The Process Thermodynamics And Mechanics Biology Essay

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Isomerisation is one of the most important processes in the current industrial world. The process was discovered during World War Two where isomerisation was used to increase the supply of isobutene in the production of high-octane aviation gasoline. Currently, a type of isomerisation process namely hydroisomerization is extensively used in petroleum refining industries. In definition, hydroisomerization is a chemical process of converting waxy n-alkanesor straight chained hydrocarbons such as pentane and hexane into isoalkanes. Hydroisomerization relies on specialized catalysts which would be discussed more in detail further on in this report. The term 'hydro' in the name of the process indicates that the process is carried out in the presence of excess hydrogen which is crucial in the prevention of coke deposition on the surface of the catalysts used. As a result, this significantly increases the lifespan of the catalyst which make up the bulk of the total processing cost of a hydroisomerisation unit.RefineryFlow.png

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Since the discovery of the process in the early 1900s, wide arrays of different hydroisomerization methods have since been developed for use in industries. Some of these methods include the Penex process and the Tip and Once-Through Zeolitic Isomerization process developed by the well-known company UOP (Universal Oil Products). It should be noted these different methods of hydroisomerization will require different operating conditions, catalysts as well as various other factors. Consequently, this provides valuable options for petroleum refining companies to choose the best methods that will best meet their financial budget and forecasted production capacities.

Before delving more into the chemistry of the process, it is important that we understand the reason why hydroisomerization is done in the first place. As mentioned before, the main reason for hydroisomerization is the formation of isoalkanes. Essentially, isoalkanes are alkanes arranged in branched structures where these structural arrangements play a major role in determining the octane rating of a fuel. The low octane ratings of the light naphtha products which are the main feed materials for hydroisomerization make them especially susceptible to knocking or auto ignitions in engines. A known reason for this is because straight chained hydrocarbons are less able to withstand the high compression pressure inside a conventional combustion engine thus resulting in rapid release of large amount of heat thus decreasing overall engine efficiency. Furthermore, the combustion of fuels with low octane rating also yields lower energy output hence making them unfavourable amongst end users.

Traditionally, to counter this problem, the light naphtha products are directly mixed with lead antiknock additives and aromatic compounds such as benzene to significantly increase its octane rating as a result of their excellent lead susceptibility. However, recent scientific discoveries on the negative effects of these substances have resulted in the banning of their use in most countries. Governments worldwide began to write legislations preventing companies from using lead additives and minimizing the aromatic compound contents of their fuel. Therefore, the ability to structurally manipulate straight chained hydrocarbons into isomers without relying on banned chemicals through the process of hydroisomerization has significantly increased its adoption rate in industries.

Henceforth, this paper is interested in discussing the chemistry behind the hydroisomerization process. This will focus upon the major types of catalysts used as well as the operating conditions and other factors that are required by these different catalysts. Apart from that, an economic assessment focusing on the aforementioned catalysts use would also be included as a measure of process viability and feasibility in the current industrial status quo. Finally, this report is also aiming to highlight relevant current researches that are being carried out on improving the process as a whole.

Process thermodynamics and mechanics

After treating light Naphtha with hydrogen, the treated chains of paraffins of low Research Octane Number (RON) pass into the isomerization unit where they are transformed into branched chains using a specific catalyst. This results in a significant increase in RON. The heavy naphtha passes into the reformer unit to increase its RON as well. However, light naphtha consisting of C4-C6 are treated separately for two reasons. Firstly, light hydrocarbons tend to crack in the reformer and secondly, C6 tends to form benzene rings. The latter is not desired due to the carcinogenic effect of benzene and the current health and safety regulations.

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The conversion of n-paraffin to i-paraffin in the isomerisation reaction is not complete since the reaction is limited by equilibrium. Usually the extent of conversion to branched chains is measured by the increase in RON. Simultaneously, due to the exothermic nature of the reaction low temperatures are favoured in order to achieve higher conversions. However, according to the Arrhenius Equation low reaction temperature leads to slower reaction rates. For this reason, catalysts are inevitable in this process.

The reaction requires a dual-functional catalyst, which has a hydrogenation/dehydrogenation function usually provided by a metal such as Platinum as well as a cracking function available at an acid site.

Two main types of catalysts are used in the isomerisation process. Platinum-loaded chlorinated alumina catalysts are considered quite active; however, these types of catalysts had several disadvantages. Firstly, corrosion was unavoidable due to chlorine presence in the catalyst regeneration. Secondly, the catalyst deactivated easily when poisoned, which required careful treatment of feed to eliminate sulphur and water. Finally, carbon tetrachloride was essential to activate the catalyst and hence was injected in the feed. Due to the high pressure of hydrogen the chlorine transformed into HCl after its elusion from the catalyst. These disadvantages lead to the development and usage of Zeolites. Zeolites show higher stability and resistance to impurities relative to Platinum-loaded chlorinated alumina (Platinum Mordenite used by Shell).

Table 1 summarizes the different operating conditions in 2 types of catalysts (Platinum-loaded chlorinated alumina and Zeolites). Zeolites are less active and hence require higher temperature ranges. Product RON with Zeolites is about 80. However, combining a separation process and recycling normal alkanes back to the reactor can lead to RON of up to 90 (Isosiv Process of UOP).

Variable

Pt/Chlorine Alumina Catalyst

Pt/Zeolite Catalyst

Temperature (°C)

120-180

250-270

Pressure (bar)

20-30

15-30

H2/HCl

0.1-2

2-4

Product RON

83-84

78-80

The reaction mechanism normally progresses through a monomolecular mechanism. Alkanes are dehydrogenated onto the noble metal site of the catalyst resulting in alkene formation. Alkenes then start migrating until they reach a Brönsted acid site, over which they get protonated. This results in the formation of a positive carbenium-ion that then undergoes one of two transformations. Either the carbenium-ion encounters a structural transformation followed by migration to a metal site where it gets hydrogenated and forms a branched isomer of the starting material, or β-scission (free radicals are formed upon splitting the carbon-carbon bond) followed by hydrogenation on a metal site. The reaction mechanism requires a perfect balance between acid and metal functions with respect to available sites. If however this balance does not exist, side reactions such as hydrogenolysis on metal sites and dimerization-cracking on the acid sites will occur.

Another mechanism is the bimolecular one which is demonstrated in the figure below along with the monomolecular one. ::Desktop:Screen shot 2012-10-26 at 9.29.29 PM.png

Figure 1: Heptane undergoing hydroisomerization via bimolecular and monomolecular mechanisms. (Adapted fro Blomsma E; Martens, J. A. and Jacobs, P. A. (1996). Mechanisms of Heptane Isomerization on Bifunctional Pd/H-Beta Zeolites. Journal of Catalysis, 159, 323- 331.)

Catalyst

As described in the introduction catalysts have been used within the refining industry since 1930s. Since that period a variety of catalyst has been used, starting with activated Clay and moving onto the modern zeolite-mordenites loaded with platinum or a mixture of platinum, palladium and Iridium.

Activated clay was first used in 1930s, activated clays is primarily calcium bentonite (Clay) treated with sulphuric or hydrochloric acid. The treatment exchanges the calcium for hydrogen ions. Due the treatment the clay becomes more porous and has an increased absorptive property. [1] The clay allows for the isomerisation reaction to be carried out at relatively low pressures while allowing for high yields. Due to the chemical structure of the catalyst reactivation can be done by heating it. It must also be noted that the clay is cheap compared to other catalysts and is chemically inert and non-toxic. However the inherent ease of reactivation leads to problems while operating at higher temperatures, the catalyst begins to release moisture to the surroundings at temperatures as low as 50°C.

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Amorphous silica-Alumina was used from late 1930s to the mid-1960 as this provided a catalyst that allowed for an increase in yield and selectivity. However there were still issues of coking and deactivation of catalysts. In order to prevent or limit the formation of coke, hydrogen is introduced at high pressures and temperatures, this solved the problem of coking but the stability of the catalyst was still in question.

In the early 1950s UOP (Universal Oil Products) and other companies discovered methods of using platinum loaded alumina as catalysts, this solved the problem of coking without the need for high pressure hydrogen (The platinum, helps in the hydrogenation of species that would lead to coke formation). However this solution is not without problems; the first problem was the low acidity of alumina, this required the process to be operated at higher temperature (400 ËšC). A solution to this was the chlorination of the surface with hydrogen chloride, but there was still an issue of the catalyst getting deactivated by impurities and this meant they were not as nearly as robust as required. Time Scale.png

Zeolite catalysts were introduced during the 1960s and used in process such as Hysomer (Shell). The robust stability, inherent acidity and high selectivity of these catalysts made them ideal for use within the Hydroisomerization process. Figure.1 illustrated the time scale and the time period between the developments of catalyst in Hydroisomerization. It can be seen that the use of zeolite is just over 50 years old.

[Current Research] Current Research - Honeywell's new catalysts, HYSOMER? Any change or advancement? How does the proportion of platinum affect the activity?

There are many types of zeolites (eg: zeolite Y, beta and Mordenite) and the difference is based on the ratio of Silica to the Alumina. The table below (Table.1) summarizes some of the types of zeolites, ratio of Silica to Alumina as well as the surface area [i] . It must be noted that within each zeolite family there is a wide range of ratios, and as these ratios increase the surface area decreases.

Name

Ratio of Silica to Alumina

Surface Area (m2/g)

Zeolite Y (CBV 100)

5.1

900

Zeolite Beta (CP814E

25

680

Mordenite (CBV 10A)

13

425

ZSM-5 (CBV 2314)

23

425

Ferrerite (CP914C)

20

400

The most commonly used zeolites are Zeolite Y and Mordenite (figure 3a &3b).

Mordenite.gifFAU_111.gif

Economics

Hydroisomerization Process

UOP Penex Process (the most cost effective hydroisomerization process for new plants)

Components

Penex reactor unit

Deisohexanizer column

Molex unit (separation of n-pentane and n-hexane)

Economic Estimates

Basis: 10000 Barrels per day

Table 1: Typical Penex Estimated Investment Costs

Options

Once-through

million US$

Penex deisohexanizer

million US$

Penex-Molex

million US$

Material and labour

6.5

12.0

18.2

Design, engineering and contractor's expenses

2.7

4.3

6.3

Total estimated erected cost of ISBL unit

9.2

16.3

24.5

Table 2: Typical Penex Utility Requirements

Options

Once-through

Penex deisohexanizer

Penex-Molex

Electric Power (kW)

375

975

830

MP steam (1000 kg h-1)

9.4

12.0

9.6

LP steam (1000 kg h-1)

-

24.2

13.4

Cooling water (m3 h-1)

136

262

277

Table 3: Costs of Utilities

Electricity (£ kWh-1)

0.031 - 0.075

Cooling Water (£ tonne-1)

0.26

Figure : Source obtained from http://www.plantservices.com/articles/2002/137.htmlTable 4: Comparison between merchant supplied and on site generated hydrogenhttp://www.plantservices.com/Media/zzzzz204.jpg

Steam Costs

where

PHPS = price of HP steam (£ tonne-1)

PF = fuel price (£ GJ-1)

dHB = heating rate (GJ tonne of steam-1)

ηB = boiler efficiency (typically in the range of 0.8 - 0.9)

PBFW = cost of boiler feed water (typically £0.50 tonne-1)

*Costs of MP and LP steam are estimated based on the HP steam cost by taking account of the shaft work

Cost of crude oil $97.95 per barrel [ii] ($2.33 per gallon) (WTI Curde Oil) and the current price of petrol $ 3.687 per gallon [iii] 

Conclusion