Vapor recompression to recover low pressure waste steam

Increasing energy cost and  pressures on improving process efficiency are forcing process engineers to minimize wasteful losses. Efforts are continually being made to minimize all such losses. In many industrial processes low pressure spent steam is let off into atmosphere and goes off as waste heat. Thermal separation processes such as evaporation and distillation are energy intensive. The need for reducing energy costs led to multieffect plants, then to thermal vapor compression and finally to use of mechanical vapor compression systems. Under steady  state conditions, sum of all energy and enthalpy inputs must equal the sum of all energy and enthalpy outputs.  It, therefore, becomes important to ensure that energy imparted to the vapors is recovered back/reused. The following options are generally  adopted in the industry for recovery of
energy:

a) Multi-effect Evaporation

b) Vapor Recompression  

• Thermal vapor recompression
• Mechanical Vapor recompression

Multi-effect Evaporation:-    
In a multi effect evaporation plant, the vapors produced in the first effect are utilized as the heating medium of the second effect and so on. This effectively reduces steam consumption in proportion to the number of effects. Ideally unit mass of vapor on condensation can evaporate  unit mass of liquid. The vapors generated at the first effect are condensed in the  second stage to further evaporate the liquid from the second stage and so on. A temperature gradient of about 7-10°C is maintained between stages for maximum efficiency. So a triple effect evaporator would consume only 35-36% of the energy in comparison to a single effect system.

Vapor Recompression:   
In vapor recompression arrangement the heat of condensation of the evaporated vapor is recovered in single effect only by raising the pressure and temperature of the generated vapor and then their condensation in the same evaporator. The vapor compression can be done by Thermal Vapor Compression or Mechanical Vapor Compression Process.
Thermal Vapor Compression:  In thermal vapor recompression steam jet ejectors are used to raise the pressure and temperature of the generated vapors. The motive steam mixes with the vapor and to maintain the steady flow heat balance some of the vapor steam mixture has to be taken to second effect for full recovery of latent heat of vapor and, therefore, excess vapor is to be conveyed to next effect for recovery.

Initially, heating steam is used to initialize evaporation. The vapors evaporated are compressed to higher pressure and temperature by steam jet ejector, condensed back for heat recovery and the residual vaporsare taken to second stage for condensation / heat recovery. The amount of surplus energy contained in the residual vapor corresponds to the amount of energy supplied for steam jet ejector operation. This is taken as additional heat input / work done for recovery of large heat content of the evaporated vapors.

Mechanical Vapor Compression:  In mechanical vapor compression, positive displacement compressors or multi stage centrifugal compressors are generally used to raise the pressure and temperature of the generated vapors.  Since mechanical compressors do not require any motive steam, all vapors can be compressed to elevated pressure and temperature eliminating the need for subsequent recovery system. The energy supplied to the compressor constitutes the additional energy input  to vapors. After compression  of vapor and subsequent condensation of the same, hot condensate leaves the system. A typical mechanical vapor recompression cycle would be as illustrated in figure below:

For mechanical vapor compressors, the  specific energy input depends upon the compression ratio (ratio of input pressure to discharge pressure). Compression ratio, therefore, must be maintained to the lowest required. 
The compression ratio is influenced by:
1. The boiling point elevation of the liquid to be evaporated. Higher the boiling point rise higher is the compression ratio required.
2. Minimum differential temperature gradient required for effective heat transfer. Indirect condensers require a minimum temperature gradient across the fluids exchanging heat. The condensers should be designed for least ∆T operation.
3. Total system pressure drop in the piping and valves. Adequate size of piping and valve selection should be done for minimum pressure drop during transfer of fluid through them.

The working cycle of Everest mechanical compressor for steam, as fluid handled, is explained under.

Enhance the performance of liquid ring pumps using Everest mechanical vacuum boosters

Liquid Ring Pumps are used throughout process industry. These pumps provide legitimate alternative to steam jet ejectors in applications requiring rugged pump that can tolerate entrained liquids, vapors and fine solids. These Pumps operate in a liquid environment, generally water and are capable of handling vapors along with non-condensable loads. They are extensively used in industrial processes such as filtration, drying, solvent recovery, distillation etc. Unfortunately they suffer from two major limitations that restrict the process
performance. They are:

• The final vacuum achievable, as it is largely dependent on the vapor pressure of the pump fluid corresponding to the working temperatures. For example, for water sealed pump, the lowest practical operating pressure for two-stage design would be in the range of 40 – 60 Torr (720-700mm Hg) for exit water temperature at 30-32 Deg. C.
• Their energy consumption per unit of gas pumped is higher since most of it is lost in handling pump fluid.

Mechanical vacuum boosters (MVB) overcome these limitations of liquid ring pump (LRP). A properly matched MVB – LRP Combinations can result in:
• Higher working vacuums – any where the range of 50 Torr – 1 Torr (710-760mmHg) or better is achievable.
• Very high pumping speeds – generally to the order of 4-8 times higher.
• Vapor/gas compression at the inlet of the water ring pump allowing use of
higher water temperature in the pump.
• Relatively very low energy consumption per unit of pumping speed.
Figure1 gives typical two stage WRP speed curve. The pumping speed is equal to the rated speed(displacement) during initial pumping and thereafter drops rapidly reaching to zero at its ultimate (690 – 720 mm Hg). In most of the chemical processes the process vacuum is in the range of 680-700mmHg where the pumping speed of WRP is merely 15-20% of it’s full rated capacity. This demands installation of much larger WRP loosing on one time pump cost and recurring energy charges. The power consumption, however, is largely constant throughout the range that makes LRP relatively less energy efficient in comparison to MVB-LRP Combination.

Curve2, Fig.1 gives a typical MVB–LRP (water-two stage) speed curve. As the WRP vacuum drops to the range of 60-100 Torr (660-700mm Hg), the Mechanical Booster boosts the effective speed manifold. As can be seen from the curve the booster exhibits relatively flat pumping speed curve in the region 10-1 Torr (750 –760mm Hg), high pumping speeds and better process vacuum is achieved, overcoming the limitations of LRP in this range. The power consumption of the Mechanical Vacuum Booster is relatively low in this range as compared to any other conventional vacuum pump. Therefore, with little extra energy, the overall pumping speed and ultimate vacuums can be greatly enhanced. In many applications, replacing WRP with a smaller one can easily offset the extra energy of MVB.

Installation of MVB undoubtly results in high pumping speeds and better vacuums. However, to get the best results in process its location is important. It can be effectively located between the condenser (Post condenser installation) and the WRP or between the kettle/evaporator and the condenser followed by WRP (Pre-condenser installation). To enable to determine most effective location process parameters play an important role.

POST CONDENSER INSTALLATION

Processes such as distillation of high boilers (kettle temp. are generally above 125°C), processes using chilled water condenser, processes having direct discharge of vapors to WRP, processes demanding vacuum close to condensate vapor pressure are generally the applications where post-condenser installations can give boost to the process, resulting in higher yields, lower process time and better product quality.

In drying applications where water vapor is exhausted from the dryer and cooling water of 10°C or lower is available in the condenser, post condenser installation would be a good choice. Since the vapor pressure of condensate (Water) at 10°C is about 9 Torr, (refer graph below) the condenser working vacuum can be estimated to about 20 Torr. Double stage WRP having fluid temperature in the range of 30-35°C would not be able to deliver working vacuum below 50-60 Torr (710-700 mm Hg). However on installation of Mechanical Booster between the condenser and the WRP would very conveniently pull down vacuum to the range of 15-20 Torr (745-740 mm Hg). Still better vacuums can be possible if the condenser & condensate temperatures are lowered further.

Dry mechanical boosters – replace steam ejector

Steam ejectors find wide use in vacuum pumping applications – so called dirty application such as in Vapour extraction, Chemical processing, Evaporative Cooling, Vacuum distillation, Vegetable oil de-odourization, Vacuum Refrigeration, Drying etc. In spite of the fact that steam ejectors have poor overall efficiency and relatively high energy consumption, they are popular in vacuum applications because of their simplicity and ease of operation. Its high time now when the industry should realize the disadvantages associated with it and switch over to efficient alternatives – Dry Mechanical Vacuum Booster being one of them. Mechanical Vacuum Booster offers an efficient replacement to steam ejector, for most of the applications, as they overcome major drawbacks associated with steam ejectors. The major advantages of Mechanical Booster being :-

• Mechanical Vacuum Boosters are more energy efficient.
• Minimum of auxiliary equipment is needed; unlike for steam ejectors, which need large condensers, cooling towers, re-circulation pumps etc.
• Mechanical Vacuum Boosters are dry pumping system and don’t give rise to water and atmospheric pollution.
• Startup time for mechanical booster is very low making them ideal for Batch process operation where immediate startup and shut down is essential for energy conservation.

Apart from the above, the operating costs for mechanical vacuum systems are low, resulting in extremely short pay back period. For example, when operating in the range of 5-10 Torr the operating cost of mechanical pumping system would be about one tenth of the equivalent steam ejector system.

Steam Ejectors:

Steam ejectors comprise of converging – diverging nozzle through which high-pressure steam (motive fluid) is forced through. (Fig.1). The ejector nozzle converts the high-pressure head of the motive fluid into high velocity stream as it emerges from the nozzle into the suction chamber. Due to increase in velocity head, there is a drop in pressure head causing partial vacuum in the suction chamber. Pumping action occurs as the fluid / vapors present in suction chamber are entrained by the motive fluid and are carried into the diffuser, by

viscous drag process.

The capacity of steam ejector is directly proportional to the weight of the motive fluid. Generally, the ratio of motive fluid to the gas pumped is high, especially under low vacuum and results in excessive demand of steam in multi-stage systems. The overall performance of steam ejector is sensitive to changes in operative parameters such as motive steam pressure and discharge pressure. A slight variation in operating parameters weighs heavily on the system capacity. Multi steam ejectors require inter-stage condensing as each stage adds to the pumping load for the succeeding stage and for reason of economy, condensation becomes important. The heat gained during condensation i.e. latent heat of vaporization, adds to the need for additional equipment such as re-circulation pumps, cooling towers etc. so that the same can be dissipated. In a steam ejector, steam comes in direct contact with gas/vapour pumped and many a time, this mixture of pumped vapour and water needs elaborate treatment before it  can be discharged / re-used. Steam ejectors, especially multistage not only require steam generation facilities but also raise demand for auxiliary equipment such as D.M. plant for boiler feed water, condensing units, re-circulation pumps, cooling towers, effluent treatment plant etc. thereby increasing total energy consumption and maintenance costs. Steam ejectors are, therefore, no longer popular as they were once because of dramatic increase in cost of steam generation, auxiliary power and effluent treatment problems. It is for this reason many steam ejector installations have been replaced by mechanical Vacuum Pumps which use far little energy for the same service and require no additional auxiliary power, cooling tower nor give rise to effluent.

Vacuum boosters for drying applications

Drying is a process of removal of a liquid from a solid mixture by thermal means. Under this article, we shall not consider mechanical methods, such as filtration, centrifuging, pressing, etc. of liquid removal from solids. Various drying process & techniques are extensively used in the various Process industry, Pharmaceutical industry, Food processing industry, Dye & Chemical industry, Perfumes & Permitted Food additive industry etc primarily to achieve one or more of the following,

• Product concentration.
• Purification by removal of unwanted volatile elements.
• Solvent recovery.
• To increase shelf life and to facilitate further processing and permit proper utilization of the final product.
• To reduce shipping costs by reducing weight of the product.
• To reduce the rate of biological decay.
• To enhance the value of by products of a process.

Drying is an important and widely used process in the industry. Often it is a major cost center in process operations. The reason for this is the high-energy requirement for the removal of water. Typically, to remove 1 kg of water, we require 540 kcals of energy [latent heat of vaporization of water] plus at least another 60 kcals to take care of sensible heat requirements. Hence, regardless of the nature of process, we must supply at least 600 kcals for every kg of water removed from the material. This, therefore, demands high-energy inputs and for this reason the process efficiency must be maintained as high as possible.

The cheapest energy source is the sun. That is why many industries take recourse to sun drying. For example, in small food industry, chillies, ginger, etc., are all sun-dried; in the textile industry, fabrics and yarn are often sun-dried; in the ceramic industry, freshly moulded bricks and blocks are sun dried. Very often, most process requirements demand continuous working, independent to the weather conditions or time of the day. So there is a vast body of ovens, dryers, drying tunnels, dehydrators, etc., to take care of industrial drying processes. While these techniques are well known, their use is now being replaced by Low-pressure, Low temperature drying techniques, which have the advantage of enabling the drying process to be carried out at low temperatures. This process results in optimum energy utilization, lesser thermal exposure & damage to the product and very often, improved quality. In fact, it is possible to dry a product at sub-zero temperatures (Freeze drying) by simply reducing the pressure to an appropriate value for carrying out the process. Freeze Drying, most widely adopted process today in food processing industry is based on the same principle. Amongst the many advantages of low pressure drying techniques over oven-techniques, some are: -

• Drying time is accelerated drastically.
• Solvent recoveries are possible, resulting in substantial savings.
• Reduces pollution.
• Minimizes oxidation losses and product degradation due to reduced Thermal exposure.
• Wide range of operating temperatures can be selected to suit the product/process requirements.

For example, conventionally Katha (an essential ingredient of paan masala and paan), is dried by traditional cold room drying process. It is kept in cold rooms for over a period of about 24 days, with cold dry air blowing over it. The moisture levels are reduced from typically 50% + to about 12-15%. The temperature in the cold room is maintained at slightly above 0 degrees C, throughout the process time. However, by dopting low pressure drying techniques, this period an be reduced substantially. This would not only save energy requirements but also reduce the huge inventory hold-ups. Katha is a high-priced product and shortening of the process time ould result in substantial savings otherwise involved. Shorter process times also reduce possibility of fungus/moulds/bacteria attacks. Similarly Drying of various other products, such as Gelatin, Meat, Milk products, Green bodies etc, can also benefit by this fast process.

A vacuum Booster, when used in conjunction with any of the above, over comes all the associated limitations and increases the overall process efficiency by increasing the vacuum and pumping speeds with relatively very little extra energy.

Optimise vacuum to improve plant performance

Vacuum Pumps and systems are widely used in the chemical process industry for various applications such as drying, solvent recovery, distillation, short path distillation (Molecular distillation), concentration etc. It is therefore, essential that the vacuum principles are understood which can be employed to maximize process throughputs, product purity and quality & minimize power consumption.

Success has been achieved in many industries such as food product, essential oils, aromatics, solvent recovery and steam jet replacements. Wide range of pumps and vacuum equipment is being used in the industry to achieve the desired vacuum and pumping speeds. The understanding of their advantages and limitations can result in optimizing their performance.

What is Vacuum?

Vacuum is simply a pressure below atmosphere. To create vacuum in a system, a pump is required to remove mass (gas/vapor) from the system. The more mass is removed, lower is the pressure that exists inside the system. Various vacuum levels are defined depending upon the ultimate vacuum as:

􀂉 Coarse Vacuum 10 – 760 Torr

􀂉 Medium Vacuum 0.001 – 10 Torr

􀂉 Fine Vacuum 10^-3 – 10 ^-7 Torr

􀂉 Ultra High Vacuum < 10^-7

Generally, the chemical industry operates in Coarse and Medium vacuum range. In this range the vacuum is generally measured in mm Hg gauge or Torr (absolute pressure). Measurements from datum as atmosphere are gauge reading, whereas the measurements referred to absolute zero are expressed in Torr. For example at sea level (atmospheric pressure 760mmHg), a system maintained under vacuum of 700mmHg, as indicated by vacuum Boudorn gauge, is said to have absolute pressure of 60 Torr. Vacuum gauges, mercury manometers, transducers etc. indicate gauge pressure and their reading when subtracted from atmospheric pressure gives absolute pressure. It is important to under stand the above since all vacuum principles and calculations are based on absolute pressure units.

Pumping speed: It is the volumetric rate of exhausting, generally expressed in Lts/min., m3/hr or cfm. It is the rate at which the inlet of the pump actually removes the gas / vapor load. It should not be confused with Displacement of the pump. Displacement of a pump is the geometric volume swept by the pump per unit time at rated operating speed. For most of the pumps, pumping speed is close to displacement value at no load conditions (FAD-Free air delivery) and changes with inlet pressure, reaching to zero where the pressure attained is said be pumps

Ultimate pressure: The Curve below, gives pumping speed for different type of pumps.

It is evident from the curve that pumping speed drops with drop in pressure. This must be taken into consideration while selecting a pump. The inlet pressure at which the pump’s speed falls to zero is termed as “Ultimate pressure or Blank-off pressure” of the pump. It is a pump characteristic, dependent on the type of pump/ pump construction. The ultimate pressure/Blank off pressure of a pump can be easily checked by measuring the inlet pressure, with inlet of the pump blanked off. At Blank-off pressures, the effective pumping speed of the pump is zero. This means that a process can never achieve vacuum better than the blank off vacuum of the pump. While selecting a pump, desired process vacuum and that achievable by a pump must be verified. Ultimate vacuum is the pump type characteristic and general conception that using a bigger Pump (of the same type) would yield better vacuum is false. The process engineer’s should establish desired process vacuum and the selection of the pump should be made accordingly. To get better working vacuum and higher pumping speed, Boosters are invariably used in combination. In most cases much higher speed and lower pressure can be achieved with a fraction of extra power, when Booster combination is used.

Lube oil purification

Lubricating oil is an important resource and a petroleum base product. The high price of oil and objective of saving valuable foreign exchange has resulted in efforts for regeneration of used lube oil. Mismanagement of waste lube oil is a serious environmental problem. Almost all types of waste oil have the potential to be recycled safely, saving a precious non-renewable source and at the same time minimizing environmental pollution. Unfortunately, most of used oil is handled improperly. Some is emptied in to sewers for going directly into water waste, adversely affecting water treatment plants. Some is dumped directly on to the ground to kill weeds or is poured on to dirty roads or is dumped in deserts, where it can contaminate surface and ground water.

Disposal of used lubricating oil into the eco system creates environmental hazards. Toughlaws are being enacted throughout the world for the disposal of waste petroleum products and every genuine effort should be made for it’s re-use. In most cases, used oil can be re-used after reconditioning with or without the addition of any additives resulting in huge saving and conservation of precious oil. Thus regeneration, reclamation or recycling of spent lubricating oils has become an important process industry, adopting various techniques for oil purification.

Regeneration of used lubricating oils is based on the fact that, “Petroleum lubricating oil are almost indestructible”. We can say, “Lubricating oil never wears out”. Lubricating oils are impaired temporarily only because of accumulation during use or handling of contaminants coming from extraneous impurities and products of oil deterioration, which can be separated from the used oil by re-refining or re-conditioning. The treated oil, then, becomes almost equivalent to fresh / virgin oil.

The major extraneous impurities in the waste oil are:

1. Metallic Impurities

2. Non-metallic impurities

3. Water, moisture and untreated acid

4. Carboneous particles

5. Fuels, impaired additives and their by products.

6. Chemical contaminates

7. Polycyclic Aromatic hydrocarbons (PAHs)

In India, re-recycling waste oil industry is mainly an un-organized one. Although some public service units (PSU) also doing re-recycling but most of the market demand is furnished by the small scale industrial units (SSI’s). Most of the SSI units, till now, were adopting “Acid Clay Process”, which now has been dis approved by the pollution control boards, resulting in their closure until they adopt new certified process.

In the conventional acid clay process the used lubricating oil is settled or filtered after collection and is dehydrated. The oil is then treated with concentrated sulphuric acid to remove polymers, asphalts, degraded additives and other products of degradation. The sludge formed is allowed to settle and removed. The oil is neutralized with activated clay, at elevated temperatures. The clay also bleaches the oil and adsorbs certain impurities not removed by acid treatment. The clay-oil slurry is filtered to remove clay and other solids. If  the raw material contains more than one grade of lube oil product further processing may be required.

Spent oils, which have not deteriorated to great extent, are often clay contracted or treated with adsorbents without any acid treatment. They are generally given a preliminary settling, filtering, centrifuging or vacuum dehydrating treatment. Insulating oils and Transformer oils are often treated in this way.

In order to ensure satisfactory operation of any vacuum process it is essential that suitable vacuum pump be used. There is generally no single pump that meets all the requirements of the process. Combination of pumps is increasingly being used to optimize the process performance. Process condensable and non-condensable loads, air leakage loads, out-gassing loads and the working process pressures are the important parameters that influence the pump selection. Various empirical load estimation charts, and leak tests must be referred for the proper selection of the vacuum system. Some of the widely used pumps for vacuum process are described below along with their limitations.

These pumps use water or low vapor pressure fluid as the pumping medium. For this reason, the ultimate vacuum achieved gets limited to the vapor pressure of the pump fluid at the working temperature. Owing to the above, water ring Pump would stall at around 60 Torr abs. (700mm Hg) and their working range would be between 60 to 150 Torr (700-610mm Hg). They have further disadvantage of being energy inefficient, because most of the power is lost in friction losses of moving the pump fluid inside the pump. This restricts the water ring pump to relatively modest volumetric pumping capacities. Another disadvantage of ring pumps is that the working fluid often has to be treated before it can be discharged or reused as it contains the carry over of condensed product.

 

Vacuum booster for distillation process

Distillation at reduced pressures is a widely used process in the chemical industry, specially used in extraction / purification of essential oils, deodorisation of Vanaspati / Vegetable Oils and purification & drying of chemicals. The advantages of low-pressure distillation process over Atmospheric pressure distillation are as under:

• Use of lower process temperatures. Under vacuum, there is a reduction in boiling points. Hence thermally sensitive substances can be processed easily.
• Shorter time of thermal exposure of the distillant. The reduction in thermal exposure time
  enables the processing of thermally sensitive items such as vitamin & harmones, whose
  properties are adversely affected by extensive exposure to heat.
• Increase in relative volatility. Materials become more volatile under vacuum and therefore more evaporation takes place, resulting in higher production rates.
• Fractional distillation under vacuum leads to easier separation of components of a mixture.
• Change in position of the azeotropic point at reduced pressure. This enables separation under reduced pressure / vacuum of hard to separate materials.
• Reduction of energy consumption by lowering of the boiling point under vacuum
• Oxidation losses of the feed stock are reduced under good vacuum conditions

Reduction in stripping steam requirements for de-odourisation process of oil due to increased specific volumes (of steam) at low pressures and enhanced agitation & stirring of the oil.



Ideal Vacuum Pump :

In order to ensure satisfactory operation of the distillation process it is essential that suitable vacuum pumps are used. While there is no single perfect pump, the ideal characteristic of the vacuum pump required for distillation process are:

• Low energy input for a given volumetric pumping capacity
• The pump should be dry type, i.e. it should not use any pumping fluid such as water, oil, steam etc. These fluids interfere with the purity of the product and limit ultimate vacuum level.
• The ideal pump should have minimum number of stages to achieve the desired vacuum levels
• No environmental pollution should be caused by pump operation. Hence there should be no material pollution due to stripping or disposal of pump fluids. Noise pollution should be at the minimum.
• The pump should have high volumetric pumping capacity at low pressures
• The pump should have low maintenance requirements
• The condensation of the vapors within the pump should be minimum so as not to effect its
  performance
• The pump should have high vapor handling capacity
• Should be able to pump out little amount of liquids, in case condensation occurs inside the pump

Va

Vacuum Booster is a Dry pump that meets most of the ideal pump requirements. It works on positive displacement principle. As its name suggests, it is used to boost the performance of water ring / oil ring/ rotating vane / piston and in some cases even steam ejector pumps. It is used in conjunction with any one of the above mentioned pumps , to overcome their limitations. Vacuum Booster pump offer very desirable characteristics, making them the most cost effective & power efficient alternative. The major advantages are :

• The vacuum booster is Dry Pump. It does not use any pumping fluid. Hence it pumps vapor or gases with equal ease. Small amounts of condensed fluid can also be pumped.
• It has very low pump friction losses, hence requires relatively low power for high volumetric speeds. Typically, their speeds, at low vacuums are 20-30 times higher than corresponding vane pumps / ring pumps of equivalent power.
• The vacuum Booster can be used to generate vacuum in range of 0.001 Torr and yet have high volumetric speeds at such low pressures. At these pressures the rotary oil and water ring pumps are not effective, as their pumping speed falls drastically when approaching the ultimate levels.
• The vacuum Booster can be used over a wide pressure range, from atmospheric pressure down to 0.001 Torr (mm of mercury), with suitable arrangement of backup pumps.

cuum Booster