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Energy and Mass can neither be created nor be destroyed, but can be converted from one form to another” The Law of conservation of energy & Mass has always been a motivation to make efficient processes and is the foundation to all our efforts in researches to improve upon existing processes & systems.
Energy conservation and complete utilization with complete mass recovery from input raw materials, eliminating wastages & effluent in any process to provide a “green solution” has always been a challenge for designers specifically for low value chemicals like liquid hydrochloric acid etc., and is the key to the feasibility of any process for production.
The demand for a greener future, to increase efficiency and lower down the pollutant wastages in any process for a better & healthy future leads to inventions to provide better and green solutions in the industry by converting wastages as by-products, which reduces the disposal cost and on the contrary results in value addition in by-products for the company, directly improving finances & company product list. In other words the performance of any chemical plant is judged in the 1 st part by its effectiveness to convert a raw material into all possible products & managing critical raw materials in-house to keep up the quality, avoid dependency to meet up the production schedule and making cost competitive products, in the 2 nd part by handling the effluent and recovery systems in terms of efficiency and economical feasibility.
In chemical industry/process the limitations become more serious due to economic feasibility of such systems, being a responsible citizen & company we cannot ignore the pollution norms as science and technology is to help us make a better future & not to poison our future, which is a major concern in all industries and leads to minimizing the waste emissions & the ETP treatment load which results in waste handling cost addition.
Dry HCl Gas Generation system is one such example, which resulted in high effluent generation.
The traditional well-known methods successfully implemented are:
1 HCl boiling route.
2 HCl- 98% Supluric acid stripping method.
In both the above methods the effluent generation in the process to form product dry HCl gas is very high and handling the effluent or treatment cost is very high.
In the 1 st process HCl boiling route, dry HCl gas is produced by simple heating in a Re-boiler - packed bed distillation column and condenser-chiller in series in the rectification section as primary gas drying section. In this process bottom product collected is HCl of 20% concentration, as 20 % HCl-water forms an azeotropic mixture, separation of HCl gas and water is not possible in simple distillation.
The following drawbacks were observed in the process.
a) HCl recovery in the form of gas from Raw Material 30% Liquid HCl is only 33% of the total HCl present in raw material, i.e. only approx. 1/3rdof the HCl is recovered in the form of gas from raw material.
b) Bottom product contains 20% HCl liquid, which mean 2/3rd remains in the bottom product as azeotropic mixture and is wasted, which increases the disposal and effluent handling cost at ETP or treatment cost to very high extent before sending it to ETP.
c) Raw material 30% liquid HCl consumption is 10 times of the product gas as only 1/3rd recovery is possible.
d) The effluent generated is 900% of the product or 9 times of the product approximately by weight.
e) Continuous boiling of the raw material takes place resulting steam consumption is accordingly high with high moisture carry over thereby increasing the condenser load and utility, as it may increase the load of final drying.
f) Boiling results in water evaporation as 70% is water in the raw material, which increases the condenser load in the rectification section for moisture removal in gas, as first stage of drying, in case if the efficiency of the condensers drops down or utility availability, the moisture carry over to final drying will be very high and result in high undesirable moisture content in final product.
g) Pressure is generated in the system due to boiling method to approximately 1.7- 1.8kg/cm2 , hence equipment design & safety needs special attention, also high safety-alert systems and precautions leads to high skill labor and overall increasing the equipment and overall operating costs.
h) As the system gets pressurized boiling point elevation is also observed.
In the 2nd process 98% sulphuric acid is added to the column directly followed by a cooler and drying section above the column similar to the 1 stmethod. The basis of the process is dilution of sulphuric acid thereby breaking the H-bonding between HCl and water & releasing HCl in the form of gas. 98% sulphuric acid is highly hygroscopic and hence a vigorous dilution takes place from 98% to around 70-65% sulphuric acid, where high heat of dilution is evolved and raising the temperature close to the boiling point of 70-65% acid. The only advantage of this process is approx. 100% HCl conversion in the form of gas from raw material & being an exothermic process, external heating is not required.
The following problems were faced in the process:
2.a) Although the HCl gas conversion is approx. 100% from raw material the requirement of sulphuric acid is approximately 1.5 times of the raw material.
2.b)The effluent generated is 65-70% sulphuric acid as bottom product and is approximately 2-2.25 times by wt of the raw material.
2.c)Addition of 98% sulphuric acid and dilution in the column to release HCl gas, raises the temperature to the boiling point of the liquid i.e. approximately 150-160 ᵒC. Equipment design and MOC is also a matter of concern to handle the exotherm on a continuous basis, as even PTFE-lined equipments for sulphuric acid dilution requires high maintenance and prone to break down.
2.d)Effluent requires special MOC for handling, storing and transportation, being highly corrosive or treatment before sending to ETP cost is very high.
2.e)Regeneration of the effluent is not feasible as sulphuric acid concentration above 70% requiresspecial MOC and design to handle vapors forming oleum, temperature and pressure handling equipment, as temperature required for concentration is above 300ᵒC etc.
2.f) Unreleased HCl from 30% HCl raw material is carried over with the bottom product, adding further impurity in the bottom product.
2.g)Maintenance and safety hazards needs special attention as the top and bottom products are highly corrosive & requires skilled labor, increasing the overall equipment and operating cost.
Calcium Chloride route:
The challenge to make a system efficient enough to handle the difficultiesfaced in the above processes lead to innovation of a new process taking Calcium Chloride as a substitute to concentrated sulphuric acid, achieving a “Green Technology Solution” for the process.
Challenge was to find a substitute to sulphuric acid for breaking the azeotrope of HClWater at 20% and at the same time the regeneration of the solute to recycle it to the system thereby reducing the effluent discharge & disposal/treatment costs. Calcium chloride being a highly hygroscopic salt is available at a concentration of 28-30% in solution form under atmospheric conditions, it cannot be handled with bare hands in solid state under atmospheric conditions as the dilution of salt is also exothermic, the most essential part was the regeneration possibility of Calcium chloride, exotherm created during dilution of salt & the optimum concentration with quantity required for addition in the column to readily dissolve in water and release HCl gas from the Raw Material 30% HCl to break the azeotrope.
Theoreticalstudy & lab analysis with practical implementation on plant level, confirmed the concentration of 50-55% as the optimum concentration with a boiling point temperature of 130-134ᵒC (varies with different grade and purity) can be feed to the column to start dissolving/absorbing water readily, being highly hygroscopic, as indicated the concentrated Calcium chloride solution under atmospheric conditions readily dissolves in water to give 28-30% solution on wt basis, accordingly the dilution of concentrated calcium chloride in the process can be expected to be around 40% which can be concentrated again to the required and recycled to the system, it is a continuous process with a HCl gas generator & drying section and Calcium chloride regenerator section.
Above block flow-diagram represents the entire green technology process of Dry HCl gas generation & drying system and calcium chloride regeneration system on continuous basis.
The beauty of the system is it separate’s both HCl(in gas form) and water(in liquid form) effectively and with minimum carry over at both ends. Due to lack in optimization of the system on plant level & inexperience in handling the system on plant level the acceptability was low initially, however today in India itself there are 4 plants running successfully with capacities ranging from 25Kg/hr to 100Kg/hr.
Following are the advantages in comparison to other 2 processes described above:
3.a) Conversion of HCl gas from 30% liquid HCl raw material is 100%.
3.b) The effluent generated is maximum 2% HCl as a condensate from calcium chloride regeneration column, which can be easily pretreated by simple caustic dosing before sending to ETP.
3.c) Calcium chloride used is regenerated and recycled to the system on continuous basis in the system.
3.d) As the dehydration is due to salt dilution, pressure is not developed in the system & hence reducing a design parameter.
3.e) As the pressure is not developed, comparatively providing safer operation, reducing leakages and maintenance possibility.
3.f) Overall cost in handling the process, including raw material costs and effluent handling is comparatively very low.
3.g) The chances of moisture carry over under any parameter change or steam fluctuations etc. are comparatively lower, as in boiling route moisture carryover is maximum to the drying section and in sulphuric acid route due to dilution of sulphuric acid the operating temperature rises very close to boiling point of the mixture, which some time may result in carry-over of oleum along with moisture resulting in highly corrosive conditions in vapor form and adding impurity to the product Dry HCl gas.
Above comparison chart shows the Raw Material Requirement & effluent generation in all the 3 processes for a basis of 100 Kg/Hr Dry HCl gas generation system.
It is clear from the above graph that the Raw material requirement & effluent generated in “Boiling route” to produce dry HCl gas is high. In “Sulphuric Acid Stripping” Method however the Raw material requirement is reduced to approximately 1/3rd but the effluent generated is approx. 5% more than “Boiling route” with a mix of Sulphuric acid and HCl, Hence in both the cases effluent treatment and handling cannot be ignored & definitely add up in the overall cost.
Comparing Calcium Chloride route for production of Dry HCl gas to “Sulphuric acid route”, raw material requirement is equivalent, but effluent generated is reduced by approx. 75% w/w and the acid concentration of the effluent is reduced to max. 2% HCl, reducing the disposal and handling cost to a great extent.
Comparing Calcium Chloride route for production of Dry HCl gas to Boiling Route, Raw material requirement is reduced to approx. 66% w/w and effluent generated is reduced by 74% w/w, with acid concentration of the effluent is reduced from 20-22% HCl to 2% HCl, here the raw material cost and disposal & handling costs both are reduced to considerably high amount. The efficiency, sustainability, safety & maintenance of the process will be enhanced by use of graphite as an energy efficient MOC and the change will help the chemical process industries to adopt green chemistry for nation’s development.
Name of the author: Mr. Dhawal Saxena
Designation of the author: Chief Executive Officer & Chief Technology Officer- Bhumistha Infra Services • Director Abha Animation and e-Solutions. • National Council Member Registrar IIChE • National Corrosion management committee Member CII 2023-2024 • Hon. Secretary IIChE MRC. • Past Vice President Indian Carbon Society Maharashtra Chapter. • Committee Member constitution Committee Indian Carbo Society. • Committee Member MED-17, BUREAU OF INDIAN STANDARDS. • Co-Ordinator Lean Six Sigma Project-based Internships at IIChE HQ. Organization: BHUMISTHA INFRA SERVICES.
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