السوفتنر والمبادلات الانيونية والميسرات

[size=undefined]Softening
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اقتباس :

[center][left][center] Ion Exchange Softening

Introduction

Softener Design

The other method commonly used for water softening is ion exchange softening, also known as zeolite softening. Ion exchange softening exchanges calcium and magnesium ions in water for sodium ions as the hard water passes through a softener. The softener is similar in design to a pressure filter, with resins in place of the filter media.

Ion exchange softener

During treatment, water enters the softener and is directed by a baffle. The water passes through a bed of resin underlain by a bed of gravel, then is collected by an underdrain and piped out of the softener.

Softener Operation

Despite the superficial resemblance between softeners and filters, the two operate in very different manners. In a softener, the bed is made up of resins, which are insoluble solids with attached cations or anions capable of reversible exchange with mobile ions of the opposite sign in the solutions in which they are brought in contact.

In the case of the ion exchange resins used in softening, sodium ions are attached to the insoluble solids of the resins. When water passes through the softener, the sodium ions are exchanged for calcium and magnesium ions in the water. The calcium and magnesium ions are retained on the resin grains. The water leaving the softener has sodium ions in the place of calcium and magnesium ions in its compounds, as shown below.

السوفتنر والمبادلات الانيونية والميسرات Softdiagram

Since sodium ions do not cause hardness, the treated water is no longer hard.

In a properly operating softener, the treated water will have a hardness of 0 mg/L. To prevent corrosion due to excessively soft water, some of the source water is bypassed and does not pass through the softener. This untreated water is blended back into with the water downstream of the softener using a valve and meter to ensure that the proper quantity of water bypasses the softener.

There are many types of resins which can be used in ion exchange units. One type of resin commonly used in water softening is zeolite resin, which is why ion exchange softening is often known as zeolite softening. When other types of resins are used, ion exchange units can be used to remove minerals in brackish water or to remove alkalinity from water.

A scientific understanding of ion exchange did not begin until about 1850 when two English agricultural chemists, Harris S. Thompson and John T. Way, observed an exchange between ammonium and calcium in the soil. Twenty-six years later, Lemberg identified the resins involved and showed that the exchange of cations was reversible. It was soon discovered that various natural minerals, called zeolites, had exchange characteristics suitable for the softening of water.

In 1904, a German chemist by the name of Gans produced the first artificial resins. These artificial zeolites had effective capacities two to three times those of the naturally occurring minerals. Synthesis of these zeolites stimulated the industrial development of water softening. Later developments of other types of resins expanded the use of ion exchange to include the removal of other substances from water.

Softener Cleaning

Introduction

When all of the sodium ions on the softener resin have been replaced by calcium and magnesium ions, the softener must be recharged. The recharge process involves backwash, followed by regeneration, and ending in rinse.

Backwashing

Backwashing the softener is very similar to backwashing a pressure filter and the purpose is the same. Although the purpose of a softener is not to filter out particulate matter, some particles inevitably get caught in the softener. By sending water backwards through the softener, this particulate matter is removed.

Regeneration

After backwashing, the softener is ready to be regenerated. This is the part of the process in which the magnesium and calcium ions on the resin become replaced with sodium so that the softener can be used to treat more hard water. In order to regenerate the resin, a salt solution, known as brine, is allowed to flow through the softener for about an hour.

The salt used to regenerate the resin is ordinary table salt (sodium chloride, NaCl), so it is easy to handle. When dissolved in water, the salt dissociates into its constituent ions - Na⁺ and Cl.^- The sodium ions replace the calcium and magnesium ions on the resin in the following manner (where "R" preceding an ion means that the ion is bound to the resin.)

RCa + NaCl → RNa + CaCl₂

RMg + NaCl → RNa + MgCl₂

So the regeneration process can be summarized as follows:

السوفتنر والمبادلات الانيونية والميسرات Softdiagram2

During regeneration, calcium chloride, magnesium chloride, and excess sodium chloride flow to waste.

Rinse and Waste Disposal

After the brine has been given a sufficient contact time, it must be rinsed out of the softener. During the rinse cycle, fresh water is passed through the unit as it would be during treatment, but with the effluent going to waste. Rinse usually takes about 20 to 40 minutes.

Both the spent brine from regeneration and from the rinse must be disposed of carefully since the calcium, magnesium, and sodium salts are corrosive and toxic to the environment. Spent brine is sometimes discharged in sewers or into streams at very high dilutions. Alternatively, the brine can be disposed of in a landfill.

In the Treatment Process

Location in the Plant

Ion exchange softening requires influent water free of turbidity since particulate matter can clog the resin bed. As a result, it is often one of the last steps in the treatment process, following flash mixing, flocculation, sedimentation, and filtration. As with lime-soda ash softening, ion exchange softening can be problematic when dealing with surface water with changing water characteristics.

Ion exchange softening is used widely in small water treatment plants and in individual homes. The process has many advantages for these installations, including its compactness, simplicity, and low cost. Operation of the unit can be nearly automatic and the chemicals used are relatively safe and easy to handle.

Monitoring

Like any other treatment process, ion exchange softening requires a certain degree of water quality monitoring. The following water quality characteristics should be tested:

Hardness of influent water influences the time between cleanings. The more calcium and magnesium compounds are present in the water, the more quickly the sodium bound to the resin becomes used up and the more often the softener must be regenerated.
Iron and manganese in the influent water will be caught in the filter bed and can plug it up. Iron and manganese should always be removed before water is softened.
Sodium in the influent water will react with the softener's resin as it would during regeneration, resulting in magnesium and calcium leaking through into the treated water.
Chlorine residual of the influent water should be monitored since an excessive chlorine residual can damage the resin.
Langelier Index should be monitored in the effluent to ensure that corrosive water is not being released to the distribution system.

Efficiency

In addition to raw water characteristics, the operation of the softener can influence efficiency. The most important factors are resin, flow, and cleaning.

Both the resin type and depth influence softening. There are many types of resins, each with different removal abilities, so the type of resin used will determine what can be removed from water. The depth of the resin bed will influence how much hardness can be removed from the water, with deeper beds removing more hardness.

Operation of the softener during the softening phase can also influence efficiency. Specifically, the flow rate of the water through the softener influences how much hardness is removed.

The efficiency of regeneration greatly affects operation of the softener during the softening phase. The salt dosage during regeneration, the brine concentration, and the brine contact time all influence how well sodium is regenerated on the resin surface. If regeneration is not complete, then the softener will not operate as long before it requires another round of cleaning.

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جهاز السوفتنر
هو عباره عن منظومة تعمل باستخدام مادة الريزن لإزالة الايونات الموجبة++ca,++mg الموجودة في مياه الإسالة

نستخدم المحلول الملحيNacl لإعادة تنشيط مادة الريزن

المكونات:

- أعمدة التبادل ألايوني من الفايبر كلاص أو البليت المغلون المطلي بمادة الايبوكسي
- خزان الملح مع الخلاط
- مضخة سحب الملح
- شبكة الغسل وصمامات التحكم
أما بالنسبة للجهاز فهو عبارة عن خزان يحتوي كمية من الريزن ( وهو عبارة عن جزيئات متصلة فيها ذرات من الصوديوم) في حال مرور الماء العسر خلال الريزن سيتم استبدال ذرات الصوديوم بذرات الكاليسوم والمغانيسوم وهي المسمى بأملاح العسرة وذلك لأنها غير ذائبة في الماء بل تترسب مما تسبب مشاكل خاصة في الأنظمة الحرارية مثل جهاز الستيم كذلك تترسب على الجلد فحين يتم استبدالها بذرات الصوديوم الذائب نجد حل لهذه المشاكل. علما أن هذا الريزن يصل إلى حالة إشباع بعد مدة زمنيه فلذلك نعمل له إعادة تنشيط باستخدام ملح الصوديوم وتتم هذه العملية اتوماتيكيا

يتكون جهاز السوفتنر من

- رأس التحكم (والذي يتحكم بعملية التنشيط)
- خزان الرزن (والذي يمر الماء من خلاله ويتم فيه تبادل ذرات الكاليسيوم والمغنيسيوم الموجدوة في المياه العسرة بذات الصوديوم المرتبطة بالريزن)
- خزان الملح ( والذي يأخذ الملح منه خلال عملية التنشيط)

طريقة فحص الريزن الموجب والسالب

يتم أخذ حجم 10 مللتر ريزن

يتم عمل تنشيط كامل الموجب بحامض HCl تركيز 4 % والسالب بالصودا الكاوية تركيز 4%
بعد الشطف والتأكد من شطف الحامض بواسطة نترات الفضه .
والصودا الكاوية بكاشف الفينول فثالين يتم امرار محلول كلوريد الصوديوم 160 مللتر من خلال الريزن ببطء

يؤخذ الناتج من الريزن الموجب ويعاير بمحلول 1 عياري من الصودا الكاوية

يؤخذ الناتج من الريزن السالب ويعاير بمحلول 1 عياري من حامض الهيدروكلوريك

ملاحظه يستخدم كاشف الفينول فثالين للمعايرة

بعد الانتهاء من اضافة المحلول الملحي يضاف 30 مللتر اسيتون على الريزن لطرد كامل كمية المحلول الملحي

يتم حساب المللترات المكافئة الناتجة من عمليتي المعايرة وتكون هي نتيجة الكفاءه للريزن بعد القسمة على 10 مللتر طبعا حيث تكون النتيجة النهائية مللترمكافئ لكل سم3 من الريزن

وللعلم فقط كفاءة الرزن الموجب تكون اكثر من 2.5 بينما كفاءة الريزن السالب اكثر من 1 اي اذا قلت الكفاءه عن هذه القيم يكون الريزن تالف
يسمى (deminerlized water plant) وليس (softner) هذا اولا اما بالنسبة للريزن المستخدم فاذا كان ريزن جديد وغير مستخدم فيتم اجراء عملية اعادة الفعالية له

(regeneration) باستخدام حامض الكبريتيك بتركيز 4% بالنسبه لcation

تخفيف الحامض ومن المهم جدا اضافة الحامض على الماء وليس العكس حيث يتم تخفيف الحامض من تركيز 98 % الى تركيز 4 % بالتخفيف عدد 25 مرة تقريبا اي ان كغم من الحامض يضاف الى 25 لتر ماء .

وباستخدام الصودا الكاويه بتركيز 4% بالنسبه ل (anion)

اما الصودا فيضاف تقريبا 2 كغم الى 25 لتر ماء وهكذا يكون التركيز المطلوب .

اما بالنسبة لكمية الحامض والقاعده فهذه يمكن حسابها من مواصفات الريزن المرفقه معه

(regeneration level) وبعدها يتم شطف الريزن للتخلص من بقايا الحامض والقاعده فيه وبعدها يتم تكرار هذه العملية(double regeneration) لهما للتاكد من اعده فعاليتهما بالكامل .

اما اذا كان الريزن مستعمل فيجب ان يكون تركيز الحامض في البدايه 2% وبعدها نرفع التركيز الى 4% وهذا بالنسبه للحامض فقط اما القاعده فتبقى نفس التركيز السابق ويتم اجراء هذه العمليه مره واحده فقط في هذه الحاله

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