Sensitivity and

Design Spec.

ASPEN XXII

GENERATION

Sensitivity

◦ sensitivity provides a mechanism for analyzing flowsheet behavior. The equation-oriented strategy computes the sensitivity between a set of independent and dependent variables. The resulting sensitivity is the derivative or gain between the variables.

◦ With SM sensitivity it is necessary to reconverge the entire flowsheet for each point on the sensitivity plot. EO sensitivity gives only the derivatives of the dependent variables with respect to the independent variables. This is the slope of the curve at the current operating point.

◦ For example, you could use an EO sensitivity block to find how much additional duty and what the change in composition would be if there were a change in the reflux flow of a column. If the problem were set up such that the reflux flow is either fixed or a degree of freedom, then this would be the independent variable. The duty variables and composition variables would be calculated and then listed as dependent variables. The results would be four derivatives or gains. The gains would have units of delta-dependent variable units/ delta-independent variable units. In this example, the gain for the duty with respect to reflux flow may have units of (MMBtu/hr) / (lb/hr), that is, (MMBtu/lb). The UOM defined for each dependent and independent variable determines the units of the gains.

Sensitivity

◦ Tools yang membantu untuk:

◦ Melihat pengaruh suatu variable terhadap variable

yang lain

◦ Perkiraan range nilai dari manipulated variable yang

dijadikan acuan untuk perhitungan yang lain (ex: design spec, optimasi)

Design Spesification

◦ A design specification sets the value of a variable that Aspen Plus would otherwise calculate. For example, you may want to specify a product stream purity or the permissible amount of an impurity in a recycle stream. For each design specification, you identify a block input variable, process feed stream variable, or other simulation input to be manipulated (adjusted) to meet the specification. For example, you might manipulate a purge rate to control the level of impurities in a recycle stream. Design specifications can be used to simulate the steady state effect of a feedback controller.

◦ When you use a design specification, you specify a desired value for a flowsheet variable or some function of flowsheet variables. The flowsheet variables used in a design specification are called sampled variables. For each design specification, you must also select a block input variable or process feed stream variable to be adjusted to satisfy the design specification. This variable is called the manipulated variable.

◦ The design specification achieves its objective by manipulating an input variable specified by the user. Quantities that are calculated during the simulation should not be varied directly. For example, the stream flow rate of a recycle stream cannot be varied; however, the split fraction of an FSplit block where the recycle stream is an outlet can be varied. A design specification can only manipulate the value of one input variable.

a.k.a : Design Spec

◦ Sebuah tool yang digunakan untuk menghitung

nilai variable tertentu dengan memvariasikan

nilai variable yang lain

It’s Time for Quiz!! ◦ Etilen oksid dibuat dengan mereaksikan etilen dan oksigen. Reaksinya

adalah sebagai berikut :

◦ C2H4 + ½ O2 C2H4O

◦ Umpan sebanyak 0,3 lbmol/detik mula-mula dalam kondisi lingkungan (300C

dan 1 atm, fase:gas) kemudian dikondisikan agar memenuhi kondisi operasi

di reaktor.

◦ Reaksi fase gas dijalankan pada Reaktor Alir Pipa pada suhu 500 oF dan

tekanan 10 atm. RAP yang dipakai memiliki 50 tube dengan panjang 2 ft

dah diameter 1,5 in. Pada reaktor terjadi pressure drop sebesar 1,5atm.

◦ Property method : NRTL

◦ −𝑟𝐶2𝐻4= 𝑘. 𝑃𝐶2𝐻40.5 𝑃𝑂2

0.5

◦

◦ k = 8x1014*exp(-50000/RT)

◦ E dalam cal/mol

◦ T0 dalam 0F

Ketentuan

• Waktu 15 menit

• Perhatikan Fase reaksi

• Tentukan sebaiknya dipanaskan atau ditekan terlebih dahulu

PFD

Komponen

Step by step:

Property

Method

Stream Input

Kompressor HE

Mixer

Input

Reaktor

Input Data

Reaksi

Reaksi ke kanan (1)

RUN !!!

Hasil:

Hitung konversi 27,39%

• Tampilkan grafik sensitivitas panjang reaktor terhadap konversi

yang diperoleh!

• Berapa panjang reaktor yang diperlukan agar konversi 70%?

Sensitivity

Aspenteam-XIX

Mencari pengaruh

panjang reaktor

terhadap konversi

etilen

Harus mendefinisikan konversi terlebih dahulu di aspen.

Note : tab fortran dipakai jika parameter yang

ingin kita ketahui tidak tersedia di aspen.

Sehingga kita perlu memformulasikannya di tab

fortran.

Mengisi tab fortran untuk mendefinisikan

variabel konversi yang ingin dihitung.

Mendefinisikan

Konversi

Dengan

menggunakan

stream variabel

berupa mole flow

dari benzene

Tab vary untuk mendefinisikan

manipulated variabel yang akan kita

gunakan.

Variasi panjang reaktor Dari 1 – 10 ft, dg increment 1ft,

Sehingga didapatkan 10 data

Reinitialize

RUN !!!

Tabulasikan apa yang akan ditampilkan

pada hasil perhitungan sensitivitas.

HASIL RUN

Design Spec

Ingin diketahui panjang

reaktor yang diperlukan

agar konversi etilen 70%

Note : deactive terlebih dahulu

tool sensitivity yang digunakan sebelumnya

Definisikan

konversi di tab

fortran

Mendefinisikan

Konversi

Dengan

menggunakan

stream variabel

berupa mole flow

dari etilene

Reinitialize

RUN !!!

Isi spesifikasi yang diinginkan pada tab

Spec (konversi sebesar 70% dengan toleransi tertentu)

Definisikan variabel yang

ingin divariasikan terhadap

konversi: panjang reaktor

Variasi panjang reaktor Dari 1 – 7.5 ft, dg increment 1ft

HASIL RUN

Panjang reaktor yang dibutuhkan agar konversi

benzene 70% adalah = 6.162 ft

TERIMA KASIH

Ada pertanyaan?

UKI