A study on acetylene reactor performance with partial catalyst replacement Table of contents 1. Abstract. 2. Introduction 3. Acetylene hydrogenation technique. 4. Hydrogenation in front end acetylene Reactor.. 5. Front End Acetylene Reactor system. 6. Acetylene reactor operation Philosophy 7. A UNITED Approach 8. Acetylene Reactor Performance Study. 9. cost benefit analysis. 10. Conclusion 9. References. 10. Author biographies. 11. Co-Authors biographies.
Transcript
A study on acetylene reactor performance with partial catalyst replacement
Table of contents
1. Abstract.
2. Introduction
3. Acetylene hydrogenation technique.
4. Hydrogenation in front end acetylene Reactor..
5. Front End Acetylene Reactor system.
6. Acetylene reactor operation Philosophy
7. A UNITED Approach
8. Acetylene Reactor Performance Study.
9. cost benefit analysis.
10. Conclusion
9. References.
10. Author biographies.
11. Co-Authors biographies.
A study on acetylene reactor performance with partial catalyst replacement
1. ABSTRACT:
The effective removal of acetylene in the front end hydrogenation using palladium based chevron Philips E-series catalyst is a challenge to ethylene producer. There are two key parameters to be considered for study of acetylene convertor performance of an olefin cracker: select the right acetylene hydrogenation catalyst and maintain stable operation of reactor. Though, poison can influence the successful operation of catalyst. Since some operational upsets will inevitably occur. The convertor catalyst utilized must be robust enough to manage these changes without degradation of the catalyst performance and unit profitability. United ethane cracker has two acetylene reactor beds in series with intercooler to control the bed inlet temperature. If, the inter cooler has adequate capacity to control the second reactor inlet temperature. Moreover, carbon monoxide (CO) generation from furnaces is optimized. Then, new operation philosophy which is adopted to maximize the acetylene conversion in the top bed and very minimal acetylene conversion is to be maintained in the bottom bed to meet the acetylene specification in ethylene product. It will provide the option to change the partial catalyst after 5-yrs of catalyst life cycle. Therefore, a provision was made to dispose off only the top bed catalyst and re-use bottom bed catalyst in the top bed. A fresh E-series catalyst is to be used in the bottom bed. The above mentioned option has been practiced in UNITED ethane cracker and successfully demonstrated acetylene reactor performance as accepted per study. This paper will present the actual acetylene convertor performance with this option over the entire life of E-series catalyst. It will provide the two fold benefits; firstly it will directly save one bed catalyst cost. Consequently reduce the production cost per ton of ethylene. Secondly it will provide the confidence of longer life cycle of Chevron Philips E-series catalyst with this operation philosophy.
2. INTRODUCTION:
Jubail United Petrochemical Company (UNITED) an affiliates of SABIC has ethane cracker, was designed to produce 1350 KTA of ethylene along with C3+ & tail gas as by products. KBR is the process licensor who has deigned to process up to 142
MMSCFD of ethane of 95% purity based on 8000 hrs to produce 4050Ton/day of ethylene.
Pyrolysis section include eight 200KTA SC-I Furnaces, are designed to thermally crack ethane into ethylene in seven (7) furnaces, while eighth furnace will be in different mode of operation, i.e. hot steam standby, decoke or maintenance. The crack gas from furnace is quenched successively in primary and secondary quench exchangers, preheating furnace feed. Finally in water quench tower where Dilution steam & heavy boiling hydrocarbon are condensed and separated. The cracked gas then compressed, treated for acid removal, dried and sent forward for acetylene conversion and product separation.
The separation system starts with low pressure de-ethanizer that removes the C3 and heavier cut from crack gas. The de-ethanizer overhead gas flow to highly selective front end acetylene reactor integrated within the de-ethanizer reflux loop. The reactor effluent flows to cold box and de-methanizer, where a hydrogen rich tail gas is separated. The remaining mixed C2s enter a heat pumped, low pressure C2-spliter that separate the ethylene product from ethane, which is recycled back to furnace. A process block diagram is shown in figure-1.
3. ACETYLENE HYDROGENATION TECHNIQUE:
In front end selective catalyst hydrogenation reactors, the acetylene reactor precede the de-methanizer in the process. As a result, these reactor feed contained a large amount of hydrogen typically in the range of 10~35 mole percent. In a front end de-ethanizer design, the de-ethanizer is the first distillation column and the reactors are on the overhead stream of de-ethanizer column. Crack gas composition as a function of reactor location is shown in table-1.
H2 +C2H2 = C2H4 (In presence of catalyst)
(Table-1.Crack gas composition as a function of reactor location)
Sr.No Crack gas components unit Value
1. Hydrogen Mol% 26.32
2. Carbon monoxide Mol% .02
3. Acetylene ppm 2500
4. Methane Mol% 8.53
5. Ethylene Mol% 41.72
6. Ethane Mol% 23
7. Methyl Acetylene/Propadiene Mol% .0012
9. Propylene Mol% 0.2
10. Propane Mol% 0.17
11 Total Mol% 100%
Process Parameter
Pressure barg 27.5
Temperature Deg.C 65~75
4. HYDROGENATION IN FRONT- END ACETYLENE REACTOR:
UNITED had loaded the first charge in August’2004 of Chevron Philips E-series catalyst, which is a palladium based on alumina with promoter. Chevron Philips E-series catalyst is proven to be the most effective removal of acetylene via front end selective hydrogenation using supported palladium based catalyst.
It is accepted that reactants like acetylene first absorb on the palladium metal site on the catalyst. The absorption process activates the reactants. Which subsequently react
with hydrogen to form the hydrocarbon product? The activity of catalyst for a particular reactant is controlled by the availability of palladium site, and selectivity depends on the preferential absorption of the reactant. Acetylene will be absorbed more strongly on palladium than is ethylene. Even, though the intrinsic rate of hydrogenation of ethylene is two time of magnitude faster than that of acetylene. As long as there are sufficient acetylene molecules are available to cover all palladium site, then only acetylene will be hydrogenated. Thus hydrogenation product will be mainly ethylene. As soon as ethylene is formed, it desorbs from reaction site and will be replaced by another acetylene molecule.
It is known from E-series catalyst literature that relative strength of adsorption on palladium as reflected by heat of absorption is as listed below.
CO> Acetylene>>Conjugated di olefins>=Alkyl acetylene>di-olefins>>olefins
Carbon mono oxide at low concentrations is a reaction modifier in the front end acetylene convertors. Both CO and acetylene will be absorbed on reaction sites. When the carbon monoxide concentration in the feed reaches to minimum level, it will prevent the absorption ethylene on reaction site. Even the acetylene concentration is reach reduced to a very low level in the course of the hydrogenation reaction. Alternatively, CO competes with acetylene for the reaction site, thus reducing the activity of the catalyst.
The absorption mechanism also explains the tempering effect of methyl acetylene and Propadiene (MAPD) in the front end de-propanizer acetylene convertor. MAPD will replace the acetylene at the reaction site as the acetylene hydrogenated. As long as there is methyl acetylene (or Propadiene) remaining, the hydrogen of ethylene is suppressed.
5. FRONT END ACETYLENE REACTOR SYSTEM:
The compressed vapor from crack gas compressor (CGC) fifth stage is preheated in Acetylene Reactor feed/effluent exchanger and then further heated up by LP steam heated acetylene reactor feed heater. The acetylene reactor consists of two reactor beds in series with a cooling water exchanger between the two beds. Reaction takes place in the cracked gas stream which contains an excess of hydrogen needed for the hydrogenation of acetylene. The high partial pressures of hydrogen permit using a
relatively low operating temperature, thus improving the selectivity of the catalyst for acetylene hydrogenation to ethylene. About 98 percent of the hydrogenation reaction occurs in the top bed. The final acetylene removal will be in the lower catalyst bed. The acetylene reactor inter stage cooler is used to control the temperature of the feed to the bottom reactor bed.
The most critical operating variable is the temperature of the process gas entering each catalyst bed, especially in the first bed. For optimum performance, bed inlet temperature must be maintained as low as possible. While achieving the desire acetylene clean up in the reactor effluent. All exchanger around the acetylene reactor are designed to accommodate a wide variety of situations, including start up, shutdown, upsets, start of run and end of run.
6. ACETYLENE REACTOR OPERATION PHILPOSPHY:
Acetylene reactor in front end hydrogenation is used to remove the acetylene which is being produced as one of byproducts of the cracking ethane in furnaces. Acetylene is undesirable impurities in the ethylene product and must be removed. The advantage of removing the acetylene in front end hydrogenation is excess hydrogen, which is required for reaction and carbon mono oxide which serve as selectivity moderator.
Since the hydrogenation of acetylene is exothermic reaction, the most critical operating variable is the temperature of the process gas entering each catalyst beds, especially first bed. Bed inlet temperature must be maintained as low as possible so long as sufficient hydrogenation can be achieved to meet the desire concentration of acetylene in reactor effluent. Temperature higher than necessary can cause the ethylene loss by converting ethylene to ethane and increase the danger of runway reaction.
First charge of chevron Philips E-series catalyst (88 Cu.m) was loaded to both the beds and was commissioned on 1st-April’2005. The reactor was operated very smoothly expect the one accident of runways. United adopted operation philosophy that maximum acetylene is to be converted into the ethylene in the first bed and very fractional amount is to be cleaned up in the second bed of reactor. The detail operating parameters are tabulated here.
(Table-2: Acetylene Reactor operating Parameters)
Top Bed Parameters unit values
Flow Ton/hr 486
Inlet temperature Deg.C 76
Outlet Temperature Deg.C 84
Delta T Deg.C 8
Inlet Carbon mono oxide PPMV 106
Inlet acetylene PPMV 2200
Outlet Acetylene PPMV 50
Bottom Bed Parameters
Inlet temperature Deg.C 77
Outlet Temperature Deg.C 79
Delta T Deg.C 2
Inlet acetylene PPMV 50
Outlet Acetylene PPMV 0.03
It is to be noted that Furnaces have special feature of online steam water decoking, which generate excessive amount of carbon dioxide & carbon monoxide (CO2/CO). Extra amount of carbon mono oxide (CO) impacted the acetylene reactor performance.
The acetylene concentration at first bed outlet goes occasionally even less than 50ppmv. Therefore, it is understood that maximum acetylene conversion taking place in the top bed and very low acetylene conversion is taking place in the bottom. It revealed the fact that bottom bed catalyst was not fully utilized.
7. A UNITED APPROACH:
It has been revealed from the acetylene reactor operation philosophy that most of hydrogenation reaction take place in the top bed of reactor , where as the bottom bed work as guard bed and was not fully utilized. Bottom reactor bed still appeared to be fresh, even after 4-year of catalyst life cycle including the continuous operation of 789 days.
Therefore, UNITED has planned to replace the one bed catalyst in the coming schedule turnaround in March ‘2009 i.e. the current catalyst in the top bed will be deposed off and 4-year old catalyst from bottom bed will be reloaded into the top bed. A fresh of Chevron Philip E-series catalyst will be loaded into the bottom bed.
It has been noted that if incoming acetylene is around 2580 ppm into the top bed, with outlet acetylene is 200ppmv and bottom bed is operating with fresh E-series catalyst. Then following question arises
1. How to maintained inlet temperature of bottom bed. Will the inter cooler provide the adequate cooling to control bottom bed temperature.
This option was discussed with catalyst vendor and their data narration are shown below in the tables. This is based past data analyzed and concluded as
Heat absorption duty required for Max flow of 495 T/hr with 2580ppm of acetylene
KW 11000
Plant capacity will be 70% during the start up, flow is T/hr 346
Heat absorption duty required for this flow at 2580 ppm of Acetylene (11000/495)X349
KW 7700
This heat duty is well below the design capacity (11517 KW). Therefore, adequate capacity is available in inter cooler to avoid the runways.
(Figure-1: Simplified Front- End Deethanizer Ethane Cracker)
Ethane from ARAMCO
Feed Saturation Furnaces Quench Water Section
Four stage Compressor Dryers Acid Removal unit 5th Stage Compressor
De‐Ethanizer Acetylene Removal unit De‐Methanizer
Tail Gas (H2, CH4 etc)
C2‐Splitter
Ethylene
Recycle Ethane
Moreover, in order to minimize the potential of runway to happen to fresh E- series catalyst of the bottom bed. It is agree to keep leaking the acetylene up to 500 PPMV and slightly high carbon di oxide around 200PPMV during the 1st week of the operation.
The start up of plant was initiated on 23rd March’2009 as per the above scenario and try to keep the bottom bed temperature as low as possible as stated above. The bottom bed temperature settle around 62 ~67 deg.C as calculated. All the parameters around reactor were settled in a couple of week. With this fact, UNITED has demonstrated that reloading of the bottom catalyst to the top bed and fresh catalyst loading in the bottom bed is possible, if, intercooler has adequate capacity to control bottom bed temperature and carbon monoxide generation from furnace is optimized.
8. ACETYLENE REACTOR PERFORAMCE STUDY:
UNITED kept monitoring the reactor parameters as listed below, even after achieving the on specification ethylene product. UNITED has achieved 792 days of continuous operation since start up of plant on 23rdMarch’2009.
1. Flow to reactor 2. Reactor top bed inlet temperature 3. Reactor top bed outlet temperature 4. Top bed inlet Acetylene concentration 5. Top Bed outlet Acetylene concentration 6. Top Bed Inlet CO concentration 7. Bottom bed reactor inlet temperature 8. Bottom bed reactor outlet temperature. 9. Bottom bed outlet acetylene concentration
It is to be noted that UNITED ethane cracking furnace has distinct feature of on line steam water decoking apart from off line steam air decoking. The on line steam water decoking is being used whenever, it was identified that few tube plugged during the early stage of furnace cracking. This technique helps to avoid furnace off line decoking. But excessive amount of carbon monoxide is being generated during this process, which disturb the reactor drastically and lead to increase reactor inlet temperature. The detailed performance of reactor is presented in the graph below.
In front end acetylene hydrogenation reactors, carbon monoxide, methyl acetylene and Propadiene commonly impact the operation of the acetylene convertors. The carbon monoxide (CO) is acetylene & ethylene adsorption inhibitor and as its concentration increases, the temperature needed to accomplish the same level of acetylene
conversion will also increase. The figure-2 illustrates this point of carbon monoxide (CO).
(Figure-2: Effect of carbon monoxide (CO) on let temperature)
This graph attempts to qualitatively outline the effects of changing the carbon monoxide concentration on catalyst that has an operating Window of inlet 5 deg.C at inlet temperature of 68 ~ 72 deg.C. As the carbon monoxide (CO) concentration rises, the inlet temperature must also rise to offset the reduced number of active sites available to the acetylene. This has been noted over entire monitoring of reactor parameters, as the CO concentration increase, the reactor inlet temperature also raise. Though, during the normal operation, we don’t observe such changes, but due to steam water decoking and excessive CO generation from furnaces has impacted to increase the reactor inlet temperature to offset the acetylene conversion. Therefore, it was agreed, not to encourage the steam water decoking, even try to operate the furnace in such manner that CO generation should be minimize.
It is quite interesting to note that how carbon monoxide (CO) influence a palladium based acetylene hydrogenation, is crucial to the successful operation of this type of reactors with this option. As CO absorbs to palladium based catalyst more strongly than acetylene, the influence that it exert on the catalyst can be extremely strong. Therefore, Figure -3 Plot displays, the variation in the carbon monoxide concentration on the right axis, while top bed outlet acetylene concentration is plotted on left axis. It is revealed from the plot that reactor maintained acetylene concentration, during the
entire operation & monitoring of the reactor. Moreover, one can also note that acetylene concentration at the outlet of bottom bed was less than 0.0ppmv. It is clear indication of potential of over hydrogenation of acetylene to ethylene.
(Figure-3: Top bed outlet Acetylene concentration as function of CO concentration)
(Figure-4: Effect of carbon monoxide & Rx inlet temperature over entire operation days)
Figure -4 Plot shows, the variation in the carbon monoxide concentration on the right axis, while top bed inlet temperature is plotted on left axis. It is revealed from the graph that as carbon monoxide (CO) concentration increase over the entire operational days due to one or another reason, the reactor inlet temperature increased accordingly. The increase of reactor inlet temperature is the offset of hydrogenation reaction. This offset was even maintained during all the operational days.
(Figure-5A: Top Bed Delta T as a function of CO concentration over the entire operation days)
(Figure-5B: Bottom Bed Delta T as a function of CO concentration over the entire operation days)
This graph shows that even carbon monoxide (CO) concentration increased due to one or another reason as indicated in the graph, the delta T across the top reactor bed remain within the acceptable operating window 5 to 8 deg.C. There are few exception persist. The steady delta T for reactors indicates that the selectivity of the catalyst was remarkably constant during the entire operation. The similarly the bottom bed has also shown consistency over the entire operation in two step.
9. COST BENEFITS ANALYSIS:
It is well known fact that catalyst is one of heavy cost item in the process plant and play vital role in complete operation of the plant. It adds cost to final product. Catalyst is to be changed as soon as catalyst life expired. Therefore, complete catalyst changed is recommended by vendor or process licensor. But process optimization & operation philosophy studied has indicated that catalyst still has potential life. It will provide two step benefits.
1. It will allow changing one bed of catalyst instead two beds as per new operational philosophy. Therefore, one bed catalyst cost will be saved.
2. It will provide the technology benefit confidence on catalyst life.
3. Cost saving estimate
1. Expected life of ethane cracker 30-Years
2. Expected life of E-series catalyst 5-Year
3. E-series catalyst required for one Bed 51 Ton
4. Total Catalyst required prior to new operation Philosophy 51X2=102ton
5. Total E-series catalyst charge required over ethane cracker life 5
6. Total Catalyst required after the new operation Philosophy 51 Ton
7. Total Cost saved as per new operation Philosophy 3.5MMUSD
8. Total Number of Time catalyst saved 4
9. Total Cost of catalyst saved 3.5X4 = 14MMUSD
10. CONCLUSION:
Today, in petrochemical competitive market as energy & utilizes price are soaring high, in turn has increased the entire petrochemical product across the globe in highly vibrant& oscillating crude oil prices . Even small saving in the production cost wills the producer to stand firm in the market. Therefore, UNITED acetylene front end hydrogenation reactor has adopted new operation philosophy and demonstrated successful operation that one bed of catalyst replacement in every turn around is possible, which will not only saved the catalyst but also reduced the production cost in the competitive petrochemical market. Moreover, it will provide the technology confidence to catalyst manufacturer and process licensor.
It has to be noted that cracking furnaces should be vigorously monitored to generate less carbon monoxide. Even there are some technology supplier offer on line steam water decoking is to be monitored and carried out at controlled rate or avoided, if furnace tube life reached to its end life. The new adopted operation philosophy and controlled parameters monitoring will always benefit to reduce the production cost.
11. REFERENCES:
1. UNITED-Acetylene Reactor data of 800 days 2. Consultation with Chevron Philips for new operation philosophy & Intercooler 3. UNITED – Technical & Operation Past experienced with Acetylene Convertor 4. UNITED- Monthly Monitoring report
12. AUTHOR BIOGRAPHIES
Abdul Wahab: M.Tech. (Chemical) from L.I.T Nagpur, India and having more than 22 years of industrial experience in Process Engineering of olefin crackers, Methyl Tertiary butyl ether (MTBE), Ethylene Glycol & Linear Alpha olefin (LAO) processes of Petrochemical and Chemical industry. Working with UNITED (SABIC), Al-Jubail KSA, as Staff Process Engineer. If you have any questions or doubts need to clarify, then author can be reached to me on [email protected].
13. CO-AUTHOR BIOGRAPHY
Nayef, A. Al–Anazi: B.S (Chemical) from K.F.U.P.M. Dhahran, Saudi Arabia and having more than 17 years of industrial experience in operation of Olefin & Air Separation Plant. Working with UNITED (SABIC), Al-Jubail KSA, as Manager –Operation of Olefin Plant. You can reached to me on [email protected]
