Simulation of Partial Oxidation of Natural Gas to Synthesis Gas Using ASPEN PLUS (Excelente Para Simulacion)

FUEL PROCESSING TECHNOLOGY ELSEVIER

Fuel Processing

Technology

50 (1997) 275-289

Simulation of partial oxidation of natural gas to synthesis gas using ASPEN PLUS M. Khoshnoodi ’ Department

of Chemical

Engineering,

Unirersi@

**a,Y.S. Lim ’

of Sistan

and Baluchesian.

P.O. Box 98135-161.

Zahedan.

Iran h Facult\

of Chemical

and Natural

Resources Engineering, Lumpur,

Univ~witi

Teknnologi Malaysia.

54100 Kualrz

Malaysitr

Received 24 July 1996: accepted 24 July 1996

Abstract Conversion of natural gas to liquid fuels is a challenging issue. In SMDS process natural gas is first partially oxidized with pure oxygen to synthesis gas (a mixture of H, and CO) which is then converted to high quality liquid transportation fuels by utilizing a modernized version of the Fischer-Tropsch reaction. This paper presents a computer simulation of the first stage of the process, i.e. the synthesis gas production from natural gas. ASPEN PLUS equipped with a combustion databank was used for calculations. Concentrations of over 30 combustion species and radicals expected in the synthesis gas have been calculated at equilibrium and several non-equilibrium conditions. Using a sensitivity analysis tool, the relative feed flow rates and reactor parameters have been varied searching to maximize the CO/O, yield as well as to minimize the undesired nitrogen compounds in the product stream. The optimum reactor temperature for maximizing the CO mole fraction in the synthesis gas was also calculated. 0 1997 Elsevier Science B.V. Kqwnrdrt

Natural gas; Synthesis

gas; Combustion

simulation:

ASPEN PLUS

1. Introduction The first commercial plant for conversion of natural gas to liquid fuels via synthesis gas was started up by Shell in Bintulu, East Malaysia in 1993. The technology is based on the classical Fischer-Tropsch reaction originally explored and developed as far back

_ Corresponding 037%3820/97/$17.00 PII

author. 0

s037x-3x20(96)01079-x

1997 Elsevier Science B.V. All rights reserved.

as the 1920s. It was used in Germany during the second world war to produce motor fuels from coal [I]. Conversion of natural gas to liquid fuels has been a challenging issue. The proposed processes may be classified into two groups; direct and indirect processes [2]. Direct processes tend to be less selective towards the desired products and are limited by the thermodynamic equilibrium: nCH, * (CHZ),> + nH,

(I)

Below 1000 “C the equilibrium is towards the CH 4 side. To overcome this barrier one should either apply a high temperature or create a sink for Hz. This may be achieved by oxidation or by addition of halogens, preferably chlorine. An example of a direct process is the ARC0 process for oxidative coupling reaction in which methane is converted first to ethane and then either thermally or catalytically dehydrogenated to ethylene [3,4]: 2CH, + l/20, - + C,H, _ C,H,

+ C,H,

+ H,O

+ H,

(2) (3)

Liquid fuels are produced by oligomerizaion and aromatization of ethylene over acidic zeolites. This process is best suited for production of gasoline and, because of a conversion yield of less than 25%, cannot compete with indirect processes. Indirect processes include conversion of natural gas to synthesis gas which is mainly a mixture of H, and CO. Steam reforming of methane (SMR) uses a nickel catalyst and is operated at about 850°C and 30 bar: CH, + Hz0 + CO + 3H,

(4)

It produces a synthesis gas with a HZ/CO ratio of at least 3. As the H,/CO usage ratio in the Fischer-Tropsch reaction is about 2, it is clear that the steam reforming will always result in surplus hydrogen production. A commercial example of such process is the Mobil plant in New Zealand commissioned in 1986 where natural gas is first converted to synthesis gas and then to methanol. Liquid fuel is produced via the Mobil methanol to gasoline (MTG) process over zeolite catalysts. Another example is the Synthol process at SASOL, South Africa. where natural gas conversion to a CO/H, mixture is carried out via steam reforming. Synthesis gas is then converted to high quality gasoline by the Fischer-Tropsch process. It should be mentioned that synthesis gas with different H,/CO ratios is used in the production of many other chemicals [5]. Synthesis gas with a H,/CO ratio of about 2 to 1.7 can be produced by the non-catalytic autothermal partial oxidation process: CH, + l/20,

+ CO + 2H,

(5)

This reaction is the basis of the Shell Middle Distillate Synthesis (SMDS) process for conversion of natural gas to synthesis gas and then to transportation fuels [6]. The oxidation process is operated at 1300 to 1500°C and pressures up to 70 bar with carbon efficiency in excess of 95% and a methane slip of about I %. The synthesis gas produced consists mainly of Hz and CO (up to 95% vol.), the remainder being H,O, CO?, NT and traces of hydrocarbons and nitrogen compounds as detailed in Section 2. The non-cata-

lytic partial oxidation of natural gas has also been widely used in the production of hydrogen. The heart of the SMDS process is a modernized version of the classical FischerTropsch reaction in which carbon monoxide and hydrogen are converted to paraffinic hydrocarbons over a cobalt, ruthenium or iron catalyst: CO+2H.+-CH2-+H,O

(61

In this reaction -CH,represents a segment of a straight chain paraffin in highly linear products. The reaction conditions are chosen so that formation of long chain liquid paraffinic molecules is favoured, whilst that of gaseous components such as butanes and lighter hydrocarbons is minimized. The Fischer-Tropsch reaction is very exothermic and operates in a relatively narrow temperature range of 200-250°C. Taking into account the requirement for massive removal of the reaction heat several types of reactors have been considered: Fluidized bed, Multitubular fixed bed. Three phase fluidized bed. and Bubble column. The basic limitations of these reactor-catalyst systems have been studied in detail [7,8]. The Fischer-Tropsch process may lead to a raw product of a rather waxy nature which is unsuitable for transportation fuels. Incorporation of a hydroisomerization and mild hydrocracking stage reduces heavy paraffins to middle distillates which are then fractionated to kerosene. gasoil and some naphtha. In the Shell MDS Bintulu plant. 100 million scfd natural gas is partially combusted with pure oxygen. produced in a 2500 ton dd ’air separation unit, resulting in 1200 bbl d-’ liquid fuels. These fuels are completely free from aromatics and sulphur compounds and upon combustion in vehicles result in much less particulate and sulphur emissions when compared with middle distillates originating from crude oil [9]. This project is a modeling investigation of the first stage of the SMDS process; that is synthesis gas production which involves more than 50% of the total capital cost.

2. Combustion

simulation

by ASPEN PLUS

ASPEN PLUS (Advanced System for Process Engineering) Release 9.1-3 (1994) was employed to simulate the partial oxidation reactions of natural gas for the production of synthesis gas. This process simulator has been developed at MIT, USA, and is equipped with up to date databanks for thermochemical properties based on the American Institute of Chemical Engineers DIPPR data compilation project as well as a combustion databank based on the JANAF Tables including 59 combustion species and radicals at temperatures up to 6000 K [IO]. ASPEN PLUS incorporates most unit operations and several types of reactors, including the Gibbs reactor [I I]. For a multireaction system such as the partial combustion of natural gas which involves numerous dissociation, recombination and elementary reactions, the Gibbs reactor was preferred because it is based on the minimization of the total Gibbs free energy of the product mixture. In this method. for the calculation of the equilibrium composition in a set of chemical reactions. the reactants with their initial amounts and conditions are specified. It is also required to specify what species are expected in the product stream. After defining reactor tempera-

ture and pressure, the composition of all species present in the product mixture is calculated by solving a set of simultaneous equations [ 12.131. In this simulation work the product synthesis gas was considered to contain over 30 species: Hz. CO. H,O. CO,. N?, CH,, H,N. H. CHN. CH,O. CHO, HO, NO. CHz, CN, 0, HN. C,H,. HNO, 02, N. N,O, H,O. CH, n-C,H,,,. C. I’-C,H,,,. NO,. I’-C,H ,?. n-C5H,, and N,O, respectively.

3. Simulation

procedure

and results

The flow diagram for the simulation of the partial oxidation of natural gas to produce synthesis gas is shown in Fig. 1. Separate streams of natural gas (14946 kg hh’), pure were oxygen (17783 kg h- ’) and some hydrogen (57 kg hh ’), for desulphurization, directed into a virtual Gibbs reactor and converted to product synthesis gas (32787 kg h- ’). Stream flowrates, temperatures, pressures, and other process parameters were obtained from the operational data provided by the manufacturing offices of the Shell MDS Malaysia Sdn. Bhd. in Sarawak. The composition of natural gas from the central Luconia field in South China sea is shown in Table 1. The main objective of this modeling project was to vary the relative feed flowrates and reactor parameters to maximize the CO/O, yield as well as to minimize the undesired trace nitrogen

(

Temperature a

>

(“C)

Pressure (BAR

)

Flow Rate ( KGIHR)

Fig. 1. Flow diagram for the simulation of the partial oxidation of natural gas.

M. Khoshnoodi,

Table

Y.S. Lirn / Fuel Processing

Technolog!

50

C1997)

275-280

?I9

I

Composition

of the natural gas from South China sea, central Luconia field Component

Mole %

I

CH,

2 3 4 5 6 7 8 9 10

C,H, co, C,H, N2 i-C,H,,, n-C,H,,, i-C,H,? n-C,H,, &HI, + TOTAL

87.975 3.543 3.543 2.194 0.97 I 0.574 0.536 0.207 0.126 0.331 100.00

No.

compounds found in the synthesis gas stream [14]. The different corresponding results are as follows:

cases studied and the

3. I. General simulation A general simulation of the partial oxidation of natural gas for the production of synthesis gas was carried out. Perfect mixing of the reactants and ideal gas behavior of the hot gases were the main assumptions. Complete equilibrium was assumed when the reactor temperature was 1390°C and pressure 46 bar. The mole fractions of the outlet synthesis gas containing over 30 species were calculated; H, = 0.5502, CO = 0.3326, H,O = 0.0954, CO, = O-0177,... to as low as C,H ,? = 1.8E - 22. A complete report for the streams specifications was generated, as shown in Table 2. 3.2. DMDS injection The general simulation was repeated with a small amount of Dimethyl-Disulfide (DMDS, 0.64 kg h-r ) injected into the reactor. Although it was advised by the Shell MDS that the additive may improve the CO concentration in the synthesis gas, the simulation results did not show any significant changes. 3.3. Non-equilibrium

simulation

Experimental data provided by the Shell MDS indicated a slight methane slip through the reactor. The synthesis gas contained 0.197 mole % of unreacted CH, which corresponded to a 0.39% departure from the equilibrium conditions of the reacted mixture. Thus, non-equilibrium simulation was carried out and departures from the equilibrium were set at 0.29%, 0.39% and 0.49%. Streams specifications reports were generated for each case. Table 3 shows the results for the 0.39% case. There is 4.944 Kmol hh ’ unreacted methane in 25 11.967 kmol hh ’ synthesis gas, corresponding to 0.1968 mole %. This is very close to the above value, read from the plant operation data.

Pamal oxidation

of tialurill

gas

StreamID