sample

        $_helper = Mage::helper('catalog/output');
        $add = new Mage_Catalog_Block_Product_View_Attributes();
        $add->setProduct($productId);
        

        $_product = Mage::getModel('catalog/product');
        $_product->setStoreId($storeId);
        $_product->load($productId); 
        
        
        Mage::register('product', $_product);
        $_additional = $add->getAdditionalData();
        
        foreach ($_additional as $_data): 

        $_attribute = $_product->getResource()->getAttribute($_data['code']);
            if (!is_null($_product->getData($_attribute->getAttributeCode())) && ((string)$_attribute->getFrontend()->getValue($_product) !='') && (!is_null($_data['label'])) && (!is_null($_data['value'])))
        {
        $produs->appendChild($xml->createElement(str_replace(" ","_", preg_replace("/[^A-Za-z0-9_ ]/", "",$_data['label'])), str_replace($replace_from, $replace_to, strip_tags($_helper->productAttribute($_product, $_data['value'], $_data['code'])))));
        } 
        endforeach; 
        
        Mage::unregister('product');