Coked Catalyst Regeneration Project

A Project Report for ChE 2013

Submitted to the Faculty of the

Department of Chemical Engineering

Worcester Polytechnic Institute

Worcester, MA 01609

March 4, 2016

Project Team: Absolute Zero

“The First Law of Thermodynamics is you do NOT talk about thermodynamics.”

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M. Bodanza

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M. Burkardt

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M. Lundgren

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R. Whalen

Summary

The process of cracking hydrocarbons over a catalytic reactor bed results in carbon deposits

on the catalyst. To preserve and restore the catalyst, the carbon can be removed by some reactive

process that forms methane from the carbon and some other compound. Our team is investigating

three different methods for the catalyst preservation process.

The first proposed method consists of a single pass conversion of hydrogen and carbon to

produce methane. Similar to method one, method two reacts hydrogen gas with the carbon buildup.

However, this time, the methane produced is separated through cryogenesis, and the remainder of

the flow is recycled through the reactor to achieve a higher conversion. As opposed to the first two

methods, the third method utilizes steam to extract carbon from the reactor, which is significantly

more cost effective. Uncontaminated hydrogen gas has an exceedingly high price especially

compared to readily available water vapor. Unfortunately, using steam increases the complexity

of the reaction, adding the necessity to account for additional reactions and other byproducts

occurring inside of the reactor. A thorough analysis of these methods is provided in this report.

Our chief objective is to minimize the shut down time.

Methodology

Part 1: Hydrogenation of Carbon for Catalyst Regeneration (No Recycle)

Preliminary analysis of this project requires us to confirm or deny our colleague’s estimate

of whether a nine hour run cycle is sufficient to fully remove the “coked” carbon from the catalyst.

To do this, the methane conversion (extent) is found through a series of Mathcad calculations.

Starting with the initial feed rate and mass of carbon, equilibrium composition expressions are

derived (A.1.1), and are related to the equilibrium constant through the definition of activity in the

gas phase (A.1.2). As the pressure remains low, the gases can be assumed to be ideal gases,

simplifying many of the calculations. Tabulated data1 is used to find the heat capacity coefficients,

enthalpy and Gibbs free energy of formation for carbon, hydrogen, and methane. From this data,

an equilibrium constant (Keq) can be derived for the reaction. Combining this value with the

equilibrium expression, the extent is found. The amount of time (hours) it takes to remove 95% of

the carbon is determined by dividing the amount of methane converted by the extent of the

reaction.

Part 2: Hydrogenation of Carbon for Catalyst Regeneration (With Recycle) (see Figure 1)

The process for finding Keq and subsequently the extent is identical to the calculations in

Part 1 (A.1). However, instead of just pure hydrogen, the combined feed of distillate and the fresh

feed must be considered as the initial composition will change equilibrium conditions. The known

compositions of Stream 2 are used to calculate the new extent and Keq. Simultaneously, a mass

balance is done around the cryogenic separator (A.2.1) to determine the composition of Stream 5.

It is assumed Raoult’s Law holds for this low pressure. With the new extent, and compositions of

Steam 2 and 5, a series of mass balance calculations (A.2.2) are used to determine all other flow

rates and compositions for the system. All material balances are done under the assumption all

chemical reactions occur within the reactor (Stream 2 to Stream 3) and not in the streams.

Figure 1: Process flow diagram for hydrogenation of carbon for catalyst regeneration (with recycle)

Part 3: Hydration of Carbon for Catalyst Regeneration

To properly analyze use of steam in the hydration of carbon as a substitute for hydrogen, an effluent

gas computation must be done to find concentrations of the feed stream (A.3.2). The strategic

method for finding the independent reactions requires forming each compound present at

equilibrium from elemental components and combining equations using Hess’s law until only

those compounds remain present at equilibrium. Three independent reactions are found, thus three

extents are necessary to evaluate the three equilibrium expressions. The extents are found (A.3)

using a similar process to that of Part 1. The three reactions are as follows:

2CO = C + CO2 (1)

CO + H2 = H2O + C (2)

C + 2H2 = CH4 (3)

To make a recommendation about the change from a hydrogen feed at 1000 K and 1.1 bar

to steam at 1000 K and 1.1 bar, a new time-frame comparison is calculated. Using calculated

extents, the amount of time necessary to completely remove the coked carbon from the catalyst is

calculated (A.3.3).

Results and Discussion

Part 1: Hydrogenation of Carbon for Catalyst Regeneration (No Recycle)

The results of Part 1 conclude it would take roughly 10.6 hours to remove 95% of the

carbon at 1000 K and 1.1 bar. We concluded that our colleagues’ estimate was far too ambitious,

differing by nearly 20% from our estimate. To fully remove the remaining 5% of carbon from the

catalyst it would take ~5% longer (an additional 0.56 hours) than the calculation for 95% removal.

The following table summarizes important calculated values for the system.

Keq 0.092

ξ (x) 78.266 lbmol/hr

yCH4 0.085

yH2 0.915

na 875 lbmol

Time 10.621 hr

Table 1: This table summarizes important calculated values for Part 1.

Part 2: Hydrogenation of Carbon for Catalyst Regeneration (With Recycle)

In Part Two, flow rates are determined for the newly proposed process, using a recycle

stream to recover the hydrogen from the reactor. This process proved to be extremely efficient due

to the flow rate of the recycle contributing to the feed of the reactor; thereby reducing the amount

of fresh feed required. For this process to be considered, a cryogenic methane recovery unit is

required to separate the methane before the stream is recycled. This ensures the methane is not all

included in the recycle, which would induce a Le Châtelier shift in favor of the reactants (hydrogen

and carbon). This report does not include a cost analysis of the recovery unit or the process;

however, it does acknowledge there would also be an initial cost associated with construction and

maintenance of the cryogenic methane recovery unity. It would be of great worth performing a

cost-analysis before decisions to the company are made to see if the capital saved in utilizing a

recycle for the hydrogen gas offsets the initial cost.

Part 3: Hydration of Carbon for Catalyst Regeneration

In Part Three, it is proposed to utilize steam (water vapor) in place of hydrogen gas as

another method to cut costs. This process, while it adds complexity to the reaction, is found to be

a time-effective method for cleaning. The total extent of reaction when using water and carbon is

found to be nearly nine times that of carbon and hydrogen. Furthermore, the process of reacting

water and carbon produces more hydrogen as a byproduct which the carbon can react with as well.

The process is found to be significantly faster than that of the hydrogen gas, requiring nearly 88

% less time (9 hour difference) to remove all of the carbon present. A downside is on top of

methane, both carbon monoxide and carbon dioxide are produced requiring more methods of

separation for the product stream. Our team believes the process is significantly more beneficial

than the hydrogen gas alone, if distillation column construction and cleaning cost are not

considered, because of the speed and the relatively cheap, comparative, price of water vapor in

contrast to hydrogen gas. The following table summarizes important results obtained for Part

Three.

Keq1 0.574

Keq2 0.404

Keq3 0.092

ξ1 (x1) 135.252 lbmol/hr

ξ2 (x2) -874.005 lbmol/hr

ξ3 (x3) 37.972 lbmol/hr

Time 1.249 hr

Table 2: The following table summarizes the results of Part 3.

Conclusions and Recommendations

Our team advises that to maintain the hydrogen gas cleaning method, a recycle stream

should be added to capture and recycle unreacted hydrogen gas, as the gas is exceedingly

expensive. Unfortunately, some hydrogen will be lost in the liquid methane stream, and while this

loss is assumed to be very small and insignificant, it may be worth investigating.

Using hydrogen gas is a more expensive process and requires a longer time to generate an

effluent gas stream that removes the carbon. In contrast, steam is relatively inexpensive and reacts

at a significantly faster rate. However, a recycle stream in the water-vapor process is not

recommended due to the variety of products. A recycle stream would require a separator to extract

the carbon dioxide, carbon monoxide, and methane which would increase the cost of the process,

and recycling the products would upset the equilibrium and slow down the process. A faster

process with a cheap feed of steam is more beneficial than adding a separator to reuse gases that

would slow down the process.

Thus, it is recommended that a cost-analysis be performed prior to any significant change

to the company’s process. First, the amount of capital saved using steam rather than hydrogen

should be calculated. Once this is determined the company should perform cost-analysis for the

cryogenic methane recovery unity.

References [1] Dahm, Kevin D., Visco, Jr., Donald P. Fundamentals of Chemical Engineering

Thermodynamics. 1st Editon. Stamford, CT: Cengage Learning, 2015. Print

Appendices

Part 1 Calculations

Part 2 Calculations

Part 3 Calculations