Naphta Hidrotreating

PENDAHULUAN

Naphta adalah fraksi dari hidrokarbon yang diperoleh dari proses pemisahan secara distilasi, yang mempunyai jumlah unsur karbon 6 – 10 (C6-C10) baik itu dalam bentuk parafin, olefin, naften, maupun aromatis. Proses hidrogenasi naphta sangat diperlukan baik di industri refinery maupun di industri petrochemical.
Hydrotreating atau disebut juga hydroprocessing adalah proses hidrogenasi katalitik untuk menjenuhkan hidrokarbon dan menghilangkan sulfur, nitrogen, oksigen, dan logam dari aliran proses. Hydrotreating biasa dilakukan untuk umpan naptha sebelum dialirkan ke unit platforming, karena katalis platforming (platina) sangat sensitif terhadap impurities seperti sulfur, nitrogen,oksigen, dan logam. Hydrotreating biasa juga dilakukan untuk umpan diesel untuk perbaikan kualitas diesel terutama untuk mengurangi kandungan sulfur dalam diesel (spesifikasi produk diesel dari tahun ke tahun semakin ketat terutama dalam hal kandungan sulfur maksimum) dan juga untuk mengurangi kandungan nitrogen dalam diesel yang dapat menyebabkan terjadinya color unstability produk diesel.

Tujuan proses hydrotreating/hydroprocessing adalah :
1. Memperbaiki kualitas produk akhir (seperti diesel)
2. Pretreating stream (persiapan umpan proses lanjutan) untuk mencegah keracunan katalis di downstream process :
• Catalytic Reforming (Platforming)
• Fluid Catalystic Cracking (FCC)
• Hydrocracking
3. Memenuhi standar lingkungan (untuk diesel sebelum dikirim ke tangki penyimpanan produk)

Pemilihan tipe katalis bergantung pada aplikasi dan aktivitas / selektivitas yang diinginkan.
• Tipe CoMo : cocok untuk HDS
• Tipe NiMo : cocok untuk HDN, penjenuhan olefin
• Tipe NiW : cocok untuk Hydrocracking, penjenuhan olefin


METODOLOGI

Teori Hydrotreating
Reaksi hydrotreating dikelompokkan menjadi :
1. Saturasi olefin (penjenuhan hidrokarbon).
2. Desulfurisasi (penghilangan sulfur) atau sering disebut HDS (hydrodesulfurization).
3. Denitrifikasi (penghilangan nitrogen) atau sering disebut (hydrodenitrification).
4. Deoksigenasi (penghilangan oksigen).
5. Demetalisasi (penghilangan logam) atau sering disebut HDM (hydrodemetalization).


Reaksi yang terjadi di unit Hidrotreating

- Reaksi Hydrodesulfurization

Umumnya reactor inlet temperature 315-340oC akan memberikan kecepatan reaksi hydorgenasi yang cukup dan tidak akan menyebabkan rekombinasi olefin dan hydrogen sulfide (namun tergantung komposisi feed, tekanan operasi, dan LHSV).
Untuk unit naphtha hydrotreater, karena heavy naphtha produk naphtha hydrotreater akan digunakan sebagai umpan unit platforming maka batasan umpan kandungan sulfur dalam produk heavy naphta adalah 0,5 ppm, agar tidak meracuni katalis platforming yang sangat sensitif terhadap impurities. Sedangkan untuk unit distilate (diesel hidrotreater), kandungan sulfur outlet reaktor dapat dijaga sesuai keinginan kita (spesifikasi produk diesel indonesia saat ini masih 500 ppm sulfur, sedangkan spesifikasi diesel yang ada di negara maju sudah ada yang mencapai 30 ppm atau bahka maximum 10 ppm sulfur. Untuk mengatur kandungan sulfur dalam produk dapat dilakukan dengan mengatur temperatur reaktor (naiknya temperatur reaktor akan mengurangi kandungan sulfur dalam produk)

- Reaksi Hidrodenitrification
Biasanya kandungan nitrogen dalam umpan lebih sedikit daripada kandungan sulfur dalam umpan. Namun, reaksi penghilangan nitrogen jauh lebih sulit daripada reaksi penghilangan sulfur, yaitu kurang lebih 5 kali lebih sulit. Untuk unit naphtha hydrotreater, karena heavy naphtha produk naphtha hydrotreater akan digunakan sebagai umpan unit platforming maka batasan maksimum kandungan sulfur dalam produk heavy naphtha adalah 0,5 ppm, agar tidak meracuni katalis platforming yang sangat sensitive terhadap impurities.
Nitrogen yang masuk ke unit platforming akan menyebabkan endapan amonium cloride di circuit recycle gas atau sistem overhead stabilizer. Penghilangan nitogen di unit naphta hidrotreater sangat penting jika naphta hidrotreater mengolah cracked feed.
Sedangkan untuk unit distillate/diesel hydrotreater, walaupun tidak ada batasan maksimum nitrogen dalam produk diesel, namun kandungan nitrogen dalam produk diesel akan mempengaruhi color stability. Semakin rendah kandungan nitrogen, maka semakin tinggi color stability-nya.

- Reaksi Penghilangan Oksigen (deoxygenation)

- Reaksi penjenuhan Olefin

- Reaksi penghilangan senyawa halida
Halida organik dapat didekomposisi di unit naphta hidrotreater menjadi hidrogen halida yang kemudian diserap oleh wash water yang diinjeksikan di outlet reaktor atau diambil sebagai stipper gas. Dekomposisi halida organik jauh lebih sulit dari pada desulfurisasi. Biasanya maksimum organic halide removal sekitar 90%, tetapi dapat lebih kecil jika kondisi operasi hanya di-set untuk penghilangan sulfur dan nitrogen saja, untuk alasan ini maka ananlisa periodik terhadap kandungan cloride dalam hidrotreated naphta harus dilakukan, karena tingkat kandungan cloride ini akan digunakan untuk mengatur jumlah injeksi cloride di platformer (cloride di platformer dibutuhkan untuk menjaga suasana asam katalis platformer).

- Reaksi penghilangan senyawa logam
Sebagian besar impurities metal terjadi pada level part per billion (ppb) di dalam naphtha. Biasanya katalis naphtha hydrotreater atau distillate hydrotreater mampu menghilangkan senyawa metal ini pada konsentrasi yang cukup tinggi, yaitu hingga 5 ppmwt atau lebih, dengan basis intermittent pada kondisi normal operasi. Impurities metal ini tetap berada di dalam katalis hydrotreater dan dianggap sebagai racun katalis permanent karena meracuni
katalis secara permanen, tidak dapat dihilangkan dengan cara regenerasi katalis. Beberapa logam yang sering terdeteksi dalam spent catalyst hydrotreater adalah arsenic, iron, calcium, magnesium, phosphorous, lead (timbal), silicon, copper, dan sodium.
Iron biasanya ditemukan terkonsentrasi pada bagian atas catalyst bed sebagai iron sulfide.


Kinerja Katalis

Kinerja katalis dapat diketahui atau diukur dengan beberapa parameter sebagai berikut :
• Analisa laboratorium kandungan sulfur, nitrogen, dan olefin (bromine number) pada produk. Jika kandungan sulfur, nitrogen, dan olefin naik pada temperature inlet reactor dan kapasitas serta komposisi feed yang sama, maka berarti kinerja katalis sudah mulai menurun dan untuk menjaga kandungan sulfur, nitrogen, dan olefin yang sama maka temperature inlet reactor harus dinaikkan.

• ∆T reaktor, yaitu selisih antara temperature bed reaktor tertinggi dengan temperature inlet reaktor. Jika ∆T reaktor menurun pada kapasitas dan komposisi feed yang sama, maka berarti kinerja katalis sudah mulai menurun.

• ∆P (pressure drop) reaktor, yaitu penurunan tekanan reaktor akibat adanya impurities yang mengendap pada katalis. Biasanya terjadi kalo feed mengandung cracked feed dalam jumlah yang besar atau feed berasal dari tangki penyimpanan yang tidak dilengkapi dengan gas/nitrogen blanketting sehingga feed akan bereaksi dengan oksigen yang akan membentuk gums pada permukaan katalis.


Deaktivasi Katalis
Deaktivasi katalis atau penurunan aktivitas katalis dapat disebabkan oleh beberapa faktor yaitu :
- Akumulasi senyawa ammonia pada katalis
Reaksi hydrotreating akan mengubah senyawa nitrogen organic yang ada dalam umpan menjadi ammonia. Jika kandungan ammonia dalam recycle gas tinggi, maka ammonia akan berebut tempat dengan umpan untuk mengisi active site katalis. Jika active site katalis tertutup oleh ammonia maka aktivitas katalis akan langsung menurun. Untuk menghindari terjadinya akumulasi ammonia pada permukaan katalis, diinjeksikan wash water pada effluent reactor, sehingga ammonia akan larut dalam air dan tidak menjadi impurities bagi recycle gas. Ammonia bersifat racun sementara bagi katalis. Jika injeksi wash water dihentikan atau kurang maka akan terjadi akumulasi ammonia pada permukaan katalis, namun setelah injeksi wash water dijalankan kembali maka akumulasi ammonia pada permukaan katalis akan langsung hilang.
- Coke
Coke dapat terjadi karena beberapa hal sebagai berikut :
ƒ 1. Temperatur reaksi yang tidak sesuai (temperatur terlalu tinggi atau umpan minyak terlalu ringan).
ƒ 2. Hydrogen partial pressure yang rendah (tekanan reaktor atau hydrogen purity recycle gas yang rendah).
ƒ 3. Jumlah recycle gas yang kurang (jumlah H2/HC yang kurang/lebih rendah daripada disain).
Pembentukan coke dapat dihambat dengan cara menaikkan hydrogen partial pressure (tekanan reaktor atau hydrogen purity pada recycle gas), atau penggunaan carbon bed absorber untuk menyerap HPNA.
- Keracunan logam
Pada proses penghilangan logam dari umpan, senyawa logam organic terdekomposisi dan menempel pada permukaan katalis. Jenis logam yang biasanya menjadi racun katalis hydrocracker adalah nikel, vanadium, ferro, natrium, kalsium, magnesium, silica, arsenic, timbal, dan phospor. Keracunan katalis oleh logam bersifat permanent dan tidak dapat hilang dengan cara regenerasi. Keracunan logam dapat dicegah dengan membatasi kandungan logam dalam umpan. Best practice batasan maksimum kandungan logam yang terkandung dalam umpan hydrotreater adalah 1,5 ppmwt untuk nikel dan vanadium, 2 ppmwt untuk ferro dan logam lain, serta 0,5 ppmwt untuk natrium.

Feed dan Produk Hydrotreating
Unit hydrotreating dapat berupa naphtha hydrotreater atau distillate/diesel hydrotreater. Umpan naphtha hydrotreater adalah naphtha yang dapat berupa straight run naphtha, naphtha dari tangki penyimpan, ataupun cracked naphtha. Jika umpan naphtha berasal dari tangki maka harus diyakinkan bahwa tangki dilengkapi dengan gas atau nitrogen blanketing. Jika tangki tidak dilengkapi
dengan gas atau nitrogen blanketing, maka naphtha kemungkinan akan bereaksi dengan oksigen (yang berasal dari udara; biasanya tangki naphtha adalah floating roof yang sangat mungkin terdapat kebocoran seal sehingga dapat menyebabkan udara luar masuk ke dalam tangki) yang kemudian akan menyebabkan terbentuknya gums. Gums ini biasanya terbentuk pada preheater atau bahkan pada permukaan katalis. Sedangkan umpan distillate/diesel hydrotreater adalah straight run diesel atau cracked diesel. Jika mengolah cracked diesel, maka perlu diketahui batasan maksimumnya karena cracked diesel membawa cracked material/olefin yang akan mempengaruhi operasi hydrotreater. Selain itu cracked diesel sangat mungkin mengandung nitrogen yang tinggi. Kandungan nitrogen yang tinggi akan mempengaruhi tingkat color stability produk diesel.
Produk unit hydrotreating dapat berupa hydrotreated heavy naphtha atau hydrotreated diesel. Hydrotreated heavy naphtha merupakan intermediate product yang kemudian merupakan umpan unit platforming. Hydrotreated heavy naphtha harus mempunyai kandungan sulfur dan nitrogen maksimum 0,5 ppmwt dan kandungan logam maksimum 2 ppmwt. Sedangkan hydrotreated diesel merupakan produk jadi siap dipasarkan dengan kandungan sulfur antara 10 ppmwt, 30 ppmwt, atau 500 ppmwt.

Referensi

Operation Manual for Unit 200 Naphtha Hydrotreating Process Unit, Pakistan-Arabian Refinery Limited, Mid-Country Refinery Project (PARCO), Mahmood Kot, Pakistan.

What's Petrochemical...?

Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane.

Two petrochemical classes are olefins including ethylene and propylene, and aromatics including benzene, toluene, and xylene isomers. Oil refineries produce olefins and aromatics by fluid catalytic cracking of petroleum fractions. Chemical plants produce olefins by steam cracking of natural gas liquids like ethane and propane. Aromatics are produced by catalytic reforming of naphtha. Olefins and aromatics are the building-blocks for a wide range of materials such as solvents, detergents, and adhesives. Olefins are the basis for polymers and oligomers used in plastics, resins, fibers, elastomers, lubricants, and gels.

Global ethylene and propylene production are ~110 million tonnes and ~65 million tonnes per annum, respectively. Aromatics production is ~70 million tonnes. The largest petrochemical industries are located in the USA and Western Europe; however, major growth in new production capacity is in the Middle East and Asia. There is substantial inter-regional petrochemical trade.

Primary petrochemicals are divided into three groups depending on their chemical structure:

* Olefins includes ethylene, propylene, and butadiene. Ethylene and propylene are important sources of industrial chemicals and plastics products. Butadiene is used in making synthetic rubber.
* Aromatics includes benzene, toluene, and xylenes. Benzene is a raw material for dyes and synthetic detergents, and benzene and toluene for isocyanates MDI and TDI used in making polyurethanes. Manufacturers use xylenes to produce plastics and synthetic fibers.
* Synthesis gas is a mixture of carbon monoxide and hydrogen used to make ammonia and methanol. Ammonia is used to make the fertilizer urea and methanol is used as a solvent and chemical intermediate.

The prefix "petro-" is an arbitrary abbreviation of the word "petroleum"; since "petro-" is Ancient Greek for "rock" and "oleum" means "oil". Therefore, the etymologically correct term would be "oleochemicals". However, the term oleochemical is used to describe chemicals derived from plant and animal fats.



Oil to Petrochemicals

Petrochemicals are chemicals made from petroleum (crude oil) and natural gas. Petroleum and natural gas are made up of hydrocarbon molecules, which are comprised of one or more carbon atoms, to which hydrogen atoms are attached.

Currently, oil and gas are the main sources of the raw materials because they are the least expensive, most readily available, and can be processed most easily into the primary petrochemicals listed on the left.

Only about five percent of the oil and gas consumed each year is needed to make all the petrochemical products.
Petrochemicals have had a dramatic impact on our food, clothing, shelter and leisure. Some synthetics, tailored for particular uses, actually perform better than products made by nature because of their unique properties.

Primary Petrochemicals:

"Primary Petrochemicals" include: olefins (ethylene, propylene and butadiene) aromatics (benzene, toluene, and xylenes); and methanol.

Olefins are unsaturated molecules of carbon (C) and hydrogen (H) that appear as short chains, of two, three or four carbons in length.
Aromatics contain a six carbon ring structure. The oxygen/hydrogen (OH) group in methanol denotes that it is an alcohol.


Intermediates and Derivatives:

Petrochemical intermediates are generally produced by chemical conversion of primary petrochemicals to form more complicated derivative products (see graphic on the left).

Petrochemical derivative products can be made in a variety of ways: directly from primary petrochemicals; through intermediate products which still contain only carbon and hydrogen; and, through intermediates which incorporate chlorine, nitrogen or oxygen in the finished derivative. In some cases, they are finished products; in others, more steps are needed to arrive at the desired composition.

Of all the processes used, one of the most important is polymerization. It is used in the production of plastics, fibers and synthetic rubber, the main finished petrochemical derivatives.
Some typical petrochemical intermediates are:
vinyl acetate for paint, paper and textile coatings
vinyl chloride for polyvinyl chloride (PVC)
resin manufacture
ethylene glycol for polyester textile fibers
styrene which is important in rubber and plastic manufacturing.

Major End Use Products:

Some typical petrochemical intermediates are:
- vinyl acetate for paint, paper and textile coatings vinyl chloride for
- polyvinyl chloride PVC)
- resin manufacture
- ethylene glycol for polyester - textile fibers
- styrene which is important in rubber and plastic manufacturing.

Control Valve

What Is A Control Valve?

Process plants consist of hundreds, or even thousands, of control loops all
networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure,flow, level, temperature, etc. within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable.
To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired setpoint. A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring,comparing, and calculating are done,some type of final control element must implement the strategy selected by the controller.
The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam,water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.
Many people who talk about control valves or valves are really referring to a control valve assembly. The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive power to operate the valve, and a varietyof additional valve accessories, which can include positioners, transducers,supply pressure regulators, manual operators, snubbers, or limit switches.

Whether it is called a valve, control valve or a control valve assembly is not as important as recognizing that the control valve is a critical part of the control loop. It is not accurate to say that the control valve is the most important part of the loop. It is useful to think of a control loop as an instrumentation chain. Like any other chain,the whole chain is only as good as its weakest link. It is important to ensurethat the control valve is not the weak est link.
Following are definitions for process control, sliding-stem control valve,rotary-shaft control valve, and other control valve functions and character istics terminology.


Process Control Terminology

Accessory: A device that is mounted on the actuator to complement the actuator’s function and makeit a complete operating unit. Examples include positioners, supply pressure regulators, solenoids, and limit switches.

Actuator*: A pneumatic, hydraulic,or electrically powered device that supplies force and motion to open or close a valve.

Actuator Assembly: An actuator,including all the pertinent accessories that make it a complete operating unit.

Backlash: The general name given to a form of dead band that results from a temporary discontinuity between the input and output of a device when the input of the device changes direction. Slack, or looseness of a mechanical connection is a typical example.

Capacity* (Valve): The rate of flow through a valve under stated conditions.

Closed Loop: The interconnection of process control components such that information regarding the process variable is continuously fed back to the controller set point to provide continuous, automatic corrections to the process variable.

Controller: A device that operates automatically by use of some established algorithm to regulate a controlled variable. The controller input receives information about the status of the process variable and then provides an appropriate output signal to the final control element.

Control Range: The range of valve travel over which a control valve can maintain the installed valve gain between the normalized values of 0.5 and 2.0.

Control Valve Assembly: Includes all components normally mounted on the valve: the valve body assembly, actuator, positioner, air sets, transducers, limit switches, etc.

Dead Band: The range through which an input signal can be varied,upon reversal of direction, without initiating an observable change in the output signal. Dead band is the name given to a general phenomenon that can apply to any device. For the valve Figure 1-1. Process Dead Band A7152 / IL assembly, the controller output (CO) is the input to the valve assembly and the process variable (PV) is the output as shown in figure 1-1. When the term Dead Band is used, it is essential that both the input and output variables are identified, and that any tests to measure dead band be under fully loaded conditions. Dead band is typically expressed as a percent of the input span.

Dead Time: The time interval (Td) in which no response of the system is detected following a small (usually 0.25% - 5%) step input. It is measured from the time the step input is initiated to the first detectable response of the system being tested. Dead Time can apply to a valve assembly or to the entire process.

Disk: A valve trim element used to modulate the flow rate with either linear or rotary motion. Can also be referred to as a valve plug or closure member.

Gain: An all-purpose term that can be used in many situations. In its most general sense, gain is the ratio of the magnitude of the output change of a given system or device to the magnitude of the input change that caused the output change. Gain has two components: static gain and dynamic gain.
Static gain is the gain relation ship between the input and output and is an indicator of the ease with which the input can initiate a change in the Figure 1-2.


Linearity*: The closeness to which a curve relating to two variables approximates a straight line. (Linearity also means that the same straight line will apply for both upscale and downscale directions. Thus, dead band as defined above, would typically be considered a non-linearity.)

Linear Characteristic*: An inherent flow characteristic that can be repre-sented by a straight line on a rectangular plot of flow coefficient (Cv) versus rated travel.

Packing: A part of the valve assembly used to seal against leakage around the valve disk or stem.

Positioner*: A position controller (servomechanism) that is mechanically connected to a moving part of a final control element or its actuator and that automatically adjusts its output to the actuator to maintain a desired position in proportion to the input signal.
Relay: A device that acts as a power amplifier. It takes an electrical, pneumatic, or mechanical input signal and produces an output of a large volume flow of air or hydraulic fluid to the actuator. The relay can be an internal component of the positioner or a separate valve accessory.


Trim*: The internal components of a valve that modulate the flow of the controlled fluid.