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THE USE OF SEMI-OPEN REFRIGERATION CYCLE FOR CRYSTALLISATION FROM AQUEOUS SOLUTION

Objective

THE WORK WILL INVOLVE THE MEASUREMENT OF HEAT AND MASS TRANSFER RATES FROM EVAPORATING DROPLETS AND CONDENSING BUBBLES TO AN IMMISCIBLE LIQUID. THIS IS A FIELD OF STUDY THAT HAS BEEN LARGELY IGNORED AND WILL FORM THE BASIS OF A STUDY INTO CRYSTALLISATION TECHNIQUES. SINCE THE PROPOSED SYSTEM RELIES ON INTIMATE CONTACT BETWEEN TWO IMMISCIBLE FLUIDS, THE SEPARATION OF THE FLUIDS INTO TWO PURE COMPONENT STREAMS WILL BE INVESTIGATED.

THE ACID RECOVERY PROCESSES CURRENTLY EMPLOYED IN THE STEEL INDUSTRY ARE HIGHLY ENERGY INTENSIVE. THIS SEMI-OPEN REFRIGERATION CYCLE WOULD, BY THERMODYNAMIC CALCULATION, REDUCE THE ENERGY CONSUMPTION BY APPROXIMATELY 97%, WITH AN ENERGY COST SAVING OF 90%. THE SAVINGS ARE ALSO INCREASED BY PREHEATING THE RECOVERED ACID RETURNED TO THE PICKLE LINE. SUBSTANTIAL CAPITAL COST SAVINGS EVER PRESENT METHODS ARE ALSO EXPECTED.
The feasibility of utilising a semiopen refrigeration cycle (SORC) for the crystallisation of inorganic salts from an aqueous mother liquor, with subsequent energy recovery was investigated.
The trial consisted of tests on both laboratory and pilot plant scales, utilising n-butane as the refrigerant. Specifically investigated during the trials were the effect of nozzle design and temperature on evaporation and condensation, the temperature distribution in the condenser, the effect of varying the condensation pressure, the separation rate for the liquid components, the rate of oxygen buildup in the system, and the physical and chemical condition of crystals formed. During the trials all investigations were satisfactorily completed and the results successfully indicate the ability of the system to achieve the desired heat transfer and separation rates.
With an alteration of condensation pressure and choice of refrigerant it should be possible to use the system for the treatment of other aqueous solutions over a wide range of temperature.
The system has a potential application in the recovery of spent sulphuric acid from steel pickling.

The feasibility of utilising a semi-open refrigeration cycle for the crystallisation of inorganic salts from an aqueous mother liquor, with subsequent energy recovery has been demonstrated. Tests were carried out on both laboratory and pilot plant scales, utilising n-butane as the refrigerant. They involved investigations into:

i) the effect of nozzle design and temperature on evaporation and condensation rates;

ii) the temperature distribution in the condenser;

iii) the effect of varying the condensation pressure;

iv) the separation rate for the liquid components;

v) the rate of oxygen build up in the system, and

vi) the physical and chemical condition of crystals formed.

The results indicate the ability of the system to achieve the desired heat transfer and separation rates. Crystals formed were found to be very fine and exhibited little evidence of contamination by the refrigerant. It was possible to reheat the purified liquor very efficiently to approximately the incoming mother liquor temperature, thus confirming the overall thermodynamic efficiencies of this system.
AT PRESENT, IRON COMPOUNDS ARE REMOVED FROM SULPHURIC PICKLE LIQUOR BY EITHER SUBMERGED BURNER OR CHILLING TECHNIQUES. BOTH THESE SYSTEMS ARE HIHGHLY ENERGY INTENSIVE AND OFFER LITTLE SCOPE FOR IMPROVED EFFICIENCY. IT IS PROPOSED TO USE A SEMI-OPEN REFRIGERATION CYCLE (SORC) AS A DIRECT SUBSTITUTION FOR THE VACCUM CHILLING PROCESS CURRENTLY USED. HYDROCHLORIC ACID REGENERATION CAN ALSO BENEFIT FROM THIS APPROACH, AS CAN OTHER CRYSTALLISATION PROCESSES FROM AQUEOUS SOLUTION E.G., SUGAR REFINING.
THE PRINCIPLE UTILISES AN EVAPORATING STREAM OF N-BUTANE DROPLETS TO CHILL SULPHURIC PICKLE LIQUOR TO APPROXIMATELY 6 C THUS CRYSTALLISING OUT THE FERROUS SULPHATE CONTENT AND REGENERATING THE ACID. THE VAPORISED BUTANE IS THEN COMPRESSED AND CONDENSED IN THE DECRYSTALLISED REGENERATED ACID, THUS REHEATING THE ACID. THE BUTANE IS SEPARATED FROM THE ACID AND PARTIALLY FLASH EVAPORATED TO COOL IT BACK TO ITS ORIGINAL LEVEL FOR FURTHER USE.
THE INVESTIGATION WILL NECESSITATE THE DESIGN AND CONSTRUCTION OF A SMALL PILOT PLANT. THE STUDY WILL INCLUDE THE MEASUREMENT AND CONTROL OF PROCESS STREAMS, PREDICTION OF HEAT TRANSFER AND SEPARATION RATES. THE EFFECT OF DISTRIBUTION PARAMETERS IN BOTH THE CRYSTALLISATION AND HEAT RECOVERY SYSTEMS AND THE DEVELOPMENT OF OPERATING PARAMETERS TO GIVE EFFICIENT RECOVERY AT MINIMUM ENERGY LEVELS WILL BE CONSIDERED. THE INFORMATION OBTAINED FROM THE PILOT PLANT WILL INDICATE THE FEASIBILITY OF OPERATING SUCH A SYSTEM AND WILL GIVE THE NECESSARY DATA TO FACILITATE THE TECHNICAL SPECIFICATION OF AN INDUSTRIAL SCALE PLANT.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Swinden Technological Centre
Address
Moorgate
S60 3AR Rotherham
United Kingdom