Environmentally Friendly Waste System

Below is the summary of the process of the methane capture system design:

We wanted to create a methane gas capture system in order to harvest the methane gas from the biogas produced from the digestion of the waste feed.




Discussion with Mr. Desmond Lim



Suggested methods to collect the methane gas:

1. Scrubber system

2. Balloon storage system



Scrubber system (also known as the inverted cup system) was recommended by Mr. Desmond Lim



Discussion with Dr Han

We presented our drawing of the modified scrubber system to Dr Han. During the discussion with him, Dr Han has improved the methane capture system design for us.



In order to start constructing the prototype, we were told to calculate the height of the column for this system.




Problems faced:

1. We went to look for lecturers like Dr Lee and Mr. Wong for help as we have no ideas on how to start the calculation for the height of the column.

2. Both lecturers have introduced correlations method to us. We have spent too much time (about 2 weeks) on finding the appropriate correlations for our calculations.





Classification of various causes leading to the main problem on spending too much on the calculations of column height for methane gas capture system is done using the relationship diagram.

Update: Mr. Kon has suggested a more simplified calculation. With the assumption of 1m radius for the base of the tank, we found out that the calculated height is over 5m tall which is not feasible.





Conclusion: Through this, we have learnt about time management and that emphasis should not be placed too much on one side especially since this project involves different aspects. We should have concentrated on the more practical side and focus on making the system workable instead of pinching too much on values. However through this, we have actually gained valuable knowledge on carbon dioxide adsorption which may come useful towards explaining the theoretical aspect of it.  

After discussion with Mr Kon, we made a rough calculation based on the values given in this website:




http://jcbmac.chem.brown.edu/myl/hen/carbondioxideHenry.html



We assume the temperature to be about 25 degrees celcius and hence take the solubility as 0.1449g of CO2 per 100g of water. We calculated that the amount of CO2 produced was about 25kg. This works out to about 17000kg of water or 17m3 of water. During the discussion, it was said that a thinner column with a smaller radius was more effective in separating the CO2 and H2S. Because as the gas bubbles through, only a small amount of water will be used to absorb them and not the whole tank. Therefore baring that in mind, we went on to estimate the height of the column which ended up being over 5m tall with the assumption of 1m radius for the base of tank. This will not be feasible. It is not easily accessible and it would be too bulky to transport.



Hence, right now, we are thinking of switch our focus to a wetland system and also possibly do a small scale setup of the digester to run instead of running the actual thing. Because due to the H1N1 situation, many camps were cancelled and there is no sludge supply for us.

Our objective is to increase the purity of methane in the gas by removing the other contaminant gases, especially CO2 and H2S which is hazardous to the environment and health of people. For effective uses of the biogas, it must be enriched with methane such that the energy value of the gas is increased to give better efficiency.




Industrial method for Absorption of water

One of the most common technique is to employ the absorption with water or otherwise known as ‘water scrubbing’ technique. The principle of this technique exploits the solubility of carbon dioxide, which is better than methane, hence allowing the carbon dioxide to be absorbed better in water. It is noted that the solubility of carbon dioxide increases with pressure so the separation is better conducted at a higher pressure.



The industrial process can be seen in the following simplified scheme:

 
 
 
  
 
   
 
In this process, the biogas is pressurized before being fed to the bottom of the absorption column where water is fed from the top. A counter-current operation is commonly employed due to better effectiveness. The water containing the absorbed carbon dioxide and a smaller amount of methane is sent to the flash tank where gas is regenerated by de-pressurizing and returned to the absorption column. Regeneration of water is performed by stripping it with air in the desorption column, the stripper. Gas exiting the stripper contains methane losses since methane is slightly soluble in water in the first place.




Absorption with water process

Absorption with water is purely a physical process. This implies that the absorption process does not incorporate chemical reaction. Instead, the mass transfer operation theory is prevalent in this process. The mass transfer from the gas to the liquid phase can be described by the two film theory. This model assumes steady state; hence it provides as a good approximation and to certain good accuracy and allows its mathematical expressions to be relatively easy to comprehend.



• Two-film theory

According to the two-film theory, the resistance to the mass transfer can be described with one or two stagnant films, the gas and liquid film. Since the solubility of the gas follows the Henry’s Law and Henry’s constant of carbon dioxide is large, this implies that the solubility of carbon dioxide is very small and the concentration gradient in the liquid phase is very large. Because of this, the significant resistance for the mass transfer is in the liquid phase and the gas film resistance and gas film itself can be neglected. As such, if the process is controlled by the rate of mean transfer through the liquid film (liquid phase controlled system).



Under these conditions, the total transfer rate of the component A (methane) respective component B (carbon dioxide) from the gas to the liquid phase in the differential volume at the absorption column (scrubber) is described by the two equations below:




Ci Molar concentration of component i (mol/m3) t top of the column

F Molar rate of gas (mol/s) Ptot Total Pressure (Pa)

QL Liquid rate (m3/s) HA Henry’s constant, (Pa m3/mol)

V Volume of the column (m3)

L Liquid phase

kL0 Mass transfer coefficient (m/s)

a Specific surface area (m2/m3)



Note: Since our design will not be incorporating the flash tank and stripper, their sizing equations will not be further discussed.





Equilibrium

Equilibrium between gas in the solution and in the vapour phase governs the limit to which how much gas can be transferred. The rate at which the gas is transferred into the liquid is governed by kinetics. Equilibrium is affected by properties of gas, temperature, dissolved solids and the partial pressure of gas. For a fixed set of conditions, the equilibrium concentration of a gas in water is proportional to the partial pressure of the substance in the gaseous phase. This relationship is linear at low partial pressures and is represented by the Henry’s law.



We refer to the two-film model proposed by Lewis and Whitman in 1924. Other models that have been used to explain the gas transfer theory include the penetration model by Higbie and the surface renewal model by Danckwertz. However, the two-film model is recommended as it simple and referred to mot frequently.



This research touches on the rate of mass transfer of a volatile substance from water to air which is generally proportional to the difference between the concentrations of the substance in the solution at the system temperature (Henry’s law). The relationship is expressed as follows:

M=KLa  (C*-C)
Where:


M= rate of mass transfer (lb/hr/ft3 or kg/hr/m3)

KLa= overall mass transfer coefficient (hr-1)

C*= equilibrium concentration of the gas in the liquid (lb/ft3 or kg/m3)

C= bulk liquid-phase concentration (lb/ft3 or kg/m3)

• Driving force for mass transfer is the difference between the equilibrium and bulk liquid-phase concentrations for the gas.


• The equilibrium conditions are defined by Henry’s law

• The overall mass coefficient kLa is a function of the gas, the process used for gas transfer and the physical parameters, such as temperature and dissolved solids. It is mainly controlled by the liquid-phase resistance.

• Gas transfer processes should be designed to maximize the liquid film mass transfer rate.



Source: Gas purification (Chapter 6- Water as an Absorbent for Gas Impurities) Referex Engineering E-Books.



The advantages of water as an absorbent for gas impurities are its availability and low cost. As such, water can be applicable for large treatment volumes because solvent losses are difficult to avoid in such installations. Water does not require a tight system, and can be used in simple scrubbing units with less concern over leakage and frequently on a once-through basis with the rich solution being discarded. Water may also be applicable to the washing of high-pressure gases where the solubility of an impurity such as CO2 which is only sparingly soluble at low pressure, is brought up to an economically high level by the high CO2 partial pressure. Impurities like CO2 may form acids in aqueous solution; therefore, the prevention of corrosion may become a problem to tackle.



In summary, the advantages of using water over using solvents like monoethanolamine solutions are:

-simple plant design

-no heat load

-Inexpensive solvent

-solvent not reactive with COS, O2 and other possible trace constituents.


Packed Tower Design


This research book does a more on pollution control techniques which require high efficiency and good removal of CO2. It is found that a packed column is commonly used to increase the efficiency of the removal. Parameters such as elevated pressure could help to increase the absorption of CO2 in the process.



It is found that the absorption of carbon dioxide in water has been shown to be almost entirely liquid-film controlled due to the low solubility of CO2. Research on the CO2-H2O system has been conducted to determine the liquid-film resistance to mass transfer when various packings are used. To support this, the work of Cooper et al. (1941) is particularly useful as it employs commercial size packing (2 by 2 by ¼ in. steel raschig rings).



Where:

m is the slope of the equilibrium line, ye/x.

GM is the molar gas velocity, lb moles/ (hr)(sq ft).

LM is the molar liquid velocity, lb moles/ (hr)(sq ft).



This parameter is known as the stripping factor and it represents the ratio of the slope of equilibrium line to the slope of the operating line (normally <1.0 for practical absorption towers and 2.0 for stripping towers). Complete CO2 removal from the gas is effected with excess water.









Note that in a packed tower design, flooding must be considered.



Water scrubbing has been proposed for the removal of CO2 from methane produced by anaerobic digestion. The first pilot unit for this type of process is called the Binax system. Water scrubbing is attractive for this type of application due to its relatively low capital cost in small sizes, simplicity of operation and maintenance, and use of readily available non hazardous absorbent.



Corrosion

Water can become quite acidic when appreciable quantities of carbon dioxide are absorbed. This results in corrosion problems. Due to the operation at near ambient temperature, the low temperature is a favorable factor for corrosion to occur. Oxygen especially accelerates the corrosion of metal by carbon dioxide. Solutions to this problem are to add potassium dichromate to the water, the use of stainless steel in areas of high turbulence, and the application of protective coatings to the interior of the absorber and other vessels. An important thing to note is that water exposed to light may also form algae.

Hydrogen Sulphide removal by absorption in Water


Hydrogen sulphide is appreciably more soluble in water than carbon dioxide. The solubility of hydrogen sulphide in water at moderate pressures has been determined by Wright and Maass (1932). Equilibrium gas and liquid compositions can be calculated from Henry’s law coefficients.
 
 
Experimentally determined vapour-liquid equilibrium data for system hydrogen sulphide-carbon dioxide-methane-water at pressures ranging from atmospheric to 1, 014 psia and temperatures from 85oF to 115oF have been reported by Froning et al. (1964). These authors found that the equilibrium constants K can be represented by the following equations:

Temperature affects both the equilibrium position of the precipitation reaction and the reaction rate. In general, solubility increases with the increasing temperature with a few notable exceptions such as CaCO3, Ca3(PO4)2, CaSO4 and FePO4 which are of importance in the water chemistry. To calculate the solubility at a temperature with the equation in the picture

According to research P = H.x


Where P is the partial pressure of the solute in the gas phase

H is the henry’s constant

And x is the mole fraction of the solute in the liquid phase.



We are having several questions and confusion about these since we do not know the total pressure of the biogas introduced and we assume that the temperature is at room temperature. We are also unable to find the mole fraction of the solute in the liquid phase which hence does not allow us to determine the partial pressure of the solute in the gas phase. Other factors for example, ionic equilibrium, has also jumped across out minds due to the reaction of CO2 and water.

As mentioned in our earlier post, the composition of biogas contains 50-75% of methane. This methane can be harvested to produce heat, generate electricity and fuel vehicles.

In our context, we are thinking of applying this system in the third world countries, the methane can be a cost effective solution for cooking and lighting.

Methane has a high calorific value of 35.8MJ/m3. However, to be able to tap on this potential of the methane, it has to be pure. This means that the other components in the biogas will have to be removed.

This is not to say that there are no negative impacts of methane. Methane is odourless and colorless. It will cause suffocation when over inhaled. It is also explosive and should be stored with care. It is also good to note that methane is not very soluble in water and it is denser than air.


The main components of the biogas that needs to be removed are carbon dioxide an hydrogen sulphide (H2S is poisonous)



We used the Buswell equations and calculated that 33.63m3 of biogas will be produced.


Below is the diagram of methane capture system designed by Dr Han:




To add on, relevant calculations have to be peformed before sizing the design of methane capture system.We were stuck with the calculations for the entire 2 weeks. We went to find lecturers like Dr Lee and Mr Wong YT for help.



Most of our researchs direct towards using Henry's Constant for calculation. However, we just could not get the appropriate mass transfer correlations for the calculations.



In conclusion we hope that these measures be put in place to ensure a system of higher efficiency and effectiveness.


(Slides to be added)


To add on, we have performed one of the QC tools that we have learnt in the module on Quality Assurance and Statistic. In this case, brainstorming is used to identify the possible ways of improving the digester operation.


For instances,
Possible ways to improve the digester operations:


1. pH probe

•  Installation of pH probe
• Addition of lime or sodium bicarbonate
• Decreasing the waste feed to digester

2. Mixing

• Installation of mechanical stirrer
• Gas re-circulation line for mixing

Update: The two proposed ideas were not carried out at last since we could not run our experimental run using the large scale digester. Furthermore, by considering the constraints of the equipment, it may be quite trouble to install the pH probe/stirrer and re-circulation line etc.

As a follow up to the previous post regarding the discussion with Mr Desmond Lim, we have continued to use the factors we have discussed and put in into design.


With aid and guidance from Mr Desmond Lim, we were able to obtain a simplified design. Through this, we have learnt not to take things to near a complicated level and certain principles can be managed practically and less theoretically.


A picture of the design can be seen below:

   
We have discovered that the Moisture Collector Reservoir is for removing any water condensation that might be present in the pipelines.




In addition to this design, we have also modified the design to tune it more towards our purpose of efficiently and effectively capturing methane gas.

 
 
 As seen, the above has demonstrated the various principles that we have discussed in the previous post. Following this, we will be performing the relevant calculations to size the design.

Meet up with Mr Desmond Lim.

We wanted to create a methane gas capture system in order to harvest the methane gas from the biogas produced from the digestion of the waste feed. With the help and guidance that he has given us, we have taken a look into several factors.

In order for us to safely and effectively collect Methane, we needed to research on its properties which is what we have been doing over the last two weeks.



We recall the biogas composition:

Compound Vol %

• Methane 50-75
• Carbon dioxide 25-50
• Nitrogen < 7
• Oxygen < 2
• Hydrogen sulfide < 1
• Ammonia < 1

As observed: the amount of H2S as compared to the Methane produced is insignificant


Properties of Methane:
-Colourless
-Odourless
-Explosive in Nature
 -light gas (less dense than air and CO2) Methane< Air < CO2
 -not very soluble in water

We were given two methods to collect the methane gas. The first, being the scrubber system (inverted cup) and the second method, being the balloon system.

SCRUBBER SYSTEM (using water to scrub of CO2 and H2S)

Advantages:

Obtain methane of higher purity which increases the calorific value.


Disadvantage:
 •For Safety reasons, the scrubber system is not advised.
 •Methane is odourless and colourless. Due to the careless usage, failure to turn off the methane supply can lead to direful consequences
 •Hence H2S is sometimes added in to give the “rotten egg” smell as a pre-warning that methane gas is leaking. Early detection before explosion occurs. (The problem with adding H2S is that it is very toxic and just a small leak could increase the concentration enough to kill)


Essential Piping:
 1) Pilot Line to burner
 - Safety feature to burn off CH4 to become CO2 if pressure in vessel too high.
 - Burn off the Air initially during startup since air provides oxygen for combustion.
 - Only begin tapping when burner can combust (indication that CH4 is mostly present for tapping)

 2) Additional piping to allow digester gas to mix with outlet gas of storage tank to induce H2S into the outlet gas.



Gas valves ONLY



- should be domed shape/ circular/ spherical in shape
- Such that pressure exerted on the container will be uniform due to equal force exerted

Most efficient in storing high Pressure gases.



Weight:
For clear indication on how much the drum has risen due to the increasing volume of methane gas entering. If not at the beginning, drum would have risen uncontrollably, not allowing use to identify how much methane has already entered.

Helps to also immerse the drum totally in water.



BALLON STORAGE SYSTEM

Disadvantage:
Does not scrub off the CO2 from the digester gas.



Properties:
-container made of polyurethane material
-should also have pressure gauge, Pressure relief valve, and pilot line to burner to burn methane off.
-Must have counter weight as a form of safety indication
 -Without weight, container is already bloated when gas enters, unable to see whether material can sustain. If weight is raised to a certain height, we know there is too much pressure and tension of material is about to be reached.