CONTROL MEASURES

How to control accidents?

·        E-engineering control (guard machines, explosion hazards, etc.)

·        E-education control (educate people about possible hazards)

·        E-enforcements control

Role of government:-

·        Provide leadership in promoting safety

·        Enforcement of legal provisions

·        Offering advisory services

·        Providing lab facilities

·        Development of safety standards, rules, etc.

·        Conducting competitions for mass communication,

Role of management:-

·       Leadership in stimulating safety.

·       Preparation & circulation of safety policies and implementation

·       Assignment of responsibility

·       Provision of safe work place

·       Creation of employees interest

·       Provision for adequate budget, medical services, etc.

Including all sail plants there is an organization called SSO (sail safety organization). Safety policy 1985 with the objectives of promoting standards of safety and health (complying with factories Act 1948), maintaining safe working conditions, providing all employees with safety information & instructions, etc. was introduced in order to meet the target – ZERO ACCIDENT.

PLANT SAFETY

Though accidents cannot be totally eliminated altogether in an integrated plant like DSP, they can be prevented to a great extent and their magnitude sufficiently reduced with minimum loss to life & property, if some basic precautions are always carefully observed inside the plant.

FACTORS OF ACCIDENTS:-

·        Operating without authority

·        Working at unsafe speed

·        Making safety equipment inoperative

·        Using unsafe tools

·        Taking unnecessary/calculated risks

·        Taking unsafe postures

·        Failure to use PPE(personal protective equipments)

UNSAFE CONDITIONS:-

·        Inadequate guard/unguarded areas

·        Defective conditions of machines

·        Unsafe design of construction

·        Hazardous work arrangement

·        Inadequate illumination

·        Inadequate ventilation

COSTS OF ACCIDENTS:-

According to survey for any kind of accident per man per day, the cost of accident is around Rs 10,000.

A>DIRECT COSTS

·        Wages and salary

·        Medical facilities to be given

·       Compensation, etc.

B>INDIRECT COSTS

·        Loss of time of supervising

·        Loss of co-workers

·        Production loss

·        Damage to machines

OXYGEN PLANT

INTRODUCTION:

          The oxygen required in various steel making-processing units, e.g. basic oxygen furnaces of DSP is produced in this plant by liquefaction and rectification of air. Since the process deals here with temperatures below -130⁰C, the technology used is often called Cryogenic Technology. The main products obtained are 99.5% pure O₂ and almost 100% N₂.

Atmospheric air contains mainly the followings:

                       CONSTITUENTS                                 VOLUME

                   1.  Nitrogen gas                 :          78.08

                   2.  Oxygen gas                                   :              20.96

                          3.  Argon gas                        :                      0.93

                   5.  Carbon-dioxide                  :                        0.033

                   6.  Carbon-monoxide              :            0.1 ppm

                   7.  Hydrocarbons, mainly C₂H₂  :               1.5 ppm  etc.

                   7.  Water vapor (H₂O)             :                0.0 – 4.0        and impurities like dust, oil traces etc.

 Air, after removal of impurities like dust, moisture, CO2, CO, hydrocarbon & oil traces etc., is liquefied by cryogenic refrigeration and liquefaction process and finally product gases like Oxygen, Nitrogen & Argon are separated by the process of distillation (according as their property of volatility). At 1 Kg/cm² B.P. (boiling point) of Oxygen, Argon & Nitrogen are -183⁰ C, -186⁰ C & -195⁰ C. Thus, nitrogen is more volatile than argon and argon is more volatile than oxygen.

CRITICAL TEMPERATURE AND PRESSURE:

With increase in pressure, B.P. of a gas increases up to a certain temperature, after which B.P. remains unchanged in spite of increase in pressure. This particular temperature is known as Critical Temperature and the corresponding pressure is known as Critical Pressure (of that particular product gas).

                                                                                      AIR             OXY          NITRO

                        Critical Pressure, ⁰C               :              38                  51                     35

                        Critical Temperature, kg/cm²     :        -141             -119              -147

PRINCIPLE OF CRYOGENIC refrigeration & LIQUIFACTION OF GASES:

In general, refrigeration is the science and art of producing and maintaining temperature below that of the surrounding atmosphere. And in particular the subject of techniques, process and equipment, which deals with the temperature at and below  -150⁰ C are broadly termed as Cryogenic Engineering and others are ordinarily called refrigeration. The liquefaction of air for producing oxygen was first done in engineering application of Cryogenics and still in use in the industry.

The thermodynamically ideal liquefaction system is the Carnot cycle in which two processes are involved, namely

                       a) Reversible isothermal compression, and

                       b) Reversible expansion at isenthalpic condition

which are shown in T-S diagram in Fig.1.For any practical liquefaction system, an expansion valve is used to produce low temperature at isenthalpic condition. For a perfect gas Joule-Thomson co-efficient is zero and inversion curve exists for a real gas from where Joule-Thomson co-efficient may be +ve or –ve.

Isothermal Compression          2                                                      1                            Isenthalpic Expansion                                                                                f                  3                           4

 

              

     

    T  

                     

                                   S                                                                                 P

                             Fig. 1                                                                           Fig. 2

          The second method of producing low temperature is adiabatic expansion of any gas through a work producing devices e.g. expansion engine, expansion turbine etc. For an ideal gas adiabatic & reversible expansion is true only in isentropic condition. Moreover it may be noted that for any gas , isentropic expansion through an engine or expander will always result a decrease in temperature whereas expansion through an expansion valve may or may not result with drop in temperature. The another important factor is that at a given pressure an isentropic expansion will always result a lower final temperature than an isenthalpic expansion for the same initial temperature.     

LIQUIFACTION OF GASES:

Low temperature gas liquefaction process involves two basic principles

                   a) on joule Thomson expansion which is usually called Linde process.

                   b) on expansion engine which is commonly called Claude process.

In Linde process the most key factor is that  the pressure and the temperature after compression and cooling should lie on or below the inversion curve , other wise the  temperature of the gas will increase with  decrease in temperature as in the case of helium or hydrogen gas.

But in Claude process , a combination of the joule Thomson expansion & expansion engine is used for refrigeration of the gases.

The followings are the different steps of cryogenic air separation process:

                   1.  Filtration

                   2. Compression

                   3. Pre-cooling & Purification

                   4. Distillation

1. Filtration:

Air with dust particles enters the augmented self Cleaning Filter (ASCF) system and is cleaned by passing through the panel-pack paper filter elements. The clean filtered air exits through the ventury into the compressor inlet. In this process only the solid dust particles are removed whereas the moisture, impurities like CO₂, CO, C₂H₂  and oil traces are left over.

2.  Compression:

To liquefy air for distillation, its enthalpy is decreased by isothermal compression i.e. compression followed by after cooling at every stage.

For any fluid,

                   H = E + PV

                             H à Enthalpy

                             E à Internal energy, a function of temp. only

                             PV à A function of temperature (PV = RT)

For any ideal gas E & PV remain const. at const. temperature and are independent of pressure and this H is const. But, for real fluids, because of non-ideality, the enthalpy (H) is decreased with decrease of E & PV, which are affected due to variation of pressure at constant temperature. Here, the variation of internal energy (E) isothermally with pressure is much higher than the variation in PV.

PROCESS:

The production of high-purity gaseous oxygen from air via the cryogenic process is illustrated in the flow sheet on the next page. The process is based on the principle of the Joule-Thomson effect, i.e. sudden decrease to pressure results in the lowering of temperature. Linde first applied it in produced O₂ industrially. This method is distillation of liquid O₂ & liquid N₂, is still in use but with modifications, most important of which is Claude, which significantly decreased the power consumption (as the expansion in his method is isenthalpic instead of isothermal) and increased the efficiency.

Filtered air is compressed to about 5kg/cm² pressure in a centrifugal compressor and after cooled. After separating out any liquid water, the air enters the reversing heat exchanger and is cooled to near its dew point in countercurrent exchange with the outgoing gas products. Carbon dioxide along with remaining moisture or other HCs’ is removed by the adsorption process in Driers where activated alumina and molecular sieves adsorbs the last traces of CO₂. The cleaned air is then fed to the bottom tray of the double column rectifier, consisting of two columns one operating at low pressure (0.5 kg/cm²) and the other operating at medium pressure (5 kg/cm²). In the double column rectifier, nitrogen being more volatile is collected as vapor from top of the low pressure column, where liquid oxygen is collected from the bottom of the low pressure column. A stream of water waste nitrogen (containing around 4.5% O₂) is vented out after proper heat transfer to the refrigerant unit.

Gaseous oxygen is produced in this plant as per DSP’s requirement and sometimes the excess liquid pure oxygen is sold to outside parties. In one unit, 9700 Nm³/hr of gaseous O₂ and 650 Nm³/hr of liquid O₂ is produced normally, the excess being storage vessels as liquid O₂ at 40 kg/cm² pressure and around -184⁰C, the capacity of which is 1000 tons of liquid O₂. Proper insulating materials like glasswool are provided at various places while in storage vessels pearlite powder is used as insulating material. The plant is fully automated, however time-to-time analysis of various outlet streams is to be done to ensure production at desired rate.

Air is compressed from atmospheric pressure to plant operating pressure (7.1 kg/cm²) by centrifugal air turbo compressor. For tonnage air separation Plant centrifugal compressors are used as it can handle a very large amount of air at low and medium pressure.

Filtered air enters the compressor first stage intake chamber through inlet guide vane. The high-speed impeller blades increase the velocity of air and impart kinetic energy to air. Part of this kinetic energy results in a static pressure rise of gas (air, in this case) in the impeller and the remaining kinetic energy results in velocity head which causes additional pressure rise in the compressor diffuser. After air passes through impeller, it enters into the diffuser and reduces velocity by increasing the flow area rapidly. The KE imparted to the air by the impeller is converted to pressure rise by reduction of velocity in the diffuser.

As,

K.E. α V² (impeller tip speed),

due to increase in K.E. and increased pressure ratio of air, temperature of air also increases.

Air is compressed basically polytropically rather than isothermally. To enable to approach for isothermal compression, air leaving each stage of compression is cooled into the intercoolers up to the inlet temperature of gas and this reduces compressor’s power consumption also. After leaving the first stage diffuser, air enters the return bend channel and finally into next stage impeller through the intercooler. Thus air leaving the last stage compression enters into the piping system through after cooler.

Each ATC,

                   Air delivery = 61,000 NM³/Hr., at 7.1 kg/cm² (abs) and 40⁰ C.

3. Pre-cooling & Purification:

The impurities of air like moisture, oil traces, CO₂, CO, hydrocarbons are eliminated through this process. It may be noted that constituents like moisture & CO₂ forms deposit in the piping system at cryogenic temperature and can cause obstruction of passage for flow. C₂H₂ and oil traces are dangerous and can cause explosive reaction with liquid oxygen.

PRECOOLING:

PRE-COOLING SYSTEM removes a major portion of moisture content. With increase in pressure the moisture carrying capacity of air decreases and in the pre-cooling system part of moisture content condenses after getting separated from gas by the process of cooling at a constant pressure.

                   The pre-cooling unit consists of:

                    i)  Air-water tower (E10)

                   ii)  Nitrogen-water tower (E11)

                   iii)  Cooling water circulating pump (P01/02)

                   iv)  Refrigeration Unit (X01/02)

i) Air-Water Tower (E10):

This is a single stage, counter flow, direct contact type heat exchanger with polypropylene packing. The compressed air (40⁰ C) is cooled to 10⁰ C by chilled water (30 M³/Hr, 8.1 kg/cm², 5⁰ C) in the Air-Water tower. A major part of moisture content is extracted along with the cooling water and finally air is passed to the Driers for further purification.

ii) Nitrogen-Water Tower (E11):

The cooling water at 33⁰ C (30 M³/Hr., 3.5 kg/ cm²) is cooled to 15⁰ C (1.15 kg/cm²) in Nitrogen-Water tower by utilizing available cold energy from waste nitrogen (21,500 NM³/Hr, 1.25 kg/cm², 13⁰ C). This is also a single stage, counter flow, direct contact type heat exchanger.

iii) Cooling water circulating pump (P01/02) & refrigeration Unit (X01/02):

The cold water leaving E11 at 15⁰ C is pumped to E10 through refrigeration unit where it is further cooled to 5⁰ C and after E10 it returns water (40⁰ C, 2.5 kg/cm²) to cooling tower.

PURIFICATION:

Air is further purified through physical adsorption process to eliminate CO₂, CO, hydrocarbons and leftover moisture, oil traces etc. Physical adsorption involves the attraction and retention of layers of molecules of a gas on a solid surface into the adsorber vessels.

Air purification unit consists of:

                    i) Adsorber Vessels  (RO1/RO2)

                    ii) Regeneration Heater (E16)

                   iii) Post Filter (F11/F12)

                   iv) Silencer (FO5)

The adsorber vessels are double adsorbent bed type. The bottom bed containing activated alumina eliminates and adsorbs moisture & oil traces. The top bed containing MOLECULAR SIEVES adsorbs CO₂, CO and hydrocarbons. While one vessel remains in operation, the other gets regenerated.

Air at 10⁰ C enters R01/R02 through the bottom of the vessel and passes  through activated alumina and molecular sieve beds respectively and leaves at   15⁰ C at the top of the vessel eliminating all impurities.

During adsorption, the operating vessel gets saturated gradually and then the vessel is changed over for regeneration and the other vessel is put into operation. In order to prevent possibility of dust particles coming from adsorbents with the leaving air, it is passed through post filter where dust particles up to 50 microns is removed, and bone dry, hydrocarbon free air is fed into the cold box.

The waste nitrogen coming from cold box is heated to required temperature by a regeneration heater and passes through drier in the reverse direction of process air. After gradual heating, the adsorbed moisture, oil traces, CO₂, CO, hydrocarbons etc. are released from the surface of adsorbents and finally these are carried with WN₂ and vented to the atmosphere. The total bed is reactivated again and cooled to process temperature by purging with simple waste nitrogen.

4. DISTILLATION:

Cryogenic separation is achieved by iterative distillation.

Principles: 

Distillation is carried out by using the property of volatility where two fundamental principles are followed.

I)   If a liquid mixture is heated and partially evaporated then the remaining liquid will be richer in less volatile component  and the resulting vapour will be richer in more volatile component.

II) Conversely, if a vapour mixture is partially condensed then the remaining vapour will be richer in more volatile component and the resulting liquid will be richer in less volatile component.

In cryogenic separation process, distillation is carried out in perforated trays (or in packed columns) inside the column where a liquid trickles down from top to bottom and a vapour goes up from bottom to top. Both the fluid make their passage through trays (as in Oxygen Plant, DSP) and come in direct contact with each other in each tray. In air separation process, these liquid & vapour are basically a binary mixture of oxygen and nitrogen. The point to be remembered is that the liquid is at its vaporization point and the vapour is at its condensation point.

          Now, let us consider one single tray T                                                ^{vapour                   lower temperature zone}

where liquid trickles down through the runner

                                                                                                      Column

of  T^t and vapour comes out from T_b . Liquid               T^t

                                                                                                              ^liquid 

is held in the tray by the vapour pressure below             T

T and overflows the weirs and runs down the                 T_b                             ^Weir

                                                                                                     Runner

runner of T to T_b. Vapour from T_b, on the other

hand, gets through the perforations of T and then                                           

                                                                                            _{higher temperature zone}

bubbles through the liquid in T to reach the bottom of T^t. During this process, the vapour, being a little warmer than the liquid, gives away heat and condenses partially. And hence becomes richer in N₂ and poorer in O₂ (Principle II). Conversely, the liquid, gains heat & vaporises partially, hence becoming richer in O₂ and poorer in N₂ (Principle I). The result is

                   I)  Vapour of T is richer in N₂ but poorer  in O₂, than vapour of T_b.

                   II) Liquid of T_b is richer in O₂ but poorer  in N₂, than liquid of T.

This way, as the vapor moves up N₂ concentration increases and as the liquid goes down O₂ concentration increases.

The distillation unit consists of the following equipments:

         

(1)  Main Exchanger (E01):   (four blocks)

GAS	MASS FLOW NM3/hr	PRESSURE Kg/cm2(a)	TEMPERATURE, 0 C
			INLET	OUTLET
Air	59,000	6.71	15	-133, -170
Oxygen	9,700 (11,000max)	1.68	-178	13
Nitrogen	15,000	1.34	-180	13
Waste Nit.	33,565	1.39	-180	13

                         

(2) Expansion Turbine (D01 & D02):

                        Type                     :   Reaction inward flow

                   Mass flow            :   12,915 NM³/hr_max          

                   Inlet air                          :   6.65 kg/cm²(a) at –133⁰ C

                   Outlet air              :   1.5 kg/cm²(a)   at –173⁰ C

                   Speed                             :   16,521 r.p.m.

                   Power generation :   200 KW

(3) Medium Pressure column (K01):

                        No. of trays                   :   44 (Al)

                   Operating Pressure        :   6.53 kg/cm²

                   Operating Temp.  :   -170⁰ C

(4) Low Pressure column (K02):

                        No. of trays                   :   85 (Al)

                   Operating Pressure        :   1.68 kg/cm²

                   Operating Temp.  :   -178⁰ C

(5) Pure Nitrogen Column (K03):

                   No. of trays                   :   15 (Al)

                   Operating Pressure        :   1.44 kg/cm²

                   Operating Temp.  :   -193⁰ C

           

(6) Main Vaporizer, or Condenser cum Reboiler (E03):

(four blocks)

Fluid	MASS FLOW NM3/hr	PRESSURE Kg/cm2 (a)	TEMPERATURE, 0 C
			INLET	OUTLET
Oxygen (shell)	33,948	1.80	-178	-178
Nitrogen (tube)	47,746	6.76	-176	-176

(7) Liquid Oxygen Filters (R03/04):

                        Mobil Sorbid : 3-5 mm balls

(8) Rich Liquid Sub-cooler (E04): (one blocks)

Fluid	MASS FLOW NM3/hr	PRESSURE Kg/cm2 (a)	TEMPERATURE, 0 C
			INLET	OUTLET
Liq. Oxygen	5,900	1.68	-178	-178
Rich Liquid	27,995	6.50	-176	-176

(9) Sub-cooler (E05):

Fluid	MASS FLOW NM3/hr	PRESSURE Kg/cm2 (a)	TEMPERATURE, 0 C
			INLET	OUTLET
LOX	650	1.53	-178	-185
LIN	9,750	6.36	-176	-189
Pure N2	15,000	1.39	-193	-180
WN2	33,565	1.44	-191	-180
PL	8,340	6.48	-175	-186

  

(10) Liquid Oxygen Pumps (P03/P04):

                   Mass Flow           :  1050 NM³/hr

                   Inlet Pressure                 :  2.80 kg/cm² (a)

                   Outlet Pressure              :  3.16 kg/cm² (a)

                   Speed                             : 1450 r.p.m.