Pinch Technology

While oil prices continue to climb, energy conservation remains the prime concern for many process industries. The challenge every process engineer is faced with is to seek answers to questions related to their process energy patterns.

Will see about "Pinch Technology"

Meaning of the term "Pinch Technology"

The term "Pinch Technology" was introduced by Linnhoff and Vredeveld to represent a new set of thermodynamically based methods that guarantee minimum energy levels in design of heat exchanger networks. Over the last two decades it has emerged as an unconventional development in process design and energy conservation. The term ‘Pinch Analysis’ is often used to represent the application of the tools and algorithms of Pinch Technology for studying industrial processes.

Basic Concepts of Pinch Analysis

Most industrial processes involve transfer of heat either from one process stream to another process stream (interchanging) or from a utility stream to a process stream. In the present energy crisis scenario all over the world, the target in any industrial process design is to maximize the process-to-process heat recovery and to minimize the utility (energy) requirements. To meet the goal of maximum energy recovery or minimum energy requirement (MER) an appropriate heat exchanger network (HEN) is required. The design of such a network is not an easy task considering the fact that most processes involve a large number of process and utility streams.With the advent of pinch analysis concepts, the network design has become very systematic and methodical. 

A summary of the key concepts, their significance, and the nomenclature used in pinch analysis is given below:

• Combined (Hot and Cold ) Composite Curves: Used to predict targets for

• Minimum energy (both hot and cold utility) required,

• Minimum network area required, and

• Minimum number of exchanger units required.

• DTmin and Pinch Point: The DTmin value determines how closely the hot and cold composite curves can be ‘pinched’ (or squeezed) without violating the Second Law of Thermodynamics (none of the heat exchangers can have a temperature crossover).

• Grand Composite Curve: Used to select appropriate levels of utilities (maximize cheaper utilities) to meet over all energy requirements.

• Energy and Capital Cost Targeting: Used to calculate total annual cost of utilities and capital cost of heat exchanger network.

• Total Cost Targeting: Used to determine the optimum level of heat recovery or the optimum DTmin value, by balancing energy and capital costs. Using this method, it is possible to obtain an accurate estimate (within 10 - 15%) of overall heat recovery system costs without having to design the system. The essence of the pinch approach is the speed of economic evaluation. 

• Plus/Minus and Appropriate Placement Principles: The "Plus/Minus" Principle provides guidance regarding how a process can be modified in order to reduce associated utility needs and costs. The Appropriate Placement Principles provide insights for proper integration of key equipments like distillation columns, evaporators, furnaces, heat engines, heat pumps, etc. in order to reduce the utility requirements of the combined system.

• Total Site Analysis: This concept enables the analysis of the energy usage for an entire plant site that consists of several processes served by a central utility system.

How a Centrifugal Pump works

This is the basic requirement for chemical engineers to know about centrifugal pump operation and its principles, This will be even useful when you go for interview.

A centrifugal pump is a kinetic device. Liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. That kinetic energy is available to the fluid to accomplish work. In most cases, the work consists of the liquid moving at some velocity through a system by overcoming resistance to flow due to friction from pipes, and physical restrictions from valves, heat exchangers and other in-line devices, as well as elevation changes between the liquid's starting location and final destination. When velocity is reduced due to resistance encountered in the system, pressure (P) increases. As resistance is encountered, the liquid expends some its energy in the form of heat, noise, and vibration in overcoming that resistance. The result is that the available energy in the liquid decreases as the distance from the pump increases. The actual energy available for work at any point in a system is a combination of the available velocity and pressure energy at that point.

Head

Head (H) is the term that is used to define the energy supplied to the liquid by the pump. It is independent of the type of liquid being pumped. Head is expressed in Feet or Meters. In the absence of any velocity, it is equal to the height of a static column of liquid that could be supported by the pressure (P) at a given point in the system. In practice, pressure is usually measured by a pressure sensing device such as a gage or pressure transducer. Head (H) is the ratio of pressure to the Density (Specific weight) of a liquid. For water at 60^oF, head (H) may be calculated, from a pressure reading, using the following equation:

This may be simplified to H = P * 2. 31. The units cancel out so that only feet remain. For a liquid with a Density other than water, divide by the specific gravity of the liquid.

Because specific gravity is an index number (dimensionless) the units remain as feet of head.

Flow Rate

Flow rate is determined by the impeller geometry and its rotational speed. Pump designers manipulate the impeller vane design to achieve an optimum throughput velocity for an impeller. The throughput velocity (ft/sec) multiplied by the usable area of the impeller inlet (ft²) yields the flow rate (ft³/sec). Every impeller has one optimum design flow rate for a given speed and diameter. This is the best efficiency point of the pump. At other flow rates there will be a mismatch between the vane angle at the pump inlet and the flow rate, resulting in increased turbulence and loss of efficiency within the pump.

Total Dynamic Head

Total Dynamic Head (TDH) is the difference in head between the pump outlet and inlet. In actual practice, readings must be corrected for piping losses, gauge location and differences in pipe diameters between the pump inlet and outlet; all unique to the specific pump/system setup.

Pump Efficiency

Pump efficiency is the ratio of hydraulic horsepower to the brake horsepower required to drive the pump. Hydraulic horsepower is the kinetic power available at the pump discharge. It is calculated by the following equation.

Brake horsepower is measured at the pump input shaft by a torque-meter coupling or similar device. The difference between brake horsepower and hydraulic horsepower is the amount of power consumed by mechanical losses, noise, heat, viscous drag, and internal recirculation.

Characteristic Curves

Curves are available from pump manufacturers that depict the ‘as new’ performance characteristics for any given pump model. These may be either generic catalog curves that represent typical values, or they may be test curves that show the actual performance of a customer’s particular pump unit. Performance curves show plots of TDH, Efficiency, BHP and, when specified, NPSHR as functions of flow rate.
The shape of a pump curve is primarily determined by the geometry of the impeller. High flow - low head pumps typically have steeper curves than low flow - high head units.
The performance of a pump when placed in a system is a function of the interaction between the pump and system as defined by their relative characteristics.

Liquefied Natural Gas

Liquefied Natural Gas is emerging technology in this decade. For Chemical Engineers, this will be the good field to learn and earn. I have added some points about LNG here.

LNG is natural gas (Predominantly methane) that has been converted into liquid form for ease of storage and transportation.
LNG takes up volume of 1/600th volume of NG. It is odorless, colorless, non - toxic, non - corrosive. Hazards include flammability, freezing and asphyxia.
The li           liquefaction process involves removal of certain components, such as dust, acid gases, helium, water, and heavy hydrocarbons, which could cause difficulty downstream. The natural gas is then condensed into a liquid at close to atmospheric pressure (Maximum Transport Pressure set around 25 kPa/3.6 psi) by cooling it to approximately −162 °C (−260 °F).
The reduction in volume makes it much more cost-efficient to transport over long distances where pipelines do not exist. Where moving natural gas by pipelines is not possible or economical, it can be transported by specially designed cryogenic ( cryogenics is the study of the production of very low temperature (below −150 °C, −238 °F or 123 K) and the behavior of materials at those temperatures. Rather than the familiar temperature scales of Fahrenheit and Celsius, cryogenicists use the Kelvin (and formerly Rankine) scales). sea vessels (LNG carriers) or cryogenic road tankers.
The energy density (Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume, or per unit mass, depending on the context) of LNG is 60% of that of diesel fuel.
Flow path of LNG process:
                                                      Treatment            
  Transportaion -- Condensate removal / Dehydration ; CO2 removal / Mercury & H2S -                     Refrigeration -Liquefaction --Storage & loading -- Transportation

The density of LNG is roughly 0.41 to 0.5 kg/L, depending on temperature, pressure and composition, compared to water at 1.0 kg/L. The heat value depends on the source of gas that is used and the process that is used to liquefy the gas. The higher heating value of LNG is estimated to be 24 MJ/L at −164 degrees Celsius. This corresponds to a lower heating value of 21 MJ/L.

The natural gas fed into the LNG plant will be treated to remove water, hydrogen sulfide, carbon dioxide and other components that will freeze (e.g., benzene) under the low temperatures needed for storage or be destructive to the liquefaction facility. LNG typically contains more than 90% methane. It also contains small amounts of ethane, propane, butane and some heavier alkanes. The purification process can be designed to give almost 100% methane. One of the very rare risks of LNG is Rapid phase transition (RPT is a phenomenon realized in liquefied natural gas (LNG) incidents in which LNG vaporizes violently when being in contact with water causing what's known as a physical explosion or cold explosion. During such explosions there is no combustion but rather a huge amount of energy is transferred in the form of heat from water to the LNG at a temperature difference of about 175 degree Celsius.) which arises from cold LNG being in contact with water