Dynamic Study of Feed-Effluent Heat Exchanger Addition on Double Bed Configuration Ammonia Reactor System within Varied Quenching Ratio Tri

Ammonia is one of the most important industrial commodity due to its wide function. Ammonia synthesis reaction is an exotermic reaction. Therefore, Feed-Effluent Heat Exchanger (FEHE) is added to increase thermal efficiency. However, FEHE could lead the process to experience hysteresis phenomenon due to interaction between equipments as one steady state T feed could result several T outlet. Hysteresis phenomenon may result asset losses like explosion, leakage, and loosing material integrity. Double bed reactor configuration allows us to use several operating parameters as variation to overcome hysteresis. In this review, quenching ratio was chosen to be that varied parameter. This study aims to determine how quenching ratio affects hysteresis zone by utilizing Aspen Hysys® V8.8 as simulation tool. Simulation showed that quenching ratio would narrow hysteresis zone yet increased extinction temperature that lower the conversion. Conversion profile showed that 0.2 quenching ratio got the highest conversion for system with bed volume ratio 2:1 while total volume was 30 m. However, the feed temperature was fallen at hysteresis zone. Dynamic simulation showed that highest conversion feed temperature (10%∆Tf above extinct temperature) was still able to preserve stability with descending temperature approach. Hysteresis itself started to occur at 1.7%∆Tf above extinct temperature


Introduction
Ammonia is one of important industrial commodity due to its wide utilization as chemical product precursor such as urea, nitric acid, and etc.In ammonia production system, Feed-Effluent Heat Exchanger (FEHE) is commonly used that aims to exchange heat between reactor effluent and reactor feed as shown in Fig. 1.Fig. 1.Ammonia reactor system configuration Such heat integration system would reduce utility requirement and increase thermal efficiency.However, the integration system could possibly lead the process system to experience significant temperature oscillation as shown in Fig. 2. Oscillations occur as the result of two or more processes that are coupled together with each other [1,7].In this case, reactors performance is closely related to how the FEHE performs.As the result, reactor possibly undergoes a phenomenon while same operating condition could produce several steady state outputs that lead the process to oscillate [1,6].This condition could cause several hazards for the equipment material and even operational staff [4,5].Hazards that occurred could dangerously result disasters such as explosion or poisoneous chemical leakage which causing economic losses.

Fig. 2. Hysteresis phenomenon on industrial scale ammonia reactor. [3]
Hysteresis area could be predicted by plotting one of steady state parameter such (T outlet) as result of T feed input as shown in Fig. 3.

Fig. 3. Curve plot to determine hysteresis zone
The 'S' curve produced showed that within range of T L and T R (∆T f ), each T feed input will result several values of T outlet.Therefore, there will be possibility for the process to experience oscillation while entering this region.
Some efforts to deal with this condition could be done by changing other supporting operation parameter for double bed system such as adding trim heat exchanger, bed volume distribution, or quenching ratio.
Therefore, this study would show parametric review and simulation regarding actions to overcome and reduce hysteresis phenomenon by choosing quenching ratio as varied operation parameter.As the quenching ratios were evaluated, conversion profile was also analyzed to map the best temperature at particular quenching ratio to produce optimum reactant conversion.As the matter of limitting the problem, this review focused on observing feed temperature, outlet reactor temperature, quenching ratio, reactant conversion, and starting hysteresis temperature.

Methods
Aspen Hysys® V8.8 were used to act as an assistive tools for simulating the dynamic behaviour resulted from altered parameter value.Simulation configuration is shown in Fig. 4.

Fig. 4. Simulation Configuration
Fluid package used in the simulation was Peng-Robinson.Ammonia reaction kinetics behaviour for calculation was based on following equation [2] .
Plug Flow Reactor (PFR) with total volume 30 m 3 was used in the simulation.Quenching process was simulated by using splitter and mixer features of Aspen Hysys® V8.8.Quenching ratio was varied with assumed 1 st bed volume at 20 m 3 and 2 nd bed at 10 m 3 .FEHE simulation was conducted with steady-state-rating mode and sized rigorously by using interactive mode to obtain certain characteristic on heat transfer.FEHE specification sheet is shown in Table 1.
Hysteresis zone identification was conducted by using Adjust feature on Aspen Hysys® V8.8.This process was done by varying the 2 nd reactor inlet temperature to adjust the feed temperature on each quenching ratio.This attempt eventually would result plot T Feed vs T Outlet correlation to identify hysteresis zone.Reactant conversion was then identified by using Case Study feature on Aspen Hysys® V8.8.This study was done by varying feed temperature on each quenching ratio or bed volume distribution and observing the mole flow of the ammonia in the outlet stream.At the end, Aspen Hysys® V8.8 Dynamic feature was conducted to observe the process dynamic behaviour at quenching ratio which resulted highest reactant conversion.Eventually, dynamic observation was used to determine starting temperature of oscillation as the percentage of ∆T f .Dynamic simulation was also used to determine process stability while feed temperature was set at value that resulted highest conversion in chosen quenching ratio.

Results
Unstable process as the result of FEHE utilization in double bed reactor is possible to create instability zone like the one in single bed configuration.Identified instability zone study resulted from varied quenching ratio and bed volume distribution of double bed reactor would be explained in the following section.

Hysteresis Zone Alteration Due to Varied Quenching Ratio
Fig. 5 shows that generally higher quenching ratio would lead to more narrow hysteresis but higher extinct temperature.

Fig. 5. Hysteresis zone alteration on varied quenching ratio
Higher extinct temperature could possibly lead the reaction to easily extincted and furthermore it would result lower reactant conversion.
At bed volume distribution of 2:1, hysteresis zone experienced trend of rise from 0.1 to 0.2 and drop from 0.2 -0.8.From here, there was an observed reversed trend at 0.2 quenching ratio.This possibly indicates that there was optimum point regarding the process parameter related to feed or outlet temperature.Closest relation to these parameters was conversion profile of the reaction.Reactant conversion was then evaluated from each quenching ratio at steady state simulation as shown in Fig. 6.

Fig. 6. Conversion profile on varied quenching ratio with 1 st bed volume = 20 m 3
Fig. 6 shows that generally lower quenching ratio will increase the conversion and it shows highest overall conversion profile at 0.2 quenching ratio.Optimum point of conversion profile at each quenching ratio was then plotted as shown at Fig. 7.

Fig. 7. Highest conversion plot at each quenching ratio
Fig. 7 shows that quenching ratio 0.2 resulted highest conversion which is 18.97% at bed volume distribution 2:1.This is the equilbrium conversion and obtained at T feed = 190 o C.However, this temperature was located inside the hysteresis zone and was close to the 0.2 quenching ratio extinct temperature which was 184.69 o C. Therefore, dynamic simulation then was conducted to determine the stability behaviour at 190 o C as the operation is basically purposed to achieve conversion requirement as high as possible.

Dynamic Simulation
In this section, Aspen Hysys® V8.8 Dynamic Mode feature was utilized to identify stable/unstable behaviour in real time.From Fig. 5, we know that extinct temperature is 184.69 °C at 0.2 quenching ratio.Hence, utilizing Dynamic Mode could possibly show minimum temperature to start hysteresis (as function of ∆T f = 107.62 0C) before the oscillation extinct the reaction.Dynamic Mode was started with ingniting the reaction by set the T feed to 300 °C (refer to Fig. 8).T feed was then lowered steppedly to 200 °C and then 190 °C.Starting from this point, feed temperature was lowered rampedly by 0.1 °C to find the hysteresis minimum temperature as accurate as possible.Dynamic simulation showed that oscillation phenomenon started at 186.6 °C.This means that at 0.2 quenching ratio and bed volume distribution 2:1, hysterisis mathematically will start approximately at:

Conclusion
Quenching ratio alteration could affect how wide the hysteresis area is while plotting T outlet vs T feed.Higher quenching ratio showed narrowing hysteresis yet increase the extinct temperature that could lower the conversion.Highest conversion was obtained at 0.2 quenching ratio (bed volume distribution 2:1) with T feed = 190 °C which entering the hysteresis zone.However, this condition was chosen to be analyzed more in the Dynamic Mode to determine whether 190 °C would already triggered the oscillation.Dynamic Mode showed that oscillation started at 186.6 °C and further extinct the reaction.The highest conversion feed temperature was then tested in Dynamic Mode and resulted stable operation.Therefore, while using bed volume distribution 2:1 (total volume = 30 m 3 ), ammonia reaction highest conversion of 18.97% (quenching ratio = 0.2) is still able to be obtained at stable operation.