Equation of Height Equivalent to a Theoretical Plate

The HETP Equation An HETP equation quantitatively describes the peak dispersion (band spreading) by expressing the �variance per unit length� of the column as a function of the linear mobile phase velocity and the various physical properties of the distribution system and the solute being eluted. The variance per unit length of the column is measured as the ratio of the column length to the column efficiency, i.e, the height of the theoretical plate or the HETP. There have been a number of different HETP equations developed, the first for packed columns was the Van Deemter equation which was followed by the Huber equation, the Giddings equation, the Horvath equation, the Knox equation and finally the Golay equation for capillary columns. All the equations, except the Knox equation, were developed from first principles, the Knox equation, however, was developed from experimentally observed relationships and subsequently rationalized on a first principle basis. All the HETP equations include functions that describe dispersion due to longitudinal diffusion and dispersion resulting from the resistance to mass transfer in both phases. All the HETP equations for packed columns include a function that describes dispersion that results from the tortuous paths taken by solute molecules as they wind their way through the intersticies of the packing. This dispersion process was given the term �eddy diffusion� by Van Deemter With the exception of the Giddings equation, the function for eddy diffusion dispersion is expressed as a constant depending on the particle diameter of the packing and independent of the mobile phase linear velocity. Giddings however, introduced a coupling function to the eddy diffusion term so that the dispersion only became constant and independent of mobile phase velocity at relatively high velocities. The Golay equation, as there was no packing, only contained three terms, one for longitudinal diffusion and one each for the resistance to mass transfer in the mobile and stationary phases respectively. The Huber and Horvath equations contain an extra term that involves a power function of the mobile phase velocity, largely to account for the curving of the HETP graph towards the base line at very high mobile phase velocities. Subsequently, this curving effect was shown to be an artifact resulting from experimental problems that arose when measuring the elution curves of very fast eluting peaks. All the equations have hyperbolic characteristics relating variance per unit length to linear mobile phase velocity. This means that the variance per unit length-mobile phase velocity curve shows a minimum at a particular velocity which has been termed the optimum linear velocity. The column will exhibit a maximum efficiency for a specific solute when operated at the optimum velocity. In practice, it has been shown that the equation that best describes the experimental data from a packed LC column is probably the Van Deemter equation.

Azeotrope

An azeotrope is a mixture of two or more pure compounds (chemicals) in such a ratio that its composition cannot be changed by simple distillation.^[1] This is because when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture of liquids. As the composition is unchanged by boiling, azeotropes are also known as constant boiling mixtures (especially in older texts). The word azeotrope is derived from the Greek words "ζειν"=boil and "τρόπος"=change, combining with prefix "α-"=no to give the overall meaning "no change on boiling".

Types of azeotropes

Each azeotrope has a characteristic boiling point. The boiling point of an azeotrope is either less than the boiling points of any of its constituents (a positive azeotrope), or greater than the boiling point of any of its constituents (a negative azeotrope).

A well known example of a positive azeotrope is 95.6% ethanol and 4.4% water (by weight). Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.1°C, which is lower than either of its constituents. Indeed 78.1°C is the minimum temperature at which any ethanol/water solution can boil. It is generally true that a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called minimum boiling mixtures.

An example of a negative azeotrope is hydrochloric acid at a concentration of 20.2% hydrogen chloride and 79.8% water (by weight). Hydrogen chloride boils at –84°C and water at 100°C, but the azeotrope boils at 110°C, which is higher than either of its constituents. Indeed 110°C is the maximum temperature at which any hydrochloric acid solution can boil. It is generally true that a negative azeotrope boils at a higher temperature than any other ratio of its constituents. Negative azeotropes are also called maximum boiling mixtures.

Azeotropes consisting of two constituents, such as the two examples above, are called binary azeotropes. Those consisting of three constituents are called ternary azeotropes. Azeotropes of more than three constituents are also known.

More than 18,000 azeotropic mixtures have been documented.^[2]

Combinations of solvents that do not form an azeotrope when mixed in any proportion are said to be zeotropic.

When running a binary distillation it is often helpful to know the azeotropic composition of the mixture

Mixing

Mixing is another important classifying feature for continuous reactors. Good mixing improves the efficiency of heat and mass transfer.

In terms of trajectory through the reactor, the ideal flow condition for a continuous reactor is plug flow (since this delivers uniform residence time within the reactor). There is however a measure of conflict between good mixing and plug flow since mixing generates axial as well as radial movement of the fluid. In tube type reactors (with or without static mixing), adequate mixing can be achieved without seriously compromising plug flow. For this reason, these types of reactor are sometimes referred to as plug flow reactors.

Continuous reactors can be classified in terms of the mixing mechanism as follows:

Mixing by diffusion

Diffusion mixing relies on concentration or temperature gradients within the product. This approach is common with micro reactors where the channel thicknesses are very small and heat can be transmitted to and from the heat transfer surface by conduction. In larger channels and for some types of reaction mixture (especially immiscible fluids), mixing by diffusion is not practical.

A simple tube can be used as a reactor. Small bore systems usually rely on mixing by diffusion

Mixing with the product transfer pump

In a continuous reactor, the product is continuously pumped through the reactor. This pump can also be used to promote mixing. If the fluid velocity is sufficiently high, turbulent flow conditions exist (which promotes mixing). The disadvantage with this approach is that it leads to long reactors with high pressure drops and high minimum flow rates. This is particularly true where the reaction is slow or the product has high viscosity. This problem can be reduced with the use of static mixers. Static mixers are baffles in the flow channel which are used to promote mixing. They are able to work with or without turbulent conditions. Static mixers can be effective but still require relatively long flow channels and generate relatively high pressure drops. The oscillatory baffled reactor is specialised form of static mixer where the direction of process flow is cycled. This permits static mixing with low net flow through the reactor. This has the benefit of allowing the reactor to be kept comparatively short.

The static mixer permits mixing with or without turbulent conditions

The oscillatory baffled reactor uses a combination of static mixing and cycling of flow direction.

Mixing with a mechanical agitator

Some continuous reactors use mechanical agitation for mixing (rather than the product transfer pump). Whilst this adds complexity to the reactor design, it offers significant advantages in terms of versatility and performance. With independant agitation, efficient mixing can be maintained irrespective of product throughput or viscosity. It also eliminates the need for long flow channels and high pressure drops.

One less desirable feature associated with mechanical agitators is the strong axial mixing they generate. This problem can be managed by breaking up the reactor into a series of mixed stages separated by small plug flow channels.

The most familiar form of continuous reactor of this type is the continuously stirred tank reactor (CSTR). This is essentially a batch reactor used in a continuous flow. The disadvantage with a single stage CSTR is that it can be relatively wasteful on product during start up and shutdown. The reactants are also added to a mixture which is rich in product. For some types of process, this can have an impact on quality and yield. These problems are managed by using multi stage CSTRs. At the large scale, conventional batch reactors can be used for the CSTR stages. At the small scale, this is less practical and Agitated Cell Reactors provide a simpler alternative.

The Agitated Cell Reactor is a multi stage CSTR

Esterifikasi

Pembuatan etil asetat

Dalam skala tabung uji

Asam karboksilat dan alkohol sering dipanaskan bersama dengan adanya beberapa tetes asam sulfat pekat untuk mengamati bau ester yang terbentuk.

Untuk melangsungkan reaksi dalam skala tabung uji, semua zat (asam karboksilat, alkohol dan asam sulfat pekat) yang dalam jumlah kecil dipanaskan di sebuah tabung uji yang berada di atas sebuah penangas air panas selama beberapa menit.

Karena reaksi berlangsung lambat dan dapat balik (reversibel), ester yang terbentuk tidak banyak. Bau khas ester seringkali tertutupi atau terganggu oleh bau asam karboksilat. Sebuah cara sederhana untuk mendeteksi bau ester adalah dengan menaburkan campuran reaksi ke dalam sejumlah air di sebuah gelas kimia kecil.

Terkecuali ester-ester yang sangat kecil, semua ester cukup tidak larut dalam air dan cenderung membentuk sebuah lapisan tipis pada permukaan. Asam dan alkohol yang berlebih akan larut dan terpisah di bawah lapisan ester.

Ester-ester kecil seperti pelarut-pelarut organik sederhana memiliki bau yang mirip dengan pelarut-pelarut organik (etil etanoat merupakan sebuah pelarut yang umum misalnya pada lem).

Semakin besar ester, maka aromanya cenderung lebih ke arah perasa buah buatan - misalnya "buah pir".

Dalam skala yang lebih besar

Jika anda ingin membuat sampel sebuah ester yang cukup besar, maka metode yang digunakan tergantung pada (sampai tingkatan tertentu) besarnya ester. Ester-ester kecil terbentuk lebih cepat dibanding ester yang lebih besar.

Untuk membuat sebuah ester kecil seperti etil etanoat, anda bisa memanaskan secara perlahan sebuah campuran antara asam metanoat dan etanol dengan bantuan katalis asam sulfat pekat, dan memisahkan ester melalui distilasi sesaat setelah terbentuk.

Ini dapat mencegah terjadinya reaksi balik. Pemisahan dengan distilasi ini dapat dilakukan dengan baik karena ester memiliki titik didih yang paling rendah diantara semua zat yang ada. Ester merupakan satu-satunya zat dalam campuran yang tidak membentuk ikatan hidrogen, sehingga memiliki gaya antar-molekul yang paling lemah.

Ester-ester yang lebih besar cenderung terbentuk lebih lambat. Dalam hal ini, mungkin diperlukan untuk memanaskan campuran reaksi di bawah refluks selama beberapa waktu untuk menghasilkan sebuah campuran kesetimbangan. Ester bisa dipisahkan dari asam karboksilat, alkohol, air dan asam sulfat dalam campuran dengan metode distilasi fraksional.

diambil dari www.chem-is-try.org

Gips

GIPSUM
Gipsum (CaSO4.2H2O) mempunyai kelompok yang terdiri dari gypsum batuan, gipsit alabaster, satin spar, dan selenit. Gipsum umumnya berwarna putih, namun terdapat variasi warna lain, seperti warna kuning, abu-abu, merah jingga, dan hitam, hal ini tergantung mineral pengotor yang berasosiasi dengan gypsum. Gipsum umumnya mempunyai sifat lunak, pejal, kekerasan 1,5 – 2 (skala mohs), berat jenis 2,31 – 2,35, kelarutan dalam air 1,8 gr/l pada 00C yang meningkat menjadi 2,1 gr/l pada 400C, tapi menurun lagi ketika suhu semakin tinggi.Gipsum terbentuk dalam kondisi berbagai kemurnian dan ketebalan yang bervariasi. Gipsum merupakan garam yang pertama kali mengendap akibat proses evaporasi air laut diikuti oleh anhidrit dan halit, ketika salinitas makin bertambah. Sebagai mineral evaporit, endapan gypsum berbentuk lapisan di antara batuan-batuan sedimen batugamping, serpih merah, batupasir, lempung, dan garam batu, serta sering pula berbentuk endapan lensa-lensa dalam satuan-satuan batuan sedimen. Gipsum dapat diklasifikasikan berdasarkan tempat terjadinya (Berry, 1959), yaitu: endapan danau garam, berasosiasi dengan belerang, terbentuk sekitar fumarol volkanik, efflorescence pada tanah atau goa-goa kapur, tudung kubah garam, penudung oksida besi (gossan) pada endapan pirit di daerah batugamping.

CHEMICAL EXPERIMENTS

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