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Looking Beyond Available Technologies
Satish Kumar Saxena, Manager and Head of Process, Mott MacDonald A chemical complex consumes lot of energy. Due to increasing energy costs, manufacturers are required to look beyond and select highly energy efficient technologies and install biggest size available unit so as to reduce the cost of production. Highly energy efficient technologies are need of the hour in chemical process industries.

The world is an open market, and as the competition increases demand for more energy efficient processes keeps increasing. Globally, the technology licensors are making huge investment in research and catalyst development to develop new energy efficient technologies. Besides innovative and advanced technologies, energy saving needs to be achieved in existing plants through minor investment, improved plant operations and maintenance programs. The "Energy Audit" keeps an important place in the energy conservation program.

The systematic energy audit of an industrial process unit helps in identifying key energy guzzlers which need real attention from energy recovery point of view.

Fired Heaters
In chemical industry, fired heater is an integral part of process units. The heaters are used for sensible heating and vaporization as per the process requirements, re-boiler in distillation column and also hot oil heating. In fired heater, fuel oil and gas is fired in a set of burners with air, to supply heat in both radiant and convection section of heater. The firing is controlled by „Burner Management System‰. The flue gas leaving the radiant zone, after heat transfer to process fluid by radiation, passes thru convection zone, where heat is transferred by convection. The process feed is first passed thru convection zone followed by radiant zone for heating. Thermal efficiency is improvised by air preheating in convection zone.

For bigger fired heaters, further heat recovery from flue gas is achieved by steam generation, BFW heating followed by air pre-heating. The fired heater thermal efficiency is defined as ratio of heat absorbed (Inclusive of process heating, steam generation, BFW pre-heating and air pre-heating etc.) to heat supplied by the fuel.

Due to continued technology advancement itĘs observed that fire heaters installed in the recent past offer thermal efficiency as high as >92 per cent. The thermal efficiency is increased by modifying heater configuration, adding steam generation, boiler feed water heating and air /fuel preheating in the convection zone and also using advanced heater accessories like improved burners, automatic burner management system and soot blowers etc.

Reduce flue gas temperature: It is obvious that reduction in flue gas temperature, at given excess air flow, improves the thermal efficiency. Some of the options available to reduce flue gas temperature are air and oil pre-heating, generate steam or BFW pre-heating, reduce temperature approach between flue gas and process fluid inlet to maximize convection zone duty. Additional checks in the heater for reduction in flue gas temperature requires adequacy of draft, adequacy of metallurgy of ID/FD fans, dew point corrosion on downstream recovery equipment and adequacy of radiant section surface area.

Minimize excess air for combustion: Excess air over the stoichiometric requirement results in loss of efficiency but for complete combustion, it is mandatory that there has to be some amount of excess air always required to ensure complete combustion of fuel. The carbon monoxide monitoring in the flue gas is some what indication of excess air.

The efficient combustion control is maintained through uniform firing in all the burners with correct amount of excess air in multi burner configuration, minimizing air leakage through hole inspection, ensuring level of automatic control is in place and through awareness about safety of furnace and redundancy requirement for automatic control.

Radiation and convection heat loss from fired heater surface: In fired heater, heat loss from the surface is conventionally in the order of 2 to 3 per cent of fired heater duty. It is imperative that appropriate refractory material be selected for various sections of furnace. The higher loss is caused by either wrong choice of refractory or inadequate thickness of refractory or local damage etc. Infrared scan is being done to pin point the hot spot in the furnace and damaged portion is repaired.

Use of appropriate auxiliaries with furnace: Faulty choice of auxiliaries can lead to frequent start-ups and fatigue damage to refractory and liners; therefore, for running the fired heater efficiently, it is important to select right accessories for the fired heater. The appropriate selection of auxiliaries depends on the right choice of burners, soot blowers, FD/ ID fans and dampers.

i) Right Burner Choice: The burners should be selected based on the following criteria:
• Ash content in the fuel oil, the burner tip should be made of suitable MOC, hard enough to withstand the erosion.
• Burners should be capable of giving requisite turndown with marginal loss of efficiency.
• The burners should be tested in the shop before on-field installation

ii) Soot Blowers: The soot blowers are used for cyclic cleaning of tubes in the convection section. The effective soot blowing ensures clean and efficient heat transfer surface. Improper cleaning results in reduction of effective heat transfer area which results in lesser heat pick-up in convection banks and further results in harder firing to compensate the loss of duty in convection bank into the radiation section. This leads to higher fire box temperatures, higher flue gas temperature existing from convection section and more heat loss.

iii) FD/ID Fans: In small capacity fired heater, draft is created by the chimney effect, but for bigger fired heaters, there is definite cost advantage of recovering heat from flue gas. The FD/ID fans are used to supply pressure so as to divert the flue gas for heat recovery equipment. In fact, efficient operation of ID/FD fan operation helps in energy conservation measures.

iv) Dampers: The Damper is used to control draft in the fired heater. In order to recover heat from flue gas, damper is used to divert the flue gas for air preheating. Mal-operation of damper effects energy conservation measures. It is very important that all the dampers in the fired heater configuration, main stack damper, air bi-pass damper and burner dampers for individual burners should have very good air tight sealing. An improper or leaking damper affects the heat recovery and thus leads to loss in thermal efficiency.

Power
In every plant electrical energy is lost in distribution and in in-efficient electric motors. The use of efficient electric motors and improved operating philosophies does provide potential for reducing electrical energy loss. To minimise energy loss during design stages, following measures need to be implemented that include power distribution philosophy, voltage levels and transformers, power factor, electric drives control and lighting system.

Distillation
For purification, distillation is one of the important unit operations in all the chemical plants irrespective of the type of unit. The efficient operation of distillation can result in better yield, improved product quality and overall reduction in utilities consumption.

Most common measures in the industry are:
a) Operating pressure optimisation.
b) Possibility of reducing the reflux ratio.
c) Improvise heat recovery from side draws and pump around streams.
d) Use of dry vacuum operation instead of wet vacuum operation (In wet operation steam is used for stripping).
e) Utilising high efficiency trays or low pressure drop packing.
f) Two stage condensation in condenser (Air cooler followed by trim cooling by cooling water).
g) Advanced control.

Besides, these conventional measures, advanced concepts such as Heat cascading, Vapor recompression (Heat pumps) and Thermally coupled columns (Split column operation) have also come into practice.

Heat Exchangers
The energy conservation or efficient use of heat by means of heat train networks of exchangers is quite common feature in the refinery, petrochemicals and chemical units. Besides this other areas also exists where there is possibility of energy conservation measures. Few such areas are as:

i) Energy Recovery at Low Level: Overhead condensers of distillation column and final product run down coolers normally operate at low temperature close to 100-120 degree Celsius. Therefore, most low level energy is dumped to atmosphere either directly thru the air cooler or indirectly thru the cooling water. In order to recover this low level energy, it is suggested that instead of rejecting the heat to atmosphere, use it for pre-heating the feed to distillation unit or boiler feed water to conserve energy.

ii) Fouling Control: Deposition of unwanted material at the heat transfer surface, not only effects heat transfer coefficient but also increases pressure drop. The cumulative effect thus has a direct bearing on the energy efficiency of the heat exchanger.

iii) Optimization of Air and Water Cooling: Air cooler rejects the heat to atmosphere whereas cooling water requires an elaborate indirect system comprised of cooling tower, circulating pumps, water make-up, blowdown, chemical treatment system etc. Though as a unit, air cooler is costly than shell and tube exchanger but if cost evaluation is done based on the total system than there is a break-even point, therefore, air cooler followed by water cooler as a trim cooler is most cost effective combination.

iv) Power Saving Measures In Air Coolers: Earlier, air coolers used to operate at constant speed, constant blade pitch and fan motor draw constant power even at part load or at lower ambient temperature. Now various means are available to optimise the power consumption in air coolers.

v) Advanced Heat Transfer Tubes: As we all aware that boiling heat transfer coefficient is a strong function of temperature d i f ference. At lower temperature difference it starts with pool boiling a very low coefficient. As the temperature difference increases, the boiling regime changes from intermediate to nucleate and finally film boiling. The nucleate boiling provides maximum boiling coefficient, therefore, it is preferred that exchanger should operate in such a fashion that boiling should always be under nucleate boiling regime. The high flux tubes are advanced heat transfer tubes, providing 6 to 8 times lower area than conventional tubes.

vi) Steam Network : The steam is an important source of energy, used in industry for heating, purging and power generation. The chemical industry is incomplete without the steam generation and its network supplying heat to various users using steam for different purpose and various pressure levels. An efficient steam network is extremely important for economics of chemical complex.

Some variable need to be taken into consideration while optimising steam networks that include steam pressure and temperature levels, heating requirements by various users, de-aerator pressure level, turbine ratings, make-up and de-superheating DM water requirement. Maximising condensate recovery is another integral part of energy efficient network and efficient steam utilization in functioning of steam traps must be considered to achieve benefits that include proper selection and sizing of steam traps, proper location, installation and configuration of steam trap piping with scheduled testing, inspection and maintenance.

Flare Gas Recovery
Flare gas system is a safety device for safe disposal of uncontrolled release of flammable vapors. Ideally, continuous venting of hydrocarbon vapors is not desirable. The uncontrolled release of vapor are normally due to faulty relief valves, control valves, compressors seals, inadequate cooling in process condensers and minor process adjustments in various units of complex. The proper monitoring of flare gas system is required to establish loss of excessive hydrocarbon vapors. The "Flare Gas Recovery" unit can be a modular construction and can be easily installed in existing flare system to conserve energy. The decision of such unit not only depends upon economic considerations but also environmental viewpoint. The scheme of „Flare Gas recovery‰ system is quite simple; Instead of allowing release of hydrocarbon vapor in the flare system for burning, these gases are compressed and routed to fuel gas network.

Cooling Water
Typically, cooling water system is either Once through typeĘ or "Circulating type". In cooling water system energy is consumed by circulating pumps and fans. These measures stated above, if adopted would definitely help in conserving energy.

Insulations
It is established fact that improved insulation provides higher return on investment. A decade back, insulation cost used to be 2 per cent of total plant cost, but now due to increase in raw material costs it is ranging between 6-8 per cent of the total cost.

Some of the insulation measures if adopted can help in energy conservation that include use of upgraded insulating materials, target surface temperature to be reduced to minimise energy loss, usage of fabricated forms of insulation blocks for difficult installations like valves, flanges and advanced techniques to protect insulation from rain, frost, fire and mechanical damage. Also timely insulation checks needs to be conducted for detection of damage and replacement.