CHAPTER 1

CHAPTER 1GENERAL DESCRIPTION

1.1Introduction

Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separation is a key part of most chemical processes, and there is a great variety of techniques to perform separation of compounds based on size, volatility, charge, and many other features. Heterogeneous mixtures consist of two or more parts (phases), which have different compositions. Meanwhile, a homogeneous mixture is known as the substance that contains only one kind of compound or element in which the reactants and products are all in the same phase with uniform composition and properties.The major equipment that will be discussed in this paper is a 2 phase separator. Separation in 2-phase separator can be done physically by exploiting the differences in density between the phases. Separator vessel thus simply means as a vessel or tank without internals that provides a phase separation. This separator vessel can be classified further into several categories based on their function. The general types of separator vessel are shown below:

Figure 1: General types of separator vessel

In this project, separator plays role in separation of flue gases from the water and formaldehyde. The separation is accomplished through an optimum temperature and pressure.

1.1.2Two Phase Separator

A vapour-liquid separator is a vessel into which a liquid and vapour or homogeneous mixture is fed and the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn. The vapour travels upward at a design velocity which minimizes the entrainment of any liquid droplets in the vapour as it exits the top of the vessel. Vapour-liquid separators are very widely used in a great many industries and applications such as: Oil refineries Natural gas processing plants Petrochemical and chemical plants Refrigeration systems Air conditioning Compressor systems for air or gases Gas pipelines

1.2Two Phase Separator

A vapour-liquid separator might consist simply of an empty vessel, which causes the fluid velocities in the entering pipe to be reduced by enlarging the cross-sectional area of flow. However the separator includes internal parts, to promote separation of the process. Below are the steps in designing a two-phase separator: Primary separation section (entrance)For separating the bulk of liquid from the gas in the feed. It is desirable to remove the liquid slugs and large droplets of liquid quickly from the gas stream, and to remove gas from the liquid.

Secondary separation sectionFor removing smaller particles of liquid by gravity settling depends to a large extent on the decreased gas velocity and reducing the turbulence of gas.

Liquid separation section (liquid accumulation section)For removing gas bubbles which may be blocked with the liquid, and for sufficient storage of the liquid to handle the slugs of liquid anticipated in routine operation.

Mist, extractor or eliminator sectioni. For removing the gas entrained drops from the liquid, that did not separate in the secondary separation section. ii. Mist extractor might be used to decrease the amount of entrained liquid in the gas and to reduce diameter of the vessel.

Vortex breaker (bottom of the vessel)Prevents potential pump suction problems during the utilization of the pump to remove collected liquids.

1.3Design Methodology

The design methodology is divided into 2 major sections; the process design and mechanical design. The process design will determine the reactor volume through Levenspiel plot while heat transfer and pressure drop determination by Erguns method. The second section which is mechanical design will be based on British Standard 5500 reference and design values were referred to data provided in the Mechanical Design of Process Equipment Data Hand Book. Overall design as follows: Selecting major equipment Two Phase Separator (T-103). Select and justify the type of reactor suitable. Collect raw data from HYSYS simulation. Determine the optimum operating conditions using Golden Section Search method. Determine the volume based on rate of reaction. Determine reactor sizing. Do reactor mechanical design. Equipment Cost Analysis. Technical Drawing for T-103 design. Start-up and Shutdown Procedures description.

1.3Separator Selection Two-phase separators may be oriented either vertically or horizontally. In some cases, it may necessary to compare both designs to determine which is more economic.Type of 2 Phase SeparatorHorizontal SeparatorVertical Separator

General FeaturesA vessel, with its cylindrical axes parallel to the ground, which is used to separate oil, gas and water from the produced stream.

A vessel with its cylindrical axes perpendicular to the ground that is used to separate oil, gas and water from the production stream.

Advantages

Disadvantages

Figure 2: Major components in horizontal 2 phase separator

Figure 3: Internal parts of a) Vertical and b) Horizontal Vapour-Liquid Separator

1.4Stages of Separation in Phase Separator

a. Primary SeparationUses an inlet diverter so that the momentum of liquid entrained in the vapour causes the largest droplets to impinge on the diverter and the drop by gravity.

b. Secondary SeparationGravity separation of smaller droplets as the vapour flows through the disengagement area.

c. Final StageMist elimination where the smallest droplets are coalesced so that larger droplets are formed which will be separated by gravity.

1.5Method of Operation

Method of operation is crucial to plan ahead on the formulation and design of the equipment. In this plant design, the 2 phase separator method of operation is classified under gravity separators. The separation process in this gravity separator occurs by settling and sedimentation and depends on gravitational force. Liquid droplets will settle out of a gas phase if the gravitational force acting on the droplet or particle is greater than the drag force of the gas flowing around the droplet or particle. Gravitational forces control separation whereby the lower the gas velocity and the larger the vessel size, the more efficient the liquid and gas separation took place. Since large vessel size is required to achieve settling, gravity separators are rarely designed to remove droplets smaller than 300 microns. Gravity separators are sometimes also called scrubbers when the ratio of gas rate to liquid rate is very high.

CHAPTER 2PROCESS DESIGN

2.1Introduction

As mentioned in previous chapter on the 3 main stages of separation process in a 2 phase separator, the following criteria should be met in designing the 2 Phase Separator Column, T-103: Force balance on the liquid droplet settling Calculation of allowable velocity for secondary separation to determine disengagement area. Separator K values

2.2Operating Conditions

Separator (T-103) inlet feed operating conditions are as follows:I. Inlet Temperature (0C) = -900CII. Inlet Pressure (bar) = 30bar

ComponentMole FractionMass Flow Rate (kg/h)Molar Flow Rate (kmol/hr)

Formaldehyde 0.2572864.89995.415

Methanol00.0250.001

Water0.2581726.94695.861

Argon084.8702.125

Nitrogen04996.628178.368

Carbon Dioxide0.0063.0160.069

Carbon Monoxide04.2430.152

Oxygen0.48084.8700

Dimethyl ether00.0010

Total1.0009680.629371.989

2.3Process DescriptionFigure 4: Block diagram of 2 Phase Separator

Figure 5: Simulation diagram of 2 Phase Separator

Diagram in Figure 4 and Figure 5 shows the block diagram and simulation representation of the 2 phase separator respectively that is used as part of the formaldehyde plant design. The feed channelled to the 2 phase separator is from the vapour product of the absorber. Both the vapour and liquid streams leaving the absorber are at about 65C. Liquid stream is the final product containing 37% weight of formaldehyde with the rest water. Meanwhile the vapour stream leaving the absorber is mostly flue gases with a considerable amount of water and traces of formaldehyde. Therefore water and formaldehyde can be further separated for recycling purposes.In order to achieve separation of formaldehyde as liquid, a lower temperature and higher pressure is needed. The vapour stream is fed to a compressor to raise the pressure to 283.3kPa. The vapour stream is fed to a compressor to raise the pressure to 283.3kPa. There will also be an increase in temperature. Since this stream requires cooling, it is directed to other streams requiring heating to conserve energy. After integration with two other streams that require heating, the temperature has been brought down to about 80C. Further cooling was achieved using a cooler to bring the streams temperature down to -90C. The pressurized and cooled vapour stream is then being fed into a 2 phase separator. In the separator, separation of flue gases from the water and formaldehyde is simple due to the optimum temperature and pressure. Flue gases leaving as vapour will be purged from the system. The continuous removal of substances from the system through this purge allows the process to reach steady state after a certain amount of accumulation. Without purge, the system will continue to accumulate and will not achieve steady state. As for the liquid stream leaving the separator, it contains mostly water and formaldehyde.2.4 Vertical Two Phase Separator

Vertical vapour and liquid separators are preferred for separating liquid from mixtures with a high vapour/liquid ratio while horizontal separators are preferred for separating vapour from mixtures with a low vapour/liquid ratio. As mentioned earlier on the process description of the separator, the feed is a pressurized vapour composition. Therefore, vertical separator is chosen to perform the design calculations.

Figure 4: Vertical Two Phase Separator

For vertical separators, the vapour disengagement area is the entire cross-sectional area of the vessel. Therefore the vapour disengagement diameter can be calculated from equation below:

Following are the steps and their associated formula for the vertical separator design procedures: