pressure loss calculation.
- scrubber height
- scrubber compression
- Piping friction loss
- heat exchanger pressure loss
The higher the pressure , the higher the suction air volume and suction pressure, and the higher the performance of the scrubber.
tank
Introducing the design of a scrubber type tank.
capacity
The most important factor in tank design is capacity .
Liquid exchange is a design element in scrubbers .
There is a choice between a method that constantly replenishes a certain amount of water and releases a certain amount , or a method that replaces it at regular intervals .
Batch type chemical plants often choose the latter.
This is because the amount of gas generated fluctuates over time, so the same process cannot always be performed.
This is because it becomes difficult to control the pH in the scrubber, or it tends to result in excessive chemical throughput.
Salt precipitation is important for determining the frequency of liquid exchange .
For example, if you store slightly alkaline liquid in a tank, the amount of acid-based gas generated will give you a certain amount of leeway and determine the replacement frequency.
Connecting fluid
Determining the type of liquid that will be connected to the tank is also a design factor.
The point is whether it is acid or alkaline.
When designing local abatement where one product handles acid gases and another product handles alkaline gases,
It is better to separate the nozzle for alkaline liquid and the nozzle for acid liquid .
dip tube
In the scrubber method, the outlet of the scrubber is dipped into the tank.
This is to ensure that the gas sucked by the scrubber comes into contact with the liquid .
The liquid and gas are mixed even in the scrubber, but because the amount of liquid is small, it is not necessarily neutralized completely.
We aim for reliable contact by submerging the gas into a large volume of water in the tank .
This type of water seal method is classic, but that’s why it’s so effective.
Be careful not to make the dip depth too deep as this will reduce the suction effect of the scrubber.
This is because the differential pressure before and after the tip of the scrubber (the pressure of the circulating water and the internal pressure of the scrubber) becomes smaller.
I think it is sufficient to dip about 100mmAq to 200mmAq.
pump
When designing a pump, first calculate the amount of water supplied to the scrubber and the pressure .
The contact pressure is easily determined by the scrubber requirements, and the amount of water supplied is determined accordingly .
The moment you decide on the scrubber, you also decide on the capacity of the pump.
What I would like to pay attention to here is the calculation of the amount of water discharged externally .
When replacing water, it is normal to pump out the water while running a circulation operation .
In this case, the pump must discharge the amount of water that is being discharged externally.
Regardless of whether the method is to replace the water constantly or the method to replace it after a certain period of time , the amount of water to be dispensed to the outside is not so large, so there is little possibility that the pump discharge amount will change significantly.
For example, if there is a 5m3 tank and the scrubber is rotating at 24m3/h, if the water needs to be changed over an hour, 5m/h should be discharged to the outside.
In this case, the pump specification is 24+5=29m3/h.
Even a 24m3/h pump may be able to deliver without overloading .
Since it depends on the frequency of replacement, it is better to increase the pump capacity a little.
Heat exchanger
A heat exchanger may be attached to the scrubber method.
In fact, you may forget it, but it can be a fatal problem.
- Neutralization heat cannot be removed
- Unable to remove pump heat input
The problem is that the temperature of the water rises steadily.
There are two things I’m concerned about:
- Water temperature rises → gas absorption decreases
- Increase in water temperature → Increase in corrosion rate
It’s not good for the water temperature to rise, so if you’re worried about it, I recommend installing a plate heat exchanger .
Material
Let’s take a look at the materials commonly used in scrubber equipment.
- FRP
- ceramic
- carbon
- Fluororesin lining
- hastelloy
All made of strong acid-resistant materials.
Of course it costs money.
The combination of each device and material is as follows.
| scrubber | ceramic | FRP |
| tank | Fluororesin lining | FRP |
| pump | Fluororesin lining | |
| Heat exchanger | hastelloy |
If you want to keep costs down, you should use FRP, and if you prioritize longevity, you should use ceramic or fluorine resin lining.
There are factories that use carbon for absorption towers, etc., but they are treated a little differently than scrubbers, so they are omitted here.
instrument
The instrument around the scrubber is also an important design element.
This is because it is an important point in determining whether or not the factory will continue to operate.
I would like to add a meter like the one below.
| scrubber | – | – | – |
| tank | Liquid level indicator | thermometer | pH meter |
| pump | Ammeter | pressure gauge | – |
| Heat exchanger | thermometer | – | – |
Tank level gauges and thermometers are common.
There are different styles of pH meters depending on whether they are attached to the tank body or to the circulation line.
It seems that the circulation line is more accurate.
Pump ammeter and pressure gauge are also common. I would like to connect it to an emergency generator.
I would like to attach a thermometer to the heat exchanger. In practice, a thermometer will be attached to the outlet pipe of the heat exchanger.
reference
lastly
We explained the concept behind the design of water scrubbers.
Scrubber, tank, pump, heat exchanger.
Design elements include suction air volume, suction pressure, piping diameter, and pressure.
It’s a pretty complicated system when you consider the type of liquid gas, whether it’s acidic or alkaline.
Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)
*We will read all comments received and respond seriously.