Reaction Kinetics - Science topic

Yes, this is related to the bond energy. You need to dig thermochemical data and do calculations. Just a small hint

The equation of Michaelis-Menten is commonly used to describe the kinetics of enzymatic reactions

Elbatoul Benidris please check your inbox

V_max depends upon and is, ideally, directly proportional to the concentration of the enzyme. Specific activity divides V_max by the amount of protein present (typically in mg, as in µmol/min/mg). Specific activity can be used to describe the level of activity of any enzyme-containing preparation, without requiring that the enzyme be purified.

k_cat divides V_max (in concentration units) by the enzyme concentration to get s^-1. Purified enzyme is normally used for measuring this parameter.

You can express the enzyme activity in whatever way suits your needs, but you should be explicit about how the calculation is made, the source of the enzyme, what protein concentration was used, and the specific activity of the preparation so that others can repeat the work.

If you keep the enzyme concentration constant throughout all experiments, and its activity remains constant throughout, then, for the sake of your calculations, you can leave V_max in the form of moles of product formed per unit of time or, dividing by the volume of the reaction, as concentration of product formed per unit of time.

At the risk of not adding a lot to the conversation and speculating about someone's thought process, an possibly being spectacularly wrong, I will point out the final bit of the Danckwerts quote has two parts: 1) This formulation is misconceived in principle and 2) and in practice the use of concentrations usually leads to somewhat better results that the use of activities. I would suggest that this second part may have been the key take-home point at the time. Given the state of computing, data availability, analytic techniques, etc. one would be better served by using the concentration approach. For part one, I must confess I don't quite follow why the term misconceived was used - the discussion of Dr. Geletii and others seems correct to me. Perhaps though the key is in the previous sentence which states that "the corresponding rate-constant will then be independent of composition" is the problem. That is, the point of concern isn't about the activities per se, but that the rate constant is found to variable (not constant) in that situation. Looking back to the second point, is this really saying that the activities of non-ideal solutions were inadequately being modeled?

There is the problem of steric hindrance in the substrate: 1-butène, which must be considered in the mechanism of the kinetics of epoxidation .

Thank you Sina Safavi

Chemdraw 2D, Chemdraw 3D, MATLAB and paint software are the good selections to perform the PES curve. Best regards.

Actually, the apparent activation energy is that in the presence of the catalyst in this case. The suitable reactor design facilitates the mass transfer inside the reactor and consequently increases the catalyst efficiency instead of having dead zones in the reactor in the case of poor design. The net result is an increase the rate of the chemical reaction

I suggest you adding internal standard to your reactor. That means you can calculate conversion using internal standard method. If it's possible, you can collect your reaction mixture on line (take little sample to analysis) after a cooling sampling line. Some instrument manufacturers could modify equipment to solve this problem, like adding quantitative loop to GC analysis.

check this link. This question has been already asked and answered on the research gate.

You can try learning from here as well. It starts from basics and has used common heat transfer examples

Toyese Oyegoke alrady the expert comments from Dr Prem Baboo has nailed it and wow loved his answers and the explanation as well thankyou Prem Baboo and being from the same industry it is always good to find like minded people

Dear Toyese Oyegoke,

I have attached some books about Surface Science and Catalysis. I hope they would be useful.

I expected relevant data to be available in the literature. However, I am thinking about the equilibrium model. The lack of equilibrium constants is not a problem when using Aspen. But treating the oxidation of carbon and carbon monoxide as reversible is unacceptable to me. It is also not understood to analyze a linearly dependent system of chemical reactions. I also have doubts about the second publication, where only the reaction rate constants are given. Were all the reactions mentioned in the publication first-order?

The use of the Gibbs reactor model in Aspen Plus/HYSYS does not require much data for it to predict the product based on the thermodynamic condition of the process. You are not expected to provide a reaction set instead, you are to provide all possible compounds that is expected to participate in the process. The simulation of the model is expected to predict the distribution of the product based on thermodynamic conditions provided in the modeled process.

Check it out Pavlos Kostopoulos

Thanks for the short but encouraging answers.

I would appreciate a method suggestion such as polarization, potentistat charging, noise or fast fourier analysis for the monitoring of fast reactions. Some methods will miss what's happening on the electrode due to long measurements

You can follow the decrease in their wavelengths unless they interfere with each other.

Dear Shaheen, based on the example that you have mentioned, HYSYS only do the calculations establish in a energy a material balance. Also, you have to write down the stoichiometry of the reaction. Then HYSYS perform the mass and energy balance in the same way that it has described in Elements of Chemical Reaction Engineering 6th Ed - H Scott Fogler or Basic Principles and Calculations in Chemical Engineering (8th Edition) for example. All calculations performed by HYSYS are of public domain and you can find also in the Perry's Chemical Engineers' Handbook, 9th Edition.

Wagner model is a rheological model developed for the prediction of the visco-elastic properties of polymers

Dear Li Deng, this question is addressed in the article shown below. The paper is available on RG.

Wave impedance: At a point in an electromagnetic wave, the ratio of the electric field strength to the magnetic field strength. (188) Note 1: If the electric field strength is expressed in volts per meter and the magnetic field strength is expressed in ampere-turns per meter, the wave impedance will have the units of ohms. The wave impedance, Z , of an electromagnetic wave is given by

📷

where is the magnetic permeability and is the electric permitivity. For free space, these values are 4 × 10-7 H/m (henries per meter) and (1/36) F/m (farads per meter), from which 120, i.e., 377, ohms is obtained. In dielectric materials, the wave impedance is 377/n, where n is the refractive index. Note 2: Although the ratio is called the wave impedance, it is also the impedance of the free space or the material medium.

In the following website you find and example and solution to calculate wave impedance.

Dear Pouyan

In my experience of testing Calcium thin films, the oxygen and water are predominant factor to react with Calcium. According to literatures, Calcium can only react with nitrogen to become calcium nitride under 200 to 450 degree C. Controlling the oxygen and water levels inside the glovebox is critical to fabricate calcium present devices. When we made devices with a calcium layer involved or pure calcium layer, the degradation can start after two hours. Therefore our devices had to be encapsulated within 1 hour. The OLED devices I can kept for 5 years were still lit up. They had hardly any black spots. The most importance is to control oxygen and water levels inside the glovebox. You mentioned that you assume the glovebox is less than 1000ppm. This figure is far far too high for any calcium layer presented devices. Our glovebox was kept both oxygen and water levels less than 0.1ppm.

Hope it helps.

Best wishes

Xuhua

You have duplicated your question. Delete this one.

Commonly, the protonation step (k-) is a diffusion controlled step. You can safely take k(-) = 1e+10 M-1s-1. Then, k can be calculated from pKa.

MnO2 accelerates the decomposition of H2O2 (catalyses). Could you use this principle? For example at a certain time collect the sample and then add MnO2 to get rid of H2O2 quickly and then look for concentration of original pollutants remaining? You may get a pseudo rate constant for the pollutant decomposition using this method of collecting kinetic data

@ Muhammad Abdullah Arslan

just from the units you gave it is not possible since there is no way to convert kg in m² without any additional assumptions.

All elementary bimolecular reactions include diffusion of reactants to form a collision complex. The kinetics strongly depends on concentrations, pH, ionic strength, etc

Hello, …… before getting the kinetic data of a reaction, think to simulate the reaction to get these data. verify your results based on the output of the reaction, eg. reaction temperature, viscosity, ….etc.

If you are going to build kinetic models you must learn how to dig the necessary data. You are supposed to know that the Nobel Price was awarded for studying the rates of acid base reactions.

The following articles introduce the most correct manner for calculation of thermodynamic parameters and contain valuable explanations about this subject.

Journal of Molecular Liquids Volume 273, January 2019, Pages 425-434; Journal of Molecular Liquids Volume 280, 15 April 2019, Pages 298-300

The C-centered radical will quickly react with O2 forming peroxy-radical, which in its turn should react with ABTS. I would suggest you to purge your solution by O2.

TEMED pKa: pKa1 5.85; pKa2 8.97. pH should >9 to have deprotonated TEMED

You can use the Arrhenius law in logarithm form to get the forward activation energy and forward rate frequency using only the ;lower temperature section of data.

Ln (k₁) = Ln (A₁) - E₁/RT

The same equation can be used with the highest temperature section of data to get the reverse activation energy and backward frequency.

For exotherm the reverse activation energy should be higher than the forward activation energy.

Ln (k₂) = Ln (A₂) - E₂/RT

There are many other equations that try to get all the data in one equation, but the results are about the same.

Hi.

As a general rule, increasing the temperature will increase the reaction rate (for exothermic and endothermic) reactions simply because it means more energy available in the system. However, if you have a reversible exothermic reaction, there's a range of temperatures where this might not be true.

Picture the reaction A+B⇌C+D (- ΔH)

The reaction rate (relativelly to A, for example) can be defined as: -rA= K1[A]^a[B]^b - K2[C]^c[D]^D

Both K1 and K2 values depend on the temperature (and the induvidual activation energies for the reactions). So, if for some temperature and concentration ranges, the term K2[C]^c[D]^D is bigger than K1[A]^a[B]^b, the reaction can even occur from the products to the reactants. The difference between the terms is an indication of the direction and rate.

A well known example is the reaction between hydrogen and nitrogen to make ammonia. Very slow at room temperature, increasing rate with rising temperature up to the point where the balance at equilibrium shifts back towards reactants.

So, study well the kinetics of your reaction and you be the judge of the possibility.

Hope this helps!

Thank you Dr. Peter Farrington and Dr. Md. Abdur Rahman for your useful suggestions.

Having the rate equation with parameter values, it seems that you want to describe the process course mathematically. At the beginning, you can try to use the shrinking core model. However, the degree of complexity will depend on the assumptions made. Regards,

Hi Enmanuel Duarte, the ODE solver will return a solution set of data points from the universe of discourse. So I think, at most, you can only get the 3-D line plot, not the surface, unless you want to obtain the slope field.

Why not use a pH-meter with pH stat capability and connect that to some dosing pump?

Hi, Chaker,

Sorry, I cannot remember which paper it refers to. I probably renamed the file already when I downloaded it and it is not possible to trace it back.

Cheers, Bin

This Link might help

The preferred industrial method for its manufacture is by the carbonylation of methanol and this accounts for approximately 60 per cent of the total world acetic acid manufacturing capacity. The carbonylation of methanol, catalysed by rhodium, was invented by Monsanto in the 1960s and for 25 years was the leading technology. In 1996 a new, more efficient, process for the carbonylation of methanol was announced by BP Chemicals, this time using an iridium catalyst. This article describes the new process and looks at the ways in which it improves upon the prior technology..

The apparatus of the present invention is used in connection with the carbonylation of methanol, and/or its reactive derivatives, with carbon monoxide in a homogeneous catalytic reaction system comprising a reaction solvent (typicallyacetic acid), methanol and/or its reactive derivatives, a soluble rhodium catalyst .

A process as provided for producing an acetic acid process stream having less than 400 ppm propionic acid and less than 1500 ppm water. Methanol or a reactive derivative thereof and carbon monoxide is fed to a carbonylation reactor in which there is maintained during the course of the process a liquid reaction composition containing an iridium carbonylation catalyst, methyl iodide co-catalyst, a promoter, water at a concentration of less than about 8% by weight, methyl acetate, acetic acid, and propionic acid by-product and its precursors. Liquid reaction composition is withdrawn from the carbonylation reactor and introduced to a flash zone to form a vapor fraction comprising water, acetic acid product, propionic acid by-product, methyl acetate, methyl iodide and propionic acid precursors, and a liquid fraction comprising involatile iridium catalyst, involatile optional promoter or promoters, acetic acid and water. The liquid fraction is recycled from the flash zone to the carbonylation reactor and introduced into a first distillation zone where a light ends recycle stream is removed and recycled to the carbonylation reactor, A process stream is removed from the first distillation zone comprising acetic acid, propionic acid by-product and less than 1500 ppm water. If the process stream comprises greater than 400 ppm propionic acid, the process stream is introduced into a second distillation zone where propionic acid by-product is removed together with an acetic acid process stream containing less than 400 ppm propionic acid and less than 1500 ppm water.

In order to measure the kinetics of one enzyme in a sequence of enzyme reactions, there are 2 necessary conditions I can think of.

(1) The reaction of the enzyme of interest has to be rate limiting (i.e. the slowest step). This can be achieved by keeping the concentrations of all the other enzymes very high. The test for this condition having been successfully achieved is that further increasing the concentrations of the other enzymes does not increase the rate of the E3 reaction.

(2) You have to know the concentration of the substrate of E3. This is challenging, but the way to do it is to run the reactions of E1 and E2 first with a known concentration of the substrate of E1, such that all of the E1 substrate is converted to the E3 substrate prior to the addition of E3. (This requires that the E1 and E2 reactions are essentially irreversible, otherwise the reaction will not go to completion.) Thus, the concentration of E3 substrate is equal to the input concentration of E1 substrate. I don't think you can run all the reaction concurrently and still be able to measure the kinetic constants for E3 because the concentration of the E3 substrate would be unknown. So substance C has to be stable for the amount of time needed for its formation to be completed.

You also have to measure the initial rate of E3 substrate consumption or product formation, as you would for any kinetics study. It isn't clear to me whether it is necessary for the D to E conversion step to be present in your scheme. If so, then the concentration of E4 (you labeled it E1) must also be high enough that the E3 reaction is rate limiting. You would then have to measure the rate of substrate C consumption or the rate of product E formation.

The rates of individual reactions (forward and backward) are of course different (omitting state of equlibrium). It should remain unchanged the form of a kinetic equation, but one must remember to change the values of reaction rate constants (one on the other) or about changing the rate constant and equilibrium constant values. The problem may also occur if in the kinetic equation there will be pressure activities instead of partial pressures.

Yes, the above kinetic equation does not make sense unless we know the value of equlibrium partial pressure of benzene and the value of the constant K_B has been adapted to this situation. Regards,

Hi Enrique!

Perhaps you can use a Calculator Block. This tool of Aspen Plus will allow you to do a program with Import Variables (such as pH) and Export Variables (such as k) and to link them with different units operations, for example, a RadFrac Column.

Teperatue rise depends on feed quality but typical conditions are 360 C inlet and 380 C outlet

Dear Shahid,

By putting b to zero, you simply degenerate the modified Arrhenius to Arrhenius equation. The temperature dependency of the exponential factor is to give a better fit to an experimental observation. In that case it can be determined by fitting the equation to the experimental data.

IR. Peaks around 2300-2400 cm-1 can't be confused with much else. not the best technique for aqueous solution though, you;d have to isolate and dry. Raman better for aqueous solution, A1 totally symmetric BH4 str breathing mode at 2310 cm-1. such peaks provide positive fully diagnostic evidence.

Are you really interested in answering the question, because I have doubts. However, if I am wrong, answer the following questions please. What is a dependent variable? Is there an inflection point on the kinetic curve? Is the equation you use for the kinetic curve approximation to be y = exp (-atⁿ⁺¹) or y = exp (-atⁿ)? What is the value of the coefficient a? Regards,

Where it is not known what the reaction is, under what conditions it occures, no relible answer is possible. The rate of chemical reaction can decrease with time, it can also increase and then decrease (autocatalitic reaction). There is also no the relationship between the order of chemical reaction and the behavior observed - no conversion changes. And I will repeat, I do not know. which once - the term "reaction order" refers to a single chemical reaction.

Or maybe the answer is the most trivial, that is, total consumption of the substrate. Regards,

Could the discussion be over?

The reaction (process) order is primarily the result of an experiment - correct experiment, but also a mathematical description. The situation you write is possible. This depends on the range of the change in initial substrate concentration. This type of behavior can also be described using hyperbolic functions, eg well-known equations as M-M or Monod (the same mathematically). Then for low concentrations the reaction may be considered as first order and for large as zero order. As low and large, the value of K_m (K_s) is decisive . However, in the whole range of initial concentration change, the reaction (process) has no order (the definition of the reaction order), which is worth remembering. Regards,

You have so high concentration of aniline that you possibly see the CV of impurities in aniline. CV is usually taken at concentrations around 1 mM

Hi Mariam, How are you doing? Sorry I am busy these days working on some projects with immediate deadline. I went through the paper you attached to your previous message. There are some inconsistencies in that model

- first you should make sure if the enthalpy change of the reaction is in kJ/ mol O2 or kJ/mol Co3O4. Because kJ/mol O2=2 times kJ/mol Co3O4.

-second, in the kinetic model the effect of oxygen pressure is not given, which make the model wrong and may result in strange result.

To correct that calculate the equilibrium pressure of Oxygen as Peq=P0*exp(A-dH/(Rg*T)) where P0 is 1bar, Rg=8.314  dH the entalpy change of the reaction.

you can estimate A by knowing that Peq=1bar at 1164 K.

Then the reaction kinetics will be Rco3o4=Rred*X*(Peq-P)/Peq , Rred is given in your Word document. you should drop Rox because is significant low (tends to zero), P is the exit pressure of O2 and should be less than Peq. in simulation you can choose value less than 1 bar. the initial condition is X=1 at t=0.

Then your source term Sh=dH(in kJ/mol Co3O4)*Rco3o4*rho Co3O4/MW Co3O4 

with  the unit of W/m3 to be consistent with the energy balance.

where rho Co3O4 is the bulk density of Co3O4 and MW Co3O4 is the molecular weight of Co3O4.

With all these suggestions in mind, your simulation will run perfectly. I hope it will help. Good work

If fluorescence intensity linearly depends on the conversion degree of reaction, then this is possible. However, you must remember that in the case of the first order reaction with the kinetic equation: r = kc_A, the degree of conversion does not depend on the initial concentration of the substrate A, but in the case of the second order reaction with the kinetic equations: r = kc_A²,r r = kc_Ac_B etc. depends. You do not know, however, before the experiment, what the reaction order is, and therefore you need to perform the experiment with several initial substrate concentrations.

Of course, there will be no problem linking the intensity of fluorescence to the conversion degree for reaction A -> B. The value of the reaction rate constant will be different from that based on concentration measurement. Difficulties may occur, however, with bimolecular reaction and then better measure the concentration.

And finally, I intetionally used the term reaction order instead of kinetic order because this is correct.

Regards,

You have written it as a reversible reaction implying that the forward rate slows down as the reaction approaches equilibrium. You didn't give enough data to compute the equilibrium constant. Zero order in B is not satisfactory for equal number of moles in a reversible reaction. Something is wrong in the statement.

Example as written a deficiency of B would not prevent A from becoming C. Other example a great excess of B would tend to make it zero order in B. Third example C might regenerate B spontaneously in which the reaction is written incompletely.

In your writing about enzymes they are catalysts which favors my third example where B is the enzyme a catalyst and hence a zero order participant.

A + B <--> C --> D + B

Hi there,

first order rate means velocity is directly proportional to reactant concentration:

v=k[A][B] and v may be expressed as v=-d[A]/dt. If considering the case where A and B are identical

v=-d[A]/dt=k[A]² which leads to 1/[A] - 1/[A]₀ = kt

in your case if t=1h, [A]=1/4 and [A]₀=1, you can calculate k (k will be 3 h^-1) and then knowing k apply the same equation at t=2h to get residual [A] (it will be 1/7)

If zero order, v is independent on reactant concentration v=-dA/dt and v=k so

[A]-[A]₀=-kt. It is only observed for enzyme catalyzed reaction at saturating concentration of substrates and v depends only on enzyme concentration. So in your case, if A is 1/4 after 1h then k is 3/4 h^-1.  So having consumed 3/4 of A in 1 hour and having v=k on the second hour it would mean that A would be fully consumed within this second hour (from the calculation A would be negative after 2h which can't occur...). In practicals zero order reaction couldn't be maintained during the whole reaction as at some point A and B would become limiting.

Kind of controversial topic

if you are following proper method and  suitable wavelength, your absorbance is always proportional to your concentration.you should prepare lower concentration standards like 10ppm,20 ppm,30 ppm,40ppm and 50 ppm, dilute your sample as per required, keep your uv-vis absorbance< 1.0 AU.

If you study equilibrium of sorption (liquid-solid system) you are looking for the relationship between q_e and C_e. If you study the kinetics, you determine experimentally the relationship between q and t (usually denoted in short qt) and then (by modeling) between r (rate) and q. The unit of variables in the model (and associated with them  values) depend solely on you. For C_e can be both g/L and g/m³. What will change - only the value of model parameters and their units.

Regards,

Dear Keshav,

For pure bimolecular reactions, transition state theory (TST) would probably be a better approach. RRKM theory only applies to unimolecular reactions. If the reaction mechanism involves a bimolecular collision activation step followed by a unimolecular rate-determining step, RRKM theory can apply. But it doesn't seem to apply to the case described here.

Hope this helps.

Dear Elizabeth,

Have you found the pre exponential factor?

If yes could you please share it with me?

You may also check the molecular weights of the species involved (in the database), as Aspen databank components data are sometimes slightly different from the data you have from literature. Then manually check the balance using the values you retrieved from the database.

Do you have recycle streams in your process? Sometimes when you have recycle you need to ask to reconcile the streams.

Another thing to do is to increase the acceptable range of error. (Simulation – Setup- Calculation Options- Check results). 

Thanks, Michael. For a long time I'm wondering what reaction rate constants should be assigned for the equilibrium H⁺ + HO^- = H₂O? This question might be worthy to discuss separately.  

Our article just accepted yesterday for publication in PCCP  may help you. The article is titled "Kinetic Analysis of Overlapping Multistep Thermal Decomposition Comprising Exothermic and Endothermic Processes: Thermolysis of Ammonium Dinitramide".

DOI: 10.1039/C6CP08218A 

For the case of perfectly overlapping exothermic and endothermic processes, you can see the following our article in JPC C.

DOI: 10.1021/acs.jpca.5b07044

Similar kinetic treatment is required for the case of overlapping mass loss and mass gain process when tracking of kintiec rate data using TG. The example  for the successive thermal decomposition and oxidation of tin(II) oxyhydorxide has been published in JPC C.

DOI: 10.1021/acs.jpcc.5b04975

I am very glad if this information is useful for you.

It is difficult to answer without any explanation of the columns in your Excel file, or of how is your symbols are related to these data (i.e. which columns are realed to Qr or Qrx, what do these symbols stand for etc.), and what is exactly that you want to integrate or to plot. If you'll provide this information the probability of getting useful answers will be increased.

Also tell, do you really want to make the plots in Excel, or in some other environment.

"The choice of a suitable reactor for the chemical process depends on: the type of the reaction/reactions (rate, yield selectivity), type of substrates, type of products, the thermal effects, the ways  of heat exchange, production volume, catalyst used, accompaniing effects as phase transition, inter and intra particle diffusion, inter and intraparticle heat transfer etc etc, etc."

This phrase can be found in any book on chemical reactor engineering.

You wrote that you know (you know, not me) heat of reaction, estimated cooling capacity and, I guess, the kinetic equation.

It's not enough to propose the correct solution.

Dear Raghavandra,  If I understand your question correctly you have used the Mettler RC1 to accurately record temperature rise as a function of time and calculate the amount of heat required for that temperature rise to occur and this does not match the calculated heat of reaction (Qrx) from literature similar to the source Prof Khalil sited.  The first thing to do is recheck the literature to make sure you understand the heat of reaction correctly.  Also you should make sure that you are looking at both the literature and experimental values properly and that you are accounting for any vaporization that may also be occurring as the process temperature rises.  The impact of heat of vaporization could potentially be large.  I hope this helps.

Here is a paper that explains reduction of Iron via partially oxidized methane the CO and H2 formed are the actual reducing agent.   Exxon had a large project to demonstrate this technology called the FeOR project back in the late 70's I was not directly involved but I knew several people who were. It was a technical success but an economic failure as I recall

Dear

Jerry Decker, Narjes

thank you for your time.

Dear Michael, I agree with all you posts. 

You are right that 

"VO + 1/2 O2 ↔ VO2 and K=(pO2)-1/2 as the standard activities of pure solids are 1 (and unitless)"

"use ΔRGϴ= - RT ln Kϴ".

My comment that " The equilibrium constants are not obligatory unitless" is also correct

Search for Benedict Ita, University of Calabar, Calabar · Department of Pure and Applied Chemistry. He is a professor of physical and electrochemistry.

I can't  open the article so I don't knw what eqns 15,11 and 17 are but you could try to model the two equilibrium reactions (11 and 17) as kinetic reactions with very high forward and reverse reaction rates at least at first to get the model going.  You could even do it in an Excel spreadsheet firs to get a better insight into what could be going wrong.

I suspect you probably have a set up problem in a way that doesn't allow kinetic and equilibrium reactions to be combined.   I have seen that problem in Hysys but I don'thave any recent experience with Aspen plus