Zizong Wang,Hongqian Liu*,Jiming Wang

1 East China University of Science and Technology,Shanghai 200237,China

2 China Petrochemical Corporation,Beijing 100029,China

3 SINOPEC Engineering Incorporation,Beijing 100101,China

Keywords:Heat integration distillation Methanol to propylene Ethylene Propylene

ABSTRACT Based on a typical gas composition from a methanol-to-propylene(MTP)reactor,and guided by a requirement to recover both propylene and ethylene,three separation strategies are studied and simulated by using PROII package.These strategies are sequential separation,front-end dethanization,and front-end depropanization.The process does not involve an ethylene refrigeration system,using the separated stream as absorbent,and absorbing further the medium-pressure demethanization,and a proprietary technology by combining intercooling oil absorption and throttle expansion.Influences of different process streams as absorbent are studied on energy consumptions, propylene and ethylene recovery percentages, and other key-performance indicators of the separation strategies.Based on a commercial MTP plant with a methanol capacity of 1700 kt·a-1,the simulated results show that the front-end dethanization using the C4 mixture as absorbent is the optimal separation strategy, in which the standard fuel oil consumption (a key-performance indicator of energy consumption) is 18.97 kt·h-1,the total power consumption of two compressors is 22.4 MW,the propylene recovery percentage is 99.70%,and the ethylene recovery percentage is 99.70%.For a further improvement,the pre-dethanization and thermal coupling methods are applied.By using front-end pre-dethanization(partial cutting)with debutanizeroverhead,i.e.the C4 mixture,as absorbent,the power consumption of the compressors decreases to 19.9 MW,an 11%reduction compared with the clear-cutting method.The energy consumption for the dual compressors for crude gaseous product mixture and main product propylene refrigeration is 16.69 MW,16.55%lower than that of the present MTP industrial plant with the same scale,and a total energy consumption of 20 MW for the triple compressors including product gas mixture compression,and ethylene and propylene refrigeration.

1.Introduction

Based on a 500 kt·a-1MTP plant implemented in a fixed-bed reactor system,guided by the first-stage suction composition of product gas compressor[3],and assuming that raw ethylene and mixed butane and butane (abbreviated as C4S) are not returned to the MTP reactor, the possibility of using medium pressure and medium temperature demethanizer, intercooling oil absorption, and throttle expansion combined technology,as well as cryogenic separation process with no ethylene refrigeration system for demethanizer tail gas recovery is discussed.It was carried out based on optional separation technologies(sequential separation and front-end dethanization and depropanization)and simulation by using PROII(™)7.0.The influences of different absorbents on the process strategy and the energy consumption of the separation process are studied, and the feasibility of thermal coupling C2removal system(dethanizer and ethylene fractionator)is also discussed.Simulation results indicate that by meeting the ethylene loss requirement in the demethanizer tail gas, the standard fuel oil consumption is efficiently decreased by combining partial cutting front-end dethanization process, medium temperature and medium pressure demethanizer,intercooling oil absorption,and throttle expansion combined tail gas recovery technology and thermal coupling C2removal system.Only propylene refrigeration system is required in this process.

2.Three Potential Separation Strategies

The optional processes for MTP crude product gas separation include sequential process, front-end depropanization and front-end dethanization. A combined technology of medium demethanization process,intercooling oil absorption,and throttle expansion is applied as a substitute for high-pressure demethanizer and-100°C ethylene refrigeration [13,14], and only propylene refrigeration system is absorbed in the process. The intercooling oil absorbents can be as follows:1)propylene fractionator bottom C3H8,2)C5removal tower overhead C4S,and 3)depropanizer bottom C4+components.Considering the characteristics of MTP product gases, the possibility of using C4+components as absorbent will not be discussed.

The simulations are preceded by using PROII(™)7.0.

The optimization procedure for the MTP product gas separation includes the following steps:

(1) Medium-pressure demethanizer,intercooling oil absorption,and throttle expansion technology are used for optional process improvement [13,14]. Operating parameters, heat-exchange network, and propylene refrigeration system are optimized through process simulation.

(2) Through absorbent selection and the optimization of process strategy, operating parameters, heat-exchange network, and propylene refrigeration system,the optimal absorbent and corresponding separation process are selected.

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(3) The energy-saving efficiency of the optimized process in step(2)with pre-cutting and thermal coupling fractionation is verified.(4) The energy-saving effect of the optimized process in step(3)is verified in an actual plant.

Fig.1.Standard fuel oil consumptions of different separation processes with different absorbents(kt·h-1).

The temperature range of the propylene refrigeration is determined by medium-pressure and medium-temperature demethanizer tail gas recovery.The propylene refrigeration temperature can be transformed into integer variables, which turns the optimization target of MTP product gas separation into the issue of discrete variables.

Process simulations of the optional processes(sequential process,front-end depropanization and front-end dethanization)with different absorbents(C3H8,C4mixture)are conducted.The operating parameters,heat-exchange networks,and propylene refrigeration system are optimized. The energy consumption of two compressors, cooling water consumption,and steam consumption are converted into standard fuel oil consumption,and the results of all optional processes and strategies are compared. Fig. 1 illustrates that the standard fuel oil consumptions of sequential process, front-end depropanization and front-end dethanization are low when using mixed C4S as absorbent.Among them,the front-end dethanization process has the minimum standard fuel oil consumption.

Table 1 illustrates that all three processes have the same propylene and ethylene recovery rates.Front-end depropanization has the highest compressor power consumption and unit energy consumption(standard fuel oil consumption).Its compressor power consumption is 9.5% higher than that of front-end dethanization, but its unit energy consumption(standard fuel oil consumption)is 10.8%lower.

3.Further Improvement of the Optimal Strategy

The previous process selection is based on the assumption of a traditional clear cutting process.C3mixture was not used in the front-end dethanizer overhead vapor,and the reflux ratio is relatively large.Moreover,when using mixed C4components as absorbent,the strategy illustrated in Fig.2 has two loops: (1)C4species absorbent recycle loop:dethanizer →depropanizer →debutanizer →absorption tower →dethanizer; and (2) recovered ethylene recycle loop: dethanizer→product gas compressor fourth stage →demethanizer →absorption tower →dethanizer.The ethylene recycle increases the unit energy consumptions of the fourth compressor,demethanizer chilling system and demethanizer.

For the front-end dethanization process illustrated in Fig.2,the following topics can be raised:(1)whether a secondary dethanizer can be added,to increase the desorption of hydrogen-methane and C2by the rich absorbent through demethanizer and for the secondary dethanizer to break the recovered ethylene recycle loop;(2)to decrease the power or duty of product gas compressor fourth stage and cold box in the tower overhead reflux loop,C3components are partially cut in the dethanizer, and part of the C3from dethanizer overhead is sent to demethanizer to reduce the energy consumption of the separation system;and(3)whether thermal coupling of the ethanizer with ethylene fractionators can be used in the MTP product gas front-end dethanizer process.

3.1.Addition of a second dethanizer

Fig.3 shows that C3components are partially cut.Moreover,part of C3enters the demethanizer from the dethanizer to reduce the power consumption of the fourth stage product gas compressor and the duty of the coldbox in the tower overhead reflux loop.The rich absorbentfrom the bottoms of the absorption tower enters the top of the demethanizer as reflux. The bottoms of the demethanizer enter the second dethanizer. The overhead of the second dethanizer enters the ethylene fractionator to produce ethylene.The bottoms are sent to the depropanizer.The overhead of the depropanizer enters the propylene fractionator to produce propylene. The bottoms are sent to the debutanizer. The overhead of the debutanizer is mixed C4mixture.The bottoms are mixed with C5+components.

Table 1 Comparison of the three separation strategies

Fig.2.Front-end dethanization separation strategy with absorbent of C4 mixture:1—demethanizer,2—dethanizer; 3—absorption tower,4—depropanizer,5—ethylene fractionator, 6 — debutanizer, 7 — propylene fractionator, 8 — condensation fraction tower(CFT).

Fig. 3. Front-end dethanization separation process (C4 mixture as absorbent, partial cutting):1—demethanizer,2—dethanizer;3—absorption tower,4—depropanizer,5— ethylene fractionator, 6 — debutanizer, 7 — propylene fractionator, 8 — No. 2 dethanizer,9—CFT.

The recycle loops of the recovery process shown in Fig.3 are listed as follows:

(1) Mixed C4mixture absorbent recycle loop:demethanizer →second dethanizer →depropanizer →debutanizer →demethanizer;

(2) Methane hydrogen,ethylene,and propylene are absorbed by the rich absorbent in the demethanizer, second dethanizer, and depropanizer successively;and

(3) Absorption of methane hydrogen recycle loop:absorption tower→demethanizer →absorption tower.

After process simulation and the optimization of parameter,heat exchange, and propylene refrigeration system, the comparison of the product gas compressor shaft power of original front-end dethanizer process and the optimization of partial cutting front-end dethanization process are carried out as shown in Table 2.The comparison of standard fuel oil consumption is shown in Table 3.For ease of comparison,we assume that the reflux rate of the dethanizer is 600 kmol·h-1.

Table 2 shows that the fourth stage of the product gas compressor axle power of partial cutting front-end dethanization process decreased after the application of partial cutting and the second dethanizer.From Table 3,the energy consumption(standard fuel oil consumption)is 8.6%lower than that of the clear-cutting process.The power of partial cutting process with two compressors is 19.9 MW,which is 11%lower.

3.2.Thermal coupling of C2 recovery system

Theoretically,thermal coupling technology such as DWC or thermodynamic equivalent side stripper has great energy-saving capability[19-28]. For front-end pre-dethanization, a second dethanizer is thermal-coupled with the ethylene fractionator, which means that a side stream from the ethylene fractionator will enter the second dethanizer as reflux (Fig. 4). After process simulation, the standard fuel oil consumption between the conventional process and thermal coupling is compared as shown in Fig.5.

Fig.5 shows that after thermal coupling the second dethanizer with the ethylene fractionator,the standard oil consumption decreased by 0.46%.This decrement is small compared with that of the whole plant,because the ethylene recovery rate is significantly lower than the propylene recovery rate in the MTP plant.This optimization can also save the cost of the condenser,reflux drum,and reflux pump for the second dethanizer.

3.3.Partial cutting of the front-end dethanization strategy

In an actual MTP plant,most of the raw ethylene will be recycled back to the MTP reactor.In the partial cutting front-end dethanizationprocess as shown in Fig.6,the raw ethylene can only be extracted from the feed of the ethylene fractionator.According to the requirements of the actual plant reactor,23 t·h-1feedstock is extracted from the overhead of the second dethanizer and is sent back to MTP reactor.The remaining 3.4 t·h-1feedstock are sent to the ethylene fractionator to produce ethylene.

Table 2 Comparison of product gas compressor parameters for the clear and partial cutting front-end dethanization strategies

Table 3 Comparison of standard fuel oil consumption for the clear cutting and partial cutting front-end dethanization strategies

Fig.5.Comparison of the standard fuel oil consumption in the conventional and thermally coupled process for partial cutting front-end dethanization separation.

After process simulation,the optimization calculation of the parameter,heat exchange,and propylene refrigeration system for the process are carried out,as shown in Fig.6.The power consumption of the product gas and propylene refrigerant compressors is presented in Table 4.

Fig.6.Actual partial cutting process of front-end dethanization(C4 mixture as absorbent;raw ethylene was withdrawn from the top of the second dethanizer column). 1 —demethanizer,2—dethanizer;3—absorption tower, 4—depropanizer,5—ethylene fractionator,6—debutanizer,7—propylene fractionator,8—No.2 dethanizer,9—CFT.

Table 4Compressor shaft work of partial-cutting process of front-end dethanization(C4 mixture as absorbent)

In the partial cutting front-end dethanization process shown in Fig.6,substantial amount of raw ethylene is extracted from the overhead of dethanizer and is sent to the MTP reactor, which lowers the duty of the ethylene fractionator.The power of the propylene refrigerant compressor is 4.39 MW,and the total power of the two compressors is only 16.69 MW, which is 16.55% lower than that of industrialized plant using Lurgi MTP technology with the power of 20 MW for the three compressors.

4.Conclusions

Based on a MTP plant with a methanol consumption capacity of 1700 kt·a-1in operation and guided by the research basis of typical composition of product gases,full process simulation,calculation,and optimization were conducted using PROII package as simulation platform for the separation unit.The following conclusions are drawn.

1) The strategy is feasible in which the self-stream is used as absorbent with no ethylene refrigeration system,and used the medium pressure demethanizer,intercooling oil absorption section,and throttle expansion.

2) For front-end demethanization, dethanization, and front-end depropanization, mixed C4species from the overhead of the debutanizer is the best absorbent.

3) Among the three recovery strategies,the front-end dethanization is most suitable for MTP product gas recovery with a standard oil consumption of 18.97 kt·h-1,compressor power consumption of 22.4 MW,propylene recovery rate of 99.70%and ethylene recovery rate of 99.70%.

4) The partially-cut pre-dethanization strategy using C4mixture from the overhead of the debutanizer as absorbent can save 11%of energy in terms of compressor power consumption.However,an additional tower will be used,and the total area and cost will increase.

5) Front-end pre-dethanization with partial cutting and the mixed C4species as absorbent, an optimal separation strategy is recommended by combining the intercooling oil absorption and throttle expansion,secondary dethanizer in C2separation system,and thermal coupling of the ethylene fractionator.Using this strategy on a practical MTP plant with a methanol capacity of 1700 kt·a-1,the total compress power of the product gas compressor and propylene refrigeration compressor will be reduced to 16.69 MW,16.55%lower than that of a present separation method of the same capacity with a total power of 20 MW for the three compressors.